Explanation needed regarding valve clearance and compression
 

hello. i have a question :)
in my friend's CBR we checked and cleaned the carbs several times, checked all the electrical+ignition system several times and they were all in specs according to the manual. but he gets a dry carbon foul in all four plugs and his fuel consumption is way lower than mine.
so my question is, can a bad valve clearance or a low compression cause a dry carbon foul in all 4 plugs? if so can some one explain how it happens please. i googled it many times but couldn't find a specific explanation for it. but they all suspect compression and valve clearance for the dry carbon fouling (assuming that the carb and the ignition system have been confirmed OK). thank you very much.

Would suggest doing a compression test to put your mind at ease,are you running the right size needles and jets, float height , is the bike blowing smoke from the exhaust, and which CBR is it

thx. this is a CBR250RR bike. i have the workshop manual for this. needle sizes are correct. jets are in standard sizes. float height is non adjustable but i checked it by letting them to float in a small gasoline container. they were all ok. no smoke but we can see kind of a heavy carbon deposit at the end of the exhaust opening. thank you.

Sounds like you may have a rich condition. Is the choke or enrichners returning properly.
One thing that can cause a rich condition is a restricted air intake. Is you air filter dirty?
Have you checked the air box to see if a small 4-legged friend has made a home in there?

hi. yea the choke mechanism inside the carb works really well. opens and closes the ports without sticking. the whole circuit is pretty clean. apart from the carb cleaner and the compressed air i pressurized gasoline in each port ( choke circuit, idle circuit, main circuit and air jets ) using a clear tube and syringe. the ports are 100% clear.
air box is clean. no dirt or any strange objects there. it has a brand new OEM air filter and a new sub filter. forgot to say that his exhaust is also an OEM one.
thank you.

Valve timing and clearances are the gatekeeper for air/fuel in the engine. If that is off, then you're fighting a losing battle. Start with checking your valve clearances on a stone cold engine (meaning room temperature for where ever you are). Don't attempt to do it on an engine that has been run at all within the last several hours.

thank you very much. i'll check the valve clearance :)

ensure that the engine is tone cold -preferably let it cool overnight and then do it . Also I'd suggest get 2 feeler gauges to eliminate any possibility of wrong readings due gauge manufacturing defects. Remember every mm counts here.
Also you need to be sure exactly what 'slight drag' is.IN my case I could never decide if the drag is slight or too much. So much so that I recently got the clearances checked at a garage (I reshimmed it myself 5k miles ago). They are all within specs thank God!
Also this would be the perfect time to get your starter valves synced. Do it just after you do your clearances.

right got it :) thank you :)

Either can cause poor combustion, which leaves lots of carbon residue. So yes.