1937 Flathead Won't Run Well
- Thread starter PecosBill
- Start date
In regards to the power valve, they can look perfect and still seep. I finally filled them with gas and let them sit on a couple bolts and you could see it well up around the pv. I took mine to a buddy with a small gun lathe and he squared up the radius on the mating surface. After that it worked fine.
I have not been able to spend much time on it lately, but I have checked a few things.
Accelerator pump seems to work. I wouldn't call it a real strong squirt, but it does squirt fuel.
One of the questions I got was whether the spark plugs were connected to the right distributor terminals. Since the wires are not easy to follow through the metal tubes, I used my ohm meter at each end. During this test I discovered one plug wire was measuring in the M-ohm range, and all the others were around 1 k. I'm quite certain that cylinder was missing. So I replaced that wire and it is now like the rest. Runs a little better but still not very good.
I tried to keep it running long enough to look down the carburetor. I don't see fuel spilling in at idle, and I do see a lot of fuel spraying when trying to rev it.
I see some wetness around the base of the carb so I pulled the carb and base. I had not removed the base before, and it will definitely benefit from a new gasket at the manifold. My kit did not include one, and the old one looks like it has metal in it, so I will probably need to buy one and not make one.
The base needs to be touched up for flattening, and I also noticed that there are low spots in the surface on each side, right over where the vacuum is ported through the base (see pics). It could be a sink defect of the casting process, but I suspect this should be flat. I think it definitely could be losing vacuum there. I may need to get a new base or use some sealant with the new gaskets just to be sure it seals up well.
I have not checked the power valve yet, that will be next.
It is still hard starting, hard to keep running, and backfires when trying to rev and keep running.
Thanks again for the tips, I'm knocking them out slowly but surely.
Accelerator pump seems to work. I wouldn't call it a real strong squirt, but it does squirt fuel.
One of the questions I got was whether the spark plugs were connected to the right distributor terminals. Since the wires are not easy to follow through the metal tubes, I used my ohm meter at each end. During this test I discovered one plug wire was measuring in the M-ohm range, and all the others were around 1 k. I'm quite certain that cylinder was missing. So I replaced that wire and it is now like the rest. Runs a little better but still not very good.
I tried to keep it running long enough to look down the carburetor. I don't see fuel spilling in at idle, and I do see a lot of fuel spraying when trying to rev it.
I see some wetness around the base of the carb so I pulled the carb and base. I had not removed the base before, and it will definitely benefit from a new gasket at the manifold. My kit did not include one, and the old one looks like it has metal in it, so I will probably need to buy one and not make one.
The base needs to be touched up for flattening, and I also noticed that there are low spots in the surface on each side, right over where the vacuum is ported through the base (see pics). It could be a sink defect of the casting process, but I suspect this should be flat. I think it definitely could be losing vacuum there. I may need to get a new base or use some sealant with the new gaskets just to be sure it seals up well.
I have not checked the power valve yet, that will be next.
It is still hard starting, hard to keep running, and backfires when trying to rev and keep running.
Thanks again for the tips, I'm knocking them out slowly but surely.
Attachments
FlatheadTed
Well-known member
Just my observation ,it appears to be starving for fuel in both carbs you have tried . ,blow out the intermediate jets with compressed air at least 100 psi ,also inlet manifold could be leaking . the plugs i think are a separate issue Long periods of cold Idling can cause sooty plugs with the fuel we have now ,
TerryOH
Well-known member
There may be vacuum leaks that are making the car run lean. Your intake manifold appears to be 1941 with the port under the carb. and the special vacuum bolt in the port. It should be fairly easy to check for vacuum leaks with the engine running poorly do check the special bolt that it is not cracked and leaking. Do leave the vacuum plate as shown in post 23 off.
Ok, so I left the base plate out and put the carb back on. I think no baseplate has to be an improvement but it still basically does the same thing. Hard to start, hard to keep running, and when I can finally get some RPM on it (which takes about 30 seconds of feathering the throttle and choke) it starts to backfire.
Several issues have been corrected now which probably could have caused any engine to run not so good. So I put the other carb on (off the spare Merc motor that has been sitting for 20 years) just to see if it may run better now that other stuff is fixed. It actually did run better. I finally got it to rev up and smooth out. And it idled well enough to check the timing which was almost 10 btdc. So I changed the timing back to 4 btdc. By then my shop was so full of fumes I had to take a break and let it air out. Later in the evening I went out to fine tune the timing a little more and the engine was back to its old tricks. Hard to start, hard to keep running, hard to rev up. My fuel pressure regulator is 2.5 - 3 psi right now.
The mufflers are Smitty glasspacks, so I don't think they are restricting me.
I will check that vacuum bolt for leaks and blow out the intermediate jets. And I still haven't checked the power valve on either carb so I guess that will be next too.
And after that maybe get a new carb kit from Daytona.
Thanks.
Several issues have been corrected now which probably could have caused any engine to run not so good. So I put the other carb on (off the spare Merc motor that has been sitting for 20 years) just to see if it may run better now that other stuff is fixed. It actually did run better. I finally got it to rev up and smooth out. And it idled well enough to check the timing which was almost 10 btdc. So I changed the timing back to 4 btdc. By then my shop was so full of fumes I had to take a break and let it air out. Later in the evening I went out to fine tune the timing a little more and the engine was back to its old tricks. Hard to start, hard to keep running, hard to rev up. My fuel pressure regulator is 2.5 - 3 psi right now.
The mufflers are Smitty glasspacks, so I don't think they are restricting me.
I will check that vacuum bolt for leaks and blow out the intermediate jets. And I still haven't checked the power valve on either carb so I guess that will be next too.
And after that maybe get a new carb kit from Daytona.
Thanks.
FlatheadTed
Well-known member
While revving see if you have a yellow spark off the plug wire ,maybe voltage drop ,
Last edited:
rustedorrotting
Member
The power valves supplied today do not set flat on the sealing surface of the carb because they have a radius where they seat Charlie's speed buys power valves in bulk and puts them in a lathe and removes the radius and sells a nylon washer that seals better.
Flathead Fever
Well-known member
When you attempt to start a vehicle that has been sitting for years you can run into multiple problems. I’ll list the things that can be checked for free. Obviously, the fuel has gone bad by now and will need to be changed but the varnish it has left behind can be so bad that it plugs the fuel line and tanks outlet. It can turn to a solid and completely block the flow or restrict it. It can be so bad a coat hanger won’t go into the steel line. The entire fuel line will need to be replaced. I bought ’33 BB truck. I could not get anything to go into that steel fuel line, it was plugged that bad. I replaced the steel fuel line, and I had the tank boiled out at a radiator shop. I even sloshed bolts and nuts around in it and it still would plug the new line within one-block of driving. Here some good advice, always road test uphill from your house so you can coast back home again. Somebody is probably asking what finally fixed it? I sold it before I had a chance to find another tank.
My dads’ flathead ’32 roadster sat for fifteen years, and the fuel turned to muck. It would idle on new fuel but not drive to the end of my driveway without plugging the fuel line coming out of the tank. I have the tank out now. Beside the rust and varnish in there, there were a rusty beer bottle caps. It ran fine with them in there until the fuel turned to vanish. I’m pretty sure it also now has a stuck valve from sitting all those years, it runs like it does. It’s on my to do list behind many other Ford projects. The neighbors’ 1913 Model T, that last ran in the 1940s received a new engine. I was the only one that new how to drive it, so I got to do the maiden voyage. It ran for a few miles and then acted like it was running out of gas because it was. I let it sit awhile and enough fuel would gravity flow into to the carb and I could go a couple of blocks and then quit again. I had to hand crank it about thirty-times and by the time I returned my already bad back wasn’t working at all. That turned into an opioid day. That day, I got the urge to own an early Model T or any vehicle without a starter out of my system. I think I’d rather have the horse the Model T replaced. Same with his ’41 pickup that sat for a few years. It would idle but would not make it up the street. Same with the ladies ’34 pickup down the street that sat for years. I drove it to her new house, a couple miles away using a can of gas and an electric fuel pump. And yes, for some reason there is a strange number of flatheads living on this street with defective gas tanks. All true stories
You could be getting just enough fuel through the line to idle but not enough to drive it. You can disconnect the inlet line to the carb and crank the engine and you should see a good flow of fuel going into a glass container. Disable the ignition but not in a way a plug wire is going to create a spark. And always do this kind of stuff outside so you don’t burndown the garage. You can loosen the gas cap and take a rubber tipped blow gun and force air down the line and into the tank. This might dislodge anything in the line or tank outlet. This would only be a temporary fix. The tank would need to be removed and cleaned. A tank that is rusty inside is the one part I would consider replacing with a reproduction. It's almost impossible to get a nasty tank like new again inside unless you can have it chemically dipped.
What I do when I suspect a plugged line is I place a can of gas with its own low pressure electric fuel pump and filter in the cab and run a rubber fuel hose to the carb and go for a ride. It the problem goes away I know there is blockage in the line or tank, or the fuel pump is bad. It eliminates all that stuff as being the problem and only leaves the carb as the culprit. At least as far as the fuel system is concerned. Just make sure to cap off the outlet fuel line from the original pump so it’s not squirting fuel or your going to have a bad day. You can also leave the gas cap loose if you suspect it's not venting the tank. If the tank is not vented the pump cannot suck fuel out of it.
Once you have established enough fuel is available to the carb you need to find out if it is actually making it into the carb. Another recent example, a freshly built flathead in a friends AV8. It ran around the block for a while and then it ran out of gas. It was the neoprene needle and seats in these new carb kits sticking closed. Either they are not compatible with the new fuel, or they are just garbage to begin with, probably both. Solid metal needle and seats in both carbs fixed the problem. This is a very common problem.
Back when I was working on a fleet of 400 vehicles, they were all carbureted engines and point type ignitors. We did a tremendous amount of carb rebuilds and they always worked when we were done. The one thing a carb should not do is be allowed to dry out. That’s not a problem on a daily driver but it is on a collector car. We had a mechanic at work that had come from a Ford Dealerships. He had been the Ford dealer’s only tune-up man in the 1960s and 1970s. He was sharp on that stuff. He taught me a lot when I was starting out. He had a blow gun with a replaceable tip so he could screw a piece of 3/16 brake line into it. He used the 150 lb. shop air to blow out the passages. Then he took a can of carb spray with the straw nozzle and forced it against every orifice to make sure it sprayed out the other ends of the passages. When he was done, he knew the internals were perfectly clean. He checked the machined surfaces for flatness. The throttle shaft for wear and the linkage for binding. He installed a quality Hygrade brand power valves and accelerator pumps, new gaskets and it worked liked new. every time. The first thing he did after a rebuild was to blow into the carb with his mouth (do not use compressed air or you will crush a brass float like a submarine at crush depth) to make sure the needle and seat was open. While he was blowing, he’d flip the carb upside down and the needle would prevent him from blowing into the carb. One of the things he did that some people might not agree with was to spray all the gaskets with silicone. That prevented them from ever sticking to the parts. On Holley 4V carbs you can spend more time scraping gaskets off the metering blocks and bowls than you do rebuilding the rest of the carb. The silicone spray made the gaskets just peel right off. Because we rebuilt those Holleys almost yearly that was a neat trick. The phenolic base gaskets he coated with white grease so that the base gasket was reusable each time.
If you made it this far you shoud have fuel getting into your carb but maybe something is still not right. You mentioned you adjusted the air/fuel ratio screws all the way in (gently) and it did not kill the engine and it should have. That tells you the engine is getting fuel from somewhere that it shouldn’t at idle. Usually that is a leaking power valve, and it would be running rich which it sounds like it is. I have also seen carbs with cracks leak fuel into the intake, and of course float levels too high or excessive fuel pressure overpowering the needle and seat. With the excessive pressure fuel will be pouring out of float bowl vent and into the venturi. It will usually drown the engine and it will die. The only fuel the engine should be getting at idle is from the idle passages which are below the throttle plate, you cannot see them from the top looking in the carb with the plate closed. The fuel the engine is running on at idle is coming from below that plate. There should be no fuel at idle coming from anywhere above the throttle plate. If you have opened the throttle plate up to compensate for another problem. Enough air could start flowing past transfer ports and suck fuel out of them. Then adjusting your air mixture screws might not make a difference. The throttle plate should just barely be open at idle. Study the basic circuits of a carburetor so you understand under what conditions each of them flow fuel.
Here is where your carb gets a little confusing to me. You also said you had to pull the choke half closed to get it to idle. What is happening when you do that is you are creating an artificially rich condition to get it to idle. The first thing you did with the mixture screws suggested the engine was too rich and then the next thing you did with the choke suggests it’s too lean. I want you to take a propane bottle, carb spray or starting fluid and just add a little into the carb as it is idling and see if it straightens out or if it makes it worse. This will tell you for sure if the engine is not getting enough fuel or it’s getting too much at idle. You can also crack open the propane bottle just a little and run it around the intake, carburetors’ base and throttle shaft.. If the idle changes you have found a vacuum leak. A propane bottle or map gas is fantastic tool for checking air fuel mixture problems.
You mentioned it backfires. If it is backfiring through the carb that can be a sign of a lean condition. A non-operating accelerator pump or one positioned to squirt a low amount of fuel would cause a lean condition resulting in a hesitation and possibly a backfire through the carb, not the exhaust. As the throttle is moved you can easily see it squirt looking down the carb. You do not need to have the engine running. The only time the accelerator pump would cause a lean condition is while the throttle is opening. Once it has opened and is no longer moving the accelerator pump is no longer adding fuel and would not be the source of any problem under steady driving
You said, you had a fluctuating vacuum gauge, that can indicate a valve problem. Diagnosing it depends on the rpm and speed at which the needle fluctuates. Each time the piston goes down on the intake stroke it creates a vacuum (suction). When all cylinders are creating the same vacuum, the gauge is steady. If one cylinder is not creating the same vacuum the gauge momentarily drops when it’s that cylinders turn to suck. If the engine sat it could have a valve sticking. I got one in my garage right now that I have not properly diagnosed with a vacuum gauge, Just the sound of the engine was enough. If it’s an intake valve that happens to stick open the piston will push the mixture back up into the intake and disrupt the incoming flow to the other cylinders. It will run horribly, which the one in my garage does and mine has a loud tapping lifter, another sign of a stuck valve. It ran perfect when it was parked, and the lifters were quiet. A sticky intake valve is my number one suspect. You did a compression check which showed the valves were closing so you do not have stuck open valve, at least not at cranking rpm. It could be intermittently sticking while running. Multipole problem can really mess with your head, but multiple things can and do go wrong on vehicles.
My dads’ flathead ’32 roadster sat for fifteen years, and the fuel turned to muck. It would idle on new fuel but not drive to the end of my driveway without plugging the fuel line coming out of the tank. I have the tank out now. Beside the rust and varnish in there, there were a rusty beer bottle caps. It ran fine with them in there until the fuel turned to vanish. I’m pretty sure it also now has a stuck valve from sitting all those years, it runs like it does. It’s on my to do list behind many other Ford projects. The neighbors’ 1913 Model T, that last ran in the 1940s received a new engine. I was the only one that new how to drive it, so I got to do the maiden voyage. It ran for a few miles and then acted like it was running out of gas because it was. I let it sit awhile and enough fuel would gravity flow into to the carb and I could go a couple of blocks and then quit again. I had to hand crank it about thirty-times and by the time I returned my already bad back wasn’t working at all. That turned into an opioid day. That day, I got the urge to own an early Model T or any vehicle without a starter out of my system. I think I’d rather have the horse the Model T replaced. Same with his ’41 pickup that sat for a few years. It would idle but would not make it up the street. Same with the ladies ’34 pickup down the street that sat for years. I drove it to her new house, a couple miles away using a can of gas and an electric fuel pump. And yes, for some reason there is a strange number of flatheads living on this street with defective gas tanks. All true stories
You could be getting just enough fuel through the line to idle but not enough to drive it. You can disconnect the inlet line to the carb and crank the engine and you should see a good flow of fuel going into a glass container. Disable the ignition but not in a way a plug wire is going to create a spark. And always do this kind of stuff outside so you don’t burndown the garage. You can loosen the gas cap and take a rubber tipped blow gun and force air down the line and into the tank. This might dislodge anything in the line or tank outlet. This would only be a temporary fix. The tank would need to be removed and cleaned. A tank that is rusty inside is the one part I would consider replacing with a reproduction. It's almost impossible to get a nasty tank like new again inside unless you can have it chemically dipped.
What I do when I suspect a plugged line is I place a can of gas with its own low pressure electric fuel pump and filter in the cab and run a rubber fuel hose to the carb and go for a ride. It the problem goes away I know there is blockage in the line or tank, or the fuel pump is bad. It eliminates all that stuff as being the problem and only leaves the carb as the culprit. At least as far as the fuel system is concerned. Just make sure to cap off the outlet fuel line from the original pump so it’s not squirting fuel or your going to have a bad day. You can also leave the gas cap loose if you suspect it's not venting the tank. If the tank is not vented the pump cannot suck fuel out of it.
Once you have established enough fuel is available to the carb you need to find out if it is actually making it into the carb. Another recent example, a freshly built flathead in a friends AV8. It ran around the block for a while and then it ran out of gas. It was the neoprene needle and seats in these new carb kits sticking closed. Either they are not compatible with the new fuel, or they are just garbage to begin with, probably both. Solid metal needle and seats in both carbs fixed the problem. This is a very common problem.
Back when I was working on a fleet of 400 vehicles, they were all carbureted engines and point type ignitors. We did a tremendous amount of carb rebuilds and they always worked when we were done. The one thing a carb should not do is be allowed to dry out. That’s not a problem on a daily driver but it is on a collector car. We had a mechanic at work that had come from a Ford Dealerships. He had been the Ford dealer’s only tune-up man in the 1960s and 1970s. He was sharp on that stuff. He taught me a lot when I was starting out. He had a blow gun with a replaceable tip so he could screw a piece of 3/16 brake line into it. He used the 150 lb. shop air to blow out the passages. Then he took a can of carb spray with the straw nozzle and forced it against every orifice to make sure it sprayed out the other ends of the passages. When he was done, he knew the internals were perfectly clean. He checked the machined surfaces for flatness. The throttle shaft for wear and the linkage for binding. He installed a quality Hygrade brand power valves and accelerator pumps, new gaskets and it worked liked new. every time. The first thing he did after a rebuild was to blow into the carb with his mouth (do not use compressed air or you will crush a brass float like a submarine at crush depth) to make sure the needle and seat was open. While he was blowing, he’d flip the carb upside down and the needle would prevent him from blowing into the carb. One of the things he did that some people might not agree with was to spray all the gaskets with silicone. That prevented them from ever sticking to the parts. On Holley 4V carbs you can spend more time scraping gaskets off the metering blocks and bowls than you do rebuilding the rest of the carb. The silicone spray made the gaskets just peel right off. Because we rebuilt those Holleys almost yearly that was a neat trick. The phenolic base gaskets he coated with white grease so that the base gasket was reusable each time.
If you made it this far you shoud have fuel getting into your carb but maybe something is still not right. You mentioned you adjusted the air/fuel ratio screws all the way in (gently) and it did not kill the engine and it should have. That tells you the engine is getting fuel from somewhere that it shouldn’t at idle. Usually that is a leaking power valve, and it would be running rich which it sounds like it is. I have also seen carbs with cracks leak fuel into the intake, and of course float levels too high or excessive fuel pressure overpowering the needle and seat. With the excessive pressure fuel will be pouring out of float bowl vent and into the venturi. It will usually drown the engine and it will die. The only fuel the engine should be getting at idle is from the idle passages which are below the throttle plate, you cannot see them from the top looking in the carb with the plate closed. The fuel the engine is running on at idle is coming from below that plate. There should be no fuel at idle coming from anywhere above the throttle plate. If you have opened the throttle plate up to compensate for another problem. Enough air could start flowing past transfer ports and suck fuel out of them. Then adjusting your air mixture screws might not make a difference. The throttle plate should just barely be open at idle. Study the basic circuits of a carburetor so you understand under what conditions each of them flow fuel.
Here is where your carb gets a little confusing to me. You also said you had to pull the choke half closed to get it to idle. What is happening when you do that is you are creating an artificially rich condition to get it to idle. The first thing you did with the mixture screws suggested the engine was too rich and then the next thing you did with the choke suggests it’s too lean. I want you to take a propane bottle, carb spray or starting fluid and just add a little into the carb as it is idling and see if it straightens out or if it makes it worse. This will tell you for sure if the engine is not getting enough fuel or it’s getting too much at idle. You can also crack open the propane bottle just a little and run it around the intake, carburetors’ base and throttle shaft.. If the idle changes you have found a vacuum leak. A propane bottle or map gas is fantastic tool for checking air fuel mixture problems.
You mentioned it backfires. If it is backfiring through the carb that can be a sign of a lean condition. A non-operating accelerator pump or one positioned to squirt a low amount of fuel would cause a lean condition resulting in a hesitation and possibly a backfire through the carb, not the exhaust. As the throttle is moved you can easily see it squirt looking down the carb. You do not need to have the engine running. The only time the accelerator pump would cause a lean condition is while the throttle is opening. Once it has opened and is no longer moving the accelerator pump is no longer adding fuel and would not be the source of any problem under steady driving
You said, you had a fluctuating vacuum gauge, that can indicate a valve problem. Diagnosing it depends on the rpm and speed at which the needle fluctuates. Each time the piston goes down on the intake stroke it creates a vacuum (suction). When all cylinders are creating the same vacuum, the gauge is steady. If one cylinder is not creating the same vacuum the gauge momentarily drops when it’s that cylinders turn to suck. If the engine sat it could have a valve sticking. I got one in my garage right now that I have not properly diagnosed with a vacuum gauge, Just the sound of the engine was enough. If it’s an intake valve that happens to stick open the piston will push the mixture back up into the intake and disrupt the incoming flow to the other cylinders. It will run horribly, which the one in my garage does and mine has a loud tapping lifter, another sign of a stuck valve. It ran perfect when it was parked, and the lifters were quiet. A sticky intake valve is my number one suspect. You did a compression check which showed the valves were closing so you do not have stuck open valve, at least not at cranking rpm. It could be intermittently sticking while running. Multipole problem can really mess with your head, but multiple things can and do go wrong on vehicles.
Last edited:
TerryOH
Well-known member
I often see an extra hole in the surface of the intake just under the Carb. That hole leads to the heat chamber fed by the exhaust. If the gasket under the Carb does not block off this hole the heat will fry the 94 Carb power valve very quickly. This will cause hard start and over rich idle. The 37 used the Stromberg 97 primarily but some Chandler Grove 94 were also used. If you have a metal gasket under the Carb, I would change it to the asbestos type which was used originally. Your intake manifold is not 37 it is 41 and up. the port you use for the vacuum gauge was connected to the vacuum wipers. The 94 power valve has to seal properly against the Carb die cast body with no leaks and the power valve has to have the proper angles at the seating surface and the threaded area. plus the die casting can warp making a good seal impossible without refacing the body. Daytona WAS the only source of the proper rebuilding kits, power valves and gaskets plus the length of the accelerator pump. After rebuild and before final assembly take the 94 primary body attached to the cast iron base without the Carb top and pour some fuel in the float bowl. Measure the level of the fuel and let the fuel sit in the float bowl with a clean paper towel under the base. 3-4 hours later look for lost fuel in the bowl and fuel stains on the clean parer towel. This should tell you if the power valve or it's gasket is leaking. If you can't get it to seal send the 94 to Charlie Schwindler he can reface the power valve seat. If your rebuilding the Carb it is best to get the proper original Carb used by Ford.
Doublepumper
Senior Member
Spark plugs wired correctly? I've seen similar issues caused by this. Just a thought. Good luck!
pistonbroke
Well-known member
You have the wrong power valve and it's leaking fuel straight down the carb. Daytona is the only kit I have found with the correct power valve with the flat face that mates to the bowl correctly. Tim
Flathead Fever
Well-known member
If you think you have a leak. Remove the carb, remove the carbs' top. Fill the bowl up and watch for fuel dripping out the bottom. Chrysler made a plastic carb that warped, melted and cracked, The ultimate piece a junk. That is how we checked them for leaks. Then we threw them in the trash and used a replacement metal carb.
Here is an excellent video on the power valve operation. It's for a Holley 4V carb but carb basics are just about the same.
https://www.youtube.com/watch?v=RvlmJoeIcpw&t=414s
If I remember correctly there are seven basic circuits in a carburetor. The float bowl circuit, the choke circuit, idle circuit, accelerator pump circuit, main jet circuit, power valve circuit and the intermediate circuit. If you treat them like individual circuits and inspect and test each circuit individually you should be able to fix a carb. Most of it is pretty simple, the float circuit is just a miniature toilet tank. You have to understand how each circuit works. It's not complicated if you're interested in it. Also, before you dive into carb operation you must understand engine vacuum. How the throttle plate opening effects the vacuum, because many of the circuit are controlled by the amount of vacuum. They use vacuum to turn the fuel flow on or off depending on how much fuel is needed under different driving conditions. Spend two hours watching videos and then go diagnose your engine.
Here is an excellent video on the power valve operation. It's for a Holley 4V carb but carb basics are just about the same.
https://www.youtube.com/watch?v=RvlmJoeIcpw&t=414s
If I remember correctly there are seven basic circuits in a carburetor. The float bowl circuit, the choke circuit, idle circuit, accelerator pump circuit, main jet circuit, power valve circuit and the intermediate circuit. If you treat them like individual circuits and inspect and test each circuit individually you should be able to fix a carb. Most of it is pretty simple, the float circuit is just a miniature toilet tank. You have to understand how each circuit works. It's not complicated if you're interested in it. Also, before you dive into carb operation you must understand engine vacuum. How the throttle plate opening effects the vacuum, because many of the circuit are controlled by the amount of vacuum. They use vacuum to turn the fuel flow on or off depending on how much fuel is needed under different driving conditions. Spend two hours watching videos and then go diagnose your engine.
Last edited:
rustedorrotting
Member
If you backfired the engine, you blew out the diaphragm on the power valve. tear it apart and start over. and the new power valves don't seal unless you get them from Charlie's speed. 97's have a mechanical power valve.
tubman
Well-known member
"Charlie's speed"? I think that explains a lot.
So is "Charlie's speed" mentioned above the same as "Honest Charley's Speed Shop"? If so, I see a lot of apparel I can buy, a couple of 97 carburetors, but not much for carburetor parts or service. Am I on the wrong site or is he just in the clothing business now? Thanks.