Rough at startup, then not

Wow. Just got home from work and have a ton of suggestions! You gotta love this board I have a few hours tonight to run down the check list I've compiled from the responses. If I don't figure it out tonight, it will be Sunday before I can get back to it.

Things I know I need to check tonight are the fuel tank (not coated, kinda rusty), petcocks, the pilot jet size, needle position, and the air screw... I swear that air screw sometimes moves on its own (weak spring???). I always set it to 1.5 turns and it seems to vibrate it's way off of that mark.

Thanks again, this has been a huge help. Will report back as soon as I know more.
-paul

cash said:

Big back-fires to me usually mean temporary loss of spark. When the spark resumes unburned fuel in the pipes is ignited with a bang. I reckon you should be looking at the ignition, coils and plug leads, you may well have a weak spark.

Best of luck.

Cash

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Who was it who said "99% of all carbureter problems are electrical"? If you approach this problem systematically, and keep your mind open, you'll be back on the road in no time!!

I think I have ruled out the carb. All the various jets and screws look good, and it doesn't seem to be starving for fuel. Next I'll tackle the electrics, starting with the plugs and moving backwards... but that will have to wait until Sunday. Time for a beer! That'll make things alllll better...
-paul

How long has the carb been on the bike as it sounds to me as if you could be to leen on the pilot jet, if you are slightly leen it may be fine in summer but as the atmospheric conditions change ie gets colder more oxygen in the air it will run leaner and your problem will be more pronounced

I didn't see this mentioned, but you might wanna check to make sure you haven't over tightened the clamp on that mikuni. If you over tighten them, they develop a vaccuum leak and cause all sorts of problems similar to what you have described here. Simple test of spraying a bit of WD40 around the intake to carb rubber while it's running. While you are at it, check to make sure there are no air leaks at the head and where the adapter bolts to the carb flange.

B.

Well I haven't had much chance to look into the bike, but I did check a few things. As for the Mikuni, it has a 35 pilot, the needle is a 6DH3 and is set at the 3rd notch, and the air screw is 1 1/2 turns out. The petcock seems fine, fuel poured out with it open and the fuel line off the carb. I took a look at the plugs and they seem to be getting good sparks. I also tried to static check the timing. I opened the crank viewing plug and bumped the engine (using the rear wheel in 4th) until I could see the mark, then looked at the timing mark on the rotor and it was just about dead on the notch. SO I am guessing if the timing is off, it isn't off by very much. When I cranked it over tonight it seemed like it wanted to start, but ONLY after I had released the started button. It would 'cough' but not catch, if that makes sense. I will try to check on the coils and the rest of the electrics this weekend, but I am getting a little demoralized... spark + fuel = go, right???
-paul

I was just reading another post by Ron L. and he mentioned that condensers could create issue. I still have the blue can hooked up, as well as the kill switch... could this be a problem?
-paul

Run on the choke shorter not longer,start on choke, switch off choke and manually hold revs at about 1500 rpm until it idles , it seems like a common mikuni complaint, but the shorter time you run on the choke the better off your bores will be :idea:
Does it start when you use the kick start :?: firing when you release the starter button sounds like a voltage loss problem or a ballist resistor,does a mkiii use a ballist resister :?:
If you run to long with the choke on you will start to foul the plugs it will then miss and fart for a mile or so untill they burn clean,around 2500-3500 seems hardest on the ignition, does it run even worse with the lights on :?:

c.cito said:

So I took her out for a 20mi ride at speed. Absolutely did not want to start from cold, had to bump start it. Ran rough again, but then seemed to settle down... until I took a corner at no throttle, at which point she died. Had enough speed to pop the clutch and get underway again, but I had to keep the revs way up just to keep going. Under throttle it ran fine, just would not start and couldn't seem to handle low revs. Took out the new Autolite plugs and they looked fine. The battery 'seems' fine (reading 12.8v), but I should probably test it under load. I had the carb off awhile back and cleaned out the pilot jet, but didn't really do much else. Tomorrow I'll tear into it again. It seems to be getting worse, so that might be a clue... Thanks for the suggestions, I need all the help I can get.
-paul

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Hi I had the same problem with a Mikuni 36 asked on this forum and was told to only use the starter enrichment circuit to start then flick it off and keep the engine idling using the throttle or some advocate adjusting the idle screw to keep a good idle until the bike warms up. Fixed my problems straight away.

Cheers Paul

A minor miracle... she started up. Took out the plugs (AP64s I think) and replaced them with NKG BP7ES (old ones at that) and she fired over. SO I guess I need to dial in the carb better so they are not getting fouled at all and start using the choke less. I don't think it's fixed, but at least I can start her, so I can check timing, etc. Thanks for all the advice, huge help!
-paul