Hi All,

I'm hoping forum members can share some possible solutions to a fuel problem that I appear to be having. I know the forum probably has the obvious answer somewhere but I haven't been able to narrow it down.

First some background, my SBC crate engine came with a Edelbrock Performer RPM street fuel pump (no return line port) and a Quick Fuel SS 750 mechanical secondary carb.

As there was no port for the return line, I went ahead and plugged the return fuel line leading back to the tank. The engine seems to run fine at idle and part throttle situations.

However, when I step on the gas to open the throttle wide open the engine pulls fine until I take my foot off the accelerator at which time the engine stalls or drops to a very low idle (500 rpm or less). After the stall if I turn the key to start it again (it's a 4-speed manual trans) the engine will start again after a few seconds of cranking the starter.

The float level on the carb at idle seems to be within the requirements as recommended by Quick fuel (middle of the sight glass). The fuel pump is limited to 6 PSI as built from Edelbrock so it should not be getting too much fuel pressure (flooding).

So I'm not sure if the problem is with the rubber supply fuel line collapsing (not using the standard C3 formed "S" line) or if it has to do with the fuel return line not being utilized (vapor lock).

Should I replace the fuel pump with a stock C3 fuel pump or ultimately does the problem stem from a carb adjustment issue(floats, jets, etc.)?

Kind of frustrating, as for the most part the engine runs just fine except this issue.

Thanks



Comments and suggestions are greatly appreciated.

 

The fact that the idle drops down to 500 RPM after full throttle seems like a distributor problem.

 

Thanks Scott for your reply, but the engine only sometimes drops to 500 rpm. Other times it stalls completely and I have to restart.

The severity of the problem seems to be in direct relation to how long I hold down the accelerator. The longer I hold down the accelerator wide open the more likely it will stall when I let off the pedal.

Seems to me to be some level of fuel starvation as opposed to a ignition issue.

Thanks

Don

 

If it were fuel starvation it would start missing at high RPM, you can drain the bowls very quickly, like in seconds at speed if you weren't getting fuel. First, is the distributor vacuum plumbed to full manifold vacuum or to metered from the carb? if not full vacuum that is the first thing to change. I would put a timing light on it, run it up to 5,500 for a ten seconds and see what the timing does when you let off the throttle, simple test. I think you are losing vacuum and your distributor can't respond correctly.

 

Hi Scott, the distributor is an MSD with a 6AL and only mechanical advance. There is no vacuum advance on the distributor at all.

Full advance is set to 34 or 36 degrees can't remember the exact number.

The engine runs fine at idle and at cruising speeds up to 70 mph as long as I gradually accelerate to speed. Problem only occurs at hard acceleration at WOT.

Thanks

Don

 

Actually, on occasion the engine does miss at approximately 2500 to 3000 RPM. This only happens once in a while, most times it runs just fine.

I think I might just adjust the primary float a little higher just to see if it will make a difference.

I believe that maybe what is happening is that as long as the RPM's are below 2000 then the primary float level is high enough but once it gets into the higher RPM and especially at WOT the front (primary) float level drops too low and then the engine idles or stalls on the release of the accelerator.

It is worth a try I think, if I adjust the float too high then it will only flood the engine and then I will adjust the level back down if it doesn't help.

 

What happens if you stay at WOT? Does it stumble or die out then? If not, then you don't have a fuel supply problem. I don't understand how the problem happens after you let off the gas. I had a fuel pump problem on my car and the engine would die like you shut off the key, at WOT. So naturally I let off the gas, then it ran normally at part throttle. But that's not how you're describing your problem.

 

Exactly.

 

Hi BKbroiler, thanks for your response.

The engine continues to pull as long as I stay in the throttle. It is when I take my foot off the throttle is when it drops down to 500 rpm and or stalls. But this only occurs after a WOT situation.

If I accelerate normally then it doesn't happen. My theory is at WOT the engine consumes almost all the fuel in the primary bowl but continues to use the fuel from the secondary bowl and doesn't quit because there is enough fuel from the secondary bowl along with the power valve circuit to feed the engine at WOT.

But once I take my foot off the accelerator then the power valve closes and then the idle circuit takes over and the fuel level in the primary bowl is now too low and the engine stalls from lack of fuel.

Or it maybe caused by both float levels being too low after the WOT and the fuel pump can't refill bowls fast enough due to a restriction (collapsed fuel pump rubber supply line between hard fuel line and fuel pump or debris at the pickup in the fuel tank).

As mentioned it is hard to diagnose, at least for me.

Thanks to everyone for the help.

Maybe LARS will see this post and lend his advice?

 

Wow. That is unusual. Do you have a spare carb you can try as a sanity check? Or set the primary float as high as possible, like you mentioned. Please let us know how you finally resolve this.

 

Yes, it is one of those problems that would only happen on a carbureted engine. Fuel injection doesn't suffer from these issues.

The only other carburetor that I have is the old Quadrajet from the original L-82. But that carb has vacuum secondaries and is a single feed verses the Quick Fuel SS 750 with mechanical secondaries and dual fuel feeds (double pumper).

So I would rather not have to reconfigure the fuel system to suit the Quadrajet just to conduct a sanity check if at all possible. I think I will try to adjust the primary float level on the Quick Fuel first and see if that will resolve the problem.

Unfortunately, tuning carbs is kind of like a science and that is why good carb tuners are hard to find.

Thanks

 

Have you checked the fuel pressure? If the fuel pressure was much higher at higher rpms it might be over coming the seat and needle valve right after you let of the throttle and flooding the engine. Did this start after the new fuel pump or the carb, or was it all replaced at one time. If it was me I would hook up a fuel pressure gage and see what it is at idle and when I bring up the RPMs. If not maybe swap back to the stock pump and see if that makes a difference.

 

Only you know how your fuel line looks or how the msd is wired up. If your primary bowl was low the pv isn't doing squat, its purpose is simply enrichment through orifices approx .045" depending on carb
which are waaaay smaller than the primaries. Has the car ever been operated with stock distributor?

 

Well I adjusted the float levels in the front and rear bowls of the carb and so far so good.

Opened up throttle to WOT in second gear and left it down for a few seconds and then lifted.

Engine RPM dropped to normal at about 900 RPM but no further and no stall, I did this a couple of times and all seems good so far and the engine is not flooding even though the float levels seem to be quite high as viewed through the sight plugs.

Will do more testing on the weekend to see if the issue is gone for good. Hopefully it should be good now.

 

Does it stall any when you brake hard?

 

Hi Boot77,

nope it doesn't stall when I brake hard. I suppose it might be how you define hard.

I braked hard enough to have my 6x9 speaker boxes slide forward and hit the back of the seats (they're not fastened down yet).

So I figure it shouldn't be much worse if I lock up the brakes at speed, but I'll give that a try on the weekend just to be sure.

 

The quick fuel carb should have the baffle vent in secondary bowl to prevent flooding during hard braking.

 

I was thinking the rear accelerator pump arm might be too tight and the rear squirter might be always a tad open, flooding it when you let out or brake. But that doesn't sound like it.

I suggest going someplace safe let it stall and then coming to a stop w/o starting it back up and pulling a plug. May give you an idea if it's going real lean or rich.

 

Delete, wrong thread.