Another 4k rpm problem...

[Jacksbugs]

Hi everybody,

I went for a ride this morning and then stopped the bike for about 15 min. Everything was fine. When I wanted to drive back home, I could not accelerate past 4k rpm. The bike was accelerating just fine below that rpm but then at 4k she felt like she was struggling, unable to go past 4k at all. The rpm would then drop down to 1-1.5k and oscillates back up a little bit with a lot of misfiring noise coming from the exhaust. The only way to get a normal ride back was to ease on the throttle and have the rpm back below 4k. Like I said, up to 4k, she was accelerating just fine.
I checked the slides and they are moving freely. I'm no expert but it doesn't feel like a single carb issue as she won't go at all in the higher rpms and it's each time right on 3.9-4k rpm when it's happening. I tried home just after starting her and she reacts the same...
Any idea where would that come from ?

For the record (all maintenance made in May):
- my bike is a 97 stock except for COPs mod.
- I did a complete carb cleaning with ultrasonic bath and changed all the parts(rubber/diaphragm) that had to be changed in the carbs.
- did a dry floats setting
- did a sync
- put 2 1/2 turns on the mixture screws
- did a valve clearance check.
- checked the intake rubber boots and changed the vboost ones.
- changed 1 intake manifold that was cracked.
- changed the fuel filter.

I had already drove almost 250 miles after these maintenance operations done before she started acting this way.
I kinda don't know where to start to troubleshoot that problem.
Thanks for your help.

 

[Traumahawk]

Were you trying to get past 4K at part throttle or full throttle, or do you remember where you were at the throttle? Different parts of the throttle control different circuits in the carbs.

 

[Jacksbugs]

I'd say both.
I cruised both on streets and on the freeway on my way back home and even at stable rpm ( 3.8k as I couldn't go past that) with almost no throttle movement, as soon as I was reaching 3.9-4k, the symptoms appeared. It really felt like a "on/off" switch.

 

[D-Max2012]

Did you happen to add fuel earlier that day or the day before, this started happening?
Reason I asked.. for almost 3 riding seasons, since i got my 91. it would stumble, starting at 4K.. it would persist till 5K.. After 5K it felt like it had power.
I did EVERYTHING you can think of. Found cracked boots, changed them, thought I nailed it down - same results. Fully broke down the carbs at least 6 times to clean - 3 of those times with an ultrasonic cleaner bath - same problem. Synced each time carbs were removed - no dice.
Even went as far as to coat the inside of my tank with that 3 step KBS Tank Sealer system to make sure there was no rust from making its way into my carbs.
Anyway - after that rant.. LOL
I did the SeaFoam cleaning into the carbs twice, but that did not help.. BUT on the can, it says you can treat up too 6-8 gallons of fuel when poured directly into your tank at fill up. SO, I poured 1/2 bottle into my tank at fill up - problem was still there.. On the 2nd Fill up and the other 1/2 bottle added, around 1/2 tank was used up, I noticed that the stumble was GONE completely.

 

[RempageR1]

Sounds like an ignition issue to me. Do you have a spare TCI (ignition box) you could try?

 

[eggohead]

I have a muscle jet kit on my 90, I don't think it's the ignition. And mine does it also.. I think I have something pluged up inside.. I am not planning on pulling everything apart to mess with them while the weather is good to ride, plus I don't have tons of work room. I have 2 sets of carbs gonna get pulled this winter and cleaned up nicely... Also I use Seafoam regulary.. didn't notice this until I started bike up this spring, also did jet kit then...

 

[adambweird]

I bet theres a clogged circut(s) in the carbs somewhere. Try shotgunning and/or the peashooter.

 

[Jacksbugs]

Well, I did fill the tank the day before and already drove 40 miles before the problem appeared. I still have some Seafoam left, so I could try that in the meantime and see if there is some improvement.
What I don't understand is why I would have a clogged circuit in the carbs after 250 miles since cleaning ? I did the shotgun and while I was at it i told myself why not do a complete cleaning/rebuild ? so I took the carbs all apart and cleaned everything thoroughly with an ultrasonic bath and ran the first tank on Seafoam after putting the ramp back on the bike. I might have some rust in the tank though, so next big thing to do is treat that.

As for the TCI box, I don't have a spare one and I guess there's no easy way to check if mine is the culprit without a spare one ?
Could the COPs mod have killed my TCI (don't have resistors on the wires) ?
I installed the mod 250 miles ago, but I thought the new boxes weren't too sensitive about the no-resistor wires.
Anyway, I was thinking about getting an Ignitech box. I guess now would be a good time...