Struggling with Mikuni

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Aug 20, 2005
Does anyone haver any ideas, I am struggling. I have 850 Commando with single Mikuni for the last 10 years. This worked fine all that time till head gasket went recently. I did that plus exhaust guides & valves, took off barrel & added 30 thou compression plate to give motor an easier time. Bores rings & all else as new.
Bike now runs great in all but the very lowest throttle settings & appears to be very rich on idle & eighth throttle, smoking, misfiring. fouling plugs. Smoke appears to be petrol not oil. Left cylinder much worse than right. Have tried plugs transposing ignition parts to eliminate these before looking at carb. Dont forget I have not touched carb. Tried smaller pilot jet & throttle slide lower needle position all making no difference. Wierdest thing is that idle becomes better when air screw is all the way in which surely makes mixture richer!
Have stripped carb checked & blown through all airways & can see nothing wrong
Pulling hair out now!

Any ideas anybody?? HELP!!!!!
Have you checked for air leaks around all the connections? (This would be the reason air screw is in all the way and it runs better)sounds almost like a small air leak possibly the left side as it is worse although this could also be other issues, valve gap etc.
Also once you decompress the motor, the carb settings would almost certainly change. I am by far not a Mikuni expert but know you can buy shims for the needle. Also they made probably 100 different needles with lengths, taper diameter etc. Takes forever to make one run well if it is out of whack.
We all are assuming electronic ignition?
Steve Jowett
If it was air leaks, mixture would be running too weak, anyway carb has been off several times & goo used on connection, so I am happy no leaks. I am using boyer, valves are checked, neddle position makes no difference, but this problem is only on vers small throttle openings. I spoke to Allens who thought small change in C/R will not vastly alter mixture requirements. Big mysteries are why is one cyl. worse when it is a single carb set up & why does more pilot air make it worse?
Hi Geoff....
Never had a Mikuni...let alone have I had one apart...but if the carb has 3 jets...a main, a midrange and an idle jet...and the main and the midrange are in appearance the same...could be that you have traded the main for the midrange when you cleaned the carb. I spent weeks fooling with my neighbours' Honda before finding that the bozo that last worked on the bike had put two mains in one carb and two midrange jets in the other.... something to check when you go back into the carb.......

if one barrel acts really different than the other.....might be two different problems...sure it's gasoline smoke/fouling? One side problems with smoke, will tend to be oil rather than gas...valve stem seal will do that.

Make sure the both barrels have about the same compression too...if one is might have to play too much with the carb to get the bad barrel to work and in the process...really screw up the adjust for the good barrel...

just some ideas.... :wink:
Thanks for your thoughts chaps

Trouble is HEw I had not disturbed carb before problem arose, so jets are not mixed up. They are different anyway. As regards poss oil smoke, this does not smell like burnt oil, but does smell of petrol & running is rough at low throttle settings only. I will do compression test when I have borrowed a tester! otherwise I think I will need to cadge a carb to try on to be sure it is definitely that or something in the engine.
None of it makes any sense.
I would start from scratch using the 'Victory Libray' Mikuni Tuning for British Twins.

P/S I am NOT connected in any way to them.
As suggested...a good tune of the carb might be the trick....and if out the carb with a known good one...will surely let you know if the carb is at fault...but my instinct says there is some difference between the barrels...let us know! Greetings from Schnitzelland... :D

Umm, only at idle or 1/8th throttle? Did you switch condensers & check charging/battery output at low speed?
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