Hitch in the giddayup, out of ideas.

[onchiman]

I'm still struggling with an annoying hesitation/sputter issue when engine is warm. 1979 GS 850G. Doesn't do it when engine is cole, but appears once engine is fully warmed up. It's a slight hesitation just above idle, just when pulling away or after a decel from 3rd to 2nd, then begin throttle up. Here's stuff I've done;
Dyna S ignition timing right on the mark.
New Dyna Coils (green)
New wires, caps and plugs
Valves adjusted to within specs.
Compression is fine on all cylinders range is 145 to 155.
Stock Exhaust
Stock airbox completely sealed with Uni replacement foam filter oiled appropriately.
New intake o-rings, boots and airbox boots
New petcock
Float heights all carbs 23mm
Stock main and pilot jets
Carbs cleaned with new orings throughout (spotless and all passages clear)
Carbs synched and re-synched after each change.

Basically the bike idles perfectly, pulls hard with no flat spots. Runs terrific except this hesitation which is a pain, especially in traffic. The spark plugs looked lean to me, snow white insulators, so I raised the needles. The clips were in the second from the bottom groove now they are in the bottom groove. Plug chops still show lean however, the problem shows up when the engine is warm which would suggest a RICH condition?! Is it possible to have the pilots too rich and still the plugs indicate lean? First carb adjustments I tried was pilot fuel screws. Started at 7/8 out going to 2 turns out. Didn't help, then raised the needles and reset the pilot screws to 7/8. No improvement, actually seems a smidge worse. I'm out of ideas now except that maybe the uni filter is creating a lean condition somehow. That still doesn't explain the off idle stumble. My gut says the pilot circuit is too rich because the problem doesn't appear when the engine is cold, only when hot. The spark plugs tell me lean so what the heck is happening?

[Steve]

On your carbs, you also have pilot air screws. How are they adjusted? If they are too far open, you will run lean (indicated by the plugs), and the pilot fuel jets might not be able to compensate.

If I remember correctly (and it's happening less frequently nowadays), the fuel screws are on the bottom, the air screws are on the sides of the intake throats.

From the carb sticky thread:
GS850 (77-79) the specs are for the VM's

idle r/min........... 950-1150
carb................... mik vm26ss
id no................... 45060
bore size ............ 26 (1.0)
float height ......... 23-25. (0.91" - 0.98")
fuel level ............ 3.0 - 5.0 (0.12" +/-0.20")
pilot air screw ..... 1.25 turns out
pilot screw........... 5/8 turns out
pilot air jet........... 1.2
pilot jet............... #15
cut away............. 1.5
jet needle........... 5DL36-2 (the -2 2nd notch)
needle jet .......... 0 - 6
pilot outlet........... 0.6
by pass ............. 0.8
main jet............. #102.5



.

[onchiman]

After I adjusted the pilot fuel screws, I adjusted the pilot air screws to the best/highest idle. They are around 1 1/2 - 2 turns out at this point. Fiddling with them does produce rpm change. I screw them in until the rpms begin to drop then back out until I get the highest rpm and no more. There's a point where turning the screws out doesn't effect idle any more but causes some missing. They are all within the acceptable range and the motor idles really smoothly and steadily, beautifully.

[mark]

The spark plugs looked lean to me, snow white insulators, so I raised the needles. The clips were in the second from the bottom groove now they are in the bottom groove.

In my opinion you shouldn't have raised the needles. For the stock set-up which you have your needles should be in stock position. I raised mine 1/2 notch for my 4-1 pipe and that's all I needed. I'd put them back and make your other adustments first. I'd try the stock setting of 5/8 turn out for your pilot fuel screw as well. 7/8 is where mine are with the pipe.

Also, when you check your plugs make sure that you check them at the RPM that you are having the problem with.

You haven't mentioned whether or not you replaced your manifold boot O-rings. Did you?

[onchiman]

The manifold intake o-rings are new as are the manifolds. Is the stock needle clip setting on these things the middle groove or the 4th from the top (2nd from bottom)?? Mine have a thin plastic shim below the clip in addition to the thick plastic shim that goes on top.

[Dave8338]

the middle.

[mark]

Mine were in the middle position as well before I changed them. The stock settings that Steve posted say the 2nd from the bottom, that's not my experience (I'm also not disputing it). All I know is that for my 4-1 exhaust with stock airbox I'm now half-way between the second and third position and it worked well.......with my rotting stock exhaust the middle position was fine.

[onchiman]

Hmmmm, the specs posted above are for the needle type (5DL36-*) and factory clip setting. For example "dash 2" (-2) which I always thought meant from the top, like -1 means first groove (top groove), -2 means 2nd groove from the top, -3 is third groove from top (middle groove), -4 = fourth groove from top and -5 = fifth groove from top (bottom groove). Is that right? Jetting spacers can give you "half groove" settings.

[mark]

Hmmmm, the specs posted above are "-2" which I always thought meant from the top, like -1 means first groove (top groove), -2 means 2nd groove from the top, -3 is third groove from top (middle groove), -4 = fourth groove from top and -5 = fifth groove from top (bottom groove). Is that right? Jetting spacers can give you "half groove" settings.

You may be right.....I'm sitting here at work (slow day) without any of my stuff. So right now you're at -5 then right? From the top then, I would be at 3 1/2 notches from the top. Notch 3 was my stock stetting (at least from what I can tell).

[onchiman]

Yes, I'm at the 4 1/2 setting. I have the clip in the 4th groove and a jetting spacer below that. That's as low as I've tried. I'm wondering if by resetting the pilot screw to 5/8 open, moving the clip to 2nd groove and then tuning, if that may help. It definitely seems like a rich stumble from the pilot circuit and the needle being set richer might just be aggrevating things. Once the air starts to flow, it all evens out.

[mark]

That sounds like a plan. Little adjustments can make a big difference....I went from way lean to overly rich just by moving the needles one notch. You shouldn't have to compensate too much from the stock settings with your pilot fuel and air screws.

[onchiman]

I tinkered with it some more last night an am on the right track! Set pilot fuel screws to 5/8, air screws to 1.25. Returned needle to stock position (2nd groove from top). Balanced, adjusted air screws for highest idle and then re-balanced. Off idle hesitation is gone! Seems a tad lean overall but I have a stock Suzuki air filter element on the way which I think will help with that. Thanks all!