Bike loses 50% power when Throttle opens past certain point


XS400 New Member
Reaction score
Minneapolis, MN
Does any one know what would cause the bike to lose significant power when the throttle is opened past a certain point? It seems to happen on all Gears but 1st.
What model bike do you have and what setup are you running. Most likely it's dirty or out of adjustment carbs.
My bike did that too. Was okay to about 3/4 throttle then misfired and lost power.
So far it seems the main jets were the wrong size. I thought it was running too lean after looking at the plugs. So I went quite rich and it was much worse. Went down in jet size and now it is okay to about 7/8 throttle.
Will report again after I get more jets and new filters installed and " Deep Purple " Seca 400 is back on the road.

Unkle Crusty
Hey Unkle Krusty. I actually was able to fully resolve this issue but I have no idea why the changes I made had the effect it did.

I put in new spark plugs and I also replaced the fuse box with in-line fuses. I noticed one was the wrong size (14pk) and the connection to it was not great.

Perhaps someone else could explain this better but maybe it will help you get that last 1/8.

Thanks everyone for the responses and help!
Spark plugs could have some effect, fuses - I can't imagine they have anything to do with it. Warming up the bike could make such a difference.

But you know, new shoes can add 5hp to bikes power! So anything is possible!
Seems to me you have an issue involving vibration. Today one of my four fuses popped out of the fuse tray, on one end, and after being really frustrated, found the fuse was simply OUT of it's "socket" at one end. Nothing more complicated than that.

Then of broke down later...but that's a correlation. If your bike vibrates a lot when you throttle up, you have two issues. loose chain, causing vibration and a wiring flaw that plays out at high vibes.

Good Luck
First....carbs,clean And clean again, put an in line fuel filter in, do you have stock air box?...exhaust?...maybe jet sizes....needle height...could you have an air leak around the carbs (throttle rod seal failure)...or carb boots...