RPM hesitates after 5k

[phatman1969]

Thats what I thought. Just wish I knew what I was missing. I have checked and checked and cant come up with what I am doing wrong. I have had the carbs apart so many times its crazy.

 

[DjSky96]

I know this is an old thread, but I have the exact same issue and it's because one of my ignition coils is going out. I'm looking for new ones but I'm having trouble.

 

[Oleschool]

I've had this problem on my 400 for a couple years and still cant figure it out. I tried increasing my main jets but its still the same. I'm wondering if the vacuum diaphragms aren't working properly??

 

[xschris]

What do the plugs look like? Any mods to the intake or exhaust will cause issues. Carbs synced with manometer, valves adjusted, compression checked (150 psi is a very good number) timing checked. Most people put too large of jets in these bikes. I would start stock with 42.5 pilot with 3 turns out on the pilot mix screws, stock slide needle without any shims and 135 main jets. Brass floats set to 26mm. Make sure to only use real mikuni jets as any others will not work well as they are made poorly.

 

[Oleschool]

I adjusted the valves, cleaned the carbs replace the torn factory air filters with new aftermarket stock style filters. I tried increasing the main jet to a 140 because it had 135s and the 137.5s were out of stock. Just took it for a ride and the same. I raised the clips on the needles from 3rd ring to the bottom ring to richen it up more but no difference. My bike has an exhaust from another bike, but I ran it without any mufflers and it did the same thing. I modified the petcock because it was leaking and have the vac ports capped. Plugs look a little black but it starts and idles fine. I haven't changed the pilot because I thought that was just for idle. Any suggestions?

 

[xschris]

I would set the needle to stock clip. Post pics of the plugs. If they are black your rich. Carbs are synced with manometer? You may want smaller jets. I have dropped from 140 to 130 on my xs360 carbs to get a better top end.

 

[Oleschool]

Thanks I'll try that. No I haven't sync'd the carbs. Is there any chance that my problem could be lack of flow through the petcock? It was leaking fuel so I modified it so it didn't need vacuum to open. The bike starts idles and runs great to about 5k-5500 rpm then it stumbles. Also if I'm cruising in 3rd at 4500ish and I crank it WOT it revs then starts to stutter. I'll try to lower the clips. There is a flat washer on top the needle and under the small spring. Is this stock?

 

[xschris]

If gas flows out of the petcock you should be fine. Carb sync make a huge difference. Post pic of you needle setup. 80 bikes should go in slide, needle, washer, spring, then the plate with two screws in it.

 

[xschris]

Like this.

 

[Oleschool]

Yes that's exactly how it is

 

[Oleschool]

I went to the extreme end of the needle and set the clips in the first groove. It got rid of my stumble. I think this is the first time the bike has been able to rev past 8k.I'll try the second groove tomorrow hoping it might be a little better. I'll also order some smaller jets.Thanks Chris