1 black spark plug

I just got a single carby (Khein 32mm) fitted on my -73 Commando 750
runs great for 10 minutes then the right spark plug goes black , left one are medium to dark brown....
I have checked the play in the valves and they are all perfect, did a compression test and both cylinders are at 13 kg
Have been playing around with main jets...no difference
I had the 2 Amal 932 before
But the performance boost from the new carby is stunning...those first 10 min....
Anyone that can help me with this one ???

Patrick

poor running on one side

Hi Patrick,
Although the valve clearance may be correct there may still be a question of valve seal/valve guide condition. Also, while compression may be good and equal I suggest you do a cylinder leakdown test, this test can be much more revealing.

GB

Re: poor running on one side

geo46er said:

Hi Patrick,
Although the valve clearance may be correct there may still be a question of valve seal/valve guide condition. Also, while compression may be good and equal I suggest you do a cylinder leakdown test, this test can be much more revealing.

GB

Click to expand...

How do u do a Cylinder leakdown test???

Cheers mate

Re: poor running on one side

Patrick said:

How do u do a Cylinder leakdown test???

Click to expand...

Leakdown test explained: http://www.mobiloil.com/USA-English/Mot ... _Test.aspx

Another thing to check is the electrical. After 10 minutes when the bike and electric's are getting warm that coil on the left side might be failing when it gets warm. Any easy test is to swap the two coils or plug wires if they reach and see if the results are the same on oppisite sides.

there is more to jetting a carb than just the main jet. at what throttle opening are you running it at? most carbs have at least 4 jetting circuts and each come into play at different throttle positions along with some overlap. most spend the largest time on the needle circuit and less on the pilot/slide than the main the least unless you are on a race track.

windy

Sounds like somewhat overrich jetting (medium to dark brown left plug - on unleaded!) combined with right-cylinder oil fouling - rings or valve guides, perhaps.

PS, my experience with Keihin carbs (detailed elsewhere) is they come pre-jetted WAAAYYY too rich. So that's probably PART of your problem. But if you're getting different plug readings off a single carb, then absent an air leak (check your intakes) or exhaust leak, chances are an electrical problem (what's that saying? - "90% of carburetor problems are electrical") or you're getting oil in the cylinder. (switching plug leads a good check for the former, leakdown for the latter). Good luck.

Hi Pat, what was the problem in the end? I have the same problem, one black plug on the RHS? Then engine has not long been rebuilt and I hope its not oil getting past the rings as some one has suggested.

patrick,didnt you have boyer wiring issues as well, put some tape on your throttle and mark it out evenly full 3/4 ,1/2/,1/4 you may be pleasntly suprised 70 mph highway cruise is about 1/4 throttle,coming off the slide onto the needle,90-100 you should be getting on to the main,but single carb even compression I would be looking at your wiring first. Try pulling plugs out, lay plugs & leads on head key on,and wobble all the wires & connections watching for a spark.Some of the single carb manifolds I have seen only bolt up on the out side bolts and as such are more subject to air leaks. Dont start pulling it apart untill you have checked every thing else,check out the 'cam shaft culprite' thread a few pages back,o dear.

Boyer, one cylinder not firing correctly; it's a pretty good bet one of the pickup coil wires from the Boyer plate is broken. You won't see it for the insulation remains intact. The wire itself is broken inside the insulation. Usually the bike will run on both cylinders except at 2500-3500 rpm. Do a search here and you'll find the fix.