I have noticed lack in power,this is going at cruising speed,and twisting the grip and the bike sputters and coughs,and if i twist the grip slowly see will in crease speed in a normal maner.Ihave changed my jets to see if any difference but not a hole lot,plugs are a tan brown,and bike runs fine other then this small problem.Running a mulkini carb on here.This problem came after my rebuild.Any ideas?

 

In my limited experience sputtering and coughing [backfire thru carb] are signs of a lean fuel mixture.

First the pilot jet: How many turns out have you set on the pilot screw? Optimum is in the range 1,1/2 to 2. This is an air jet not a fuel jet so screwing it in makes the mixture more rich. The procedure to set it is in the Mikuni Tuning Manual, section 5-3 page 10 ...

http://www.mikuni.com/pdf/vmmanual.pdf

After the pilot jet is set to optimum then we can look at other options.

 

first check all the tune stuff the dist advance unit for wear / points almost closed or burned out - the valves could be tight and that will cause that - and intake sucking air seals leaking - check the low speed jet for dirt clogging as well -- jz

 

+1

Check pushrod adjustment ...

Ironhead Push Rod Adjustment
http://xlforum.net/vbportal/forums/s...ad.php?t=44143

Check for intake and exhaust leaks ...

Intake And Exhaust Leaks Tests
http://xlforum.net/vbportal/forums/s...ad.php?t=39766

Check points setup ...

gapping points and static timing video
http://xlforum.net/vbportal/forums/s...d.php?t=231711

After doing the above three items you can proceed with my previous suggestion.

 

Large displacement singles and twins benefit from an accelerator pump-equipped carb. When the throttle is opened quickly, there's an instant, sizable increase in airflow due to the suction of the large piston(s), but a slight lag in fuel flow. An accelerator pump injects extra gas into the carb throat to counter this. Without an accelerator pump, the mixture is lean for a moment, causing the sputtering you describe. I'm guessing the Mikuni doesn't have an accelerator pump, so you have to roll the throttle open gradually to let the gas metering keep up with the airflow, instead of snapping it open quickly. There's really no good cure for this, as it's inherent to using that type of carb on and engine like the Sportster.

 

True, the Mikuni VM38-9 carb does not have an accelerator pump. But the reality is that it does not need one. I run this carb and do not experience this problem.

I expect that the OP's problem is with the fuel flow, either carb parts need cleaning or it needs re-jetting; but that the basic tuneup items must be checked first.

Note the 6 fuel circuits ...

 

that is not as popular as it once was in a sporty carb - back in the day all the choppers had those or the bendix as the air filters were small and out of the way and one cable was the norm

i dont remember ever having a flat spot in one and we put them on almost every day we sold another one -- as Mick said its a tuning issue and a complete carb cleaning is in order before any guessing what might be going on - jz

 

Dang, you mean I typed up my masterful response for nothing? I'll defer to the guys who have experience with the Mikuni...I suppose experience trumps theory every time...