My MC21 is only running on the top cylinder,pulled plug it still has compression,changed out coil still nothing,where should I go next ?any help would be great,Thanks
Have you actually got a spark there? Remove the plug and with it still connected to the cap, hold it againt the head and kick the engine over. You should get a nice blue spark.
Get back to us... _________________ MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball
I'm having the same problem - just air going through one of the cylinders... Funny thing is the plug sparks fine outside the cylinder. Power comes and goes (cylinder goes on and off periodically ) so ignition connections might be screwed. Guess I'll have to check wiring system - going to take a long time cause I have no idea what is normal/abnormal. Checked leads as far 'up' as coils - resistance as per shop manual, any other suggestions guys? _________________ '87 MC16
Ok this is what I have done, lower cylinder does not run uless rpm is at 2000 or better ,did a compression test,both cylinders at 100 psi when warm. ,changed pgm and coil , plugs,started to think carb but squirted some fuel in carb while running did not fire off cylinder,i'll take any suggestions for help could it be throttle position sencer or could it sill be carb,i dropped the needle thinking it was to rich because it was smoking way to much, if its electricle were else should i look ?
100psi sounds like it needs some attention. From what I have read it should be at least 110psi. Be sure to carefully test the compression. The first time I tested mine, I had attached the extension bit of the guage - which gave me a lower reading as apposed to the direct attachment (see image if you don't get what i mean). Also make sure it's screwed in pretty tight, and kick it a few times.
I've got the exact same problem on the same cylinder (lower) as you do.
I have just been given another regulator/rectifier to swap out and test (Thanks Bryan ) to see if that is the problem. I'll let you know how i go with the reg/rec... when i get around to it
But I suspect it's a blocked slow jet as someone has suggested. A few times when I have come to a stop, it almost stalls and I've had to give it a bit of throttle to bring the revs back up over 2000rpm. I've yet to check the carbs as I don't have a clue about them
Hope you get it sorted. _________________ Jerry Lai
NSR250 [MC21SP]
Jezza's right, 100psi is a little low. Take another reading to be sure, and if it's still low, it may be time for a new set of rings.
Was it running ok before you changed the needle position? _________________ MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball
I'll try another compression test to be sure,but even if its low it should fire on both cylinders,I dont have a spare regulator,rectifier ,someone said they were going to swap I would like to know what happens,Dave this bike has never run right since I have had it ,I think its the carbs,slow jet or something,havent taken them off yet,is ther a gasket kit or something I should get and where?
Low compression can cause all kinds of running problems.
There's no gasket kit for a carb remove / refit, as ther're attatched by rubber boots.
If you haven't had it run right so far, then I'd start by removing and thouroughly cleaning the carbs. If you don't know the condition of them, it's the safest thing to do - blocked jets could sieze the engine.
I know it's a bit of a pain, and swapping electrical parts may be easier, but I think you need to do it anyway.
How much charge is in the battery when the engine isn't running? When you do get it running, and hold it above 2000rpm, how much charge is there?
This will tell us if the charging circuit and battery are ok. _________________ MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball
Ok think I finally found the root of my problems - one of the cdi units is not working . Suggest you swap round these units on your bike Lonestar to see if the plug fires when attached to the other unit... p.s. if anyone has a spare CDI for an MC16 give me a shout ). _________________ '87 MC16
Yeah I suppose... Parts manual lists it as 'CDI' but its just a palm-sized black box with two plug connections- one with 4 'teeth' and the other with 2. There are two of these boxes bundled in a rubber sheath. I think I have seen others referring to it as 'PGM'. _________________ '87 MC16
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) so ignition connections might be screwed. Guess I'll have to check wiring system - going to take a long time cause I have no idea what is normal/abnormal. Checked leads as far 'up' as coils - resistance as per shop manual, any other suggestions guys? 
) to see if that is the problem. I'll let you know how i go with the reg/rec... when i get around to it 
). 
Parts manual lists it as 'CDI' but its just a palm-sized black box with two plug connections- one with 4 'teeth' and the other with 2. There are two of these boxes bundled in a rubber sheath. I think I have seen others referring to it as 'PGM'. 