Don Terrill’s Speed-Talk

Yes, very basic tool .....I was watching a guy check the compression on his race bike....He cranked over the engine 10 compression strokes per cylinder to get a reading...I asked why so many strokes of the gauge needle? He said 10 is the industry standard.......
I disagree, I use three strokes ,with the compression rise on the first stroke being most important....
Your opinion? ... besides saying to use a leak down tester

The needle should jump to 90 on the first stoke. After that I give it 5 strokes and it should be at maximum compression.

If you have to keep cranking on it seems like something isn't right.

Depends how fast you are spinning the engine. If all of the plugs are out and the carb is off and the battery is fully charged then 3 to 5 strokes should do it. 10 seems like overkill but maybe his setup required that.

Was he using a starter or by foot?

Spinning it more times than needed will not cause any problems, but not spinning it enough will not show the correct number.

Stan

Stan Weiss wrote: ↑Tue Oct 31, 2017 11:33 am Was he using a starter or by foot?

Spinning it more times than needed will not cause any problems, but not spinning it enough will not show the correct number.

Stan

Ha! I didn't think about him kicking it over. I'm old school but as old as I am now, if that has a kicker and no giggle switch to make it start I ain't kicking it.

Yes, vintage race crap like mine with a kick starter....If the cylinder is sealed up good , one kick will show about 75 psi depending on engine tune ... I just don't see the need for cranking any engine past the point where the gauge only goes up 10 psi per compression stroke..

The idea is to try it out lots of times using different methods then you know what gives you what readings.


It's a bit like ricers who insist that the throttle has to be wide open otherwise you get lower readings.

Then me.......take all the plugs out........it doesn't matter about the throttle.
The numbers are the same in the end.


If you've broken the ringlands through detonation, there's oil on the threads of the plug and the compression numbers end up higher, not lower.


But you can't get through to ricers ?
But it must be good because the numbers say so.

Andy F is right on a point that is critical ,' how fast you spin it ' ,comp tests on a late model brand name bike show immediate consistently high readings compared to identical earlier models and the only difference is the starters cranking speed, a friend of mine that does my dyno testing checks cranking pressures all the time before tuning and has found this observation to be common.
That all said , if a person has a ritualistic way of doing their testing [or any test procedure] and it gives them consistent results that work for them and when they adjust to their results get a positive outcome , it is best to stick with the ritual so variables don't creep in .
With an electric start in sound condition testing the product i am involved in [throttle open] i would expect a relatively high reading on the first bump then a couple of little nudges on subsequent revolutions , with kick start i would leave the prognosis to the guy that knows his gig.
Compression test tools that have the check valve in the combustion chamber face tip give the most accurate results ,as does fitting the tool to the same plug bore point as the spark plug shell.
Cranking pressure tests in conjunction with leakage tests on a fresh stabilized engine give invaluable information for further diagnosis.

Cheers.

My old BMW 600 twin with a tiny single slide carb per cylinder scared me badly because the readings were so low when I did a compression test with the throttle closed. Electric start. Like Joe said, None of my cars ever did THAT.

Opened the throttle, readings were fine.

Because it's one throttle per cylinder, not shared with others.

andyf wrote: ↑Tue Oct 31, 2017 11:13 am Depends how fast you are spinning the engine. If all of the plugs are out and the carb is off and the battery is fully charged then 3 to 5 strokes should do it. 10 seems like overkill but maybe his setup required that.

I agree but the thing I look for is what the reading is on the 1st pump to the gauge. That is all the engine gets when its trying to start and if I see a big variance in the 1st hit then I know there is a leak some where in the rings or valve train areas that will need attention.
As Stan mentioned the crank speed can also play a big part in the variance hole to hole so be sure the battery is up when you do the tests

And if the tester's Schrader valve fails, don't replace it with a tire valve unless you want to lose 20 psi or more.. (don't ask me how I know this.. )

Probably doesn't matter, as long as you do it the same way every time.

how many times you will have to spin to get a full reading.... will depend on the relative volume of the cylinder verses the volume past the one way valve of the gauge.

I do not know if there is a standard, but I have a few different gauges.
Put the big gauge with the extra long hose on a weedeater I bet it will take ten pumps, or more.

Cliff notes on this story... Before running the 650 vintage Triumph at the LSR track in Maine this summer , I checked the compression although it's something I don't usually do..The bike had been sitting for a few months so ....I checked it cold, 70 PSI....Hmmm, so I checked the valve adjustment and it was ok...I poured fresh fuel in the tank, and the engine starts on the second kick.....So I tell my Brit bike friends and they all say there's something wrong with the gauge or I'm doing it wrong or the engine needs a valve job or new rings...No way an engine can start on 70 psi compression....I asked if they ever checked compression on a lawn mower. I figured the oil had run off the cylinders while sitting and the rings lost their seal?.Anyways, I pull the head and cylinders to have a look to see lots of light scoring on the cylinders and pistons (no air filters).I remove the rings and clean the pistons with a Scrubbie pad. Using a flex hone , I hone the cylinders while holding them between my feet on the shop floor using brake fluid for lube. Clean the parts and assemble the engine using the old rings. Sounds like a story from PDQ67 ,but it's true....So the bike goes to the track and off the trailer runs 133.1 MPH ,faster than any 650 push rod gas naked modified production bike has ever gone at any LSR track......