Carb setup tips, Mikuni BST40

carlosvalderr... · Two Stroke Sniffer Joined: 05 Mar 2015 Karma :

Hello, first post, new member, and in need of assistance from someone with more experience.

I have a Suzuki Goose 350 which I'm fixing up, modifying and playing with.

It was practically a non runner at the point of purchase, water had got into the tank, rusted it and the carb was full of muck. It started and rolled about though. I rebuilt it all, and got it rolling properly again, and now because I must, I started to tweak it.

I've removed the airbox, and fitted a Ramair sock filter, and now I'm trying to get it to run. I've done some very light carb tweaking in the past, and it seems to be recommended to start with getting the main jet right, and working my way down to the pilot jet and mixture screw.

For reference it's a Mikuni BST-40 as used on the DR650, and KTM LC4 engines. As standard it runs a 125 main and a 42.5 pilot jet.

However, I can't start with the main as it simply will not rev past 5000 or so, or use more than about 35% throttle angle.

It starts and idles badly, but just about manages it with persistence. I suspect it's rich, as it starts more easily with no choke, even when cold. I'm not sure about this as I'd think without the airbox the depression across the butterfly would be greater, and as such, more air should be passing through the pilot circuit..?

However, any fast throttle opening sees it cut out when cold, but with care when warm and about 25% throttle it'll slowly rev to 5000 or so, then seemingly hit a wall. More throttle and the revs die away. I'm not sure what to make of this, as even quite lean or quite rich, with no load I'd expect to be able to rev it out. It makes enough power to ride to 40-50mph or so, but needs coaxing to not cut out.

The float height seems to be OK, but the only guide I can find is for a KTM.

The needle and needle jet are in good shape, not ovalised or worn. The main and pilot jets are new and from a reputable source.

I've been playing with the pilot jet and mixture screw to get the idle reliable enough to start with, but I'm getting nowhere.

Sorry for the big post, any tips gratefully received. Smile

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

Sounds like the main jet is blocked too me.
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carlosvalderr... · Two Stroke Sniffer Joined: 05 Mar 2015 Karma :

As far as I can tell, it's clear. It's a new 125 jet, and the bowl and body were both cleaned well out to remove muck. The original 125 jet makes no difference to the running.

I've tried a 127, 130 and a 132 main, and running at the richest needle setting which raised the cut out rpm limit and the amount of throttle possible, but only slightly.

It could be that the original airbox is horribly restrictive, and it needs a 145 main jet, but I've never fitted a jet more than about 2 sizes from standard. I can't help but feel that richening it up monstrously is just masking an issue elsewhere.

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

What does the plug look like?
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Well, you know what they say. If you want to save the world, you have to push a few old ladies down the stairs.
Skudd:- Perhaps she just thinks you are a window licker and is being nice just in case she becomes another Jill Dando.
WANTED:- Fujinon (Fuji) M42 (Screw on) lenses, let me know if you have anything.

temeluchus · World Chat Champion Joined: 01 Oct 2008 Karma :

If you have removed the airbox and fitted a pod filter you will need a significantly larger main jet for it to fuel properly. Often it will need to be 25% larger and sometimes more.
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The Shaggy D.A. · Super Spammer Joined: 12 Sep 2008 Karma :

https://www.advrider.com/forums/showthread.php?t=347184
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carlosvalderr... · Two Stroke Sniffer Joined: 05 Mar 2015 Karma :

The plug is quite sooty, but it's new and it's barely run off a rough idle, so I'm not reading too much into it yet.

The BST bible is a brilliant write up, that's where I got the float setting from! It appears to be the same for the DR650 though.

The signs are pointing to trying a main of up to 150, but that seems enormous! Worth the few quid it'll cost though.

Cheers!

temeluchus · World Chat Champion Joined: 01 Oct 2008 Karma :

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

Ariel Badger · Super Spammer Joined: 02 Dec 2006 Karma :

Go up 25 on the main jet and see what happens, then you can go to tweaksville.
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temeluchus · World Chat Champion Joined: 01 Oct 2008 Karma :

carlosvalderr... · Two Stroke Sniffer Joined: 05 Mar 2015 Karma :

carlosvalderr... · Two Stroke Sniffer Joined: 05 Mar 2015 Karma :

YES PLEASE. Very Happy

A 145 main jet, a 40 pilot and 1 turn out as a start point, and it barked straight to life and now revs like the little barnstormer it is!

Idle is a little rich still as the revs hang a bit, so I'll try a 37.5 and 2 out, in neutral there's not much crackle or pop when revved, so the main and needle are probably fairly close, but I'll need to get some load on it first.

Many thanks for the tips. I'm still shocked I needed +20 to even start.

willyd · L Plate Warrior Joined: 29 Jun 2018 Karma :

Hey Carlosvalderrama, I just PM you would love know what exact setting you ended up at!

Excuse the ignorance but is there reason that without the airbox you would end up with a smaller than standard idle jet?

Cheers

carlosvalderr... · Two Stroke Sniffer Joined: 05 Mar 2015 Karma :

Just PM'd back.

I think I ran around on the 147.5 for a little while, but never got it quite working.

I know that the idle screw taper was damaged, I suspect that threw the idle jet out.

However, I still think that with what is effectively an infinite volume airbox, you'll get less of a depression and the carb will have an easier time drawing through.

In the end I decided to simplify things by fitting a Mikuni TM40 pumper carb. I find tuning a flatslide a far easier proposition than a CV carb.

I honestly couldn't tell you the settings, 2-3 years ago now, but if you were to go for a TM40, let me know and I'll have a look.