Mikuni VM30 Tuning for xs400

[TimBuz]

So I'm in the final stages of assembling my xs400 build and I've come to some carby issues. I've cleaned my stock bs34's and was in the process of re-jetting them to suit my pods and free flowing exhaust. The bikes been started and taken for a quick spin on stock jetting and its obviously running very lean. Anywho I took off the carbs and have realized that either I or the PO has snapped off a mixture screw in the carb body which I cant seem to get out. On top of this it seems there are no seals for the screws, my throttle shaft seals need to be replaced, the screws holding the butterflys to the shaft are also stuck fast and i cant seem to get them out for fear of stripping them, all the screws on the locating bracket have also been loctited and no amount of heat or acetone is working on them.

TL;DR I've bitten the bullet and bought a flashy pair of New VM30's. Based on info out there they seem to be about the right size for the bike.

I've searched high and low for tuning threads on these for xs400's, cb400's cb450's and kz400's and there is SOME info as to jetting specs but most threads seem to die off.

Anyway as a starting point I've found a jetting guide for a street tuned kz400 with pods and exhaust:

KZ400 tuning*** colder climate
**change supplied main from 250 to 230 (hex jets)
**change pilot from 40 to 27.5
**change throttle valve cutaway (slide) from 2.5 to 3.0
*Jet Needle : 6F5 on #4 (1 clip richer)
keep needle jet stock P-5
Keep air jet 2.0

I should have the carbs, new filters and throttle cable in the next few weeks and will continue with the thread once I start tuning.

How does that combo look for putting me in the ballpark for a reliable tune?

Cheers, Tim

 

[Wolfe_11B]

Main is going to be a bit too big I think... But better to start there instead of too small. If I had to venture a guess, main would be around 150

In all honesty, unless you've done some head mods, VM30's are probably too big for the XS400.... 26 or 28 probably would be perfect. But I'm no expert, so take it for what it's worth.

I just grabbed a set of Keihin PWK's for my XS, so I'll be in the same boat here shortly.

 

[TimBuz]

Well my reasoning for the VM30's were that most guys with cb350's get advised to run 28's and most cb450's are running 32's. I agree 28's could have been a slightly better choice, but I suppose sme of us have an idea in our heads that one day head mods or a bigger bore could be on the cards even if its just a pipe dream.

I thought the main sounded a little large too, but I've seen that alot of cb350's run these with around a 160 jet. So who knows.

How come you went the flat slide route? I've heard they can be a bit tricky to get right

 

[Wolfe_11B]

I chose the PWKs due to local parts availability, and I've messed with them on 2strokes so I already know my way around them.

I think you'll be good to go with the vm30s. Just study the mikuni vm tunning guide, they explain everything really well. Hardest part is getting the needle jet and jet needle size correct, as it slightly affects the overlap into WOT and the main jet

 

[TimBuz]

Turns out alot of retailers are waiting on these to get shipped from mikuni. Mine won't be getting shipped until mid july and even then i wouldnt be surprised if partzilla delay the order again