Model B flat spot

T

tiger.1000

Well-known member
Model B has a severe flat spot with the GAV closed even on a hot engine. If the GAV is open half a turn it runs fine, idles evenly and no flat spots.

Is it OK to drive with the GAV open?

I know we've been down this path before ?
 
My BB wil not run well with the GAV off, exibiting lean run characteristics. Runs best 1/2 to 3/4 turns open. Seems to me that a GAV should be able to turn the mixture off if closed and working properly.
 
BB....... I don't think the purpose of the GAV is eliminate the supply of petrol. I was under the impression that it was for cold start purposes only?

Does anyone know its' true function?
 
David,

Its original purpose was indeed for cold starts only. That's confirmed in the owners' manual. The Bs that I've had/have run fine with it closed once the engine warms up.
 
David,

That's what I thought, so any clues why I need the GAV open when running? I hasten to add that it was running fine with the GAV fully closed and this fault has appeared
"overnight".....?
 
David,

I agree with JimTN as to the likely cause, namely a foreign body restricting or obstructing the idle jet, B-9542, or the passages in the upper and lower castings leading to and from that jet. (A friendly reminder from one of the Service Bulletins was to not use anything metallic to clean a jet, but rather use "broom straw" so as to not distort the opening in the jet.)
 
I remain quite confused about the function of the GAV on the Model B carb. It has often been stated that the valve should be closed once the engine is warm. Mine has never worked that way, it runs better with the GAV open slightly like the original post in this thread. I dismantled the carb to check the 9542 (idle jet) as suggested and found no problems. My question is, why should the "dash controlled main jet adjusting valve" be closed when running? Additionally why should the main jet adjusting valve affect the operation of the idle jet? I'm thinking the air adjustment screw affects the idle jet.

Help me out here! There must be something I'm missing!
 
As DavidG states probably a plugged jet.
Have you flow tested your jets??
Most jets available from the vendor's are not right. Renner's has GOOD flow tested jets for the A's and B's. Reasonably priced. FWIW
Paul in CT
 
Cleaning a B carb in good condition. Does anyone have a exploded diagram of the B carb? Also how do I add a picture with my post?
 
Moose....hope this helps ?
 

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