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The Magic Numbers

  • Thread starter Thread starter tfb
  • Start date Start date
T

tfb

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G'day all,

Well, I've been farting around with my GS1000ST's carbies for weeks now, trying to get them running right. In the meantime, the bike has had a top-end rebuild, so there should be no excuses for me getting the old girl running sweetly, right?

Huh. Try as I might, and following the excellent PDF article on tuning VM carbs here on the GSR, it just wasn't coming together for me. Between them, the pilot screw and the fuel screw on each carbie had me licked. How many turns out should they be? I'd tried my little heart out, but it still had me stumped.

UNTIL... until I finally got my mitts on an original you-beaut Suzuki manual for the GS1000 series. What a mine of info. it is... and then, flicking through the relevant section for my bike, what did I spy but the MAGIC NUMBERS:

Air screw ... 1 3/4
Pilot screw ... 5/8

Now I understand why Archimedes (as legend has it) leapt out of the bath and ran down the street naked hollering "Eureka!" The reason is because he had given up on his Haynes or Clymer manual or whatever it was, with its pesky "PRESET" entries for the mystical screws, and he'd got himself a Suzuki manual.

So, I set the screws to the prescribed settings, and the difference is astounding. A bit of fine tuning, a carbie balance, and all should be well!

Cheers,
Mike.

PS. If anyone would like the suggested air- or pilot-screw settings for other GS1000 models, let me know and I'll see if they're listed. It saves a lot of pain.
 
So your pilot screws under the carbs are 5/8 out? I guess it should be the same or close to my 750 since it shares the same carbs. I think mine are like 1 1/2 out. I think i will test it at 5/8. Theres a little notch near the screw, does it have to be relevant to that?
 
I'm pretty sure the settings are model specific... so the settings for the GS1000SN, for example, differ to the GS1000ST.

By all means give the settings a try, but I would be a bit surprised if they're correct for your GS750.

Re. the little notch at the perimeter of the air screws... OK, as I understand it, the quoted setting (eg. 1 3/4) is a 'nominal' setting, and then some fine tuning was done at the factory, and the notch then stamped on the carb. body at the perimeter of the screw head.

On my machine, the screw slots have all lined up fairly closely to the stamped notches. I might try turning each screw slightly until it lines up with its notch... but really I'm just so glad to be 'in the ball park' that I think I'll just leave them alone until boredom/ adventurousness takes over.
 
tfb said:
G'day all,

Try as I might, and following the excellent PDF article on tuning VM carbs here on the GSR, it just wasn't coming together for me. Between them, the pilot screw and the fuel screw on each carbie had me licked. How many turns out should they be? I'd tried my little heart out, but it still had me stumped...

[/I]
Click to expand...

Please advise whether yours are VM 26 or VM 28 carbs. And what year is the bike? I would have thought the "T" model would have been a 1980 model with CV carbs. Thanks

Tomcat
 
Tarbash 27 said:
So your pilot screws under the carbs are 5/8 out? I guess it should be the same or close to my 750 since it shares the same carbs. I think mine are like 1 1/2 out. I think i will test it at 5/8. Theres a little notch near the screw, does it have to be relevant to that?
Click to expand...


The optimum for my 79 750E was pilot fuel screw 3/4 to 7/8ths turn out and airscrew 1 3/4 turns out. I put about 40k miles on the bike with it set up that way before selling it.

Earl
 
Good point, tomcat. Actually, they're VM30 carbies, not 26's or 28's.

And yes, I know that the GS1000ST's exported to America had CV carbs fitted as standard. Here in Australia, however both the '79 (SN) and the '80 (ST) came with VM carbies. The '79 had VM28's, and the '80 had the VM30's.

I have had such a pain of a time trying to get the pilot circuit set up properly for these carbies. Trying to do it with a tired top end probably didn't help either. So now with the fresh top end, and the MAGIC NUMBERS dropped into my lap by Suzuki's own manual for the GS1000, I'm in clover. :-D Of course there will be fine-tuning called for, but now we're 'in the ball-park' there is the smell of victory in the air.
 
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Well I have vm26's with pods and exhaust.I have 20 pilot jets and 115 main. I will test different positions with the pilot fuel screw once I get the bike back together. Im in the process of valve adjustments.
 
OK, I went through the manual systematically, and here are the settings for the various models it lists:

GS1000SN ('79) --- VM26SS-equipped models --- Air screw 1.0, Pilot 5/8
GS1000SN ('79) --- VM28SS-equipped models --- Air screw 1 1/4, Pilot 3/4

GS1000L ('79) --- VM26SS-equipped models --- Air screw 1.0, Pilot 5/8
GS1000L ('79) --- VM28SS-equipped models --- Air screw 1 1/4, Pilot 3/4

GS1000ET ('80) --- BS34SS (ie. CV carbs) --- Pilot 1 5/8

GS1000ST ('80) --- VM30SS (in Australia) --- Air screw 1 3/4, Pilot 5/8

GS1000GT/GLT ('80) --- BS34SS --- Pilot 1 5/8

GS1000GX/GLX ('81) --- BS34SS --- Pilot "PRESET" Ah yes, there you go, the dreaded "preset" word. But don't worry, in all other respects (jetting, float height, etc.) the carbie data for this model are the same as for the '80 GS1000GT/GLT (above), so setting the pilot at 1 5/8 will be the way to go.

*Naturally* these are just nominal settings, and assume that everything is stock-standard and that your engine is in good condition, clean air filter, etc. So that if you have pods, after-market exhaust system, bored-out engine and so on, you will need to fine-tune things to suit your setup.
 
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Stock bikes generally have the pilot fuel screws set from 1/2 to as much as 1 1/4 turns out, but right about 3/4 is the most common. Differences in head/cylinders in each engine are why they are set over a certain range. It's common to find them set a bit differently from cylinder to cylinder. They assist the pilot jet through a secondary passage. They are for fine tuning the pilot circuit and are set at the factory using an emissions analyzer. That's why they warn that you can't set them exactly if you ever move them and lose track of their initial setting. You can get them close enough though with tinkering. The small nick on the carb body is stamped by the worker after they're set. The nick lines up with one side of the screw slot.
The side air screws should be adjusted to fit your specific riding conditions. Elevation, humidity, etc. You simply adjust them using the highest rpm method. As I've always posted, they generally end up right around 1 3/4 turns out on VM 26 and 28's, intake/exhaust mods or stock. Just like the pilot fuel screws, there's no exact setting. Just set them using the highest rpm method.
 
Keith, do you think one of those Colortune's would be good for setting the pilots? Have you ever used one? Or maybe good for setting the side air screws as well?
 
I took my pilot fuels screws out today since Ive been cleaning up the carbs. I noticed, every one is bent and smeared on the tip. Seems as if someone tightened them in so tight that they bent and smeared.
 
Thanks for that advice, Keith. Illuminating as usual! ;)

The thing that had me stumped, was finding the settings which would get the bike running acceptably, and would then be a platform in the quest for better settings.

But, I just couldn't even get started. The good thing is that the manual gave me a place to start with my particular model and its particular type of carbies (and being one of the few, if not the only, markets in the world with VM-carbied ST's, I was getting nowhere fast).

On we slog.
 
Tarbash 27 said:
The only ones I can find are a new carb kit on ebay. The guys wants $70 plus $4 to ship for all 4. I dont think I want to spend that much on 4 fuel screws. I think I will try to sharpen mine up for now.
Click to expand...
I don't know how much you need to re-shape yours, but remember they are fairly precision and if you change the taper or the tip sharpness that much, they'll operate poorly.
 
tfb said:
Thanks for that advice, Keith. Illuminating as usual! ;)

The thing that had me stumped, was finding the settings which would get the bike running acceptably, and would then be a platform in the quest for better settings.

But, I just couldn't even get started. The good thing is that the manual gave me a place to start with my particular model and its particular type of carbies (and being one of the few, if not the only, markets in the world with VM-carbied ST's, I was getting nowhere fast).

On we slog.
Click to expand...
Thanks for giving others that info. I can say that if you searched, I'd think you'd have found several posts of mine that would have made it easier for you. By now, I'd think just about any carb question would be covered by me, or someone else. I suppose the search feature can still be time consuming to browse through.
 
renobruce said:
Keith, do you think one of those Colortune's would be good for setting the pilots? Have you ever used one? Or maybe good for setting the side air screws as well?
Click to expand...
Never used one so I can't say nothing good or bad about them.
At least in my experience, I can't see a reason to use them for the side air screws on the VM's. I have no problem at all adjusting by the highest rpm method. If you had bad hearing it would change things though.
I can see where they would help with the pilot fuel screws though, if they really are accurate. Could save time I suppose. Again, with me, I've got the pilot fuel screw adjustment ritual down to the point I'd never bother with the tool unless someone dropped it in my hands to try out.
If a certain bike, say a stock bike, needs them adjusted after a rebuild or whatever, I just set them to about 3/4 or even 1/2, which is the leanest I've seen on a correctly running bike. If pilot circuit tests say lean, then I go straight to maybe 1 1/4. Doesn't take that long to find the right setting between the "extreme" lean and rich settings.
On bikes with intake/exhaust mods, again, I just go with past experiences known to work and add a little richness. If the pilot jet is still stock, I usually add about 3/4 turn to the above settings which would be about 1 1/4 to 2 turns out. Then I test and do what the plugs/performance say. It doesn't take that much tinkering if you narrow it down this way.
It's when you get problems that happen when the different circuits are at solid overlap that you have to make more decisions. The cut-away is the transition between pilot and jet needle and since we rarely change the cut-away, even though it may actually be the problem, we have to work around this and come up with a compromise of the pilot and jet needle settings. For me, this means getting the more important jet needle right and if I have to lean out the pilot circuit a bit to eliminate a rich problem during overlap, so be it.
Things happen when you make intake/exhaust mods and you can't always expect the carbs to accept these changes. But you can usually tune them to have that power increase you wanted and still have decent street manners. Just don't expect perfect manners every time.
 
KEITH KRAUSE said:
Stock bikes generally have the pilot fuel screws set from 1/2 to as much as 1 1/4 turns out, but right about 3/4 is the most common. Differences in head/cylinders in each engine are why they are set over a certain range. It's common to find them set a bit differently from cylinder to cylinder. They assist the pilot jet through a secondary passage. They are for fine tuning the pilot circuit and are set at the factory using an emissions analyzer. That's why they warn that you can't set them exactly if you ever move them and lose track of their initial setting. You can get them close enough though with tinkering. The small nick on the carb body is stamped by the worker after they're set. The nick lines up with one side of the screw slot.
Click to expand...

When rebuilding the 26's on my GS1000, I noticed that when all the pilot fuel screws were lightly seated that the tips were visible when looking into the carb throat. Two of them were very touchable when running finger over them. One was barely touchable and the fourth one you could not feel at all.....Swapping screws around gave same results...I came to the conclusion that perhaps the carb bodies themselves are the reason they are not all set the same. Something to do with casting/machining differences in the passageway and hole into the carb throat.....So what I did was make a note in my manual about the different tip positions with the fuel screw lightly seated. Kind of like this..
#1 very touchable
#2 very touchable (same as #1)
#3 slightly touchable
#4 NOT touchable (but visible)

When I set the fuel screws, I used these notes to fine tune from the basic (majic number) setting of 5/8 to 3/4 turns out...This assumes that #1 and #2 will be leaner than the others with the screws at the same number of turns out.......These were my observations anyway......BadBillyB
 
Right Billy. I've seen the same thing with tip exposure in the carb throat. There are differences in the carb body that can result in slightly different settings for each screw. Even if your set of screws expose the same at a given setting, flow differences in the head/cylinder may require us to set each screw a little differently.
Thanks for pointing out another good reason why you should always keep all the parts in each carb separated from the other carbs. All the parts equal one unit.
 
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