Just wondered if you could help me with an irritating problem I have with my bike. The bike in question is a XR400 R4 2005 with approx' 4,000 miles on the clock. I'm the second owner of about 18 months, the previous owner is a close mate of mine who tells me he hadn't 'tinkered' with the bike except for opening up the tailpipe tip and removing the air-box snorkel, other than that it's 'bog standard'. I suspect the problem has existed from new as he had never competed on it or really taken it off road and therefor, possibly not noticed the problem!
The Symptoms...
Road Riding- When cruising along at 1/4 to 1/2 throttle and suddenly give it full throttle then chop it back to 1/4, it'll 'burble' for a second or two as if the choke has been switched on. I agree, that is pretty aggressive and an unlikely riding style. However when Trail or Enduro riding the symptoms above are exaggerated 10 fold, as the nature of off road riding is comparatively aggressive to road riding, as |I'm sure you are aware! Obviously this makes the bike unusable off road.
After two new plugs were fitted I moved to the Carburettor
-The snorkel and exhaust restrictor were returned to standard. Due to no change with the bikes performance I un restricted it again
-Checked the float height according to the Haynes manual
-Thoroughly blown the carb' through with an air line on several occasions
-Reset the float height according to John Rushworth.com website- quite different to standard!
Getting more desperate, I put a strobe gun on the HT lead and found a very obvious miss-fire from tick-over up through the rev range. As the revs rise it advances as the Haynes manual suggests it should.
-CDI unit replaced (luckily, 'sale or return')
-Stator/generator replaced (upgraded to a 100w)
-Coil, HT lead and plug cap replaced (second hand, e-bay!)
Now getting really desperate...
-Compression test showed that all is o.k with the top end and rings etc. (115-120 psi)
-Drained all fuel and replaced with a different brand
-Pilot/air screw tried from fully in to fully out and every where in between
-Pilot/air screw removed and o'ring checked
-Air cutoff/accelerator pump o'ring and diaphragm cleaned and checked
Obviously all the above has had no effect. The only other things |I can think of is rectifier, flywheel, switches, loom or a manufacturing fault with the carb'.
Any help would gratefully received, no matter how trivial. Believe me, I'll try anything!!!
When the throttle is quickly partially closed this happens, the air column that is rushing through the carb has a lot of momentum and suddenly finds it has a smaller space to squeeze through. The air behind the carb momentarily compresses and the air rushing under the slide speeds up. This draws up a lot of extra mixture, hence the burbling and running rich.
The opposite also tries to happen, if you quickly snatch the throttle open, the air column can stall causing the bike to cough. I expect most people have experienced this at sometime with a 4 stroke single.
Most carbs have lots of compensation systems built in to try and help prevent symptoms like this being excessively troublesome.
There are 2 carb systems to try and stop this, a emulsion tube and a capacity well, these are built around the main jet tube. American tuners will often change these and the slide profile to get the carburation spot on. Us Brits usually just tinker with the idle jet and main jet.
It is possible that the jetting is slightly rich in your case, as this will cause the effect you describe to worsen. Have you tried dropping the needle 1 notch, this may help a little as it will block some of the extra mixture being drawn through. But beware, as it may cause other symptoms. IE. Running weak, + associated issues.
Happy tinkering.
Adrian
-- Edited by Biker on Wednesday 17th of March 2010 10:23:42 AM
