Commando won't run with Mikuni "choke" off

Yup ,you could have the choke action reversed ,it's happened before.

This may or may not be relevant but I'll post it anyway as it may be of help. I have a 750 with a brand new single Mikuni. I used to have similar problems from initial installation until I developed a starting routine. The bike always starts first or second kick and runs fine. What I found the problem to be in my case was that the bike would run fine on choke and then die or run like crap for a minute or two when you flipped the lever off until it cleaned itself out. I'm assuming, possibly, my idle circuit is too rich and the plugs are loading up. Other than that bike starts and runs great. Any way, without having done any physical changes to the jetting, if I start the bike on choke I let it idle for a minute or so to warm up and when I'm ready to go I bring the revs up to about 2000 rpm and flip off the lever. I hold the revs there for a second or so and then give the throttle a couple of blips to clear the cobwebs out and I'm ready to go. If I let it idle too long on choke, the symptoms are worse. I know the bike well enough to know when it's time. This works well for me and I just accept it as part of my starting drill.

Fast Eddie said:

Before you change it, check properly that it isn't partially blocked.

My 850 has a single Mik with 35 pilot. It was partially blocked. Now I've unblocked it, and it is too rich. So Id say, 35 is too big.

I soaked mine in thinners and sprayed carb cleaner through it, and blasted it through with a compressor... all with zero result.

I had to unblock it physically, using weld tip cleaners.

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This is the only way to insure pilot jet is open IMO and experience.

Some say it's wrong, because you can damage the orifice, but if your careful it will not.

I bought a set of tiny drill bits at a Hobby Shop that gave me a range of sizes for tuning jets between sizes. This works very well for opening plugged jets as well as changing size of orifice. Rotating the proper size bit through the orifice by hand will clean it completely without changing the size or damaging the orifice. A little carb cleaner helps to soften the old crud and it will clean out perfectly every time.

NOTE: You have to take the jets out of the carb to get them cleaned properly. Spraying carb cleaner and blasting air in a jet in the carb rarely gets them clean.

Plugged or dirty pilot jets are one of the most common issues with the new crap fuels.

The MIK choke lever opens up another fuel circuit that delivers too much fuel causing ultrarichness and sooty plugs. Flipping lever back to normal can change back to a too lean state if not warmed up enough combined with sooted plugs can cause a dead-stall.

What's the old saying ...........99% of all carb problems are electrical ............just saying :roll:


Tim_S

what mostly blocks pilots and its called zn oxide so no solvent can touch it just acid or base wash and physically knocking it clean. On the electrical note might try a range hotter plug, though a proper C'do tune is lean enough warm it should need tickler choke or enricher to start cold. The 2 Miki's I've had on two Combats and the one on Wes's '71 all worked a treat - flip lever to start then few seconds later flip off while blipping enough to keep smooth running and few second later ride off on low off idle throttle stumble free. Pilot 25 in ours.

stuck amals said:

Update,

Fired it up with the choke on. It idled good. I let it idle for quite a while then turn the choke off. It would run for about 30-45 seconds as log as I pumped the throttle every few seconds.

Then I got the crazy idea to adjust air adjusting screw from it's recommended 1&1/2 turns out. The further IN I went the better it got. I'm now all the way in and it idles smoothly with the choke off.

I"m running a #25 pilot jet. It seems I need to go to #30 or #35.

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Screwing the " further IN I went the better it got." (there's a joke there) is reducing the air at idle !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! so it getting too much air ! .... You have an "air leak" somewhere... and yes i see that you have "checked"......... but ..

stuck amals said:

My 1974 Norton with a single Mikuni will start and run with the choke on. ( I know it's not really a choke) When I turn the choke off engine will die. I can't find anything wrong with the carb. In fact I took the Mikuni carb from my Combat and I get the same results.

I sprayed carb cleaner around the rubber carb mount and 2 into 1 manifold to see if it was sucking in air from somewhere. No Luck.

Any suggestions?

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Check that the carb to manifold rubber is ok. I was having similar problems to you and eventually traced it to damage on the manifold side of the carb rubber, probably caused by me overtightening the mounting screws. Once I replaced the rubber the bike returned to a sweet starting/running machine.

sam

My mk3 was running fine with this.. W0uld only been a matter of time till the carb "fell off"... Now has new flange rubber and a small support/hanger bracket to hold weight of air filter and carb.. The following post is same bit..... i cant do 2 pic in one post ...yet [album][/album]

olChris: I had the same problem a few years back. 'bout time for a new set, maybe.

I took out the #25 and put in a #30. This gave me more adjustment on the air adjustment. I went on a 2-3 mile test run and everything was fine. I waited until it cooled down and checked the plugs and fired it up on 2 kicks. This was enough to get me through inspection at the DMV so I could get my expired plates renewed.

Come warmer weather I'll head back to the garage.

I appreciate all the advice and comments.