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Compression test procedure

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bondjbond Avatar
bondjbond Jim Kelly
Reqiured Town, MA, USA   USA
Hello- I haven't done a compression test in decades and plan to do one (when things get a little warmer). I 'm thinking that the following procedure should do it:

- shut off elec. fuel pump
- disconnect coil
- with engine stone cold, pull all plugs
- connect tester, finger tight
- take & record dry pressure for all cylinders
- put about a teaspoon of oil in each cylinder & repeat wet pressure for all cylinders

One question I have is - should the throttle be wide open when cranking for pressures?

Any suggestions are appreciated. I'm getting a little smoke after idling for a minute or two then hitting the gas (like at a red light) and I'm trying to determine if it is rings or valves. I suspect valves as the engine burns practically no oil.

Thanks

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Andy-Sherry Avatar
Andy-Sherry Gold Member Andy Martin
Portland, OR, USA   USA
Yup wide open



Andy&Sherry
1974 Spit 1500 Carmine Red
1977 Spit 1500 Pink Panther Pink

Always learning something

Lizzard d id
san jose, CA, USA   USA
With all the plugs out it doesn't matter if the throttle is open or closed . It will be impossible for there to be any real vacuum in the intake with no spark plugs .
On the intake strokes the three/five other intake valves will be open and the other three/five spark plug holes will be open , if there is any vacuum in the intake , air will flow in the spark plug hole into the cylinder , backwards out the open valve and into the intake .


"- with engine stone cold, pull all plugs"
Some people do "hot" compression tests . Get all the tools out and ready , take it for a drive , get it up to running temp , pull in to the spot for the test , pull the plugs real quick , run the tests .

And then you might think about running a vacuum gauge .
https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge/



Edited 1 time(s). Last edit at 2018-02-08 10:11 AM by Lizzard.

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bondjbond Avatar
bondjbond Jim Kelly
Reqiured Town, MA, USA   USA
Thanks - The vacuum article is very helpful.

Yellowhawk Valley Avatar
walla walla, WA, USA   USA
1969 Triumph Spitfire "Walla Walla"
1969 Triumph Spitfire "Portland"
1972 Triumph Spitfire MkIV "Spokane"
1975 Triumph Spitfire 1500 "Dayton"    & more
Be sure to let it do several revolutions to bring it to the max pressure.
Dan

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
If you are using the push in type you should use a little oil on it's schnoz to help it seal.

aebreard Armand Breard
Monroe, LA, USA   USA
You are on track; no ignition & throttle open, but do this with the engine warmed up to where water temp needle is in the middle. Write each cylinder pressure down
then do it again to make sure reading is good. The a squirt of engine oil, & repeat the procedure. I use 150# & up as a measure; 165 on my Sprite when racing, and now have a '79 Spitfire with 185 psi on each cylinder!

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mkivmarty Avatar
mkivmarty Marty Yanik
N.E.Ohio, USA   USA
Keeping a battery charger on while you run your tests will help keep your readings consistent. Run your tests 1-2-3-4, then run them 4-3-2-1 and average your results.

Marty

spitfire50 Avatar
spitfire50 Paul Mugford
Rochester, N.H., USA   USA
Jim,
Don't expect to match numbers from any other gauge. Compression gauges aren't precision instruments. There can be considerable variation from one to another. The thing you should look for is readings that are within 10% of each other from cylinder to cylinder. Closer is better.
All the best,
Paul

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bondjbond Avatar
bondjbond Jim Kelly
Reqiured Town, MA, USA   USA
Thanks all - if the weather here holds up I'll give this test a try (vacuum also) over the weekend & post results. Advice is much appreciated.

GeorgeOhr Nonya Business
Yes, confused, USA   USA
In reply to # 1512138 by Yellowhawk Valley Be sure to let it do several revolutions to bring it to the max pressure.
Dan

Damn hose on mine is is like 3 feet long and has to be as many CCs as the cylinder itself. I spin mine till the needle stops climbing and starts to bounce a little.

clshore Carter Shore
Beverly Hills, FL, USA   USA
Remember, what matters is consistency ...
Do each cylinder as exactly the same as you can.

The absolute number means very little, the cylinder to cylinder differences are what you are interested in.

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