4-5.5k stumble on hard acceleration

[dbeardslee]

I've been working on my brothers 80 standard for the last couple weeks, and I finally got it running yesterday (after it sat for about three years.) I set his valve clearances and thoroughly cleaned his carbs, adjusted and synched them. I also did a black mikes xs coilectomy and replaced them with 3 ohm Dynas, new plug wires, and almost new plug boots (5k ohm resistor type.)

His setup is 4/1 header, individual pod filters, 79 carbs jetted with 147.5 mains, 45 pilots, floats are dead smack on 25.7mm, dyna 3 ohm coils, and NGK BP6ES plugs gapped at .035, solid core wires and 5k ohm NGK caps.

The problem I'm getting is that it stumbles from 4-5.5k on hard acceleration. It also starts to stumble up around 8k. It kind of acts like it's starving for gas. If I roll the power on slowly, it goes through the 4-5.5k stumble zone without a hitch.

When I had the carbs apart the diaphrams were in good shape, as was everything else after cleaning. The needle position has never been touched on this bike, and I did not take them apart, but it used to run like a scalded dog. I pulled the plugs to check their condition, and they were perfect - little black soot ring around the outside and a nice medium brown in the middle.

I checked the resistances on the coils at the coils, across the high tension wires, and at the TCI, as well as the resistances on the pickup coils, and everything was what it was supposed to be. I also disassembled and cleaned the gas cap to make sure it wasn't stopped up.

He replaced the stock petcocks with the manual mikes xs universal variety (IIRC). I had one issue yesterday when I first started it, and it ran like crap. Misfiring, backfiring through the carbs, and only hitting on a couple cylinders. The thought occurred to me that the inline fuel filters may not have liked their extended vacation, so I pulled them out of the lines and it started running pretty good.

So that's the story. Anybody got any ideas regarding the stumble?

[TopCatGr58]

Hey there,

Are the plugs colors done with a throttle chop, or did you just pull them after running it a bit and them coming to a stop/parking??

The 4-4.5 zone is where it kicking into the mains pretty strongly WITH the WOT approach. You say it stumbles, but then gets through. Yet with an easy roll on technique it goes thru the same rpm zone without a stumble.

147.5 mains sounds a little rich to me, and when you are hitting the throttle heavy the slides are rising a lot letting in much more fuel thru the mains which might be bogging due to too much fuel until the rpms can increase and then the vacuum drops, slides drop and becomes less rich.

Also, POD filters, are they mounted on Velocity stacks or just directly to the carbs? Quality Pods, or Emgos with the annoying restrictive lip? No matter which type of pods, would suggest using the velocity stack mounts to help straighten out the airflow before it gets to the carbs, vs. the turbulence of coming in sideways thru the filters before turning and going into the carb throats.

When doing the slow roll on, less throttle input, less rise of vac. slides, less overall fuel getting through and so it doesn't bog/choke on too much fuel!?

However, you say that it seems to run out of gas at the top of the rpm range?? 8 k is a bit high with these OLD engines, OLD valve springs? You probably need to do a THROTTLE CHOP at or near that rpm range with load for 30 seconds or so to really see what the plugs are doing.

IT's recommended to tune the top end first, then the adjustments for the lower range...ie. changing vac. slide clip position.

T.C.

[dbeardslee]

I didn't do a throttle chop - just checked the color after a 5-10 mile ride around the neighborhood and a little highway. When my brother last rode it he was having trouble with it fouling plugs, so I just pulled them to see what they looked like.

On the pods, they kind of look like the Mikes XS pods - oblong but a bit smaller. I pulled one to check, and sure enough there's a lip on the inside. Could I just cut the lip out of the way so it doesn't obstruct the air jet as opposed to making a velocity stack?

[dbeardslee]

I just answered my own question on cutting the lip. Can't do it on these pods. I'm not sure if my brother still has the old air box, so I could use a nudge in the right direction for building some stacks from scratch.

[3Phase]

Doug,

Nevermind, lips in the pods.

.

[BA80]

Sounds familiar. Never had with my 3J6 engine though.

http://www.xs11.com/forum/showthread.php?t=44282

[dbeardslee]

I took another look at the pods, and when they're seated to the stops the lip doesn't actually cover the air jet, but it is in front of it with about a 1/4" gap. I'm going to pull my Mikes XS pods off Betsy tomorrow and try them on my bros bike and see if that solves the problem.

[Bonz]

Check out this thread.

http://www.xs11.com/forum/showthread.php?t=8269

[dbeardslee]

Thanks for the link Bonz.

I swapped out the pods with my Mikes XS's this morning, and it appears I had the same problem with my brother's bike that I had with my ex-wife. Just too much lip. She ran like Hillary Clinton in a crowd of reporters.

There's still a tiny stumble around 3k if I really crank it hard, but she'll push all the way to redline with plenty left. Although the jetting is correct as far as the Jetting Recommendations page, I think I'll try TC's advice and drop the mains back a notch to 145's.

Then I'm going to work on Betsy, 'cause that was just WAAAAY too much fun!

Thanks for the help fellers.

[dbeardslee]

Mystery of the stumble solved

I changed out the jets for 145's, but I still had that annoying stumble around 3k under heavy throttle. While synching the carbs it kept farting through the carbs, which got me thinking. Dad always said timing is 90% of everything, and evidently it works for motorcycles too. I checked the timing and it was set on the "T" mark instead of the "F". I put it where it's supposed to be and that baby will suck your brains out and throw 'em on the road behind you now. Nice.

[Jeffintampa]

Nice

I cant wait to get mine running 100 percent and see how it feels, nice post enjoyed reading it, and makes me wish I was that good working on these bikes or had you living next door lol

[dbeardslee]

I cant wait to get mine running 100 percent and see how it feels, nice post enjoyed reading it, and makes me wish I was that good working on these bikes or had you living next door lol

Glad you enjoyed it. There's nothing like the feeling you get when you're tracking down a gremlin and you finally get that bugger cornered.

There's plenty of expertise running around here, and a lot of times it's just a matter of careful observation. The better you can communicate your issues, the better the advice is going to be. Before I took my four year hiatus from the forums we had a group that used to get together in my mancave pretty regularly. They used to pack my driveway with XS's, and we had some pretty good mechanics to wield the wrenches. We even organized a few "rescue missions" to other towns.

Best advice I can give you is don't give up. The more you wrench on it, the better you'll get. Just remember to ask your questions before you turn the wrenches. You'll save a lot of headaches that way.

And check around. I met a lot of good XSives locally by making a post to see if anyone wanted to take a ride around and look at the leaves turning colors (they do that up here ). Got a lot of good times started that way.

[dbeardslee]

After action report

Well, I spoke too soon. While the timing adjustment was a HUGE improvement, I still got a stumble around 3K if I really hit the throttle hard. Something TC said kept rattling around in my head regarding the slides, and although my brother swore up and down they'd never been touched, I decided to disassemble them anyway. Luckily I've got the deep reach snap ring pliers to do the job, but I digress...

Bigger than do-do the darn slide needle clips were one notch off. I put them in the center notches where they're supposed to be, and now I can twist the throttle as hard as I want without a stumble. So between too much lip from the pods, timing that was retarded by 10 degrees, and circlips in the wrong notches, this one is finally put to bed.

Lessons learned:

1) One notch off on the slide needles really makes a big difference.

2) Never take the owner's word for anything - like the sign that used to sit on my dad's desk said, "Those things are done well that the boss checks."

[BA80]

There are 5 notches in the needles. Were they set too high or too low?

[dbeardslee]

There are 5 notches in the needles. Were they set too high or too low?

If you held the needle in its normal operating orientation, the circlips were one notch down from center, which causes the needle to sit one notch higher and opens the mains sooner. It also takes away from the top end. It refused to redline with the clips in that position, but it will sure do it now.