Mikuni VM32's Jetting - 79 XS400

Campbell45

Campbell45

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Hey guys been awhile since I've been on here but I'm back at it with the build.

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 VM32s. I've got them and there working well except I need to dial them in. Here are some specs

Plug: NGK B8ES
photo2-1.jpg


Carbs with MikesXS filters
photo1-1.jpg


Main Jet: 250
photo4-1.jpg


Pilot Jet: 40
photo3-1.jpg


Here is a video of it running:

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. Also the plug is usually much more black than that I had cleaned it earlier and then ran it for awhile. I've yet to be able to take it on a drive since I havnt gotten it all together yet. I'm not sure where to go now, any advice from the carb gurus would be wonderful.





Thanks - Campbell
 
Actually now that I'm searching for info these may be VM30's :banghead: I had a guy order them while I was out of state and I thought he said they were 32's
 
I'm stumped by the black plugs, but if you need your choke to run, you are lean. Start by going up a size on your jets and see if it will idle on it's own. Once it will run without the choke, you can fine-tune from there...

Pushing the lever down, the choke is on...Lever up, choke is off...
 
Just watched the vid. Didn't hear it pop, which is usually a sign of being lean. Perhaps the black plugs are a sign of being too rich. Do the plugs look sooty?
 
I'll get it running again and take some better pictures of the plug. Since I just cleaned it that pictures pretty much no good. If I remember correctly it was black and dry tho

Am I correct in saying that it only starts with the choke on when I pull up on the tabs?

From what I've looked up the VM30s this is what it comes with when bought so I guess I've got VM30s I cant find any stamping that has to do with the sizing
 
Guy's, the sparks are black because the choke is on the run them.

are you sure you got 250 mains in there? that's a VERY BIG main jet and not even 1000cc bikes have them!

I would say pilot needs upping to 45 and start from there.
 
Heres my jetting after just checking:

Main jet: 250

Pilot jet: 40

Needle jet: 159 P-5

Jet Needle: 6F5

Slide Cutaway: 2.5

Air Jet: 2.0


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



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.
 
Based on the numbers and whats been said about choking being off when up, im going to say when its off its running rich, and when you push it down its flooding it, you should try a smaller main... that thing is huge.
 
gotta consider the stock carbs are 34s you're running 30s, you have restricted the air flow a big so the engine is gunna suck harder to get what it needs, so that means alot of fuel coming through, id try the 200s if you think you need that big, but really the stock carbs on an XS with wide open pipes & filters usually only needs 160, im not an expert though
 
I called up sudco and they recommended mains at 150 and pilots at 25 so we will see how this goes. From there I will plug chop
 
I have a VM 30ss on my Dt400 dirt bike[Yamaha,which came with a VM32]And it has a 160 main and 40 idle jets. A 250 main would be from an MX???,and from the "ring"around the main jet would suggest that you have an MX carb. The MX bikes have the transfer ports enlarged,thus allowing for more gas.The 2 stroke carbs have more velocity and run richer compared to 4 strokes.The "choke is actually an enricher port,which delivers an overly rich mixture to help cold starts and warmup. Pushing down the lever opens the valve and releases more gas,and closing is accomplished by pushing it down. Good luck. lha
 
corruptmind said:
gotta consider the stock carbs are 34s you're running 30s, you have restricted the air flow a big so the engine is gunna suck harder to get what it needs, so that means alot of fuel coming through, id try the 200s if you think you need that big, but really the stock carbs on an XS with wide open pipes & filters usually only needs 160, im not an expert though
Click to expand...
The engine on an xs400,with 2 cylinders is only 200cc per cylinder. My 400 2-stroke,1 cylinder has a 32mm carb with the slide controlling all aspects of throttle except idle. The BS34 carb is operated by a butterfly valve and a vacuum operated slide. There are a lot of differences to take into consideration. A 2-stroke must put out enough gas so that the gas/oil mix won't be too lean and hole a piston. lha
 
Bringing this dead old thread back to life. Has anyone successfully jetted vm30s for the 400s yet? I have yet to even ride my 400 this season

The last time I rode it it liked to bog under heavy acceleration to the point of almost dying. Although slow pull on the throttle and it will pick up speed. Whatever I jet it to it likes to read rich and then when I get it to where I think it should be its hard to start and I get to thinking its too lean.

I was wondering if switching over to a 2 to 1 carb would be a good fix for this. I was thinking with 1 carb and 2 cylinders the 1 carb would have more vacuum and have a more controlled flow.

But then again I might just suck at tuning :shrug:
 
I threw my VM30s at the wall as they turned out to be cheap Chinese ones and the second time I changed jets all the o-rings expanded and I couldnt put the bowl back on.

We put on a set of TM28s and I had big inlet adaptors made to increase the runner length. We barly had to change the jetting the bike came with and it picked up a good 20-30% power than the stockies

Old Ebay VM30s


Genuine TM28s
 
If you're going to go with a 2-into 1 intake,I'd use copper tubing,a y,and two 45* fittings. If you can't find a Y,get two 45* fittings and split them,then braze them together. A Vm30ss from a Dt250/360 would work well,and get an emulsion tube for the jet needle,and a bleed type pilot jet. This type carb has a bolt mounted main jet,so to change mains,you simply remove the bolt,change jets,and you're off. Parts are easy to find and kits are available by Keyster. I ran My Dt400 with one from a Dt360,and it ran like a striped ape!! There are a lot of slides,needles,etc available,so it's easy to dial in,though stock settings would be a good starting point. The carbs use a 180 main,a 50 pilot,so it should start and run from there. a smaller diameter carb has greater velocity,and will give a lot of low-end torque,and still deliver cruising performance,with very little changes[needle jet and maybe pilot jet.I have a 32mm Vm carb that I will use after I get aall my 2-strokers rebuilt and sold. lha
 

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