Mikuni VM32's Jetting - 79 XS400

Campbell45, What Jets do you have in it now?

lcgtr, is that 20-30% in the low end, or at peak?

Also, I've seen a little debate over carbs and performance increases. Can you really get 20-30% performance incerases from Carbs? Even in a small portion of the RPMs this would be a noticable difference I would think.... I have no current plans to change out my stock carbs, but I would like to know what the reasoning is, in case I have some spare change to invest in engine performance...

I feel the same Here is what a set of 32mm flatslides do on a xs650 compared to a fully jetted and tuned stock set of carbs. http://www.mikesxs.net/parts/docs/dyno_48-9010.png

I'd take a 2.5 hp across the board! !!! Not sure i'm willing to shell out the $, but good to know ...

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Gosh I suck I brought this up from the dead then forgot I did sorry guys

Anyway I took the xs for a cruise today and it does okay but somethings just not right. Its definitely drivable but it will stumble under hard acceleration and then finally pick itself back up.

This may be an issue with getting fuel to my carbs too as my petcock is pointed forward. So the fuel has to go forward and up slightly and then back down to my carbs. When I have a full tank this is less of an issue.

My jetting is:

Main jet: 150

Pilot jet: 30

Needle jet: 159 P-5

Jet Needle: 6F5

Slide Cutaway: 2.5

Air Jet: 2.0


It looks like Millennium Falcon has had some success with jetting his VMs but his jets are what mine were when I first bought the carbs (had a 250 main and 40 pilot). It would run but was stupid rich.


Anyone have great success with vm30s? If not I do like the idea of TM28s Ive heard good things about the TMs

drewpy said:

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.

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My At3,125cc uses a 230 main jet and 35 pilot,and by the way,the choke must be pushed down for on,up for off. It's a lever that lifts the enricher valve when pushed down. lha

I've bought VM32s for my XS400 and I was alerted to the fact that they don't behave like a normal CV carb since the slides are connected directly to the throttle and don't rely on vacuum. That means they do bog down if you pin open the throttle too quickly.

Can I ask what intake boots you are using for yours? I'm having trouble finding any to suit the 67mm spacing on the XS400 head and the 40mm carb spigot diameter. The standard intake boots I have are 42mm inner diameter and don't clamp onto the VM32s tightly enough.

Any help greatly appreciated!

The VM carbs should work well on these bikes,as they've been used on 4-stroke bikes for years. You can't equate jet size with CCs,as a '73 At[125cc] has a 35 pilot,240 main,and a '73 Dt125 uses the same carb with a 22.5 pilot,and a 140 main. It's got a lot to do with slide setup,airflow,etc. Try experimenting with the needle position. Your needle is set at the last clip on the bottom[rich],most are positioned on the middle clip position. The main jet may have to be changed a few times[richer]. Since the VM didn't come on a XS 400,there are no specs on their application. lha

RyanJL said:

I've bought VM32s for my XS400 and I was alerted to the fact that they don't behave like a normal CV carb since the slides are connected directly to the throttle and don't rely on vacuum. That means they do bog down if you pin open the throttle too quickly.

Can I ask what intake boots you are using for yours? I'm having trouble finding any to suit the 67mm spacing on the XS400 head and the 40mm carb spigot diameter. The standard intake boots I have are 42mm inner diameter and don't clamp onto the VM32s tightly enough.

Any help greatly appreciated!

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Hey Ryan, glad you finally joined this website. There is a huge amount of XS knowledge on here. I suggest you create an intro thread & post some pics.

Are you using stock clamps to secure your carbs or just standard hose clamps. I think the stock clamps should squeeze down a couple of mm. Also, are your carb boots new? Could be worthwhile replacing them as my new ones are heaps tighter than the old ones on my bike. Got mine from yambits or wemoto in UK (can't remember which one). If you want you can bring a carb over and see how they fit on my carb boots.

Cheers,

Nick

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Sudco also sells carb holders as well as on Ebay,you can find some of them. lha