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79 GS carbs

  • Thread starter Thread starter Anonymous
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Anonymous

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The 79 GS850 was the only year of 850 to use a regular slide valve carb, all future models used a CV carb. Here is my question/concern. On the bottom of the carb body is a "pilot" screw. Normal throttle valve carbs did not ever use this, but for some reason (something to do with emissions perhaps?) the 77,78 and 79 GS multi cylinders all had this feature. The carbs do have the normal idle air mixture screw in the normal position. All service literature including the factory service manual provide zero information about the pilot screw. It simply states that this is a factory set adjustment, and should not be tampered with. The pilot screw is vertical, coming up from the bottom front of the carb, and it is used in conjunction with the actual pilot jet inside the float bowl.
So, heres the problem.
These bikes are old, and many have had the carbs taken apart for cleaning, the pilot screws removed and many times damaged upon reinstallation. These mix screws are verrrrry soft, and a ridge is very quickly put into the tip if they are even slightly overturned.
So, my question is this......does anyone know FOR CERTAIN (please, no guessing) what the approximate setting is for this pilot screw, and what was the actual purpose/intention of this screw.
I have owned 3 79 GS850's, all of them extremely cold blooded animals and one of the bikes I actually played around with the setting on the pilot screws and it did seem to help with cold starts, but ended up with a fairly rich idle mix, which was not easily overcome by using the "normal" air mix idle screw.
Hoping someone has the correct info....

Cheers!

Rick in Canada
 
There is no exact setting for the pilot screws. The factory sets these using an emissions analyzer. Because no two cylinders are exactly the same, the factory uses these to fine tune the mixture for each cylinder. So it is common to find them set differently from each other. Example: 7/8, 3/4, 1, 1 1/4 turns out. You can adjust them CLOSE to factory settings, but it would take a lot of tinkering and testing. They are "generally" 1/2 to 1 1/4 turns out from the factory. If the screws have never been moved, the factory almost always stamps a "nick" next to one side of the screw slot. Then they apply a colored coating. If you moved the screws without keeping a record, this may help you determine where to put them back. Of course, there are two ends to each screw slot, so it can still be a guess.
The pilot screw helps regulate fuel mixture for the pilot circuit. The pilot jet supplies fuel for a main passage to the carb throat. There is a second passage to the carb throat that splits off the main passage. This second passage is controlled by the pilot screw. This screw allows for fine tuning the mixture.
For the pilot circuit to operate correctly, the side air screws need to be adjusted to achieve the highest possible rpm's and then you re-set the idle to 1,100 rpm's with the idle screw adjuster knob.
In your case, if the bike is stock, I would try setting them at 3/4 turn out and adjust the side air screws as said and test/take plug reads. For the pilot circuit reads, I run the bike in 4th/5th gear at a steady 35/40 mph for several miles. I then chop the bike off and get plug reads. Before doing the pilot circuit reads, I'd run the bike at 1/2 throttle or more for a good 1/2 minute to help clean the plugs and get a more accurate read of your latest jetting adjustments.
 
Thanks for the great info. I do realize that each of the fuel pilot screws is individually set at the factory, but there would still be a "common' starting point. So again, thanks for confirming my original thoughts. I have them set at 1 turn out, and she idles quite nicely. I may turn them out another 1/4 turn to help out with the colder weather idling, but otherwise, she purrs like a cat on nip.
It is very odd that Suzuki/Mikuni took that approach on that carb, using a fuel screw similar to what you would find on a CV carb, but I guess thats what make our SuziQ's so unique! :)

Cheers!
 
Keith basically said everything I was going to, so thank you Keith, a nice wealth of knowledge there. :)

Don't guess at your settings with the pilot screws. It took me about a year and a half to fine tune mine since I've been using this rack of carbs. They ran very rich when I got it, and it's been a small adjustment here, and small adjustment there that have gotten them to perfect now. Your biggest tool is reading your spark plugs. As Keith said, no two cylinders will run the same, so setting them all to the same number of turns is good for a starting point, but it wont work in the long run, most likely.

As Keith suggested, run the bike for a few minutes at a certain speed (do this at idle, mid range, and high RPM's) and check the plugs for color. You want the ceramic around the electrode to be coffee colored. You may find that a certain cylinder wants more or less fuel.

Don't assume the bike needs more gas in the winter, again, use the plugs to tune. Opening the pilots 'for the sake of it' can be a waste of gas, foul plugs, or cause driveability problems.

~Adam
 
rickskeena, I think the pilot circuit design in the VM carbs is the best and simplist way to fine tune mixture amounts. That sharp tipped screw regulating a second fuel passage allows for very sensitive adjusting.
 
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