Reasons for low compression?

mizike77 · Post by **mizike77** » Thu Sep 10, 2009 4:16 am

Upon kicking the bike over, im getting about 115 psi inthe vert and 120 in the horz. Bike has high comp .40 pistons, and runs. A leak down test has just been done and it looks perfect. Could valve timing be off a tooth somewhere?

MartinMille BANNED · Post by **MartinMille BANNED** » Thu Sep 10, 2009 4:32 am

I wouldn't worry about 5psi at all . as a common rule with engines a 10% discrepancy is exceptable

Why do you ask ? is it really jumping all over the place?

Cheers Marty

mizike77 · Post by **mizike77** » Thu Sep 10, 2009 12:53 pm

I ask because it kicks over very easily, easier to kisk than my stepfathers 860gt even though I have high comps. Shouldnt I be up around 160psi? What are others getting for compression?

Craig in France · Post by **Craig in France** » Fri Sep 11, 2009 12:58 am

mizike77 wrote: <snip> Shouldn't I be up around 160psi? What are others getting for compression?

Compression readings are really a comparative, not an absolute, measure (but I guess you knew that already? :-). However, FWIW (and I quote):

"Calculating cylinder pressures

The target reading is calculated by:
• Ambient air pressure x the compression ration of the cylinder.

So, if the pistons are a notional 10.5:1 compression and the air pressure is 1100 milibar, the compression reading should be 11.5 bar. For 11:1 compression, 12.65.

Note: ambient air pressure changes with both altitude and weather conditions."

Also, in my experience, it can be pretty difficult to get 'good' readings using just a kicker. And you say a leak-down test has just been done ... if this was done correctly, it will be more informative than a simple compression check.

That said, have you tried the basic check of a teaspoon of oil down the bores and then taking the readings again? This will show up any ring wear, at least.

HTH

Craig

wdietz186 · Post by **wdietz186** » Fri Sep 11, 2009 4:57 pm

The valve timing is pretty easy to check.It involves pulling the timing side cover and the bevel covers.You wil need to remove the alternator rotor and the timing support plate to get a good view of the dots on the bevel gears.With the vert. cyl. on TDC the dots on the crank,bevel drive and bevel gears should all line up.Refer to the manual on this site for a good illustration.If your leakdown is ok that would eliminate rings and valve sealing as problems.NOTE the bevel support plate and the gears have shims that can get displaced easily if you aren't careful when removing the plate.

vargas1 · Post by **vargas1** » Mon Sep 21, 2009 7:03 am

If you have a Darmah, it's a desmo. Let's assume for the moment that your valve timing is correct. Desmo engines appear to have poor compression when kicked over. Compression increases when the engine is running and the gas pressure inside the combustion chamber snaps the valves onto their seats, achieving a gas tight seal. In my experience, desmo valve seats can degrade quite quickly. When you next come to do the valve clearances, you might consider taking both heads off and lapping the valves in — unless this has been done recently. This is also an opportunity to check the valves in their guides, a notorious weak point on all the bevel engines. If they are very sloppy, the valves tend not seal properly. Once you have achieved a good, gas-tight seal with the closing valve clearances set at zero, you should meet with good resistance at the kickstart lever. If not, then the piston rings and/or cylinder bores may be worn beyond service limits.

Having said all that, I went for a fast spin last weekend with a friend on his immaculate 900SS. His engine is alarmingly easy to kick over but there appears to be no loss of performance! What's your performance like?

Dave