carb backfire
- Thread starter jerdan
- Start date
scicala
Well-known member
If the ignition timing and distributor are in proper condition, then a good chance it has an issue with the accelerator pump in the carb, or sloppy linkage for it that causes a delay in the squirt. There are also two holes in the throttle lever for the accel pump link. The outer hole gives a longer and quicker stroke, if it's not in that hole already.
Also, if it's been backfiring through the carb, it may have ruptured the power valve in the carb. If it has a rich idle now, that may be the issue.
Sal
Also, if it's been backfiring through the carb, it may have ruptured the power valve in the carb. If it has a rich idle now, that may be the issue.
Sal
Tinker
Well-known member
I agree as sal said... timing or lack of fuel. prefire vs backfire. Spark finds the fuel. Lean condition. Pre or carbfire can also be due to a valve issue. If valves are good then timing or lean fuel. As Sal mentioned on the 94s you probably blew out the pv when firing out the carb. Have to love them 94 pvs.
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Flathead Fever
Well-known member
I agree with Sal too. A lean condition will definitely cause it.
Does it backfire through the carb when it is just idling, accelerating or cruising? There is the "backfire" where the carb becomes a flamethrower and spits back through the carb and removes your eyebrows and then there is the "popping noise" backfire in the carb but no flamethrower. They mean two different things. You'd rather see the flame thrower! If its the popping noise it could be a burned intake valve. A cam lobe that has gone flat or a broken valve spring will cause all kinds of problems. It screws up the flow in the intake to all the cylinders and it won't hardly run. It sounds like several cylinders are having major problems at the same time. A vacuum gauge is how you would check for valve problems. If the needle is steady the engine is happy. If the needle has a consistent drop your not going to be happy. Sometimes you need to remove valve covers or an intake manifold on a flathead and physically look to see all the valve are opening the same.
If its only backfiring when you start to open the throttle then the accelerator pump is probably not squirting. This causes a lean condition while the carburetor is changing from the idle circuit to the main metering circuit. There is not enough vacuum to draw fuel out of the idle port once the throttle blade opens but it takes a moment for that air to get moving fast enough to start pulling fuel from the main metering port. For split second the engine is not getting any fuel. That extra squirt of fuel from the accelerator pump is to keep things running smoothly during the change between carburetor circuits.
Without the engine running look in the carb and see if its squirting fuel when you move the throttle. Next check the timing. Also if you have a carb backfiring keep the air cleaner on it to prevent fuel from being sprayed out and causing a fire. If it backfires and shuts off and you now have a fire inside your carb that your trying to put out try starting the engine. That will put it out. Also, anytime you have an engine that you think might spit back through the carb or a new engine that you are starting for the first time move the car outside just incase it catches fire. It happens.
What started this problem? Did it run good at one time with the combo of parts your now using or did the problem start with the aftermarket ignition system. Did you have the distributor out and maybe put it back in a tooth off? What changed to cause this problem?
Does it backfire through the carb when it is just idling, accelerating or cruising? There is the "backfire" where the carb becomes a flamethrower and spits back through the carb and removes your eyebrows and then there is the "popping noise" backfire in the carb but no flamethrower. They mean two different things. You'd rather see the flame thrower! If its the popping noise it could be a burned intake valve. A cam lobe that has gone flat or a broken valve spring will cause all kinds of problems. It screws up the flow in the intake to all the cylinders and it won't hardly run. It sounds like several cylinders are having major problems at the same time. A vacuum gauge is how you would check for valve problems. If the needle is steady the engine is happy. If the needle has a consistent drop your not going to be happy. Sometimes you need to remove valve covers or an intake manifold on a flathead and physically look to see all the valve are opening the same.
If its only backfiring when you start to open the throttle then the accelerator pump is probably not squirting. This causes a lean condition while the carburetor is changing from the idle circuit to the main metering circuit. There is not enough vacuum to draw fuel out of the idle port once the throttle blade opens but it takes a moment for that air to get moving fast enough to start pulling fuel from the main metering port. For split second the engine is not getting any fuel. That extra squirt of fuel from the accelerator pump is to keep things running smoothly during the change between carburetor circuits.
Without the engine running look in the carb and see if its squirting fuel when you move the throttle. Next check the timing. Also if you have a carb backfiring keep the air cleaner on it to prevent fuel from being sprayed out and causing a fire. If it backfires and shuts off and you now have a fire inside your carb that your trying to put out try starting the engine. That will put it out. Also, anytime you have an engine that you think might spit back through the carb or a new engine that you are starting for the first time move the car outside just incase it catches fire. It happens.
What started this problem? Did it run good at one time with the combo of parts your now using or did the problem start with the aftermarket ignition system. Did you have the distributor out and maybe put it back in a tooth off? What changed to cause this problem?
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