cylinder compression

[RobH]

Hi guys, hopefully someone will pick this one up, posted the same question in a previous post but not as the subject. Please forgive the repetition!

I'm trying to identify a ballpark figure for cyl compression pressure, ideally someone who has checked their pressure and knows it to be good, a 'guestimate' will just confuse things.

The reason for this is my pressure seems to be extremely low on the two outer cyls (can't get my adapter on cyl#2 for obvious reasons :-(). The bike isn't running right but I have no reason to suspect it's compression other than the readings are low, 108 psi and 102 psi (!!!!!). I haven't dropped any oil in yet so I don't know if I'm looking at rings or valves, engine was warm when tested.

Any help appreciated.

Cheers

H

[RobH]

Hmm...I seem to make a habit of answering my own posts...

Spoke to triumph today, they say they wouldn't publish such info themselves as it is so subjective - huge variation depending on what test you do (wet, dry, warm, cold, leakdown etc) and what equipment you use to do it. The best indicator being each cylinder within 10% of the others - this generally shows good.

I still think a benchmark could be set for given test conditions - the usual is warm engine and dry. My figures look poor on the surface but without applying the same measuring technique to another bike it's impossible to say...

Back to the drawing board.

Cheers

H

[JetdocX]

A leakdown check will actually tell you WHERE you have a problem in addition to IF you have a problem.  

In aircraft engines, it's 80psi in and 60psi holding minimum.  Listen for leakage and you know where to go to fix your problem.

[BR]

The last time I checked the compression on my '98 steamer it was about 170psi across all three holes... pretty good in my book.  I have a non-Triumph shop manual and I can check what their listed spec and post it later.  Btw, my readings were taken with engine cold, cylinders 'dry', throttle wide open, crank it until gage reading peaks out.
If yours are around 100psi, that is indeed quite low...  Adding oil will indicate where leakage is occuring so either your rings are leaking, or perhaps your valves are a bit tight....

Brendan

[RobH]

Cheers guys. Adding oil does improve compression on the engine when it's cold (haven't tried it warm) but then I'd expect that of a 'good' engine as well...according to Triumph it would be burning quite a bit of oil and smoking heavily if it was a bore problem.

The valve clearances are all spot on, all I can think of is burnt valves or maybe weak valve springs (bounce?). It does still sound pretty poor (rattle) from the top end when it's warmed up. On the lookout for a decent, complete head.

I had a ride on my mates '96 model a few weeks ago and mine is actually quicker - his is relatively low mileage and sounds much nicer than mine. Maybe I should quit whining and just ride it!?

Cheers

RobH

[JetdocX]

Compression is also a function of how quickly your starter is able to spin the engine.  If your battery or starter motor is tired, your compression readings will reflect that as well.

[Mustang]

The valve clearances are all spot on, all I can think of is burnt valves or maybe weak valve springs (bounce?).
RobH

I doubt you have burnt valves ...... , if intakes were bad it would backfire thru the carbs constantly and generally run very poor . and if it were exhaust valves that were burnt it would constantly backfire out the exhaust pipes and even shoot flames out the exhaust cans from unburned fuel making it's way past the exhaust valves

are you sure the coils are good ?
also plugged up pilot jets will make a steamer do funny things at rpm's below 3k