Compression test results - worth worrying about? ('88 Sunny)

Couple of questions for you.

How many times did you test each cylinder i normally do each one, then repeat each at least one more time. If results are same (or very close) that will do, if not do again until you get pretty constant readings.

Did you hold the throttle fully open while cranking (you get low readings if you do not do this) which is a common mistake made by people. It basically isn't letting enough air in for the engine to compress. Also were all 4 plugs out to aid the engine cranking speed?

Also you do not know how accurate your gauge is compared to someone elses. So take that into consideration when comparing max figures, the difference between cylinders on your gauge is the main thing to look at. I'd say 30psi missing is something to look into, its not a huge loss so not a major failure, but its more than i would like to see between cylinders.

Start with testing all again with throttle (and choke if you have one) fully open to let engine breath. DO each one a couple of times and write down the exact figures (not rounded up/down to the nearest 10psi)

/\/\/\ Plus watching how the pressure is arrived at can tell you whether it's rings or valves. If it's 3 or 4 progressively smaller jumps then that's normal and if lower than the ideal pressure it's likely to be rings, especially if you puts some oil in and repeat and it improves. Valves are indicated if it goes in 1 or 2 biggish jumps and then a series of 10 lbs jumps.

Those figures are not the end of the world but are indicative of the decline of its health. I would agree with you concerns about fitting a better carb wouldn't be worth while.

Colin

OK, so just got in from this afternoon's tinker and have the results of the second test. As suggested - I did two tests on each cylinder,

Cylinder 1: 155 & 155 PSI

Cylinder 2: 140 & 145 PSI

Cylinder 3: 127 & 127 PSI - But then I ran this again and it come out with 140 & 140 PSI - maybe I hadn't screwed in the guage properly.

Cylinder 4: 145 & 145 PSI

So a lot more consistent - 15 PSI difference across the lot. Thanks for the throttle tip, Mantasport.

colnerov - alas, I didn't have a spare pair of eyes to help, so couldn't say how the readings were reached. Good to know though.

steveo - plugs are all the same and look fine. The MOT emissions test came back well within the limits. Doesn't smoke either.

So given all that, do you all reckon I should go ahead with the carb swap?

Yeah thats looks better, i'd carry on with the carb swop with those numbers.

Its very easy to get a small difference with how tight you screw the gauge into the plug hole

Oh and for clarity when i said do them twice i meant test 1,2,3,4 then start again with 1,2,3,4. That way you have removed the gauge from the hole and re-fitted it before the 2nd test on the cylinder. Which should eliminate the posibility of not having the rubber seal fully down.

So your above test would have given 127psi then 140psi on cylinder 3, which would have had me check a 3rd time due to the large difference (removing and refitting said gauge) then if we got a 140 again it would show the 127psi was a testing issue rather than engine fault.

Also bear in mind that the manufacturers figures are for a calibrated pressure gauge. 'DIY' gauges can be a long way out (15 - 20 psi is not uncommon). It's quite conceivable that the figures are closer to what they should be than the gauge is actually indicating.

Also can I just stress you cannot check engine compression if you take the plugs out, come on guys!!