Questions for engine experts

[Rich Bowes]

Yesterday a friend and I conducted a series of diagnostic checks to see what work the motor in my '30 Roadster needs. Currently the engine is using some oil, as seen on the dipstick as the oil level decreases, and a puff of blue smoke from the exhaust pipe during gear shifts. Here are the results of the tests:

Leak down test results(conducted on cool engine, starting with each cylinder in turn at TDC, verified with digital inspection camera, then pumping 20 PSI into each cylinder in turn):

#1 cylinder -0 % leakage;

#2 cylinder - 20 % leakage, piston pushed down and engine turned backwards, air coming out the carburetor;

#3 cylinder - initially less than 3 % leakage, with air coming out exhaust pipe, then the leakwent down to 0 %;

#4 cylinder - less than 5 % leakage, piston went down and engine turned forward, could not detect where air was leaking from, checked carburetor, exhaust pipe, and radiator.

Camera also revealed carbon buildup atop pistons, and around the edges of some of the valves.

Vacuum test results (conducted on warm engine):

Drifts rapidly at idle between 13 and 20, stabilizes at higher RPM between 18 and 20.

Compression test results (conducted on warm engine):

DryWet

#1 cylinder -6060

#2 cylinder -6060

#3 cylinder6063

#4 cylinder6060


The leak down and vacuum tests indicate to us that the engine needs a valve job.

The oil usage, and blue smoke indicate oil in the cylinders, but that is not indicated by the compression tests, except maybe in cylinder #3.

Also, we don't know why the leak down test caused the piston in #2 cylinder
to go down and turn the engine backward, or why it caused the piston in #4
cylinder to go down and turn the engine forward.

Could someone please help us understand the test results, and let us know if they indicate that the engine needs more than just a valve job?

[Rich Bowes]

The compression test results were:

#1 - dry 60, wet 60
#2 - dry 60, wet 60
#3 - dry 60, wet 63
#4 - dry 60, wet 60.

[Mitch//pa]

when you put each cylinder on TDC and inject air you cannot let the piston move any as this will open a valve which is what happened in your case when air came out the carburetor.
i hold the crankshaft still at TDC using a wrench on the crank nut...

do a retest holding the crank still.
this wont cause oil burning but also remove the radiator cap when doing the leak down looking for air to come up..

[J Witt]

If the engine is free turning and the other three plugs are out, car in neutral, the 20psi in the cylinder is enough to push the piston down. which way depends on which side of TDC the crank is on. Put it in 1st gear and put on the hand brake.

Puff of smoke when changing gears can be a sign of loose valve guide clearance -- oil is pulled by the valve stems when the throttle is closed and burned when the throttle is opened again.

Sounds like a valve job to me, but hard to tell since the pistons moved. Perhaps an A&P mechanic among us can give a better opinion.

John

[Tom Wesenberg]

Third gear with the parking brake on gives more holding power than using first gear.

I like Mitch's idea of using a wrench on the cranking bolt.

[harleytoprock]

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I agree with Tom in that 3rd gear will hold better.

[Mitch//pa]

If you go that way chock the wheels also

[29 model-a]

Yes u need to stop the crank from turning

[BILL WILLIAMSON]

Compression's not THAT bad, me, I'd go a trifle heavier Synthetic Oil & DRIVE IT!
A "little bird" tole me that Synthetic burns CLEARER, if not, re name the car "SMOKEY JOE"
Bill W.

[Russ/40]

I don't see any problem with your engine. Perhaps guide wear, but an occasional oil puff, wouldn't make me dig into an engine, unless I was just looking for something to do and to spend some money. Your leak down is totally bogus, sorry to say. Pistons must be on top and unable to move. Your probably losing more by leaks than combustion.

[Mike V. Florida]

Vacuum test results (conducted on warm engine):

Drifts rapidly at idle between 13 and 20, stabilizes at higher RPM between 18 and 20.

Movement between 13 and 20 is usually indicative of a sticky valve.

[Dave in MN]

I have completed many leak-down tests; if the crankshaft rotates during a leak-down test, the piston/crank position was not at top dead center. From your compression tests and leak-down results along with you noticing smoke, I would only complete a valve job to eliminate the smoking. If you can check, try to move the valve around in the guide to see if there is noticeable movement. If so, new springs, guides, valves and seat machining should take care of your issues. I also suggest...Check your bearing clearances while you have the engine open.
Good Day!

[mcorrell]

I agree with what others have said. The compression results are great if you have a stock head. I'd say the valves are OK too. If there was a significant leak at any valve the compression would be lower. A leak down test is usually done to determine the cause for low compression.
The vacuum fluctuation could be due to carb adjustment. You might try tweaking the low speed mixture screw in/out. If not it could be a sticky valve as others have mentioned. Do you notice any backfiring or popping? How is the overall power?

[Patrick L.]

I too think your engine is OK.
Compression readings are good.
As said, the engine has to stay still on the leak test. I think your bad #2 cylinder leak reading is only because the engine turned and started to open the intake valve.

Now, to clean the pan and check the bottom end along with the side cover and checking the lash.

[BILL WILLIAMSON]

Just "THINKING",
If you had an otherwise good running engine, with stock valves & lifters, BUT 1 Exhaust valve was, say, .004" TOO TIGHT--Has ANYONE ever tried to SQUEEK in some type of "file" or Diamond strip, etc, to give it .004" MORE clearance, without pulling everything apart?
Bill W.

[dave11]

I's been a lllong time I know, but I am wondering how you made out with the smoking issue? My 30 has the same symptoms.

Dave Eddie

[Patrick L.]

Its kind of a pain when folks don't follow up with a result isn't it.

[tbirdtbird]

it sure is.

I myself would have done nothing to that motor with those compression results. So what if there is an occasional blue puff? Worn valve guides....not valves... with 60 compression

[Mitch//pa]

Some synthetic oil will burn clear

[Special Coupe Frank]

If the engine doesn't foul plugs or spew a constant cloud of smoke, and the oil consumption is "reasonable" (not more than 1 qt in 300-500 miles), then I think I would leave it alone and keep an eye on the oil level.

A few puffs on shifting is nothing to be ashamed of with a 90 year-old car.

Watch some silent movies with cars in them, you'll be surprised out how much smoke some them spewed when they were less than 30 years old...