Vacuum test after compression test -what does it mean?

[Jim_Chesapeake_VA]

Pondering my next step as I diagnose a power problem with my 1929 Tudor. About two years ago I replaced the head with a 6:1 head after we found a crack in the water jacket of the head. I lapped the valves and inspected them at the time. The car is driven weekly and has been great including starting the first time almost every time. I have been noticing two problems with the car over the last year. 1. The car vibrates way more than expected (compared to other club members), and 2. A loss of power over about 40mph. The car could easily obtain 55 mph in the past. I did a compression test when the car was warm (full throttle) and got 85, 95, 85, and 100 PSI which is concerning because they aren't within 5-7 PSI of each other. I did a vacuum test today and at idle it is around 21 but bounces constantly between 18-23. I believe it is an indicator of either sticky or loose valves but looking for advice.

 

[old ugly]

timing

did you advance it when on the hyway.

 

[Flathead Fever]

You can also do a leak down test. It will let you listen for a leaking valve. We had one of these testors at work but usually the problem was so bad at that point we didn't need to check it we just pulled heads or rebuilt the engine. Leak down tester just keeps the cylinder pressurized so you can listen to see if the valves are leaking or maybe the rings. If it's worn guides the valve may or may not leak because it's not running. If it's bouncing between 18 and 23 at idle it's not consistantly sealing

Leak Down Testing

https://www.youtube.com/watch?v=-oLwZmJpOMk


I also found this one. This leak down test shows valves leaking on a motorcycle engine.

https://www.youtube.com/watch?v=grl_br1Wu6M

 

[Wick]

Check your valve lash. When you lapped the valves, you may have decreased the valve lash.
Make sure when gas pedal is pressed all the way that the carburetor is wot.
Remove fan belt, does engine still vibrate?

 

[Bob Bidonde]

Reinforcing what Wick said, that vibrating vacuum gauge needle is indicative of excessive valve clearance in your case. Also check your throttle linkage. Also, a 6:1 cylinder head needs less spark advance than the stock head. I time the ignition with the spark lever 6 notches down with the #1 Cylinder at TDC. I typically run with the spark lever 5 notches down, and 4 notches down when the engine has a heavy load such as climbing hills.

 

[Marshall V. Daut]

For your top end speed to drop off that drastically tells me the problem is more fuel related than ignition. If the ignition were to blame at that speed when the engine is working hard, it would tell you by bucking or misfiring.
Unless the gas tank has been cleaned out - and I mean REALLY cleaned out - after almost 100 years, gunk collects at the bottom. This crud consists of old gauge cork bits, rust particles that have flaked off the inner tank lining, impurities in cheap gas over the years and even sand that some kids dumped in an abandoned "old car". You have a 1929 Model A, so the lowest point in the gas tank is where the on/off valve is located. This crud tends to collect over this valve and eventually gets into the valve, effectively clogging free fuel flow. This is especially critical in a fuel system that relies upon gravity to deliver the fuel to the carburetor. Enough fuel will trickle through this crud for the engine to idle and even attain speeds up to 25-30mph, possibly a little higher. But as speed demands increase, the flow of fuel can't keep up, so the engine doesn't receive the amount of fuel it needs to reach higher speeds.
I have posted this test a couple times over the years and had people PM me that it worked and their Model A's now have their old speed back. If nothing else, at least you will know that fuel delivery is not the problem. But your description of lower top speeds compared to the past indicates your on/off valve is getting more and more plugged. I apologize if long-time readers have to wade through this description again, but it just might help others with a similar problem.
First, remove the gas cap. With the fuel valve lever in the closed position, unscrew the fuel line fitting at the carb and only loosen the one in the sediment bowl. Swivel the fuel line 180 degrees so that its carb end is now on top. Tighten the sediment bowl fitting to hold the line in this position. Turn the fuel back on. Because the line is now so high, little or no fuel will trickle out of the open end. Put the open end of the fuel line into your mouth and blow as hard as you can. You may feel resistance at first, but keep blowing until you hear a gurgle inside the gas tank. That is the sound of the gunk being blown back into the fuel supply. The on/off valve is now clear and will remain so for a while. Turn the fuel valve off and put the fuel line back into the carb, tightening both brass fittings. Put the gas cap back on and take a test drive. If you heard that gurgle and felt the difference in resistance when blowing into the fuel line, you can be sure that gunk was compacted inside the on/off valve. If you hear no gurgling, the on/off valve is still clogged. Maybe try compressed air onto the open end of the fuel line. But if you don't hear and feel that gurgle, you have gained nothing: the valve is still clogged.
This is not a permanent cure and will only temporarily solve a fuel delivery problem. Alternately, if your fuel level had been really low, you could have drained the gas tank and removed the on/valve to be cleaned out. At least that would have gotten rid of some of the gunk. It would help in the future to keep the fuel level inside the tank fairly high so that the gunk you blew back into it will be suspended and distributed throughout the gasoline. If the fuel level is kept low, the gunk will be more concentrated and more likely to end up in the on/off valve.
Try this procedure and let us know if it solves the lower top end speed problem. If it doesn't, you can at least cross off fuel delivery as a suspect.
Marshall

 

[Marshall V. Daut]

Another thought, although I am still leaning towards a fuel delivery problem.
Ignition points that we are obliged today are not the best quality, both original style and modern. The rubbing block wears down quickly in the first 50 miles, necessitating re-adjusting the gap. It helps to lube the distributor cam with Vasoline or the special cam lube offered by vendors, but the fact is that the rubbing blocks on points made today are softer than they had been in the past. When the points close up like this, the ignition timing will be affected. You may notice that where the spark lever used to be most effective is no longer the same. You might have to move the lever to find the new sweet spot. That's a sure sign that the points are closing up. Eventually as the points continue to close up, moving the spark lever is of no help. The block will harden by 100 miles and usually it is not necessary to reset the points after this. But the first 50-100 miles with new points can be a pain and a headscratcher if you don't know what to look for.
Have you changed the points in the time since you have noticed a drop-off in top end? And was it necessary to use the spark lever more than you used to? Check the points gap and set it between 0.018" and 0.022". Recheck it after every drive during the first 100 miles since changing points.
Marshall

 

[Jim_Chesapeake_VA]

I appreciate all the feedback. I had big fuel issues when I first got the car four years ago. It had about a 1/2" of tar in the gas tank. Black ooze. I ended up spending three months getting it all out including removing the tank using various chemicals and mechanical means to remove. The tar picture is when I thought I was done and put an inspection camera behind one of the baffles. Boy did that depress me. Eventually got it all out. The "clean" picture is what I ended up with. I did install a pencil filter in the tank and a new cutoff valve in the tank. The fuel flows pretty easy from the sediment bowl although I could have an issue with the carb or the points. Main issue I am concerned with is the difference in compression between the cylinders.

 

[Flathead Fever]

A compression test or a leak down test that shows a problem has nothing to do with fuel or ignition. It's simply a test to see what percentage of air goes in the cylinder and where it might be leaking too. The engine is "not" running so if the gauges show a problem, it's a mechanical issue. The lost percentage of air while doing the test, not running, can only have a few places it can go, valves, rings or a crack. You could check the valve lash like Wick said, make sure there not too tight.

 

[philosofriend]

Fuel delivery test: Remove the drain from the carburetor, open the gasoline shutoff. Gas should come out like a giant urinating. If it is more like a gerbil, you have a problem. This tests the entire fuel system including the float valve.

The hardest part of this test is catching the gas in a container. You need an assistant to open and shut the valve while you try to catch the gas.

If your car is going 60 miles per hour and getting 15 miles per gallon at that speed, the carburetor needs to get 4 gallons of gas per hour which is 8 1/2 fluid ounces of gas per minute.

This is probably not the problem but it is an easy test.

 

[Wick]

Back to the compression difference and vacuum guage bouncing.....when you lapped the valves, did you return the valves to the same cylinder you removed them from? Did you return the valve guides with its valve, or mix them up?
New valve springs? Did you cut the valve seats with a cutter? And lastly did you wire wheel the valves to a like new finish?
One more thing to check on power loss is your brakes, can you spin all 4 wheels buy hand?
When you let the wheel go,will it make a couple of revolutions before stopping? Dragging brakes kill power.

 

[Jim_Chesapeake_VA]

Thanks again for the help. I have attached a picture of the results of the leak down test. Cylinder 1 and 3 aren't looking great per the leak down test. I put the results from the compression test on there too which corresponds. I did use the rubber glove method to check the inlet and exhaust and for #1 they are both leaking air and for #3 the inlet was leaking more than the exhaust. I went ahead and got new valves and a head gasket. For some reason split valve guides are hard to get right now. I still have more to do on the fuel side and adjustments but I am pretty sure the valves are the issue. I also attached a picture of the #3 plug but all seem similar.

 

[Diastole]

Did you perfrom the Leakdown test at TDC or at the SMP. I was sent this article about where to position the Fordf Model A engines cycle for LeakDown testing and it changed my mind on my engine's problem.

D

https://drive.google.com/file/d/1BX1Srn1f4fELnfuUdMUQDUfiIEMwgC_d/view?usp=drive_link

 

[Jim_Chesapeake_VA]

Wow that's a great resource. I did it at TDC. I used a scope camera to check the valves but this looks like a better way.

 

[50Trucking]

I tried a leakdown test on my model A and when you would get a cylinder at top dead center and apply the air test it would the car and come off dead centyer and of course leak air. Could not stop the engine from turning when testing.

 

[Dick M]

FYI
There is a tool for an easy removal of split valve guides. They are not expensive and are easy to use. Most vendors carry this tool.

 

[Diastole]

If you are 1 degree past TDC the engine will advance when u pressurize the combustion chamber.
If you are 1 degree BEFORE TDC the crank wrench will stop engine movement when the piston is in the upper half of the cylinder. Trying to use the crankshaft nut-wrench past TDC risks unscrewing or loosening the nut (dangerous).
And, if you are not at TDC on the compression stroke then a valve will be open, and you’ll have a big leak.
Advance the engine till the rotor points to the SMP and place the wrench on the crankshaft nut. Pad the fender and do the leakdown test for that cylinder. Then advance rotor to the next SMP and repeat.
This allows the leakdown test to check the walls of the cylinder, gasket, rings, lands, piston, head, and valves all at one position. If you are at TDC you will not be be testing the cylinder walls, rings and lands (there is a small ridge at the top of every cylinder the seals the rings and wall at TDC). Plus the engine will not stay at TDC as you proved.

PM, email, or call anytime if I can help or you have a question.

Good luck,
Diastole.

See the article in post #13 if I was unclear.

 

[rotorwrench]

I still have my old Eastern Technology E-2 cylinder differential compression tester that I've used many years. They were only used in aviation circles for many years but there are other units available today that are less expensive such as this link. I don't know what orifice size they have though. It should be around .040" in diameter. Larger orifices don't give as good a reading.
https://www.harborfreight.com/cylin...MIvpGmmKzBlwMV4L0DAB1GGhQREAQYASABEgJ_QvD_BwE

All use two gauges for differential readings across a measured orifice size. The orifice allows the input air pressure to bleed so it will maintain a set pressure. The input psi used in aviation is 80 psi. With a propeller, it is easy to set #1 at TDC and get it to hold at a low input to start. Once it holds TDC, the pressure can be increased slowly to 80 PSI. The other gauge reads the differential in leak down. Lycoming always stated in their SI-1191 that the differential pressure must read at least 60 to 65 psi with all readings within 15 psi of each other. A really good set of rings can be 78 or 79 PSI but once the engine has a lot of time on it then it will drop the 74 or 75 psi consistently. If one is low and it hasn't been run for a while, run the engine and pull some power with it. It will clear any debris that might be on the valve seats. If you oil a cylinder and the compression rises then it's showing some ring wear signs. If pressure builds in the crank case then it's just the leakage by the rings. If you hear or feel air at the carb inlet then it's an intake valve. Leakage at the exhaust is the exhaust valve leakage. I follow firing order for the rest of the cylinders. A trick I use is to just apply a little bit of pressure to the cylinder to start. As the crank is rotated, the psi will show an increase. After it hits TDC, it will stop showing an increase so you know your there. Sometimes a person can feel it sort of click into the sweet spot but don't count on it.

The BIG Caution was to be careful around the propeller in case it slips off TDC in one way or the other. I worked on helicopters so we didn't have a prop. We used a turning tool to turn the crankshaft to TDC. There is a sweet spot at TDC where the con rod is in a spot where the downward pressure will hold it from turning either way. It can be hard to find that sweet spot but I've never had an engine that I couldn't get there during the test. It may take several tries but it can be done. If a person tries to hold it, it can be done but you better have a long turning tool and even then, the nut can come loose if it goes the wrong way. The aircraft had 5" cylinder bores so there can be has a LOT of pneumatic force there.

 

[Big hammer]

When I was a teenager and working at the airport the mechanic/boss was doing a differential leak test , being an inquisitive kid I wanted to see what he was doing , walked to close and he said get out of the propeller area ! I did and just then the propeller spun ! I never will forget that !!!