[Healeys] 67, BJ8 Acceleration problems

[Bob Spidell]

re: 
"The float bowls take care of immediate fuel needs" 


Huh? Are you saying everything will be fine until the float bowls empty out? Carburetors--excepting the pressure type--require a constant fuel level in the float bowls (it's not like flushing a toilet ;). Remember, the level of the fuel in the jet is the same as the level in the float bowl. If the float bowl level is being reduced faster than than it can be replenished because of, say, a fuel delivery issue, you will be effectively running with an incorrect mixture until the float bowls catch up. I'm not sure which way the mixture will go, but since you'd be lowering the fuel level it would probably be equivalent to raising the vacuum piston, which produces a momentarily lean mixture (the 'traditional' way to test mixture on an SU carb). A lean mixture on acceleration would cause sputtering and coughing, which is why carburetors have a) accelerator pumps (constant venturi carbs) or b) vacuum piston damping fluid and springs (SU carbs). 

This is why getting the float levels set correctly is critical on any (non-pressure) carburetor, and why needle (or ball) valve operation is also critical. The proper operation of any carburetor is all about having a correct, constant fuel level in the float bowl(s) and all the other parts properly adjusted with respect to that level. 


bs 

-------------------------------- 
Bob Spidell - San Jose, CA 




<snip> 
Sounds like fuel delivery (acceleration requires more fuel than steady 
cruise). 
<snip> 

I don't think it is a fuel delivery to the carburetor problem. 
The float bowls take care of immediate fuel needs. A weak fuel pump or 
clogged fuel filter would evidence themselves by the car being able to 
accelerate to a certain speed (by consuming fuel already in the float 
bowls) but not being able to maintain that speed (because fuel was being 
consumed faster than the float bowls could refill). 
On the other hand, it could be lack of sufficient fuel delivery 
between the carbs and the combustion chambers. 

- Bernie Johnsen 1967 BJ8