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How-To tune 3x Mikuni on 280?


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how would you tune 3x mikuni carbs? it's a 280 block in a 73 w/ a 4sp if that makes any diff.

seems to sputter @ anything over 4k RPM. maybe cause the flatspot is around 3-4k from all the freeway mi. is there anything that can be done short of dyno tuning? under WOT from dead stop, it sputters till 2.5-4.5k range and does it till limit. w/ less of a lead foot it's not as bad but still does it in higher RPM's.

seems to run very rich cause it smells like fuel inside the car. also the black carbon speckles in the driveway can't be a good thing.

my uncle had this car in storage for a few yrs. till i picked it up 2-3yrs ago to make it my daily commuter. i think i'm getting maybe 10MPG which i think can't be right considering i'm not driving it too hard (though i know the 4sp is hurting mileage on the freeway)

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I'm a Webber guy so this might not apply, but the carbs looks simular in construction so they probably work much the same way.

My first guess is that the float level isn't set right or the needle valves are leaking bad. Ever get any gas dripping out of the front of the carbs? The only adjustment besides syncing them is the idle mixture which is out of the picture at 2K.

If they have a "choke" which is really a mixture riching valve maybe they are stuck open or you are running the choke lever in the wrong position, this would give you a way rich mixture all the time.

If you have been running it this way for a while the plugs are probably pretty fouled and could be part of the current problem, though it sounds like changing them will only be a partial fix.

Those are my guesses....

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doesn't have the choke on AFAIK. the lever is there but i don't think it's working. there's no change in RPM if i have it in either position.

i've never seen gas dripping from the carbs either.

maybe it matters, maybe not, but would very dirty air filters contribute to this problem? i don't know when the 3x carbs were installed so can't say how long it's been run in the current config but they do look pretty dirty. i wouldn't doubt that they're still original.

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I would think that they would have to be pretty dirty, but then they may be! An easy test would be to pull them off and take a quick spin down the road. I won't hurt anything to run around the block once without them unless you live on a dirt road :)

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  • 1 year later...

where was the car in storage and where are you driving it now. And what are the jet numbers air, fuel, and main? Some where in those answers will tell you what you need to know. If you dont want to dyno, then youll be looking at purchasing alot of jets trying to feel out the best mixture. This is all assuming the carbs arent disfunctional

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