Fuel Starvation Under Braking - SPF40

[Tim Terry]

I've read several threads about fuel systems, pumps, swirl pots, baffling, etc. I'm currently running a single mechanical (Carter) pump to the Holley carb on my Roush 427SR. Both the pump and Holley were supplied with the crate engine.

My symptom is that the engine has died twice recently, under heavy braking after spirited acceleration. Imagine really getting on it, through the first few gears, then having to slow for a driveway. Brakes on hard, clutch pedal down, car dies.

Before I go down the road of electric pumps and swirl tank/pot, is there anything with the mechanical pump or Holley that I should check right now? Car runs and idles very well. I haven't touched the carb or timing since Roush did the dyno runs at the factory.

Thank you.

 

[Jac Mac]

In the situation you describe I would suggest you need/should have the Holley bowl vent extension 'whistles' to prevent fuel being forced up the bowl vent tubes and dumping in 'this case' rear barrels creating an excess rich condition that kills the engine at idle RPM. They can be fitted to both front/rear metering blocks.The Holley parts may not fit your carb if its got side hung float bowls without modified or aftermart floats. Pics of Holley vent extension and a pic of a DIY extension tube that is simply pushed into or over the factory vents above the barrels, trying that to check if it cures your issue for a start, but be aware it might cause some other problems under acceleration/cruise conditions.

 

[Tim Terry]

Thank you, Mr. Jac Mac! I'll look into it!

 

[Tim Terry]

I've ordered the Holley Fuel Bowl Vent Baffle, part 26-89 and the Float Kit with Main Jet Extensions, part 116-10. The extension tube has also been mentioned by Roush. I'll report the results.

Tim

 

[Mike Drew]

I've ordered the Holley Fuel Bowl Vent Baffle, part 26-89 and the Float Kit with Main Jet Extensions, part 116-10. The extension tube has also been mentioned by Roush. I'll report the results.

Tim

Did you first check to see if your carb came with them from the factory? I think most double pumper have the vent baffle standard.

Could also be that your rear float level is set too high....start with the easy stuff!

Good luck, and let us know how it goes!

 

[flatchat(Chris)]

Here's another version of vent valve # H026-342
http://i.ebayimg.com/thumbs/images/g/SfQAAOSwUuFWw~jd/s-l200.jpg

 

[Tim Terry]

Mike,
Thanks for that reminder. If I made a bad assumption, I'll be glad for Amazon's easy return process.

Chris,
Those are in case the car turns over, no?

 

[Jac Mac]

Carb is vac secondary, not double pump. As described Tim is creating worst case scenario, hard acceleration followed by immediate deceleration without any real chance for engine to stabilise mixtures.

 

[Andrew Robertson]

Hi Tim,

another thing is that some Holleys do not have a secondary power valve, meaning the secondary main jet area is relatively large, which increases the chance of raw fuel dribbling from boosters under hard braking. I found vent baffling, attention to float level, and installation of a secondary power valve and jetting down the secondaries fixed the problem for me. I got quite scientific and ran the car with a camcorder filming the top of the carb - the fuel was coming from the secondary boosters not the bowl vents.

Cheers, Andrew

 

[Tim Terry]

Interesting, Andrew!! Thanks for that insight.

I have installed the jet extensions, floats, and vent whistles. Now for some adjustments to floats and I'll report back!