GSXR400 losing power at 8000rpm

[Toppie]

I'm having trouble with my 1986 GSXR400. It seems to start losing power around 7000 or 8000 rpm. The maximum it will rev to is 10000. The red line is 13000. I have once managed to rev it out properly in 3rd and 4th gears before it had properly warmed up. When this happened, I cracked open the throttle, it backfired once then felt like it had a massive boost of power when it hit about 9000 revs. Also, it idles a bit rough when its warm but much better when its a bit cold.

I can't seem to source the problem. My thoughts at the moment are either the coils or carburetors. I have heard the coils on these bikes can give out after a while but could that actually cause this type of problem?

I'm told it has the same motor as the GSX400X impulse but with different carbs. The carbs on this have a diaphragm that lifts some slides. They are supposed to be constant velocity or something. I am pretty sure they are the standard ones for this bike.
Please help!

[AllanB]

I'd go with carbs.

Could be gummed up a bit and you only really notice it when being nailed. Or, and I've had this happen on a older bike, the rubber on the diaphragms has perished over the years - a few holes and it will not rev cleanly (if CV carbs). Problem is they are expensive to replace. I have a link somewhere that offers after-market ones a lot cheaper than factory.

Pop the tops off and have a look. be careful with them as they are easy to damage.

[Toppie]

Cool, thanks for that. I will try cleaning them out tomorrow if I get time. I did have them apart about a month back just after I bought it and thought the diaphragms didn't look too bad. They did look old though. Please let me know if you remember that website, I have had a bit of trouble sourcing parts.
Thanks

[westie]

My zxr 400 is doing something very similar. Inspection of the engine showed it was fine, carbs and tuning fine however the sparkplugs............
Shove some new sparkplugs in and give it a hand full before you get too far along.

[robinm]

Has it had any mod's at all, ie any mod's to the exhaust for example? The early GSXR's are very fussy on their jetting, basically any changes from std will require rejetting. What you said in an earlier post that it revved out once before you had warmed it up suggests a carb issue. The best way to find out is to have the bike tuned on a dyno, but that is not cheap. You could try a plug chop, with a new set of plugs. That is run the bike at max (on the road) for a minute or two, then clutch in and kill switch at the same time, stop the bike and remove the plugs. The colour of the plugs will give you an indication of whether it is running lean ( plugs will be white) or rich (plugs will be dark or sooty).

[Toppie]

Awesome, thanks guys. I will give that a go. And, no it has the standard exhaust. I only bought it recently and haven't modded it myself. From what I can tell, it hasn't been raced and there are no visible modifications. I haven't checked the plugs yet though. Sounds like a plan.

[Katman]

When were the valve clearances last checked. Smooth idling when cold but rough idling when hot is a classic symptom of insufficient clearance.

[Toppie]

I checked the clearances a week ago. It was my first time checking clearances but I think I got them right. I did set them closer to the small tolerance than the large one as I was getting quite a bit of tappet noise.

[Katman]

Were they tight when you checked them? Sometimes valves (particularly inlet valves) start to recess themselves into the seats. You can set the correct clearance and within a short space of time the gap will close up again.

[quallman1234]

spark plug caps.

[Toppie]

Ok, thanks Katman. Some of them were a bit tight actually. What can I do if they are recessing?

Quaillman, what could be wrong with the spark plug caps? Can they sometimes have more resistance than they are supposed to or something?

[Katman]

Recheck them to see if they've tightened up again. If they have, it means pulling the head off to replace valves.