Compression test

[WPVT]

I'm trying to do a compression test on a 1954 Dodge 218 flathead engine. When I pull all the plugs and use the starter, the Bendix drive kicks out, because of the speed. I can put two plugs back in and that slows it down enough to do the test. I'd like to test with no plugs in, though. Has anyone else run into this ?    

[A. Ballard 35R]

Put all plugs in and then remove one at a time to test individual cylinders.

[WPVT]

1 minute ago, A. Ballard 35R said:

Put all plugs in and then remove one at a time to test individual cylinders.

Thanks. I did that but it cranks more slowly and gives a lower reading. Everything I've read says remove all plugs.

[edinmass]

I would install one plug, and be sure to hold the throttle wide open. It’s more important to see the numbers across the board.........to compare them to each other. You could also do a leak down test and a cylinder power balance test.........I usually start with the latter. Also you can scope the secondary and look at burn time.

Edited November 21, 2020 by edinmass (see edit history)

[Mikefit]

1st replace the starter drive. Or 1 plug or 5 plus in open throttle or closed Just do each cylinder the same you are looking to get a base line nothing more nothing less 

[Mark Shaw]

If the engine has not been run for awhile, don't forget to add a little oil to each cylinder before testing.  

[edinmass]

Do it dry first........then wet. Don’t add anything till you do a dry test.

[Larry Schramm]

7 hours ago, Mark Shaw said:

If the engine has not been run for awhile, don't forget to add a little oil to each cylinder before testing.  

 

My standard procedure to run a compression test is to remove all of the plugs, usually not necessary, but I do it to make it easy to move from one cylinder to the next and less drag on the starter.  

 

After removing all of the spark plugs and without adding any oil to the cylinders  you are ready to do the first round of testing.  This is the "dry test".  Check the compression of all of the cylinders turning the engine over three times.  You should see the compression gauge go up three times as the engine is turning over.  Record all of the values for each of the cylinders. 

 

Charge the battery or keep a charger on the battery while you are doing this.

 

Next, add about one or two squirts of oil into each of the cylinders.  This is called a wet test.

 

Redo all of the compression tests on the cylinders and record the values.  Remember, do it the same with at least three moves of the compression gauge while cranking the engine.

 

Now compare the two values before and after adding the oil.  If they are close, that is good.  The values of all of the cylinders should be within 10% of each other.

 

If the compression is higher after adding the oil, then the rings are worn.

 

If you have a cylinder that is the same and lower than the others before and after adding oil, then you have a leaking valve. 

 

Fairly straightforward.   If you have any questions, let me know.

 

PS:  Leave the throttle open or closed on all of the cylinders when testing.

10 hours ago, edinmass said:

be sure to hold the throttle wide open. It’s more important to see the numbers across the board.........to compare them to each other. 

Edited November 21, 2020 by Larry Schramm (see edit history)

[WPVT]

1 hour ago, Larry Schramm said:

 

My standard procedure to run a compression test is to remove all of the plugs, usually not necessary, but I do it to make it easy to move from one cylinder to the next and less drag on the starter.  

 

After removing all of the spark plugs and without adding any oil to the cylinders  you are ready to do the first round of testing.  This is the "dry test".  Check the compression of all of the cylinders turning the engine over three times.  You should see the compression gauge go up three times as the engine is turning over.  Record all of the values for each of the cylinders. 

 

Charge the battery or keep a charger on the battery while you are doing this.

 

Next, add about one or two squirts of oil into each of the cylinders.  This is called a wet test.

 

Redo all of the compression tests on the cylinders and record the values.  Remember, do it the same with at least three moves of the compression gauge while cranking the engine.

 

Now compare the two values before and after adding the oil.  If they are close, that is good.  The values of all of the cylinders should be within 10% of each other.

 

If the compression is higher after adding the oil, then the rings are worn.

 

If you have a cylinder that is the same and lower than the others before and after adding oil, then you have a leaking valve. 

 

Fairly straightforward.   If you have any questions, let me know.

 

PS:  Leave the throttle open or closed on all of the cylinders when testing.

Thanks for the straightforward advice. That's basically what I had planned to do. My starter (foot actuated) kicks out from the speed of turning the engine without plugs. I assume that's the same mechanism that kicks it out when the engine starts. I was curious if anyone else had experienced this problem.   

[JACK M]

If your foot starter is like the last one I worked on the drive wouldn't be able to kick out unless your foot got light.

This one was doing something similar (spun before it was engaged) and wanted to be adjusted so that the starter pedal would engage the flywheel a little deeper and sooner.

Is your foot starter just a switch?

Edited November 22, 2020 by JACK M (see edit history)

[WPVT]

39 minutes ago, JACK M said:

If your foot starter is like the last one I worked on the drive wouldn't be able to kick out unless your foot got light.

This one was doing something similar (spun before it was engaged) and wanted to be adjusted so that the starter pedal would engage the flywheel a little deeper and sooner.

Is your foot starter just a switch?

 I know the kind you are speaking of, where the foot pedal both electrically activates the starter as well as physically moves to engage the gear. 

 I'd have to check my '54 again to be sure what  have. I believe that it must have a Bendix drive, as it kicks out even if you keep the starter pedal pressed down. That would make sense, because it also keeps the gears from grinding once the engine starts. As I recall, the foot pedal activates a lever that only serves to make the electrical contact, not engage the gear. 

 

So the problem that I am having is just the Bendix doing its job. I'm curious because I would think this would happen to anyone with a Bendix type starter trying to turn the engine over with no plugs in it.

[Oldtech]

I suspect the bendix is kicking out just because the engine spins so freely with the plugs out that it unloads the bendix. If it is not kicking out with the plugs in, it is probably fine. 

 

[Rusty_OToole]

Are you using a 12 volt battery?

[WPVT]

My system is 6 volt.

I agree that the Bendix is working fine.

My question is whether anyone else had run into a problem trying to do a compression test with all of the plugs out. I've never done a test that way before myself. I decided to read up on compression testing and the consensus was that it be done with all of the plugs out. Wouldn't any engine with a Bendix starter kick out , cranking with no resistance? 

[Bush Mechanic]

59 minutes ago, WPVT said:

My system is 6 volt.

I agree that the Bendix is working fine.

My question is whether anyone else had run into a problem trying to do a compression test with all of the plugs out. I've never done a test that way before myself. I decided to read up on compression testing and the consensus was that it be done with all of the plugs out. Wouldn't any engine with a Bendix starter kick out , cranking with no resistance? 

 

 No. I always remove all of the plugs, but have not had the Bendix unload and release on any that I have tested. Not much help, but it answers your question.

[WPVT]

51 minutes ago, Bush Mechanic said:

 

 No. I always remove all of the plugs, but have not had the Bendix unload and release on any that I have tested. Not much help, but it answers your question.

Thanks. That's exactly what I was wondering. 

I'm sure that the design and rating of the Bendix drives varied from one model to another . Yours was probably just designed with a stronger spring than mine.   

[JACK M]

I cant say that this has ever happened to me.

I would conclude that your bendix is week.

I also would just do the compression test with the plugs in place.

All you are looking for is consistency. If you have a low hole then get a leak down tester.

[WPVT]

6 minutes ago, JACK M said:

I cant say that this has ever happened to me.

I would conclude that your bendix is week.

I also would just do the compression test with the plugs in place.

All you are looking for is consistency. If you have a low hole then get a leak down tester.

Thanks. The engine spins pretty fast with no plugs, so I figure the Bendix is just doing its job. It may also have to do with the gear ration between the starter and flywheel. All of which is immaterial to the compression test. I get pretty even numbers with the plugs in. Not much difference with a wet test. I'm getting around 100 instead of 120 that the specs call for, so I thought with plugs out it might make for a higher reading. Bottom line is there's nothing wrong with the engine...it runs fine.  

[Rusty_OToole]

I know your system is 6 volt, that is why I asked if you are using a 12 volt battery. If you were that would account for the excessive speed of the starter and kicking out.

[WPVT]

1 hour ago, Rusty_OToole said:

I know your system is 6 volt, that is why I asked if you are using a 12 volt battery. If you were that would account for the excessive speed of the starter and kicking out.

I've never seen the need to change it. It starts with less than one revolution of the engine. 

Thanks for your question though. Nobody designed the starter system to operate without plugs in the engine. 

I'll do the test with one or two plugs in, just to slow it down a bit. 

[C Carl]

36 minutes ago, WPVT said:

 

I'll do the test with one or two plugs in, just to slow it down a bit. 

 

5 hours ago, WPVT said:

. I get pretty even numbers with the plugs in. Not much difference with a wet test. I'm getting around 100 instead of 120 that the specs call for, so I thought with plugs out it might make for a higher reading. Bottom line is there's nothing wrong with the engine...it runs fine.  

 

With decades of compression tests with plugs out and wide open throttle, I have never had an issue. With the other plugs in, all other things being exactly equal, sure the compression readings COULD be a little lower. But your readings are UP AND EVEN under two different test parameters. IMHO your compression testing has been successfully completed. Go drive when conditions and discretionary time on your part permits. Enjoy your car ! And just monitor starter/bendix condition for any signs of deterioration.  👍, 🙂,   -  CC 

 

[WPVT]

A final note...I used a leak down tester and tested the engine cylinders today. All were at 90-95%, so I'm satisfied that everything is OK. I think the leak down test eliminated some of the variables I was dealing with using the compression tester. 

Thanks for all of the helpful input.

[edinmass]

Also, a cylinder power balance test while the car is running is fast, easy, and costs nothing. My favorite quick test on pre war cars.

[WPVT]

On 11/27/2020 at 5:25 PM, WPVT said:

A final note...I used a leak down tester and tested the engine cylinders today. All were at 90-95%, so I'm satisfied that everything is OK. I think the leak down test eliminated some of the variables I was dealing with using the compression tester. 

Thanks for all of the helpful input.

Well....not quite the final note. Turns out my leak down tester had a design flaw. Once I sort out all that I've read about leak down testing, I'll try the test again.  It's turning into a useful education. 

[edinmass]

I also have often times received an education from a car............To be honest, I rather remain ignorant!

[WPVT]

I've been doing a lot of reading regarding compression testing and leak down testing. Although there are differing opinions about many aspects of both, I think I understand the basics at this point.

Although the engine runs pretty well, my testing shows I seem to have a problem with cylinder #5. It's consistently 10-15 lbs lower than the other cylinders on a compression test. A leak down test I did last week showed it leaking more than the other cylinders...80% as opposed to 15% in the other cylinders. On all the cylinders the slight leakage hiss can be heard coming from the crankcase and no where else. 

So I decided to try it again this week, focusing on #5. I hooked up the leak down tester and found air escaping rapidly at the intake. I figured I had the crank positioned wrong, and checked and rechecked.  Still air pouring out the carburetor throat, though I definitely had the crank positioned correctly.  So I tried a compression test on #5. Still at 75 psi. Low, but not extremely so. So I tried the leak down again. This time I was back to 80% leakage....not great, but all the hiss was from the crankcase, as it had been before. Consistent with the 75 psi compression. 

It sounds to me like spinning the engine with the starter freed up an intermittently sticking intake valve.  Vacuum is steady at 19, with a barely perceptible vibration to the needle. Maybe the valve only sticks when the engine is rotated slowly by hand. 

Does my diagnosis seem correct, and should I be concerned ?

[Oldtech]

Since you obviously have an intake valve leaking I would run it for a while and see if it smartens up.  Intake valves run cool so you won't likely burn it and if it is a sticky problem in should fix itself...hopefully. 

Nothing to loose anyway, 

[WPVT]

1 hour ago, Oldtech said:

Since you obviously have an intake valve leaking I would run it for a while and see if it smartens up.  Intake valves run cool so you won't likely burn it and if it is a sticky problem in should fix itself...hopefully. 

Nothing to loose anyway, 

Thanks. The sticking valve has never manifested itself while the engine is running, or even when the engine is cranked by the starter during a compression test. I only experienced it when I was slowly rotating the engine by hand to do the leak down test.

[Grimy]

The anti-snake-oil set will scoff, but the two Pierce Series 80 L-heads I've had would develop sticky valves over the winter, and would manifest themselves in the first 10 miles during the spring.  I mean, one valve would hang-up-open level of "sticky."  I began a process of adding 4 oz of Marvel Mystery Oil (a gallon is about $25 at WalMart or O'Reillys) to each 10 gallons of gas every other tank, putting the MMO in the tank before the fuel nozzle.  Problem solved.  Haven't had another sticky valve in almost 25 years.  And it keeps the elephants away, too!

[37_Roadmaster_C]

What @Grimy said above is very good advice. The Marvel Mystery Oil will not hurt anything and it has some very interesting benefits! Give it a try for a while and see how it works for you. As for the lower compression in the #5 hole, did you try a compression test both dry and wet (with a squirt of oil in the cylinders)? If so, did the compression become higher and more like the other cylinders with the same test? If so, then rings, and that is what I suspect for #5. There may be other things also, but that is for when/if you open up the motor... Keep us posted with what you find.

[WPVT]

1 hour ago, 37_Roadmaster_C said:

What @Grimy said above is very good advice. The Marvel Mystery Oil will not hurt anything and it has some very interesting benefits! Give it a try for a while and see how it works for you. As for the lower compression in the #5 hole, did you try a compression test both dry and wet (with a squirt of oil in the cylinders)? If so, did the compression become higher and more like the other cylinders with the same test? If so, then rings, and that is what I suspect for #5. There may be other things also, but that is for when/if you open up the motor... Keep us posted with what you find.

Thanks.  I suspect there could be wear in an intake valve or stem, which may have caused the valve to hang up when I slowly rotated the engine by hand.  It isn't a problem that manifests itself while the engine is running, or even while it's being cranked at starter speed.  

But thank you for your interest and for your opinion.  

[Grimy]

OR... there can be a build-up of carbon, perhaps from evaporation of the volatiles from a rich mixture.  That was the problem on my two cars--apparently.

 

IMHO, the best thing you can do is to drive it as much as possible, and put even a slightly richer dose of MMO in your gas, every tank, for the next 1,000 miles.  It's certainly cheap enough.  If your rings are gummy, the MMO will help clean them up a bit.  MMO is naphtha-based and was developed in the 1920s, when lousy gasoline required removal of heads for "decarbonization," often with chisels, every 15,000 miles.  MMO claimed then that it had the effect of breaking hard carbon into soft carbon which could then be ejected through the exhaust system.

 

I would not do a valve job anytime soon with the results you've reported.  Had this vehicle been inactive for awhile?  Especially if that's the case, just DRIVE it!