Compression testers

[davidblythe]

playing around in the garage decided to have a try with a compression tester on a couple of scoots

got readings of 110, 120 and 140 psi

do these readings mean anything in the real world

ie is the compression ratio too low on the scoot reading 110, do I need to run 98 octane on the one reading 140 psi (which I do as the geometric compression ratio is well over 9.5 with a ccr of 6.5)

or is it merely a test on the condition of the rings

numpty seeks elucidation

[Ian Hepworth]

The yanks seem to swear by them.

[byron]

never used one on a scoot, but when using on my van, I never get too hung up on the actual readings... as long as they are all about the same, and consistent with last time i checked I think that's fine.
so maybe check the engines periodically and note any drop, suggesting further investigation required....
I'd test my scooter, but I ran over my tester and it's now broken...

[davidblythe]

never used one on a scoot, but when using on my van, I never get too hung up on the actual readings... as long as they are all about the same, and consistent with last time i checked I think that's fine.
so maybe check the engines periodically and note any drop, suggesting further investigation required....
I'd test my scooter, but I ran over my tester and it's now broken...

thats how I used the compression tester on old sheds I onced owned such as a reliant regal and morris marina's

am aware that the septics go mad on them, just as they are always jetting for altitude

just wonder if we are missing a trick here or is it snake oil

[sean brady scooters]

the use of compression testers is really not such a bad idea ...and i have used these myself on scooter engines.

on car engines ,four cylinders or more ...there main use is for comparing one cylinder to another.....
if all four cylinders provide similar results then all is generally fine...within say a 10% variation.
if one is down,then it usually points to low comp due to say burnt out valves or blown gasket etc.

on a single cylinder two stroke scooter engine,you first really need some sort of Datum figure as a starting point,other wise you cannot allow for worn bores/pistons /rings..etc.

higher comp ratios will of course give higher PSI readings.

but another thing to bear in mind is the difference between just cranking your motor over and testing PSI and what is actually happening at high speed running conditions....due to the plugging effect of expansion chambers....

[RICSPEED]

i agree with Sean ,if you know the psi of your newly built ts1 for instance you would then have that figure to check against possibly making it easier to fault find on non runners

[J1MS]

There is some mention of these compression testers being used on some competition two strokes motorbikes, one engine (cant remember which) had a compression test figure of around 200psi... Several quick kicks in succession were used to achieve this figure...

I have used a compression tester on my own Lammy engine builds in the past (130+ psi seemed a good result on a cast iron road tuned 200 barrel with a cold engine) as soon as there built then again after around 200 miles... but the connecting hose on mine seemed to get damaged by the rock of the engine as the kick start is pushed down, this caused an air leak on mine and I now need a new one... Also if its a center plug head, forget it...

Gear ratio on the first (kick gear) combined with the choice of sprockets, will and does change the PSI figures, as the engine spins slightly faster or slower as its kicked over, or at least thats what I found on mine... also a slipping clutch when kicking over can give a low reading

[dirtyhandslopez]

I think the reason I like to have one and use one (Snap-on jobbie) is that although I can tell just by kicking a scoot over whether it has good compression/low comression or stupidly high comresssion, that sometimes wont sit well with a customer. If I can tell them, without a doubt, that the motor is only putting out 90psi so it won't run right or 150 psi that will be breaking kicksart shafts, the customer is a lot more inclined to throw some money down and get things fixed/changed. I do know that Lammy's like at least 100psi to be starting reasonably easy.
Another reason I like using one is say I have had a run of Lammy's to work on(usually end up with three in a row of everything) and say a V90 turns up, the feeling on the kickstart pressure on a 100psi Lammy feels a whole lot different than the kickstart pressure on a 100 psi V90, so the guage will let you know what's going on, provided the same method is used.
BTW, I ain't no goddam Yank, ya'lls crazy

[Special X]

A compression tester will quickly tell you if the bore/rings are worn. If the reading is low but is higher when you add some oil to the cylinder it's an indicator of bore/ring wear.

[Lamaddict]

I've been told the first thing Charlie Edmunds does on any engine he's going to dial in is compression test it and from his track results there must be something in it. I mean what's the point in trying to tune an engine with an air leak or crap compression.