a little help please ive been trying to diag. my 91 1000 for a misfire for some time now. my timing i believe is correct, both ignition and valve. my ICM is good. I think my compression is low it ranges from 175 to 125 psi. any help is greatly appreciated.

 

Anytime you're doing a compression check on a motorcycle engine make SURE that the throttle is NOT on the idle stops. It MUST be open to get a correct reading. This is true of all engines with individual short intake runners. Just open it wide while cranking. Many an engine has been torn down to no avail due to an incorrect procedure on a compression check.

Actually, any engine should be done with the throttle open but it's not as critical with a common manifold as most auto engines are. On a car, the readings may still be low but the intake fill from cylinder to cylinder won't tend to vary and therefore may still accomplish the intent of the test, to find cylinders with mechanical leaks.

What happens on MC engines is that even minor variations in idle synch will show as a low cylinder due to the flow restriction of the throttle plate and the low volume of air in the individual intake. The combination will vary the filling of each cylinder which can show up as wide variations in compression pressure. A common manifold has more volume and will at least balance all cylinder's intakes because they are all drawing through the same set of butterflies, where the individual intake runner and butterfly isn't. Hope that helps.

 

thank you for that. i assumed i should do the compression check with the throttle open. in fact i did it at wide open throttle and engine hot. i was at my local honda store and got to talk to a tech, finally, and he suggested that i could have smacked a couple valves together in the low cyl. he suggested checking my valve clearences and go from there. dont forget this bike has 31,000 miles on it and it was bouncing off the rev. limiter when it stopped running correctly, i missed second. so that is were i am going to check next. i know there is a lot of time between my posts but i have about four progects going at any given time, plus grandparents to take care of. so ive not given up on her yet but im gettin there. also i really do appreciate the time taken out of your busy lives to answer my silly questions. thank you

 

Ouch! Sounds like your local tech is correct. Time for a teardown. I'm waiting for the Honda mechanic to finish doing the valves from my '91. Getting the head off takes some digging around, but can be done with the engine in the frame. The tech was correct when he told me I was doing the hard part in removing and replacing the head and he had the easy part in doing the valve job.