Help! Limited RPM problems

[Tumbles]

Ok, I have a fairly major problem going on with my Yamaha SRV 250. Background info is I have conducted a full rebuild of this bike, from the gearing out. I have modified the intake system by installing a K&N Pod air filter, and turned the exhaust into a 2into1 system. Basically she now sucks and blows a lot easier than it used to. Spark plugs show the front cylinder is running slightly rich, and the rear cylinder slightly lean. But both within limits. Carb is 2x Mikuni BDS26.

My problem is that in first gear, at around 7500RPM, the rpm tops out, a coughing sound comes from the exhaust, and power surges occur at about a 1s interval. In 2nd gear it does this at 600RPM, and gradually reduces to just under 5000 RPM in 5th gear. This means the top speed I can get out of the bike is 80Km/Hr as opposed to the 140Km/Hr I could get previously. It is interesting that the engine will run true right up to the 8000Rpm redline in neutral. But as soon as you apply load (i.e. ask it to do some work by turning the weels in gear) this starts to happen. It is for this reason I find it had to believe this could be a carb issue. I also struggle to believe it could be an ignition system issue because its electronic. This leaves me completely stumped!

Has anyone come across this happening before? Or at the very least, give me a list of most likely to least likely causes and therefore a priority list for me to start looking at?

[kiwi cowboy]

Ok, I have a fairly major problem going on with my Yamaha SRV 250. Background info is I have conducted a full rebuild of this bike, from the gearing out. I have modified the intake system by installing a K&N Pod air filter, and turned the exhaust into a 2into1 system. Basically she now sucks and blows a lot easier than it used to. Spark plugs show the front cylinder is running slightly rich, and the rear cylinder slightly lean. But both within limits. Carb is 2x Mikuni BDS26.

My problem is that in first gear, at around 7500RPM, the rpm tops out, a coughing sound comes from the exhaust, and power surges occur at about a 1s interval. In 2nd gear it does this at 600RPM, and gradually reduces to just under 5000 RPM in 5th gear. This means the top speed I can get out of the bike is 80Km/Hr as opposed to the 140Km/Hr I could get previously. It is interesting that the engine will run true right up to the 8000Rpm redline in neutral. But as soon as you apply load (i.e. ask it to do some work by turning the weels in gear) this starts to happen. It is for this reason I find it had to believe this could be a carb issue. I also struggle to believe it could be an ignition system issue because its electronic. This leaves me completely stumped!

Has anyone come across this happening before? Or at the very least, give me a list of most likely to least likely causes and therefore a priority list for me to start looking at?

hi tumbles.
i dont no mutch but my two cents is it may be jets.
you say you modifyed the inlet and exaust but have you opened the jetting up? as from whot ive herd you need to if you let it breath easer.[i e more air more fuel bigger exaust=winner but standard jets would make it run lean at full throttle]wot i mean is with no load[in netral]you wouldnt be on main jets when revving out but the higher the gear the more the engine is working the more fuel its trying to suck but gets to the point that the jets arnt big enuf.
shit i hope this makes sence cheers.

[Tumbles]

Haha, Anthrax you are obviously a non-believer! Cheers handle, this is pretty much the same thought process I am going through. Good to have it confirmed by someone else. So far I've checked compression (165PSI front, 170PSI rear), stripped the carb, and checked the fuel tap/filter. Fuel tap and filter are mint, so no dramas there.

I was always going to re-jet the carbs for exactly the reason you described above. I hope this is also the cause of the limited RPM also. One diaphragm has been repaired with silicon at some stage in its life, so I'm replacing that ($95!!!), giving it a good old huck out, and redoing everything from pilot jet to main jet. I'm obviously thinking the main issue will be the main jet given the RPM range we are talking about, but I want to tidy the rest up due to the mods anyway.

IF after this, the problem is still occurring, is ther any other potential causes anyone can think of? Would timing be affected by the mods? -More air and fuel = bigger/longer bang?- She's not backfiring that I can tell, so I don't think so. Spark plugs are new, but what about if the rotor/pickup/stator/wiring can't put out a big enough spark? would this be a likely symtom of that?

[FJRider]

hi tumbles.
i dont no mutch but my two cents is it may be jets.
you say you modifyed the inlet and exaust but have you opened the jetting up? as from whot ive herd you need to if you let it breath easer.[i e more air more fuel bigger exaust=winner but standard jets would make it run lean at full throttle]wot i mean is with no load[in netral]you wouldnt be on main jets when revving out but the higher the gear the more the engine is working the more fuel its trying to suck but gets to the point that the jets arnt big enuf.
shit i hope this makes sence cheers.

What he said...BIGGER breaths (in and out), need MORE fuel, to keep the correct fuel/air ratio. Try next size up main jet.

[FROSTY:Me]

dude its easy _ I think you really know it
It was running just fine before you touched it --right?
So it has to be what you have done thats caused your problem?
If it was me I wouldn't piss around.Get the beasty on a dyno.
I'd call Adam at henderson Motorcycles and see if he will do you a deal
Basicly His man runs the dyno you do the jet changes

[Tumbles]

I agree in principle, however what I have done is a full bottom and top end rebuild + new intake + new exhaust which = ???? I am keen to chuck her on the dyno once I have the carb parts i need though. Anyone know of a place in Wellington?