Any help/direction would be appreciated...

[Guest]

Hey folks.

My GS400 has been running great. Today it developed a hesitation or misfire at 4500 rpm when using mild throttle.

Originally thought I was running out of fuel as I was climbing a sizeable hill. But it has continued.

No hesitation (or none that can be felt) at WOT, only when accelerating slowly. Happens in every gear except first. Haven't noticed it at all in neutral.

Carb issue? Air leak?

It starts and runs like a dream.

Any thoughts or opinions would be appreciated.

Thanks.

[bonanzadave]

Does the fuel petcock have a PRI position ? If so, set it there and try it again. That might answer any fuel delivery questions.

[ontario.chris]

Does the fuel petcock have a PRI position ? If so, set it there and try it again. That might answer any fuel delivery questions.

Could be a petcock issue?

[bonanzadave]

Could be a petcock issue?

Maybe. Alot of these old bikes use a gravity/vacuum fuel delivery system. The vac hose has to be connected to the petcock in order for its internal diaphragm to open and allow fuel to flow. The PRI position bypasses this.

If your vac hose gets kinked or develops a leak you wont get full fuel delivery.

[ontario.chris]

Ok not a petcock issue

So I tried the petcock... There doesn't seem to be a crimp or leak....

Bike is still running great, except when accelerating through 4500 rpm. It stumbles/hesitates/misfires? Then rights itself near 5000 rpm.

Doesn't do it when idling. Doesn't do it at WOT. Have put some seafoam through it. No change.

Any advice appreciated.

[08rangerdan]

sounds to me like you could be due for an assessment of your bike. id start with a plug check.. see if any cylinders are cold or hot. see if there is any wet plugs..maybe time to change them? i wouldn't know your maintainance schedules. air cleaner could be excessively dirty? doesnt take much to throw it off.

if you have some variances in the plugs id count on doing a carb sync eventually. but first id consider rebuilding the carbs if they havent been done in a while.

limit any problems before you set up because you will be wasting your time syncing carbs if you have a plugged up jet or an air leak. common air leaks start at the rubber intake boots behind the carbs,, if your engine is anything like my 1982 850.. o rings behind the boots against the head were all cracked in mine.

[spyug]

Some good thoughts from Mr. 08. Have the carbs been cleaned and rebuilt recently? If not I'd suggest that is where you should look. Seafoam is good but you may need to use quite a bit and probably several tanks of gas to get any results. Its probably quicker to do a strip, dip and rebuild and don't forget fresh O-rings including intake boot O-rings if fitted. it just takes a little bit of an air leak to throw things way off as he mentions.

When it comes to carb synch, I can help with that as I have a Morgan Carbtune, you just need to come in to the Hammer

Good luck with it.

[ontario.chris]

Thanks folks.... I'll check the plugs , and air filter and then move onto the carbs.

Spyug, thanks for the offer. Might end up taking you up on the offer some day.