74 Mk2A,single VM34 Mikuni 250 main 35 pilot,Sparx ignition,Dual output single coil.Ok guys here's what used to happen.Put choke on,kick over,starts, put jacket, helmet, gloves on,wheel bike out of garage,shut door,get on bike turn off choke ride away.Now all of the above up to turn off choke,now when i turn off choke engine stutters and dies unless i hold the revs up.If i have to pull up at lights it'll cut out until i've been going for 5 mins then all seems well,no loss of power still pulls like a train!! Things i've checked,carb cleaned(about 4 times)checked for air leak on rubber manifold no signs,checked fuel flow fine,checked battery fine ,checked Sparx fine,checked coil fine ,changed plugs,checked timing all ok.Now i'm thinking leaky exhaust valve? Any thoughts before i pull the head??..........Thanks Andy...
What's going on? Valve job?
- Thread starter arc
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Has this just started happening? Has this been the case since you bought the bike? Have you done anything new to the bike? I know nothing about mikuni carbs, but what about float level in bowl?
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I second Rio.
You are loading up the spark plugs.
I have the exact same set up as you.
Cold start: choke on with no gas on starting kick, bike fires, immediately reach down and shut off choke while giving more gas to hold idle around 1200.
Helmet, gloves, etc already on before start drill.
After about one minute holding rpms with throttle only, take off and let it continue to warm up riding slowly.
The choke is not a warm up device, it is for starting ONLY.
You are loading up the spark plugs.
I have the exact same set up as you.
Cold start: choke on with no gas on starting kick, bike fires, immediately reach down and shut off choke while giving more gas to hold idle around 1200.
Helmet, gloves, etc already on before start drill.
After about one minute holding rpms with throttle only, take off and let it continue to warm up riding slowly.
The choke is not a warm up device, it is for starting ONLY.
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highdesert said:
exactly.. you can go up a bit on your pilot jet (30 to 35 etc) and it will make it a little less likely to die when first starting off when cold, but i'd first try the turn off choke rights away and hold rev's up with throttle till warm (can take a bit of time if bike driven infrequently or its cold out there)
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Thanks for the refresher on choke and warm up behavior. A well tuned Amal should not start w/o a choke or a tickle of richness, then best to attend to throttle till idles on its own. Sometimes, in my case too often, if I'd forget to unchoke and ride off into traffic till coughing almost to stalling, my plugs would not work well after wards.
After about 6 months on my 1st Combat I removed the chokes and my part of human error to use em or always be checking that first on any hick ups.
Its possible a stations gasoline formula delivery was not what you had prior tuned for.
New age stuff is learner mix on same carb set up as old days or even last season.
After about 6 months on my 1st Combat I removed the chokes and my part of human error to use em or always be checking that first on any hick ups.
Its possible a stations gasoline formula delivery was not what you had prior tuned for.
New age stuff is learner mix on same carb set up as old days or even last season.
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hobot said:
well tuned amal = resleeved (or new) in most cases. i have seen people go on about how their amal equip bike doesn't need any time for warm up- typically caused by worn slides and over rich setting resulting from the wear. I have new amals on by BSA and while they don't need the choke to start (tickle it once), they do need a minute or two or three as you suggest to run proper.
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hobot said:
I think you're discounting the effect of ambient temperature. I think a well-tuned Amal will start with just a tickle in warm weather, but remember that not all owners live in the warmest climates and choke may be called for.
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Before new gasoline a well tuned Amal should need choke or tickle to start or tends to be too rich once warmed up. I coldest I've started Peel was in upper teens at night. Took good tickles and lots of bliping mainly because oil so thick would drag engine down to barely kick over but also gas not vaporizing well. Boyer then so likely low voltage of 1.2 ah battery was screwing with Boyer brains.
My buddy just visited. Been on ride half the day 150 miles and one kick starts, till he went to leave here and took half dozen kicks with Micky carb enrichment on then a few more with it off to finally get going. Seems like mood and moon phase comes into play. But I've been so spoiled by 2 Combat that I'd not waste a 2nd kick on till I found a reason for no start the first time and almost always did to get back to spoiled. Nothing for it but keep plugging away with all the suggestions till success.
hobot
My buddy just visited. Been on ride half the day 150 miles and one kick starts, till he went to leave here and took half dozen kicks with Micky carb enrichment on then a few more with it off to finally get going. Seems like mood and moon phase comes into play. But I've been so spoiled by 2 Combat that I'd not waste a 2nd kick on till I found a reason for no start the first time and almost always did to get back to spoiled. Nothing for it but keep plugging away with all the suggestions till success.
hobot
arc
My Mikuni has always behaved that way, I've used the idle screw to keep the rev's up for the first few minutes after shutting off the choke. It's been said that if an engine will idle stone cold then the idle mixture is too rich. What do the plugs look like after you let it sit and idle for a few minutes after it's warmed up? Do you have ethanol blended gas to deal with?
My Mikuni has always behaved that way, I've used the idle screw to keep the rev's up for the first few minutes after shutting off the choke. It's been said that if an engine will idle stone cold then the idle mixture is too rich. What do the plugs look like after you let it sit and idle for a few minutes after it's warmed up? Do you have ethanol blended gas to deal with?
I would also fall into the camp that says the enricher is being left too long. I'm at the point I don't even use it, especially in warmer weather. Just kick it over and then let warm for a minute. If you choke, only do it for 15-20 seconds and then maintain with throttle if needed. I had the same issue as you and I was fouling the plugs following the same routine you described. Only issue I ever have is it not wanting to start until after 4-5 kicks after sitting for a week or two. Doesn't matter if the enricher is used or not, same result. After that, I'm golden.
maylar
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britbike220 said:
From the original post it seems something has changed. Stalling without the choke means it's too lean. I know nothing about Mikunis.. where can they leak air from?
And as far as need-or-don't-need the choke, every bike is different. My 850 wants the choke to run when cold, my brother's 750 doesn't. Go figure.
My choke is on for 90-120 secs max and as i said i would turn it off and ride away no probs been that way for 2yrs now the bike all of a sudden starts to falter as though not enough fuel??? until it's a certain temp? what's changed?........................Cheers Andyhighdesert said:
DogT
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Are you getting enough fuel, just like you say, check the gauzes in the fuel lines, any filters, petrol taps, junk in gas? Sounds like it is starving for fuel. That's easy to check, and usually cheap to fix too. I like to try the easy stuff first, but I always seem to forget and go for something expensive first.
Dave
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Dave
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As Dave says sounds like a fuel delivery issue or fuel air ratio issue at start.
Is your air filter clean or soaked by oil?
Makes breathing tough on start up.
I'd start cheap work expensive as well.
Pull the air box and clean or replace.
Blow compressed air through both air and fuel inlets at the mouth of both carbs.
Make sure these are clear. Smallest amount of dirt can act as a partial clog intermittedly.
Clean any carb bowl screen and blow out main jet since your in there.
The smooth idle will return after this and yes we will have to do it again in a 1,000 miles due to
modern fuels. The stuff just clogs everything up.
Marshal
Is your air filter clean or soaked by oil?
Makes breathing tough on start up.
I'd start cheap work expensive as well.
Pull the air box and clean or replace.
Blow compressed air through both air and fuel inlets at the mouth of both carbs.
Make sure these are clear. Smallest amount of dirt can act as a partial clog intermittedly.
Clean any carb bowl screen and blow out main jet since your in there.
The smooth idle will return after this and yes we will have to do it again in a 1,000 miles due to
modern fuels. The stuff just clogs everything up.
Marshal
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I don't run Mikunis; still on the original Amals, but my 850 started playing silly b*gg*rs recently with similar symptoms: Would start on being flooded, but wouldn't hold on for very long at all. Fortunately I have a long memory for things I'd rather not remember, and pulled the carbs apart and cleaned the pilot jets out - hey presto; problem solved 
I spent a few years in the bike trade, and 90% of all rough running bikes coming through my doors were due to clogged pilot jets.
I think Concentrics are the worst offenders because the pilot jets sit above the fuel level, and are consequently very susceptible to furring up (mine got blocked between late May and mid-July, in a dry garage). I'm not sure if this applies to the Mikunis though.
Final observation - the Amal pilot jets can be cleared by removing the pilot mixture screw and prodding with fine wire, as the jets are pressed into this drilling. I realised this after I'd taken the carbs off
I spent a few years in the bike trade, and 90% of all rough running bikes coming through my doors were due to clogged pilot jets.
I think Concentrics are the worst offenders because the pilot jets sit above the fuel level, and are consequently very susceptible to furring up (mine got blocked between late May and mid-July, in a dry garage). I'm not sure if this applies to the Mikunis though.
Final observation - the Amal pilot jets can be cleared by removing the pilot mixture screw and prodding with fine wire, as the jets are pressed into this drilling. I realised this after I'd taken the carbs off
B+Bogus said:...cleaned the pilot jets out - hey presto; problem solved
Second that - take the carb off and unscrew/remove the pilot jet. It might still be plugged even if you cleaned out the carb. The jet's opening is bigger than you might think... Anyway, my bike did this same thing when I first got it - wouldn't idle without the enricher on. After cleaning out the pilot jet, I just use it to start and turn it off right away, as has been said here.
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