VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 cranking sound

First off, I had some foolishness giving the Green Hornet a tuneup. I replaced the cap and rotor, started good. Then the condenser, started good. Then the points and nothing. I spent an hour and a half messing with the points. Eventually it dawned on me that one should put the rotor back on to get the fire going and it worked great. I always learn something, I didn't really have the points base quite right so it was probably a blessing in disguise.

In the middle of all of this, I got a good listen to a cranking engine that doesn't fire. It sounded like one cylinder had a lower frequency sound. On the old junky '70s cars of my youth, that usually meant one cylinder with lower compression. Do other cars cranking sound even? I'd adjusted the valves recently, hopefully didn't have one too tight. She purrs like a kitten with lots of power now. Idle is nice and even.

Thanks,
Mark

Hello mshaw,
Wouldn't a remote kill switch be easier than removing the rotor for security? Just kidding. I don't know if there would be any issues because of sound differences, seeing as you say that your engine is performing well. For curiosity, you might do a compression check and see if there might be a cylinder lower than the others. Let us know what you find. Rotor on, motor on.

Mark,

Thanks for giving me a great chuckle for the day!! Takes a good man to fess up to his mistakes.

Yup, I agree, a compression check when in doubt. It will ease either your mind or your hearing.

Jim.

Guess what Daddy got for Father's Day? A tach/dwell meter, new feeler gauge, and a compression tester all from Sears. It's been over 30 years since I used these, but here's what I found:

Dwell 36, RPM 750, Volts ~6.8V (I should've tried my DVM to compare, says ~6V when not running).

Compression, here's where things get interesting... Engine was fairly warm, not hot when I did this.

#1 70, #2 72, #3 76, #4 79, #5 75, #6 60
I tried it again since learning the tool...
#1 75, #2 78, #3 80, #4 80, #5 75, #6 60

Both times, there was a funny squeak when the tester was on #5. I figured I'd gotten the valves too tight. Or it was leaking into cylinder #6, but if that were the case, I would think #5 should have been low compression. So I readjusted the valves with the new guage since the old one was junk (don't buy Harbor Freight for precision tools). Things were a bit tight, but these are tricky. My adjustment was with the engine off. I went across the valves probably four times checking and rechecking...

It was all cooled down by the time I got done....
#1 74, #2 72, #3 80, #4 78, #5 78, #6 59-60

The squeak was gone after I readjusted, maybe it was just learning curve. It seemed that the last time I put the gauge on #6, it screwed in more. The compression tester has a 5 lb granularity, so there are estimate here.

Just like it sounded, one cylinder is not like the others. It still runs pretty good. I replaced the spark plugs with one's I'd bought. Previous plugs looked good, no sign of excess oil or burning. #4 wasn't tan on the bottom or the electrode/insulator, but that's not the cylinder in question.

I hasn't been dumping water. Getting warm at stop lights, but the belt was a little loose and the license plate in front of the radiator. Changed that, will see next week if it helps.

Thoughts, suggestions, opinions on the low compression on one cylinder only?

Thanks,
mark

Mark --

Just my opinion here, but if it were my car, I would continue driving it and than check the compression on #6 occasionally. Sounds like you are happy with the way it runs presently.
Congrats on all your new tools!! Just like Christmas when I get a new one!!

The other thing you might check is to add a few squirts of oil in #6 and than take another compression test. If compression comes up, than rings are going, but if compression stays the same, than probably a valve starting to leak some. This will give you a better idea of what to do or not to do. As I stated earlier, if it were mine, would continue to drive it more. Who knows, stranger things have happened, and the compression may come up some with more driving??

Best to you, and thanks for the update.

Jim.

You can plan on doing a valve job soon. When the time comes you will know it due to a rough idle.

Hello mshaw,
I'll agree with jvd123 about putting some oil into #6 to seal the rings some for the compression check. If no change, maybe another tool to add to your collection should be a borescope to take a look at the valves and seats. Seeing as #6 does not appear to leak into #5 and you say there are no air bubbles in the cooling system, check for leaks around #6 gasket area with soap. Not too likely but you never know. If you could pressurize #6 while at TDC compression stroke, listen for air leaking into crankcase, intake or exhaust systems. That should pin point the leak. For safety, put car in gear and block wheels and stay away from the fan, belt and pulleys when pressurizing cylinders.

Just curious, did you mean to put the car in neutral?

Hello AntiqueMechanic,
No, actually I did mean in gear. My reasoning is that with the vehicle in gear and the wheels blocked, there is not much potential for one pressurized cylinder to spin the crankshaft over freely if there is a change from TDC. Of course you are right to ask how do you rotate the engine over to find TDC to begin with, but after TDC is located, then comes the in gear and blocking just for safety. Then pressurize the cylinder. I'm thinking of the resistance throughout the drive train and the wheels blocked to prevent movement while TDC is maintained and accidental engine rotation is minimalized. My opinion.

When you pressurize the cylinder it will push the piston down. This is desireable as then both valves are closed. I have never heard of it being done any other way by professional mechanics. As it is pushed down it would make the car move a little if in gear.

Hello Chev Nut,
Thank you for the input. I believe that if the piston is at TDC and then the cylinder is pressurized, the piston will not move downward in either direction. This location being where the compression test is to be taken. If the piston moves downward, the compression reading is inaccurate. If pressurized and in gear and the vehicle moves, you are not at TDC. If pressurized and in neutral and the piston moves, you are not at TDC. I'm saying that in gear and wheels blocked helps to hold the piston in place if set to TDC.

Well, I learned something, how an Aircraft Mechanic does a compression check. Dang, and I will be flying next week.