1980 xs400 breaking up at high rpm

[justcallmeV]

Hi all, finally got my xs400 working well enough to ride regularly and I've immediately come to a problem. When commuting its perfect but when I try to get on it a little bit and enjoy the ride it breaks up past 6-7k at about 3/4 throttle, feels like its missing (slowing while on throttle then flashes of power) and at full throttle it holds its rpm and just maintains its speed. I've been through the carbs multiple times and even just cleaned them today, I've also read that this could be a spark issue. Wondering if someone here can tell if its the carbs or ignition. Should also be mentioned that when I'm not riding it I can push it to redline no problem. Video link showing bogging with full throttle

 

[CaptChrome]

Sounds like a fuel issue. More info would be helpful to point you to some solutions:

1. Since you have been in the carbs, what are the jets and needle numbers that are installed?
2. Does this happen regardless of the gear you are in?
3. Have you set the fuel level in the float bowls with the clear tubing?
4. Have you synched the carbs?
5. Do you know you are getting a solid flow of fuel to the inlet of the carbs (no petcock or fuel filter restrictions?)
6. Do you have a stock exhaust and intake on the bike?
7. Run the bike and cause the issue. Immediately shut it down and pull the plugs. What color are they when you read them?

Bike sounds pretty good otherwise. If the other stuff checks out it might be that you need to go up one size on the main jet.

 

[XRKawboy]

Hi all, finally got my xs400 working well enough to ride regularly and I've immediately come to a problem. When commuting its perfect but when I try to get on it a little bit and enjoy the ride it breaks up past 6-7k at about 3/4 throttle, feels like its missing (slowing while on throttle then flashes of power) and at full throttle it holds its rpm and just maintains its speed. I've been through the carbs multiple times and even just cleaned them today, I've also read that this could be a spark issue. Wondering if someone here can tell if its the carbs or ignition. Should also be mentioned that when I'm not riding it I can push it to redline no problem. Video link showing bogging with full throttle

I've had similar issues on other bikes. It turned out to be a loose or corroded electrical connection. Double check all of your connections before you go chasing carb issues, but as CaptChrome advises, make sure the carbs are set up and adjusted correctly.

 

[ehud42]

My XS500 would bog at 5-6K / 80kph. Ran great toodling around town. Just got weak knees on the highway.
Carbs were clean, points clean.
The TL;DR was that someone had drilled out the stock main jets. By a lot. The result was flooding in the power range.

 

[justcallmeV]

Finally had the chance to change the jets out as I suspected it might be rich from the bogging at full throttle, when I bought the bike it had 125 jets in it and I put it back at 142.5 (stock) when I rebuilt them. Turns out previous owner was correct, took her for the first ride to the gas station with 125s in and doesn't bog at full throttle anymore, not breaking up anymore either. I paid for the whole tach great to finally be able to use it