Mikuni VM26 Pilot Air Screw Access - to drill, or not to drill...

[AndyC]

Hello all & greetings from sunny Guernsey!

I have a '79 GS Thou, which I'm running with foam pods, 4 into 1 pipe and LEDAR air corrector jets fitted.

I'm looking to fine tune the fueling using my trusty color tune, and following some research on this awesome forum, it appears tge right way to go about this is to set the fuel mixture screw at 7/8 turns out, and fine tune the pilot air screws from 1.5 turns or thereabouts.

Historically, I've simply tuned the mixture just using the fuel screws just behind the manifold, but I wanted to try with the air pilots too.

My issue is, that I don't seem to have access to these on my bank of carbs: they appear to be blanked off under pot metal.

I've attached a pic, showing the place where the screw should be.

I suspect this is for emissions control, and tge screw is sitting happily behind this, away from tinkering fingers...

Is it safe to drill through this to access the screw, or is this a different design of carb body?

Cheers!

Andy C

 

[Agemax]

The air screws on your carbs are not hidden, they are not there. Those carbs are not original GS carbs, probably an early Kawasaki, which many models came without the air screws, just the fuel pilot screws underneath.

 

[AndyC]

Thanks for the quick response!

Well, that was close ��.

That makes sense - the bike was a bit of a parts bin special when I got it.

Also explains why gs1000 cables don't quite fit: the outers are always a few inches short.

Given the above, I'll tune her up in the usual way.

Thanks again!

 

[tatu]

The air screws on your carbs are not hidden, they are not there. Those carbs are not original GS carbs, probably an early Kawasaki, which many models came without the air screws, just the fuel pilot screws underneath.

We had exactly this remember?
I'm still kooking for at least a pair of 29s..

 

[Agemax]

We had exactly this remember?
I'm still kooking for at least a pair of 29s..

Yes i remember. you mean 28's? 29's are smoothbores.

 

[Nessism]

The Kawasaki version VM carbs hold a little more value than the GS variety. Some have air screws and some fuel screws, but none have both like GS bikes. The VM28's came on later year KZ1000's and the ones I've seen have had an accelerator pump on them. While this may seem good in theory, in practice the pump can be a pain in the butt and it isn't needed if the carbs are jetting properly. Kawasaki used the pump on some versions of the VM26 too. One other difference between the KZ and GS VM carbs is that the KZ's typically used 2.0 slides, although some models used 1.5's instead. I think all the GS's used the 1.5's. As I recall the 1.5's are leaner, and GS carbs came with smaller main jets so apparently GS bikes like a leaner mixture than KZ engines.

 

[tatu]

Yes i remember. you mean 28's? 29's are smoothbores.

Sorry yes 28s of course. I ideally need two inside carbs.. Used to be heaps about now there are few and always expensive...

 

[Agemax]

Sorry yes 28s of course. I ideally need two inside carbs.. Used to be heaps about now there are few and always expensive...

Bore out a couple of 26's easy to do on your lathe.

 

[tatu]

Bore out a couple of 26's easy to do on your lathe.

It might come to that, I don't think i have any 26'es either... When I get back to UK i'll pick it all up again. 9 months since i was in my shed... I think i looked at the throat of the carb with that in mind and thought that the hole looked oval on the other side of the slide... Anyway the need for carbs is a long way down the road at this point....

 

[AndyC]

That's really interesting information, always useful to have some context. Sometimes working on these lovely old bikes is like being on an archaeological dig!

Cheers

AC

 

[GregT]

. One other difference between the KZ and GS VM carbs is that the KZ's typically used 2.0 slides, although some models used 1.5's instead. I think all the GS's used the 1.5's. As I recall the 1.5's are leaner, and GS carbs came with smaller main jets so apparently GS bikes like a leaner mixture than KZ engines.

Higher numbers on the slides mean higher cutaways. Higher cutaways mean more air at part throttle which equals leaner. And vice versa.
Generally, if it's a lean cutaway, there'll be a richer pilot jet and often a richer needle too.
There's more than one way to get good throttle response.