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Quirky stumble/sputter on takeoff.

  • Thread starter Thread starter onchiman
  • Start date Start date
O

onchiman

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I've got a '79 GS850 with the VM carbs. I've gone through it and upgraded to a Dyna S ignition with new dyna coils and plug wires, diassembled and dipped and cleaned the carbs. New o-rings throughout including intake boot o-rings. Stock aircleaner that's sealed up good and tight with new weatherstipping on the box and filter carrier (new uni-foam filter element). Stock exhaust system with no leaks. Clean gas tank, new petcock flows perfectly. Checked and adjusted the valves, basically all the stuff's been gone over really well. I haven't replaced the intake boots though. They seemed okay.
Now the problem. She starts fine when cold on choke. Runs very well until it get's fully warmed up then the weirdness begins. It develops a stutter or stumble upon just opening the throttle under load. I can rev the engine no problem at standstill but when taking off in first gear it stumbles for a moment then proceeds just fine. Like it catches or clears it's throat or something. It also does this between shifts and is especially bad if I decelerate using engine braking, then go to power back up. If I hold a steady 4000 rpm in third gear, then slightly back off the throttle I can feel the stumble there too. Spark plugs 1,2 and 4 appear somewhat lean to me, white color insulator tip that actually looks like plugs are new. #3 is normal coffee colored. I adjusted the air screws to the highest idle, I balanced the carbs with a mercury manometer.
The bike idles perfect at 1100rpm and holds it, accelerates like mad when I get on it. It's certainly quite drivable as it is but this little bug is annoying me to no end. Compression is fine in all cylinders by the way. It's definitely a fuel system problem of some kind. The only way I've been able to improve it was to screw all the air screws in a quarter turn. That did help the stumble. It didn't go away completely but it did help. The idle rpms dropped. Upon readjusting the air screws using the highest rpm method, the problem returns!
My thinking is this, could the pilot fuel screws not be opened quite enough? As I understand it, the pilot fuel screws should be somewhere around 1 7/8 open, then the air screws adjusted to the highest idle. So my thought is that the pilot fuel screws need to be opened more? But if I do that, I'll readjust the air screws and won't that just lean it out again? Or will there be more mixture available? I guess I need to know is this, if the air screw just adjusts the mixture, does the pilot fuel screw then act as an adjustment of the overall AMOUNT of that mixture? Or do both screws COMBINE to form the mixture? Jeeze I hope someone understands this....
 
Can't help you much with details on how many turns out to have which screw, but it almost sounds like the pilot passages are not quite clean. I know you said you dipped the carbs, but did you then poke a small wire through all the passages in the bodies, and through all the holes in the jets (especially the emulsion tubes)? After poking around with a wire, did you then spray carb cleaner through the passages, then blow it all out with compressed air?

Dipping the carbs does wonders, but for really stubborn deposits in small passages, it only softens them up, they need to be forced out with a poker and spray.


.
 
For an easy short term fix you could try adjusting the idle up a couple of hundred revs & seeing if it helps....

From what Keith has told me 17/8 turns on the mixture screws is quite rich, he recommends 1 turn, 1 quarter at most with stock airbox & exhaust.

Dan :)
 
That was a typo on my part, the pilot fuel screws are 7/8 out , Not 1 7/8. Sorry. I don't think it's clogged pilot circuits because I really was quite meticulous about cleaning them with dip, wire, spray, and compressed air. Since the stumble improves when I turn the air screws in, I think I'll open the pilot fuel screws a little and see if that helps.
 
Could be several things. It sounds lean to me. What also bothers me is why one of the plugs burns an acceptable color and the others look lean.
On a stock bike, I'd try up to 1 1/4 turns out from lightly seated on the pilot fuel screws underneath. Because these screws are meant to fine tune each cylinder, they may or may not end up set a little differently from each other. But approx' 1 1/4 is as much as I've seen stock bikes have them out. A stock bike shouldn't need adjustments beyond that or something's wrong elsewhere. I can only assume the small holes in the carb body that the screw tips regulate are clear?
The side air screws generally end up from between 1 1/2 and 2 turns out when adjusted using the highest rpm method. If adjusting for best idle gives poor performance, something else is wrong with the pilot circuit or float levels.
 
Over the weekend I readjusted the pilot fuel screws giving them about another 1/8 turn out. I then readjusted the air screws for high idle and performed a vacuum sync. It's better but there is still an ever so slight stumble just off idle. It seems most pronounced after an engine brake deceleration to a stop, then take off in 1st gear. This is the point where the mix is leanest so I'm thinking it's still running a touch lean. I wonder if the replacement Uni-air filter is more free flowing. I oiled it per instructions with filter oil. I guess I tear into the carbs once again and re-check float levels. Aside from that it's very smooth and steady throughout the entire range with no other flat spots. Pulls well and purrs at idle.
 
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