I recently switched from a Holley to a Demon and have this weird issue... I noticed that when I let the fuel pump run with the motor off the fuel bowls will completely fill and fuel will begin to dribble out of the carb onto the throttle blades thus flooding it and making it unstartable. But, when I start the motor with the fuel pump off then once its started turn the pump on the bowls are normal and theres no flooding? With my old Holley the bowls would also over-fill but it would never start to dribble fuel out? My fuel pressure is at 5-6psi, Holley Blue pump and Mallory return regulator. Once the motor is started the fuel bowls return to their proper level and everything runs fine.

I always thought that maybe this was normal until I got the Demon Any ideas on this? Its pretty annoying.

 

This is NOT normal, with either a Holley carb nor a Demon carb, and you should fix it ASAP. Why...because you could hydraulic the motor and bend a rod. Just saw this the other day on a friends car. Not nice!!

Most likely cause is too high of fuel pressure. Do you have a fuel pressure guage plumbed in at the carb. Is it accurate??

Most Holley ( Brand name Holley...not aftermarket !! ) needle and seats should hold 6.5 to 7psi with the engine off... without the float bowls overfilling. A slight rise with the engine off is normal...but no way should the fuel level rise to such a level that it starts dripping into the airhorn.

Your fuel pressure is either too high...or the needle and seats in the Demon are leaking.

 

Well, last night I took out the N&S and sprayed them with carb cleaner. Now, they still overfill with the motor off but not completely and they don't leak. I have 2 fuel pressure gauges 1 mounted straight off the regulator and 1 at the carb. The one at the regular reads a steady 7PSI and the one at the carb kinda bounces back and forth between 5 1/2 to 7psi

I dunno, I find it weird that over the years I've had the same exact problem on 3 different carbs, 2 different fuel pumps and 2 different regulators... Thats why I kinda figured it was normal. I guess its alright now, the fuel bowls will fill almost to the top of the sight plug with the motor off and then go down to the bottom/middle while the motor is running.

Thanks,
Chris

 

If you run pressure against the carb for long enough with the engine off it WILL overfill the float bowls. Fuel will SLOWLY leak past the inlet valves under pressure no matter where you set the floats. This is SOMEWHAT normal. Nothing seals up perfectly. Once the engiene is running the tiny little bit of fuel leakage past the inlet valve will just be gobbled up by the engine idling and it will hold a constant (and true) fuel level in the bowls.

In short, set your float levels WITH THE ENGINE RUNNING.

Remember I'm talking about a TINY little bit of fuel leakage past the inlet valves with the engine off. If you run the pump for maybe 30-60 seconds, engine off, there should not be enough leakage past the inlet valves to dripple out of the venturis or ANYWHERE out of the carb. Let the fuel pump run, engine off, for a long time and fuel will eventually start to ooze out of everywhere.

 

Same thing was happening to me not long ago. One of the things I found though was some junk on the secondary side needle and seat. Fuel was just dribbling down into the engine even with it running. I haven't seen it leak since but then again, haven't had the fuel pump going for long ammounts of time with the engine off.

 

[quote]I recently switched from a Holley to a Demon and have this weird issue... I noticed that when I let the fuel pump run with the motor off the fuel bowls will completely fill and fuel will begin to dribble out of the carb onto the throttle blades thus flooding it and making it unstartable.[/b]

The fuel dribbling out of the boosters is NOT a good situation under any circumstances and can lead to disaster if not corrected. This is a sure fire way to either wash out the rings or hydraulic the motor.

New Holley Brand Viton needle and seats will not leak if the fuel presuure is not set too high. Cheap non-holley brands ( such as in typical jobber rebuild kits or Mr. Gskt crap ) will leak.

Knock the fuel pressure down at the regulator to 6.5 or even 6 PSI. That should be more than sufficient. Remember...it is sufficient volume...not pressure, that is the most important factor in the fuel delivery system.

As mentioned, set the level with the car running. Fuel level at the middle of the sight glass is too high. This could be part of your problem. Proper fuel level when runing should be:

1) Primary side. Slightly below the sight plug threads. About 1\16" below threads is about right.

2) Secondary side: About 3\32 " below the sight bowl threads.

These settings will reduce fuel slosh and are particularily usefull when Road Racing or Autocross. Running the secondary side a tad low stops fuel slosh from stalling the engine under hard braking. Also reduces fuel slosh out of the vent tubes. You may have to adjust jetting one size richer to compensate for the reduced level. Fuel level affects fuel mixture.

If you must run 7 PSI and you keep getting fuel level creep to the extent that fuel starts dribbling into the airhorns, then hook the fuel pumps into an electrical oil pressure switch, so that they turn off when the engine is off. Then wire in a fuel pump kill switch so that you can manually switch the electric pumps off or on ( Manual overide ) for priming. Handy for situations where you might need to leave the ignition turned on for checking something.

 

In the Speed Demon manual, the baseline float level is in the middle of the sight glass. They do say a little above or below is fine though Mine have been set in the middle (when things are working right) ever since getting the carb and had no problem. Do Holley and Demon have the sight plugs/glass in the same locations? I know with the Demon, they're a lot larger than what was on my old Holley 670.

Here's a pic of the 750 on the old 305 hehe

 

How about doing yourself a favor and not keep running a blue pump? I assume you've always had the problem while running a blue pump right?
I've seen lots of people have issues with controlling fuel pressure and keeping it stable with those pumps.
You might want to try a Carter that runs at a lower pressure. The pressure should stay much more stable and you probably won't even need the extra regulator with it.

 

As far as the sight glass, you do want it at the middle on a demon. They seem to be located different on demons(probably because the come stock with the glass plugs). I have a mechanical carter pump, but it is a 8 or 8.5 psi pump, I dont remember which. Anyway, it seemed to run good with it at first, but for some reason after a couple days it started over filling the fuel bowls. My problem seemed to be pressure related though because when I put a regulator on and brought downt he pressure a little, it stopped doing it. Since I I have a mechanical pump, I dont know what it would do at the lower pressure with the motor not running and the pump running.

Ben