Jetting VM30 - XS400

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campbell45

New Member
Hey guys been awhile since I've been on here but I'm back at it with my build on a 79 XS400 bobber

First off here is the build if you'd like to know anything:

http://www.xs400.com/forum/showthread.php?t=4796

My problem is my jetting. I know that not many people with the 400s (besides cb's) have ventured over to the mikuni VM30s. I've got them and there working well except I need to dial them in. Here are some specs


Main jet: 250

Pilot jet: 40

Needle jet: 159 P-5

Needle position: 3rd

Jet Needle: 6F5

Slide Cutaway: 2.5

Air Jet: 2.0

Here is a video of it running:
http://youtu.be/3j_VPP4EFWU


I did some searching and found that a KZ440 with VM30s were running this:

Main: 230

Pilot: 40

Needle jet: 159 P-8

Jet Needle: 6F5

Slide Cutaway: 2.0

Air Jet: 2.0

The only time I can get the bike to run is with the choke on (I think the chokes on metal tab saying "choke" is up pushing rod down). If I push the tab down the bike instantly dies. It will idle very well and seems great although its very random to get the thing to start up and run, its like it only wants to run when it wants to. I have brand new coils, wires, caps, plugs and my spark is wonderful. I've yet to be able to take it on a drive since I havnt gotten it all together yet. I'm thinking about calling sudco and buying a jet kit off them. Unless I can find so more info. I feel like its all a shot in the dark. I've got a manual for the VM carbs but its not really helping me dial this in. I'm not sure where to go now, any advice from the carb gurus would be wonderful.


Thanks - Campbell
 
Choke is "ON" when the lever is down and teh plunger is raised and that allows fuel to flow.
 
Okay I called up sudco and they recommended 150 mains and 25 pilots. I got those in now and its still rich. I've yet to sync the carbs but I already know I'm still too rich.

Plugs are black and sooty and falls on its face when giving throttle quickly and given full throttle. The idle is close but as said before I need to sync them and get the idle down

does anyone know how to tune the mid range? jet needle and needle jet? From what Ive gathered the VMs come with a primary type and need a bleed type.
 
Well this is starting to get frustrating..

Trying to check the pilot system I put new plugs in and started it up and let it idle for about 5 mins. Killed it checked the plugs and they still looked new. Can you check the pilot system off the plugs?


After that I tried messing with the air jets and didnt really get much so I turned them 1 1/2 from seated.

Then I raised the clip on the needle one notch wanting to lean it out. Before when I gave it 1/2 throttle before it would have some black smoke with some fire shooting out on backfires.

Once I changed the needle height I noticed things got a lot crisper, throttle flowed easier, and didnt have black smoke anymore (but still backfired some fire), so I took it for a drive and performance was much better. Right before I got it back home though it died and I couldnt get it to start back up. Pushed it home and pulled the plugs, they were white.

I finally got it started back up by adjusting the idle waaay up. Granted I didnt go on a very long ride but I thought it wouldve shown something



I'm not really sure where to go from now...
 
At idle, plugs really don't get hot enough to show any meaningful color. I have seen plugs from a drag bike that got NO color at all into the plugs in an 1/8 mile run.

reading plugs works best at high load when the plugs are really really hot.

Air jets have maximum effect at high rpms, and almost no impact at low to mid revs. If you mean the air screws, they should make a difference. If they can be screwed in all the way and still not make a difference to idle speed, the pilot jets are too small. Air screws only change the mixture at small throttle openings, and will have little to no effect above say 1/4 throttle.

First step is to get it started. Did you run out of gas in the tank or in the carbs? Loosen the drain nut and see if there's any fuel in the bowls - do that outside with a fire extinguisher close by. You need to find and fix whatever that was first.
 
I ran it with the tank. I went up in pilots from 25 to 27.5 thinking it was lean from the clean plugs at idle.

Doing that, idle to 1/8 throttle is so much better! but beyond that it fumbles around. I took it for a ride and came back and the plugs looked like this:

e1c1e4d2.jpg



That is white on the top btw. I think its because of the mid range and main being out of whack but I could be wrong

What do you think?
 
Raised the needle back to the middle position and what a difference! It has such a smooth power delivery now. I dont really understand this now though since I was already in this position before but with lower pilots? Maybe the transition was too much? Anyway after a good long ride I think I'm still lean

428cee52.jpg
 
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