Weird misfire/stumble between approximately 4K-5k rpm

[Red Ballz]

Hi everybody,

I’ve run into a weird situation and could use some input. The bike runs great from idle to redline if I just roll on the throttle. It does great if I snap the throttle open going from low rpm to redline. If I’m at high rpm, 5k+, and go WOT, it is fine. The issue is if I’m between 3.5/4k - 5k rpm and snap the throttle open. The bike seems to lose power momentarily and I can feel it misfiring and stuttering for a few seconds. It will then recover and pull as hard as it should. If I’m in that rpm range and I roll on the gas, it seems fine. It’s not noticeable in first or second gear, hardly in third, but is easily noticed in 4th and 5th.

Mods:
K&n without prefilter
Cobra pipes w/ baffles
38 slows
115 mains
Factory pro needles w/ clip in 3rd pos
Pilot - 2 turns out
Desmogged

I am very confused as to what’s going on. I’ll try moving the clips to different positions on the needles tomorrow. But again, it seems to run great everywhere but this specific situation. I bought the bike with 81k miles on it and will be doing a timing belt change and valve clearance adjustment soon, but I can’t imagine either of those have anything to do with this problem. I’ve also run several very concentrated tanks of Techron through it.

So yeah... thoughts? Should I order up some new interstate springs? They promote better throttle response from the stock standard springs correct? I also can’t imagine a vacuum leak would affect it under these narrow parameters and not have an effect somewhere else.

[Blackduck]

With Factory Pro needles and 115 mains it sounds like you could be on the rich side, have a buddy ride behind and see what you exhaust looks like when it happens.
I had tried Factory Pro with larger jets and they were just too rich, think they were designed more for a close to stock bike.

[Leathel]

+1 on thinking to rich

Do a run at those revs then hit the kill switch coast to a stop and pull a plug, I am picking its black

[Pete]

Based on your observations, it is encountering a temporary rich or lean condition so try both directions. One should make it worse and one should make it better, do not be surprised if the change affects somewhere else in the rpm/throttle band.

[Red Ballz]

I guess I’ll go ahead and move the needle clip position around today and see what kind of effect that has on it. Also, even if I get it worked out, I’m going to stick it on a dyno this week just so I can see what all is going on with it.

[Chrisj CMA]

Maybe just stop SNAPPING the throttle open at low rpm would be my advise. That sounds like abuse, the fat girl is telling you to be more caring.

[indybobm]

If this has just started, I would go to WalMart and get a can of Berryrman B12. Put 1/2 can in full tank and ride it. Do it for two tanks full. Do this before you cange anything.

[Ricky-D]

Well, you've completely modified the air pump nature of the engine and also the fuel delivery system likewise.

I think the first thing to do would be to index the throttle in order to get a handle on exactly where your engine problem resides.

Using rpm's as an indicator is folly and will not accurately detail where the problem actually is.

Another good measure would be to use an exhaust gas analyzer to help delineate your problem but that is probably too difficult to do.

Indexing the throttle would be the best starting point.

***

[Red Ballz]

I moved the needle clips up a position and it seems to have helped a bit. I mean, the issue is still happening under the same conditions, but is much less pronounced. If it wasn’t raining here, I’d have had a better test ride, but yeah, power is improving and the stutter is decreasing. I figure after lunch I’ll adjust it again. It’s still just strange. I mean, if it were a problem with the main jet, I would imagine it would be pronounced through the high end of the rev band. It such a shame this thing doesn’t have a proper fuel injection set up.

“Maybe just stop SNAPPING the throttle open at low rpm would be my advise. That sounds like abuse, the fat girl is telling you to be more caring.”

In my experience fat girls like it rough, it’s the skinny ones that need it gently.

[Red Ballz]

Fixed! Moved the needle clips up to the top notch and it’s running great.

Also, because I can’t leave things alone and had to see what it’s capable of in noise... I pulled the baffles next. God’s brother wept. 102 dB at idle. I’ll leave them out for the week to indulge in it, but they’ll go back in. Maybe.

[Tfrank59]

Well did it start running great before or after you pulled the baffles out? With them out there's a great deal less back pressure as you must know.  And may I ask what you're doing with your bike--are you racing or just hot-rodding around with it because honestly I've never had to rev my bike above like 4500 for the kind of riding I do. Just curious.

[Red Ballz]

Before. Pulling the baffles out was just for craps and giggles this evening. Most of the time on the streets I don’t go over 4500 or 5000, but the average cruising speed on Houston freeways is a good 80 mph. So yeah, I’m regularly up in the higher end of the rev range. Seriously, if you’re on I10 on the west side of town and not doing well in excess of the speed limit, you’re likely to get rear ended by a jacked up bro truck.

[Tfrank59]

Yep, used to live in Houston I know what it's like down there. Ride safe.

[Red Ballz]

Fixed! Installed the 110 mains in lieu of the 115s. The needles are in position 3, and the pilots are out 2.25 turns.

The bike runs excellent now. It’s hilarious twisting the throttle hard and running it up to a hundred with my feet up on the highway pegs.