I just did a compression test on my mc21 and I got 110 psi on the front cylinder and about 80psi on the rear cylinder.
Is that normal or should they both be in the range of 115-120 psi?
The bike is use for the street.
Firstly, both should be about the same (like 5psi out), so it sounds like you have a problem there.
Secondly, the pressures should be 15-120, but anything over 110 is good enough.
I'd get that low one checked out though, sounds like a worn ring on it. Pop some oil into the spark plug hole and try it again, if the pressure goes up then you need new rings. If not, you've another problem somewhere... _________________ MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball
Sorry, my name is Cuong. Thanks Dave for the advice. I went and put a little oil in the spark plug hole, give it about 4 kicks and the compression went up to 115psi. Guess I'm gonna need new rings then.
Glad that trick worked. At least it's only rings and a base gasket you'll need, and nothing more drastic!
When was the engine last apart? It may just be that the rings are gummed up, and not expanding to fill the gap. Only way to check is to remove the barrel (leave the head attached - it'll save the price of a head gasket).
Let us know how ya get on... _________________ MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball
Well, I just bought it about 4 months back. And the person I bought it from said the top end was done recently.
I just took out for a ride for first time yesterday and everything was running well for the first 20 km. It was running great. Then the thing died while waiting at the traffic light. I was able to restart it, but it looks like only one cylinder was working as I was crawling home. When I got home, I checked the plugs. The front was ok, but the rear plug appear as if piston had rammed against it. Part of the tip was gone and the the rest is welded or pressed again the core. Both plugs looks wet. The plugs are different also. The front is BP9ES and the rear plug is BR9ECM. What's the norm?
The only thing I did was install the HRC jet kit. What do you make of it? Do you think I should do the whole top end on the rear, piston, ring, etc. or will replacing the rings do the trick.
That plug sounds like it has melted. What colour was the end? White, brown or black?
What sizes did you put in from the jet kit? _________________ MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball
The color of the plug looks brown/tan on both plug, but it looks pretty oily.
Here is my jet setting:
Front cylinder: MJ-140;SJ-38;PJ-0;needle-B-2nd ring from bottom; new emu tube;new needle holder
Rear cylinder: MJ-142;SJ-38;PJ-0;needle-B-2nd ring from bottom
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What colour was the end? White, brown or black?