R&D vm38-9-yam500 Free info, no charge.

Got a great deal on a vm38-9-yam500. The reason appears to be that the fuel feed to the needle seat was obstructed. Blew it out with compressed air and away we go.

Start with a 166 type P4 needle jet, 6F8 needle, 260 main and 35 pilot jet.

I have had to go to 25 pilot. Idle is fine.

Main issue was/is the chattering when cruizing city strees. It's not really popping or cutting out, just that sort of not excellerating nor de-excellerating type chatter. I set the needle clip to center as defaut. It sure seemed like this was on the edge of a mid-range leanness so I dropped the clip (raising the needle) and this issue is 75 persent resolved .

The carb came with an extra R0 needle jet which is a bit richer so I can give that a go and maybe lower the needle again if needed.

So basically I'm leaning the pilot and richening the midrange.


FWIW. This carb slapped right on the current adapter and the fuel line require no modification. Throttle cable needed the shething/outer cover cut back 1 1/4" to expose more cable to accomodate the 45 degree fitting/adjuster out the top of the carb. That's it.

Excelleration is great and it literally pops at throttle opening with no hesitation all the way to redline.
I'll leave it on for a while to see how it shakes out.

Glad to hear the Mikuni VM 38-9 is cooperating and you are making satisfactory headway on tuning. In hindsight it was serendipitous that the topic of Mikuni VM and HS series idle circuits was kicked around recently, for without that conversation you may have been hunting for parts that weren't on the carb (idle air jet), turning the air screw the wrong way and going in the wrong direction with pilot jet changes.

Let us know if you get the chatter out of it. When I first heard your term "chatter" (which is a great term!) I was thinking you were referring to a slighty rich stutter when the engine was running along with minimal/no load on it. Obviously, it was not rich if you are richening it and improving drivability. Before the needle raising did it give any indication of slight surging, as can occur with a slightly lean condition? We like the NC info

pete.v said:

I have had to go to 25 pilot. Idle is fine.

Main issue was/is the chattering when cruizing city strees.

I set the needle clip to center as defaut. It sure seemed like this was on the edge of a mid-range leanness so I dropped the clip (raising the needle) and this issue is 75 persent resolved .

Click to expand...

Cruising, even at steady freeway speeds, is still primarily on the idle circuit. You're entering into the transition between idle and slide cutaway. What's your slide number? I would try dropping it 0.5 from where it currently is. You'd then probably have to up the idle jet to re-establish the idle mixture.

Nathan

Nater_Potater said:

pete.v said:

I have had to go to 25 pilot. Idle is fine.

Main issue was/is the chattering when cruizing city strees.

I set the needle clip to center as defaut. It sure seemed like this was on the edge of a mid-range leanness so I dropped the clip (raising the needle) and this issue is 75 persent resolved .

Click to expand...

Cruising, even at steady freeway speeds, is still primarily on the idle circuit. You're entering into the transition between idle and slide cutaway. What's your slide number? I would try dropping it 0.5 from where it currently is. You'd then probably have to up the idle jet to re-establish the idle mixture.

Nathan

Click to expand...

The standard VM38-9 has the 2.5 slide where I have the 3.0. That seems to make good sence and will find a 2.5.