'98 400F won't run after getting started. Acts like it's not getting enought gas.


I've been riding and working on bikes and cars for over 30 years, and NEVER had to take one into the shop yet....but this one has me stumped.

I've owned my 400 since new and it's alwasy run like a champ. All of a sudden it runs like crap after I get it started....well, here are the symptoms:

Must use choke to get it started and totally warmed up.

No effect when adjusting the idle screw...all in....all out. No difference

When riding mid-throttle, it surges...falls off....surges....falls off....etc.

Let off on the throttle and it backfires.

Let it idle...blip the throttle....it dies. Then choke to get a hot bike started again.

Sometimes when I have the idle adjustment way up, it starts at a high idle for 1 second, then fall off like the gas is going away.

It smells lean and acts like it's not getting enouth gas. Yes, I have cleaned the carb....twice, but did not remove the slide. Yes, air cleaner is spotless. No varnish in carb and gas is new...so is the spark plug. TCS checks out on the ohm meter. Have cleaned the accelerator pump and diaphram is not torn.

Valves check out set and cams have never been removed so I know they are in sync. Yes, the fuel is flowing to the carb...no obstruction.

OK....that's it. Can anybody help this pool ol boy out. Hate to take it to the shop...but I wanna ride!!!


Check the vacuum release plate on the back (engine side) of the carb. They will wear, then chip or crack, loose a corner, etc., and cause all kinds of misbehavior.

If the slide plate checks out, then it still sounds like something going on with the pilot circuit. I know you said you cleaned it, but they can be a royal PITA to get completely cleaned and sometimes you have to dig a little deeper. I struggled with the same problem where I cleaned it a bunch of times and the cleaner would blow right through every passage and jet, but it still wouldn't run right. Eventually found I had a partially clogged passage. Most of the time it's something with the jet or the fuel screw. Another possibility is an air leak between carb and head. Some guys have had a tight/damaged intake valve problem. Maybe try doing a compression check.

Thanks guys for the advice. I got consensus that it was either fuel or vacuum.....not electrical or spark. I went to the intake manifold first and guess what I found. The manifold is held to the cylinder with one clamp, and to the carb with the other. Little did I know that the two clamps ARE NOT THE SAME DIAMETER. I had them reversed and was creating a vacuum leak.

So I am back running and I do appreciate your help on this forum.:ride:

Donnie in Las Vegas.

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