Carb Technical Question

[digginfool]

My Prairie has a Keihin CVM carb on it.  It's recently developed a problem where it begins to starve for fuel when running at speed and tank is about half full or less.  I thought the problem might be the fuel pickup screens since the quad sometimes sits for months at a time without being used.  I pulled the fuel valve today and everything is clean there.  So, now I have to assume there's crud in the bowl.  Pull the carb and everything is clean in there.  No sign of any residue or dirt.  I'm not real familiar with constant velocity carbs so I thought I would ask all of you if you have any suggestions or if you've ever run into this problem before.  Could this be an issue with the diaphragm or spring?  Anybody know anything about this type of carb or problem?

[TRX450R_Racer]

Check the vent hose to the tank and make sure it isn't clogged. You need to clean out the jets not just the bottom of the bowl. You need to remove the main jet and the pilot jet and clean them.

[digginfool]

I didn't mention it, but I cleaned the main and pilot jets as well as all fuel and air passages.  I didn't think to check the fuel tank vent hose.  I'll have to run back over to the shop.  It's really a strange symptom because the engine dies just like it ran out of fuel but by the time you put it in neutral and stab the starter button, it fires right up again.  It's maddening.  Also, the floats don't seem to be the issue either.  I don't have the spec for where they're supposed to be set at but they do move freely and there's no fuel in them either.

[qt314nfla]

have you had it in water?  i've had the problem where i had to drain the water out of the carb.  is the fuel hose attached?  another prob i had.  how about the fuel filter?  is the butterfly reacting to the throttle cable pressure?

[eldiablo64]

personally I don't think yer dirty diaphragm has any thing to do with it   lmao  j/k  I will take my gutter brain to the other site now  lmao

[TRX450R_Racer]

Try running it with the gas cap off. If you can do it without making a mess.

[Turboderf]

dose the bike have fuel pump or diaphragm that pumps fuel to the carb

[JackL]

Look for a chunk of crap in the fuel tank.

[mudnuri]

I have been to carburator Hell and back. Sorry you are having trouble. When you clean the carb it is important to use compressed air. You cannot blow hard enough (well?) to dislodge a dirt buger from the carb holes.

[digginfool]

Thanks Brian.  I think Frank may have nailed it with the fuel tank vent.  I removed the cap and tried to blow through the hose.  Initially, it was totally blocked but then there was a pop and I could blow freely through the vent tube.  I disassembled the cap and found two ball bearings and a spring loaded rubber diaphragm.  Cleaned everything in there.  The motor seems to be running good now but haven't been able to take it out and run it hard for more than about 100 yards.  As far as the carb cleaning, I use a non-chlorinated aerosol cleaner to clean out the passages.  Always seems to work well.