Compression test

[Steve J]

Just remember that on a 4 stroke engine, compression starts with the piston at BDC (more or less). On a two stroke, you'll not get any compression using the kickstart until the exhaust port closes.

[johnnyXS]

yes... but a 2x stroke engine fires every revolution so it produces compression every stroke unlike a 4x stroke engine which has valves and produces compression every second revolution. A single kick on a 2x stoke should turn the engine over 3-4 revolutions .
If you prod the kickstart 3-4 times it should turn the engine over 9-14 revolutions which should be more than enough to give a maximum compression reading.
http://www.animatedengines.com/twostroke.html

What is your compression like Steve ?

[Steve J]

It's not really the point - as the piston rises on a two stroke (on a non-running engine, just turning it over with the kickstart) the area above the piston is effectively at atmospheric pressure (via the exhaust system) until the rising piston closes off the exhaust port. The point at which compression will start is dependent on exhaust port height - even with a standard Lambretta engine, the exhaust port is about 17mm high, so with a 58mm stroke, compression starts with the piston about 41mm below TDC. So a 7:1 theoretical CR is reduced to around 5:1 due to the shorter distance of piston travel from the point the exhaust closes. On that basis, a figure around 75psi might be reasonable. Obviously, once the engine is running, pressure waves running up & down the exhaust will change all that....

Since you asked, I've never measured the cylinder pressure on my GP200, but the last time I checked it on my Triumph Spitfire (with 10:1 CR & high lift cam), I was getting around 198psi!

[johnnyXS]

I take your point but you don't need a lot of piston travel to develop compression .
You only need to look at the force required to pump up your bicycle tyres with a hand pump and thats against soft rubber inner tube and tyres which have give in them.

Yesterday I found that just 3x kicks of my kickstart gave me a max reading of 125psi .

[Triffid]

I have little confidence in the accuracy of my cheapo guage - the thing is the relative readings between a well running scooter and one that struggles to fire up, let alone settle down to a decent tickover. As for trying it on my car, being a diesel that option is ruled out. Hohum!
Anyway, the barrel and piston will come off today to check the piston/bore gap and whilst there, I'll try to measure the squish clearence. Thanks for all the feedback - it's the value of the forum to get others thoughts/experiences.

[johnnyXS]

I have little confidence in the accuracy of my cheapo guage - the thing is the relative readings between a well running scooter and one that struggles to fire up, let alone settle down to a decent tickover. As for trying it on my car, being a diesel that option is ruled out. Hohum!
Anyway, the barrel and piston will come off today to check the piston/bore gap and whilst there, I'll try to measure the squish clearence. Thanks for all the feedback - it's the value of the forum to get others thoughts/experiences.

If your gauge isn't faulty I would say that you have a compression leak somewhere on the SX to have such low compression readings. Probably a leaking head gasket.
Before you strip the engine it might be interesting to re-torque the head to the correct torque then re test the compression to see if there is any improvement however slight.

Having a piston thats slightly undersized would not account for your low compression reading on the SX. if your rings are still in one piece you should still get a good ring to bore seal and better compression that you've got.


It would be very useful to test your squish BEFORE you strip your engine otherwise you'll lose the opportunity to get some valuable information about the present state of the engine and also have no baseline to measure against once you've rebuilt the engine and test the squish.
Do you know which cylinder ,piston and head you have on the on the SX ?

[JETEX]

Exactly Johnny. Anything less than 100 psi on a petrol engine is too low ( in my opinion ). Although I've seen some plastic Chinese things with very low compression ratios, so much so that they refuse to start after a few months.