Stumbling Hard

[68conv4sp]

My '68 is getting better at producing some pretty respectable Autocross lap times, but is starved for gas as it exits a turn and stumbles badly at throttle application. Current carb is Edel 500 cfm that runs perfectly on the street. What carb do you guys recommend to solve the starvation problem?

 

[68conv4sp]

Should have mentioned that it has a stock 289 4 speed.

 

[krash kendall]

Is the stumbling at throttle application full throttle? Mechanical secondaries will cause that due to the sudden vacuum drop when the butterflies all open suddenly resulting in decreased fuel charge flow.

 

[68conv4sp]

Krash, I tried part throttle exiting a couple of hard turns, but it still acted like there was very little fuel. I'm not sure I understand your comment on the mechanical secondaries. Do you mean that when the secondaries open suddenly, there is insufficient fuel to mix at that moment? During hard straight line accel/decel there is no problem.

 

[68conv4sp]

Would "fuel slosh", whatever that means, cause the apparent lean condition? BTW, the fuel tank was 3/4 full during the race.

 

[krash kendall]

Do you mean that when the secondaries open suddenly, there is insufficient fuel to mix at that moment?

Yes. In greater detail what I'm saying is that when you floor it with mechanical secondaries, particularly from a standstill, you are opening up the intake to the atmosphere and you go from high vacuum to low vacuum very quickly resulting in the decrease of flow through the intake. I'm sure you've the sound of that brief hesitation before. Its the "Bu" part of "Bu-Whaa!"

You may have a float level problem.

How's the fuel filter?

 

[Gailahan]

Maybe the fuel tank needs to be filled with fuel cell foam to prevent from sloshing if it happens after a hard turn.

 

[mustangdave]

There are carbs. with a different design for autocrossing to prevent loss of fuel; I would check into that.

 

[BornInAFord]

Shelby used a center-pivot float Holley similar to the newer 4150 to minimize starving, flooding and fuel slosh. I'm not sure how to tweak the Edlebrock you now have to mimic this...

Also, have you changed your oil pan to one with baffles so you don't cavitate or lose your oil pressure? That would be a lot worse than just a hesitation caused by the carb. :o

Daniel

 

[68conv4sp]

During the first autocross event a year ago, I finished w/ a ticking motor and also noticed that under hard cornereing the oil pressure went to near zero. I installed a high capacity baffled sump and that issue is cured. Someone else mentioned a "center pivot" float, but I know little about carbs and am not sure what that is. Sounds like I need to take a trip to the local speed shop.

 

[FirecrackerKTM]

There are carbs. with a different design for autocrossing to prevent loss of fuel; I would check into that.

Are those similar to the ones with the center-hung float design like the 4x4s use?

 

[68conv4sp]

I have never taken a carb apart. Is the float positioned so that if the fuel all sloshes to one side of the carb it is not available for use? I don't know how the fuel gets from the float/bowl to the venturis - gravity?

 

[FirecrackerKTM]

Crap, I haven't been inside of any but motorcycle carbs in a long time and they're significantly simpler. But yes, I believe side-hung floats can cause problems during hard cornering or if you were on a steep enough slope.