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Mixture screws on GS1000ST (1980)

  • Thread starter Thread starter tfb
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T

tfb

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G'day all,

I bought a GS1000ST about 3 weeks ago, and have been having lots of fun fixing it up.

The tank was majorly rusty inside, so while I had the thing off and put a POR-15 tank cleaning and lining kit through it, I also took the opportunity to clean out the carbies. Good thing I did so; they were full of rusty mud. :roll:

Now I know that the manual screams at you not to touch the air mixture screws because they are set at the factory, etc. etc. ... but as they had obviously been fiddled with already, I thought I'd pull them out and give them all a clean into the bargain. Alas, I now realise it might have been a good idea to record how many turns out each one was, because the bike has obviously been tuned with them set the way they were. :oops:

The long and short of it is, the bike is running, but maaate she's rough (was relatively smooth before) and does a lot of surging at lower engine speeds, etc. My question is, can anyone help me with re-setting the mixture screws?

Now I should say I am talking about two kinds of mixture screw on each carbie. The first are located on the underside of the 'exhaust' outlet of each carburettor, ie. towards the engine. They simply allow air into the inlet tracts. [Edit: but I just read somewhere that winding them out lets more fuel through and so things are made richer, is that right?]

The other mixture screw (well, I assume it is to do with the carbie mixture), is on the 'inlet' side of the carbie, ie. on the airbox side. They are easily accessed as they are located on the upper & outer side of the carbie. They also have a small reference mark of some kind, stamped/ punched into the carburettor body at the edge of the screw head.

Now then, the first kind of screw I have set at 2 turns out from fully seated. This is seems to be the setting where the bike runs 'happiest', but I have to say it's still pretty darn rough, and I have had to wind up the idle speed knob to stop it from stalling.

The second kind of screw I have set about 1/4 turn out. I tried aligning the screw slot with the reference marks, but this didn't seem to improve things any.

Needless to say the manual is no help on this score... "do not disturb" and "set at the factory" are not the kind of words I like to see.

Any clues? Or should I just pull out the carbie vacuum gauges, balance them and be done with it?

Cheers,
Mike.
 
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Sync

Sync

Hey Mike, I think you should definitely plan on synching your carbs. The air screws on top of the carbs are pretty easy to deal with. Just adjust #3 until your idle speeds up and adjust the others accordingly. (You may have to turn down your idle speed in the process of doing so.) Those air screws will probably end up between 2 1/2 and 3 1/2 turns out.

Take a nice spin and then 'read' your plugs...you know the drill. They should look healthy and be tan of a little lighter..(these bikes seems to run a little lean). Turning them out will enrichen your mix.

As for the other screws, I can't figure out what you're referencing.

Good luck...you'll have it running great in no time!:-D
 
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Thanks for that, chuckycheese. I'm getting it sorted bit by bit. Found a great article on VM carb cleaning and tuning here on the GSResources, too. Tomorrow I'll go step-by-step and I'm confident I'll have it sorted.

In the meantime, the lesson is: count and write down the fuel screw settings before dismantling! Trust me to be so gung-ho and read the instructions after the event... :shock:

Cheers,
Mike.
 
Yes

Yes

You're welcome! There is some very good information on carbs here on the GS website.

I thought of one other thing: you mentioned that you took the screws out when you cleaned the carbs...that was a very good idea because, otherwise, the 'carb dip' will ruin the little, tiny "O" rings...are they still there and in good shape?? (In addition to the screws, there should be a spring, a washer and the little rubber "O" rings.)
 
What has worked well for my VM carbs is this, I set the air screws two full turns out and adjusted the idle mixture with the lower fuel screws (fastest idle) then if you have a off idle flat or lean spot turn the air screws in a 1/4 turn at a time until it goes away. You will have to re adjust the fuel screws each time you turn the air screws. Don't turn the fuel screws all the way in, they have a nasty habbit of breaking the tips off. Also the motor must be fully warmed up for flat spot testing and use plug readings as a tuning tool. Dan
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Cheers for that, Dan. I will give your method a go, because following the standard procedure here doesn't seem to be working on my bike.

What I basically ended up with this morning, after another hour or so of fiddling around, was an engine that idles so high that you run out of adjustment on the idle knob, and you just can't get it idling at the right speed.

So I thought, there must be something else more basic wrong. It shouldn't be an intake leak (although I will check), because I just put in new O-rings between the head and rubber inlet manifolds, too.

I did a quick compression check of one cylinder, it only read about 100psi, so I think I should do a top-end rebuild before I try and do any serious tuning. Also the bike is, I think, choked with an exhaust system that is way too restrictive, so maybe that's playing a part, too.

But I'll give your method a go, Dan, and see if it runs any better.

If any of the other GS sages around here have any suggestions for me, fire away!

Cheers,
Mike.
 
If it idles too high the slides are up too high or there is a leak somewhere. Dan
 
Dan, you must be a prophet. [-o< The problem turned out to be slide #2 was OK, but #1, #3 and #4 were all set too high. Got them all seated, reinstalled the carbs, set the mixture screws to the 'nominal' setting of 1.5 turns out for the air screws and 1 turn out for the fuel screws, and she fired right up.

Made a few lean 'popping' sounds, so I wound the air screws in 1/2 a turn each. Re-adjusted the idling, and went for a ride. Not a bit of hesitation, not a bit of popping, just smooth power everywhere. \\:D/ I'm sure there's some fine-tuning to be done, but this is a great starting point -- now we're cookin'.

The crazy thing is, the bike must have been set like this for years. That meant that they had to find some way of bringing the idle speed back down. So they set the idle mixture super-rich. Which explains the four sooty black plugs and the atrocious fuel economy (10 kms per litre... the same as a car).

Thanks chuckycheese and Dan for your patience, input and wisdom. Much appreciated.

Mike.
 
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