Carb problem

[bruce]

Need some thoughts on this one, I'm helping a pulling buddy out and have built a "modified" stock carb for his tractor. It is blowing fuel out of the bowl vent so somehow it is getting pressure in the venturi area and pushing the fuel back down the nozzle which in turn forces it out the bowl vent.

Now I do not know the specs on the cam and suspect it might be the overlap? Other thoughts would be exhaust pipe size to restrictive? The engine ran well over the weekend on the track and I was told it lugged really well which is what I was after.

I modified the nozzle for a higher fuel signal so it is much more sensitive than a stock nozzle so I'm sure this is highlighting this problem. I'm going to play with the float level and drop it down a tad.

What I design for is this; his engine runs on a governor at 4000rpm's as the engine pulls down the governor keeps the throttle plate wide open but airflow slows down and the engines run lean as they no longer pickup enough fuel to match the airflow. By playing with the fuel signal I am able to match the fuel signal to the airflow at lower rpm's/wide open throttle plate.

Any thoughts?

[jfholm]

I had a similar problem once where we improved the airflow through the engine drastically and the carb was too small and started pressurizing at the venturi - was just not enough carb

John

[John]

Does this problem occur across the whole rpm range or only at certain points? This sounds like something called fuel standoff or reversion.

[bruce]

I only played with this engine for a few minutes on Saturday afternoon, he needed it to work for Sunday. I'll have to get some additional info. I'm also leaning towards opening up the venturi some but, it's a fine line in the rules :;):

Quite possible it's valve float, I know it's got a wild cam in it for this class and stock springs I'm just not sure what the specs are.

I want to get it on my flowbench and see what it's doing before I cut anything.



Edited By bruce on 1240253986

[49-1183904562]

[color=#000000]Bruce; Brass or Nitril float? I have had this issue with similar tractor motors of a major VTwin brand found on motorcycles

[bruce]

It's my nozzle design for increased fuel signal, was using my float seat design but changed it back to stock when I thought that might be the problem.

Float is brass, seat was a ball design, now it's back to a stock brass needle. It was shaking really bad!

I'm going to have to work through one thing at a time to eliminate the problem. I'm getting the carb back in my shop this week sometime.

[slracer]

I've got to follow this one closely! Last year at Bonneville I ran to the 3 mile mark (or tried to) and got a fuel starvation problem that sounds like this one. It would starve, nearly die, then crankup again. Only in the 3rd mile! The little Honda is turning near 9000 rpm and the outside of the carb was covered with the purple color dye used for fuel ID. Hope you find the problem! -- Doug

[slracer]

Bruce, Anything new on this problem / solution? Just interested! -- Doug