The Stovebolt Site Forums The Shop Area Engine Shop tune ups and compression tester...

my friend jack came over this morning to assist in a compression check, valve adjustment and gasket refit. we also did points, plugs, condenser, rotor and distributor cap.

we did the compression check first as a bench mark for where the original 216 is currently at, not knowing if a valve adjustment had ever been done since my grandfather bought it new in 1950, we were both curious about the condition of the engine/valve train.

the first cylinder read 45psi...compared to what the shop manual indicates, this is far below the 110psi expected reading. the following cylinder read as follows. #2-50psi, #3-40psi, #4-50psi, #5-40psi, #6-50psi. concerned or more accurately confused, my friend jack, a retired motorcycle mechanic who's father owned a gas and auto repair business his whole life, surmised that the gauge was not reading correctly. his feeling was that as good as the truck runs (daily driver by the way) and the condition of the plugs, all looked a pretty grey with only the number two cylinder plug looking a little black, that the gauge was not reading accurately.

checked the valve lash next and not a single valve required adjustment. all were well within acceptable tolerances. makes ya feel real good when you consider that this engine has never been apart and sees less than trailer queen treatment...not abused, but this truck works hard.

we put all the pieces back together, fired it up and it runs like a finely tuned sewing machine.

after jack left i hooked up the compression tester to my air supply line to check the calibration of the tester in comparison to my dryer/regulator set-up. the compression tester gauge read 7lbs. lower than the gauge on my regulator with the regulator reading 95psi and the compression tester reading 88psi.

short story long, does anyone have an opinion on why the compression tester read so low regarding the engine test but was only off by 7lbs. the only thing we came up with was that we weren't getting enough air intake to make the appropriate compression. we did follow the shop manual as far as using the throttle knob to open the carb facilitating the air intake.

any thoughts are welcomed...

p.j.,

How many times did you spin the engine over on each cylinder for the compressions check. It sounds like you checked the valves with a cold engine. If this is true, did you make allowances in your lash readings? If the engine starts and runs well, I'm sure those are bogus compression readings.

Stuart

spun the engine a minimum of 6 times per cylinder to insure optimum, reading. engine was not cold. i followed the shop manual procedures. 30 minutes warm-up to reach normalized engine temp (including normalized oil temp).

not really that worried if it runs this well, but just baffled by the readings...

I am sure you did this, but sometimes even experienced mechanics can overlook something. Did you have the throttle wide open when you did the test? If the gauge agrees with your air compressor gauge, (which could be off, but probably unlikely) something would seem to be wrong somewhere.

well it passed the main test they were all with in 10lbs. of the adverage cyl.

Was the compression test repeated after "injecting a liberal supply of engine on top of the pistons with low reading cylinders"? (quoting from the shop manual.) If the compression comes up to near normal, worn rings would be the suspected cause.

Was the compression gauge a screw-in or hold-in with a rubber seal? I recall some gauges that did not seal too well with the small sparkplugs in most 216s. Of course, a leaky gauge is usually obvious, and the readings are usually erratic.

I've conducted at least 100 compression tests on stovebolts (216, 235, and 261), and I do not recall ever seeing readings that low on an otherwise good running engine. Did pulling out the throttle knob really open the throttle to near full open? Was the choke wide open? I would usually open the throttle wide open by hand and lock it open with a visegrip or have a "helper" push the pedal to the floor.

As long as it runs well and does not burn a lot oil, I would not worry much, if at all.

Hoyt

Usually there will be some carbon or other debris in the schrader valve of the tester not letting the compression build. Also, all of the plugs should have been removed with the throttle blades blocked wide open. A six volt system cranks real slow so adding a battery charger to it will help get accurate readings. My experience thinks that 75 lbs. per cylinder or less will cause a misfire, so being in the 50's or less, the truck wouldn't even start. As long as it runs well and doesn't burn oil, let the sleeping dog lay,
Scott