Lean or rich in the pilot circut?

1f2frfbf · New Member Joined: Jun 26, 2009 Posts: 9

Okay, I've read the carb guide, but I can't get my head around two things: what's the difference between "stumbling" and a "flat spot"?

y 79 just got the carbs rebuilt again, and now at 2500 it sounds like Porky Pig with a shotgun. It stutters up and down until it reaches 3000 and then pops loudly and takes off. It will fall back down through the same RPMs smoothly with only a little hanging. I can start it off the line okay, if I feather the clutch.

y current carb settings are: BS38s jetted to #140 main and #35 pilot with the mix screw out 2 turns. I know this is something that is in the pilot circuit, but is it lean or rich? Is there something else I can do to figure this out? TIA, all.

acbanks

Were the needles adjusted? Typically with those jets I think you would drop the needle one groove(raise the clip one groove) to lean out the midrange. (The lean needle compensates for the rich main jet, the rich pilot jet then has to compensate for lean needle. It's like you're chasing your tail, crazy I know.) It's a pain to get to them and you'll need a pair of snap ring pliers.

Then again this s*** is all black magic so I could be totally wrong.

1f2frfbf · New Member Joined: Jun 26, 2009 Posts: 9

I think the needle is still in the middle position, I didn't adjust it this time around. I'll check that this evening...

mgreaves

my 77 has '78 BS38 carbs, pod filters and Supertrapp 2 to 1 exhaust. Settings are Main- 137.5, Pilot 30, needle in the middle. I can see going up to a 140 main, but the 35 pilot sounds too rich unless you have a big bore kit?? dropping the needle may give you the same issue you have now, just at a slightly higher RPM. What air filters do you have? I have experienced a stumble/flat spot caused by after market "universal' airfilters where the rubber lip disturbs the air getting into the small jets around the mouth of the carb. Rolling Eyes

1f2frfbf · New Member Joined: Jun 26, 2009 Posts: 9

I have the K&N knockoffs from Mike's, I've been using them for a while with no issues. I switched to the larger pilot since I had to have my mix screw at 3.5+ turns out with a 30 and I felt like that was too far.

What Happened to 650rider.com?

Yamaha xs650