Minimum cylinder compression value? (Have Haynes got it wrong?)

[chrishiggs]

Hi Folks,

Just checked the compression after trying to bump start bike whilst hydrolocked. Both cylinders are reading ~139psi so thankfully I've not bent a rod. Acording to the Haynes manual the minimum compression specified is 142psi for both cylinders but having searched previous threads I found a minimum of 114psi mentioned. Does anyone know what the Suzuki workshop manual says? I'm hoping the figure in Haynes is wrong as the bike's only done 11k and otherwise seems absolutely fine (wee bit of smoke at startup only). I should mention that the valves were checked, motor warmed and throttle held open. Thanks in advance!

[The Buddha]

Hot compression is what counts. I have run a bike that read 100 or less cod and 150's hot.
Cool.
Buddha.

[chrishiggs]

Thanks. The motor was hot when I got 142. Just wondering what Suzuki specify as the minimum before piston rings/ valve regrind or whatever is recomended? Cheers.

[chrishiggs]

Sorry, meant to say got 139psi when motor was hot!
As I say, I' m really just wondering what Suzuki specify as the minimum before piston rings/ valve regrind is recomended? Thanks in advance.

[beRto]

Hi Folks,

Just checked the compression after trying to bump start bike whilst hydrolocked. Both cylinders are reading ~139psi so thankfully I've not bent a rod. Acording to the Haynes manual the minimum compression specified is 142psi for both cylinders but having searched previous threads I found a minimum of 114psi mentioned. Does anyone know what the Suzuki workshop manual says? I'm hoping the figure in Haynes is wrong as the bike's only done 11k and otherwise seems absolutely fine (wee bit of smoke at startup only). I should mention that the valves were checked, motor warmed and throttle held open. Thanks in advance!

This was taken from the 1990 section of the Suzuki service manual. As the engine design has never really changed, these values should apply to all model year bikes.

Standard compression values are 142 psi - 199 psi, but the service limit is 114 psi. 

It is also important to ensure that you do not have more than 28 psi difference between the compression values in the two cylinders.

[beRto]

Just for reference, I think the Haynes manual says the same thing:


In other words, at least one of the cylinders must have a compression of 142 psi. This means the lowest compression allowable in the other cylinder is 114 psi (i.e. max 28 psi difference).

[The Buddha]

I will put oil in it and gas in it, set the valves, check plugs and run it till it well ... quits.
If your tranny is good, a top end job is all you'd need, its easy and not that $. Bad tranny and you might as well buy another motor.
Cool.
Buddha.

[Mandres]

open the throttle all the way when you take the reading, it can make a difference.  Really, since you're close to spec and the figure is the same on both sides you have nothing to worry about.

[chrishiggs]

Thanks for the help folks. Yup, think I'm not going to worry til I start burning significant amounts of oil. Will check the compression again in a while to verify- did have throttle wide before tho. Going to enjoy the bike now- one of the most flickable bikes I've ridden plus light and low- perfect for slick scottish winter roads! Thanks again.      Chris

[The Buddha]

Throttle WFO helps, but having carbs completely off really works well. Slides, air filter ETC all can be a PITA.
Cool.
Buddha.

[chrishiggs]

Thanks. Also it occured to me that the compression tester I bought may be inaccurate. It cost all of about $35 so probably not the best quality! May try to borrow a good one from a car mechanic friend so I can check its accuracy. Having been out on the bike again today I noted that there was no smoke at all even at startup. Runs great and no smoke whatsoever. It could be that when the bore filled with petrol a while back it's taken the carbon build-up off the piston crown and rings and lessened the seal with the bore. (The virtually brand new oil came out black like it had been in the bike for years.) Either way it runs great so I won't worry. Just waiting for delivery of a CRF tailpipe off ebay then I can worry about rejetting and getting rid of that damned mid-range flat-spot! Cheers.

[Mandres]

yea if it's running well with no smoke and no excessive oil loss over time I wouldn't give it a second thought.