Any help gratefully recieved..running issue GSXR 250

[colt1836]

Hello to all...

I'm looking for some guidance in relation to a gsxr250 with some running issues. The bike starts and runs fine - on the sidestand weather hot or cold. It starts running on what seems like 3cylinders once under load ie accelerating from a standstill. At this point it is definitely missing on at least one cylinder. The moment it gets to about 6k rpm it then revs cleanly on a wide open throttle until the limiter at 18K - any ideas? The list of what I've looked into is fairly comprehensive having rebuilt a fair few of these specialist bikes - am I overlooking something simple - any help greatly appreciated, many thanks....

[Fizzer Thou]

The slides in the carbs do not start to lift until around the rpm that you say it runs on all four cylinders.Have you had the carbs off so as to investigate if all of the pilot jets are clear?If you swap the pilot jet from one carb to another does the fault move?

Check to make sure that the spark plug caps are in good condition and not cracked.I had a problem on my Exup where the engine would run ok when cold but misfire on one or two cylinders once hot.I tried all sorts of things but in the end it was two of the plug caps expanding with heat and the resulting crack on at least one allowed the coil energy to discharge through the cap to the cylinder head.This was particularly noticeable when it was raining
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[brains_t]

May be relevant to your issue. I had an intermittent loss of power on my GSXR750 at low revs, fine at speed and on motorways. Everything pointed to vaccum leak but it was the side stand springs not holding the stand firmly so the switch with it's weird diode was tripping on bumps, new springs resolved issue.

[Robby]

Low to medium power you're using the little jets and the needle. This means you're relying on the small holes to meter the fuel correctly, these holes are the easiest to block up. In this instance just riding the thing should sort it out sooner or later - petrol is good carb cleaner.

It could also be a float problem, either from the float height being wrong or the needle valve not working.

Essentially, low to medium power is where fuelling problems happen. At tickover its all down to the pilot jet/mixture screws and you won't necessarily notice if something is wrong - the mix could be way off but the idle still hold steady. At big power outputs you're mostly on the main jet, which is least likely to block. Its a big hole that spends its time submerged in petrol.

[Ariel Badger]

Identifying the cylinder that is not firing is easy just feel for the cold pipe. Then strip the corresponding carb and clean all passageways out.
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[Vincent]

Checked the fuel tank breather?
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[colt1836]

First of all thanks to all you guys who have posted - really appreciate it. I also think its a fueling issue that I'm chasing, coils are checked already together with new plugs/leads and caps, coils are bang in the middle of the manufacturer specs. The weird side stand cutout on these models has also been ruled out as I bought a NOS part which hasn't changed things one jot grrrr! As per the exhaust headers being hot, all warm up and are hot to the touch within a minute or so of start up, but cylinder 2 takes slightly longer. After a run I've taken the temperatures again using a thermal infra red temperature guage - Cylinder 1 temp (at header) 180c, cylinder 2 184c, cylinder 3 172c and cylinder 4 179c. Would I be right in assuming that the fueling issue is likely on the lowest temperature reading cylinder or should I start with the carb that supplies cylinder 2 which takes slightly longer to warm up?
To all - I've taken the carbs off numerous times - the first to clean them out albeit they didn't look too bad, the second time to replace all the seals as some were long past it - and the last time to clean them again thinking some residue may have remained. I've since balanced the cabs on the bench and thereafter fine tuned them on the bike. Compression figures using a screw in guage are 170-170-184-186psi across all cylinders so I know this should be a simple fix as the basic engine is good...
NOT tried the fuel tank breather yet, Ive got this weekend off so It'll be the first think I check - and thereafter start with whichever carb you guys reckon to check first - thanks again for all the help, cheers simon

Forgot to ask - is it worth getting the carbs ultrasonically cleaned while there off, the main reason being I want to make sure they are 100pc perfect so I can rule them out!!!