78 - 79 stumble

[BA80]

Ever since I put this 79 engine in my 80 there has been that irritating stumble at WOT from take off at about 3 - 4K. It takes off smooth then hits that stumble and then takes off like an Acme rocket like the coyote used to try and catch the roadrunner.

I'm pretty sure it's in the cam grind but I was just wondering if anyone has been able to tame that down through tuning.

I've tried different carbs and different jets and no difference.

I can live with it but I would rather have a smooth transfer into that power band.

[IanDMacDonald]

Hey Greg- If you'll be at the U.P. ride next month, I welcome you to take my bike for a trip to see if mine does the same thing. I ride so many differemt bikes, it's hard to remember how each one behaves.

[BA80]

I'm planning on being there and looking forward to meeting you and some other new people.

I've seen other 79's do the same thing but I just was wondering if anyone came up with a cure. Probably just the nature of the beast though.

The 80 - 83 engines don't seem to do it, that's why I think it's in the cams.

[IanDMacDonald]

Well . . . I got a tricked-out advance. Not sure if that helps with the stumble. Will ride the bike Wednesday and report back. Willing to bet it's either the vacuum advance, or something related to it. Part of the reason I went back to my locked-out advance. It just feels like instant power, where-as the un-modified advance felt sluggish.

[BA80]

Oh, it's NOT sluggish. It just kinda says "hang on here we go!" then tries to yank the grips out of my hands. My old 80 engine never did that, but then, this 2H7 engine will run circles around it.

[mack]

I don't

know Greg, my Sf has never had a stumble at any RPM, but there is a definite kick when it transitions onto the full on mains (needles retracted all the way). And it's always done that right from the showroom floor. After I rebuilt it, it was missing that punch because I'd used K&L jets on the rebuild, but as soon as I switched back to the original mikuni jets that kick is back. But definitely no stumble going from 1050 thru to 8500.

[BA80]

The ONLY time I get that stumble is when I roll on WOT. It's especially noticeable in 1st gear. The E I had did it too but not quite as bad.

I have a NOS 2H7-11 centrifugal advance unit I may swap in there to see how it reacts.

[Bartman]

I have a 79 with air box and factory exhaust. The carbs that I was given with the bike to use had 145 mains. After a lot of reading and taking the carbs on and off about twenty times I realized that the stumble I was getting happens when the big mains were coming on (3~4k). I had been blaming float level but I dropped the needles one click and it runs fine now. Slow speed in a parking lot in first or second is smooth and no hesitation to red line in any gear. I Do have a four barrel kick at around 5k!

[mack]

Four barrel kick

Thats the best description I've heard for it. Mine also has the airbox and original exhausts. I wonder Greg if it's not related to the exhaust or intake configuration you have?

[Rickrod3]

Not sure if this helps but:

I had a stumble around 3K prior to going though my carbs. I went back to the brass floats since In an attempt get the needles to seat better. I set them at the recommended float height and the bike never ran better.

YRMV
Rick

[BA80]

I have a 79 with air box and factory exhaust. The carbs that I was given with the bike to use had 145 mains. After a lot of reading and taking the carbs on and off about twenty times I realized that the stumble I was getting happens when the big mains were coming on (3~4k). I had been blaming float level but I dropped the needles one click and it runs fine now. Slow speed in a parking lot in first or second is smooth and no hesitation to red line in any gear. I Do have a four barrel kick at around 5k!

I was using these same carbs on the original 3J6 engine with the stock jetting and it didn't have the issue. I've had them on this engine since I put it in and it's done it from day one. I just put a different set of carbs on last Saturday and it still does the exact same thing

Thats the best description I've heard for it. Mine also has the airbox and original exhausts. I wonder Greg if it's not related to the exhaust or intake configuration you have?

Yeah, that's kinda what it is, a kick but with a garbled transition. I used to be able to adjust it out with the adjustable secondary air valves on automotive carbs.

I had a stumble around 3K prior to going though my carbs. I went back to the brass floats since In an attempt get the needles to seat better. I set them at the recommended float height and the bike never ran better.

YRMV
Rick

I've always had the correct internals in mine. I've gone from the 137.5 mains to 140 now up to 142.5 to address a lean condition but that hasn't made any difference in what I'm speaking of.

[DiverRay]

Greg,
Are you running the '80 or the '79 advance and TCI? That could also make a difference. My "Quail Bike" has a stumble and kick, but it also has the Kirker header. It's running an '80 engine with the '79 carbs and advance.
Ray

[BA80]

The engine and advance are all what was on it when I pulled it out of the F. I've never had the ignition advance apart to see what might be in there. It has the original SG TCI, 2H7.

[Rickrod3]

I was using these same carbs on the original 3J6 engine with the stock jetting and it didn't have the issue. I've had them on this engine since I put it in and it's done it from day one. I just put a different set of carbs on last Saturday and it still does the exact same thing



Yeah, that's kinda what it is, a kick but with a garbled transition. I used to be able to adjust it out with the adjustable secondary air valves on automotive carbs.



I've always had the correct internals in mine. I've gone from the 137.5 mains to 140 now up to 142.5 to address a lean condition but that hasn't made any difference in what I'm speaking of.

I'm running 137.5 mains and 42 pilots. Which I believe is one up from stock I could not get a blue flame using the color tune so I went to the 42's. Stock air box with K&N filter. After market unknown exhaust 4 into 2 slash cut open. they are not too loud. What size are your pilots?