The Magic Numbers

[KEITH KRAUSE]

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.

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.

[KEITH KRAUSE]

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.

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.

[KEITH KRAUSE]

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?

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.

[BadBillyB]

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.

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

[KEITH KRAUSE]

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.

[SabreJim]

tfb I sure appreciate your thread it has been helpful to me thank you all so much.

[Tarbash 27]

I found this on google. It says how to tune the pilot fuel screw on a 4 stroke dirtbike. Could this be a good reference to tune my vm26's?

http://www.thumperfaq.com/fuel_screw.htm

start at question 11

[KEITH KRAUSE]

I found this on google. It says how to tune the pilot fuel screw on a 4 stroke dirtbike. Could this be a good reference to tune my vm26's?

http://www.thumperfaq.com/fuel_screw.htm

start at question 11

I didn't read everything, just a bit here and there. He says 4 strokes don't have an air screw but then contradicts himself later.
He says to set the pilot fuel screws at 1800 rpm's, but why would you set them at that rpm? If you want to adjust the pilot circuit, you adjust it at the factory recommended idle rpm, which will guarantee no overlap effect from the next jetting circuit, the cut-away, and to a lesser degree, the jet needle, which has an earlier effect than many believe. Also, it's much easier to hear the rpm's max out if you use the factory idle rpm as your base. The air screws, on carbs that have them, are adjusted at factory idle rpm and I see no reason why you'd use a higher rpm base to adjust the pilot fuel screws. Same goes for the mixture screws on our CV carbs.
I've posted several times how to adjust the pilot fuel screws and side air screws on your VM carbs. But you're free to believe who you want to believe. His advice doesn't specifically mention VM carbs. His advice is based on a very different engine.