I have poor power response at the upper end.When cruising and giving the throutle a total twist the bike sputters and has no power,when doing this slowly it seems ok and keeps picking up speed.I have a muikine carb,and have tryed diff jets,only gets wourse with bigger jets. I have a 300 in there at the moment,went up slowly from a #250,my alttude is 4500 FT.Any ideas ?How about changing the clip on the long jet needle?it has 4 or 5 positions.

 

Sputtering is typically a symptom of lean fuel mixture.

I do not have any experience tuning the Mikuni beyond a thorough cleaning and setting the pilot screw and pilot jet. I believe that getting these right should proceed any other carb changes.

For cleaning i use one of those needles used for inflating soccer *****, footballs, etc. I use a fitting from a hardware store to attach the needle to the hose of my air compressor, and blow thru every orifice i can find, along with spray can carb cleaner thru the same.

I follow the Mikuni Tuning Manual for setting the correct pilot jet and pilot screw turns out [section 5.3, page 10 & 11]. I aim for a pilot jet that will yield 1,1/2 to 2 turns out on the pilot screw [closer to 2 if i can] ...

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

Hope this helps some,

 

What ever made you put a #250 main jet in that carburetor?
The standard main jet for the VM38 is a #330.
I suggest you change what you have in there to a #330 and see what happens.

If you are cruising at 3/4 throttle when you open it up then it is either a main jet size problem or an exhaust pipe issue...... or maybe both.

The ironhead needs a certain amount of back pressure to respond to a throttle being cracked open suddenly.
If you have mufflers, or straight pipes with a baffle then this is not going to be an issue.

If however, you are using large... open at the end pipes, then lack of back pressure may be part of the problem. Especially at the elevation you are at. The solution here is to install mufflers or baffled drag pipes.

For the carb, I would definetly start with the #330 main jet and move up or down one size to make the response better.

The clip should be in the middle notch on the needle. That is third from the top or third from the bottom.
Moving the clip down raises the needle further out of the main jet and will enrich the mixture slightly between 1/2 to 3/4 throttle opening.

So at 4500 ft elevation you could try dropping the clip one notch to the fourth slot or raising it one notch to the second notch and see how it affects the running.

But do only one thing at a time. Otherwise you will have no idea what action was needed to correct the problem.
pg

 

It runs treible with a # 330 jet in there ,way to rich for this altitude,or 4500 ft.At the moment i have a #270 with the needle all the way down,runs not bad,stiil a wee bit of a sputter.