Spluttering...

CBR400_Paul · 07-08-2009, 9:49 AM

I have a 1990 cbr400, but theres a problem.... I bike has an aftermarket exhaust that was fitted by the previous owner.... I bike revs up to around 7-8000 rpm, after that it splutters through to about 12,000 rpm onwards then its better. I was thinking carbs needs re-jetting??? Any idea??

Cheers.

Milzy · 07-08-2009, 9:57 AM

hi mate,

try balancing the carbs first.... than should help it run better.

its not a big job and doesn't require a specialist dynojetter so should be cheaper.

i think its approx £30-60 (1-2hr labour)

CBR400_Paul · 07-08-2009, 10:02 AM

Is that done electronically, i've heard its very expensive :S . btw, its got a full new system on it, not just end can.

Milzy · 07-08-2009, 10:17 AM

na, nothing on bikes of that age are electronic....

you take the tank and airbox off the bike, then put a gauge on the carbs. this will then tell you whether any of the carbs are throwing in more or less air then any of the others.

it is then just a case of adjusting some screws to get them all the same.

it is electronic and expensive when you have fuel injected bikes. everything can be programmed to minute detail then.

my 93 blade can have most things adjusted using a basic tool kit!
and i do most things myself cause im cheap!

btw

i just saw ur join date!!!

CBR400_Paul · 07-08-2009, 5:32 PM

Ive been told by a mechanic that that it could be a resistor behind the clocks that needs to be removed ???? Any ideas???

Milzy · 07-08-2009, 5:39 PM

sorry,

that is out of my range!!!

Surffrog · 07-09-2009, 1:03 AM

Quote:

Originally Posted by CBR400_Paul

Ive been told by a mechanic that that it could be a resistor behind the clocks that needs to be removed ???? Any ideas???

That's part of de-restricting and won't help solve your problem. I'm more inclined to believe that you should start with cleaning your spark plugs or replacing them, followed by the same treatment for the air filter. Then, if that hasn't helped, take the carbs out and give them a clean without changing any of the settings.

With the carbs out, you'll be able to see if the inlet rubber tubes are letting in air where they shouldn't, if fuel has been sitting in the carbs long enough to 'gunk up' or if the needles have got ridges developing on them. Sometimes, dirt build up reduces the moment of the needles as they slide up and down through the carb cylinder or jet. Also check the rubber membrane at the top of the needle holder as a small hole here will affect the vacuum and cause similar problems to those you're experiencing.

CBR400_Paul · 07-09-2009, 6:04 AM

Well the guy i bought it from, 'apprently' had just serviced the bike, with new plugs n fluids, oil filter looks new as well, havent checked air filter. I was thinking that the full new system could be at blame, as the increased airflow would need more fuel as well, e.g. bigger jets maybe.

Surffrog · 07-09-2009, 8:50 AM

Quote:

Originally Posted by CBR400_Paul

Well the guy i bought it from, 'apprently' had just serviced the bike, with new plugs n fluids, oil filter looks new as well, havent checked air filter. I was thinking that the full new system could be at blame, as the increased airflow would need more fuel as well, e.g. bigger jets maybe.

Well, he may have done these things but people tend to 'say' a lot. Until you visually check for yourself, you won't really know. And, in fairness, even if he did I would put good money on the carbs not being cleaned. Lets face it, if your going to re-jet the bike anyway you may as well pull them out and give them a scrub down inside and out.