Mikuni advice

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Jul 10, 2004
:?: My 72 (Mikuni & BB Ign) has been running quite well - starts on 1 or 2 kicks and idles well when warm - but the last few rides have been strange due to sticking choke. I sprayed the little plunger with some choke cleaner and it freed it nicely. While I was at it I unscrewed the top of the carb and sprayed some cleaner in there. Seemed like a good idea. Evidently I did not reassemble it quite right - now it starts and revs up to a very fast idle, and takes many seconds to calm down. Before I make it worse I'd like to get more information....

I have had carbs apart before - but I have no instructions or diagram for this one. Also, I assume the smaller flush screw on the right side is the idle adjust, and the larger screw with spring on the left is the idle adjust???

Any tips would be appreciated.

Stuart Ostroff
The screw with the spring is the idle speed adj, this simple lifts or lowers the slide. (you'll notice a little ramp on the slide that it runs on if you remove the slide again. The other screw is the mixture screw (generally 11/2 turns out but never more than 3 turns out). The mikuni doesn't realy have a choke in the strict sense of the word (a choke restricts air flow thereby enrichening the mixture) but rather the plunger operates an enrichening circuit. Often times the plunger itself sticks a little. Is your problem a difficulty closing the choke after starting/warming the bike? if so a little well placed, light weight lube on the plunger may do the trick. Press the lever down, thereby engaging the choke and exposing the plunger and wipe a little lube on it then operate it a few time to work the lube in.

The fast idle may be due to the slide binding a little after taking it apart, take the top back off and make sure the slide will move up and down in its bore easily, make sure the needle is seated properly under the spring. Just taking the top off should result in the need for any adjustments.
For a great source of info on Mikuni carbs download the MAPCYCLE catalog, it devotes about 5 pages due tuning mikunis and supplys exploded images for your reference.
I meant to say shouldn't result in the need to adj speed or mixture. Also check to see that the cable doesn't bind. Move the bars side to side and make sure no change in idle speed occurs.
If the enrichener is sticking it should rev to high when warm rather than calming down after warmed up. Check the slide before making any adj, ensure its moving freely and seated properly when the throttle is closed.
Progress? Not

I swung the carb over to one side so I could open it up and really inspect the bore, etc. Everything looks OK, cleaned it with carb cleaner and reassembled carefully. With the engine not running, the slide snaps shut smartly, spring feels OK, cable is free and has good slack, etc.

When the bike starts with the choke the idle is high and rough - so I took it out to warm up and reassess. Once warm the idle is just fine, but it feels as though something is keeping the slide open for a long time, like 5 or 10 seconds before it settles down. A few times it came down quickly, but mostly the engine speed stays up, during upshifts, etc. Very wierd. Really feel like it needs a stronger spring. Runs fine - not popping or backfire, just stays open like I didn't let go.

WHAT would cause this to happen, after the carb seemed fine for the last 2 months. All I did was lube the choke slide, open the top of the carb and look inside.

Perplexed, Stuart
Solved, for now. . .

:) After a chat with Stan Smith at Rocky Point Cycles I decided to add a second spring in the carb. Stan sent me one along with a new air filter. Immediately upon starting the engine I could tell that the problem was fixed - it returned to idle instantly. Have been out a few times - first ride I needed to get used to the increased throttle effort, not really too much, just different. By the second outing my shifts were smooth again.

So - why would opening the carb and spraying in some cleaner cause the original spring to not be adequate? I'm a little perplexed but I have found a solution that works, so on to other issues.

Stuart Ostroff
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