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79 GS850 VM carb tuning results / Plug question

  • Thread starter Thread starter ender22
  • Start date Start date
E

ender22

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Brand new plugs, maybe 20kms on them. This is after a run at 1/4 throttle then shut off and then I pulled plugs 1 and 4. The hanging over thing seems a little white to me and I think it's still a little lean? The screw in part looks like the right colour though?

Pic is clickable to large sized original if it helps. Thanks for your time


Below is my write up to help anyone that's doing or thinking of doing the same thing to a VM carbed GS850
Hopefully this helps someone else out,

I added pods and exhaust and started tuning with 110 mains, clip down 1(#3), Pilot jets at 17.5 as it was recommened.

set the fuel screws to 1.5 and the air screws to 1.5, and sync'd the carbs. It was really lean at 1/8 to 1/4. better but not great at 1/2 throttle.

I upped to the Pilot Jets to 20 and still felt a lot of lag on accel at 2% to 25% throttle. So I ordered a size 22.5 Pilot jets, they won't be here until tuesday though.

So yesterday I downed the needle clip 1 more to the second from the bottom. upped the main to 112.5, adjusted the fuel screw to 1.5 and tuned the pilot air screws as outlined in this document. And it was way better at all throttle positions.

-Rob
 
My solution with VM was rather lean on pilot mixture screws and 1 clip position down (rich) on needle jet. Spark plugs gaped 0.31 (wider). Am happy with the results, the dreaded 1/8 throttle lag disappeared.
 
hmm wider gap on the plugs. I'll have to read about that tomorrow.

thanks for the reply!
-Rob
 
Before you start messing with it MAKE sure you have 12 V to the coil
 
bhm said:
My solution with VM was rather lean on pilot mixture screws and 1 clip position down (rich) on needle jet. Spark plugs gaped 0.31 (wider). Am happy with the results, the dreaded 1/8 throttle lag disappeared.
Click to expand...

I've been poking around trying to find jetting/tuning advice for my 1979 gs850. Right now, with the stock airbox and open mufflers (old mufflers rusted out), the stock jetting is doing OK, minus your aforementioned 1/8 throttle lag. I've just been tweaking the fuel screws, but the lag has thus far persisted.

Do you think adjustments similar to yours, changing the needle position and spark plug gap, will be able to minimize the lag or would a change in jetting be required?
 
I'd definitely try the clip on lower. Then re-sync the carbs :)
 
FYI, I dropped the clip to the middle position (down one notch from stock) and it cut down on (but didn't eliminate) the lag. This is with otherwise stock jetting.

Good tips, I'm going to keep running with this and tinkering it, but I'm quite happy to be on the right track. I had previously thought just the fuel screws were all I had to work with.

Thanks!
 
You might want to go 2 or 3 sizes up on the mains as well. After that try another clip down or just a spacer.

Try a spacer or another clip down :)
 
ender22 said:
You might want to go 2 or 3 sizes up on the mains as well. After that try another clip down or just a spacer.

Try a spacer or another clip down :)
Click to expand...

Moving another clip seems like a good idea to me. It has the advantage of being free to do.

Did you reset the fuel and air screws to factory each time you changed the clip position or main jet? or did you leave them where they were? I know that the three main carb circuits affect each other, but I can't fathom the main having a big effect on <1/8th throttle.
 
I think you will find the Pilot fuel circuit responsible for barely off idle and low speed characteristics (1,500-3,000 rpm). It is probably a mistake to use anything but the stock 15 pilot jets. Stuttering in this range is usually too rich on the pilot fuel screws. Don't be surprised if it seems to work backward in your adjustments. The pilot fuel screw adjustments require air screw adjustment for highest idle. I think if you can find the pilot fuel setting that gives you close to two turns out on the air screw adjustment for highest idle, you will get closer to dialing it in. When the pilot fuel circuit is working right, it doesn't take much of a turn on the screw to make a fairly large change.
 
I would put a 115 main in and then tune the needle. Last thing is to tune the pilot circuit (idle)
 
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