Could use some help diagnosing a problem

[dan juhasz]

patient is my 66 hemi coronet 4 spd. I removed the holley sniper set up I ran last year and put the dual quads back on. When I did the re install I had the carbs rebuilt by a professional, bought a new jegs branded distributor that is setup to plug into the mopar electronic ignition it uses. Re installed the old fuel pump with a new brass tip pushrod. New fuel filters too.
So the problem is that under full throttle at about 4500 rpm it starts to hit a wall, feels as though it's starving for fuel, mildly surging ,hunting until I shift around 5500. Up until 4500 it's pulling very hard.
I've tried multiple base timing settings from 10btdc to 18btdc with total advances ranging from 28 thru 34 degrees. non of this changed anything. Today I hooked up a fuel pressure gauge and a long enough hose to bring it into the compartment so I could monitor pressure. Cruising it would hover around 8 to 9 psi, floored it would drop back to 6, might have dipped to 5 once. Didn't do any sort of a volume check so can't comment on that.
I didn't have this issue with the sniper set up so I'm thinking it's not a mechanical issue like weak valve springs floating valves. I'm wondering if the rebuilder just set up the carbs with low float levels, cause it kind of feels like it's doing the best it can to deliver fuel as it's being dumped it the carbs.
What do you guys think? am I overlooking anything else?

 

[rakort]

IDK, to me it sounds like ya running out of fuel. I remember I had an old truck that drove sort of fine most of the time. When pushing it a little it would fall on it's nose. It was a semi-plugged fuel filter.

Messing with timing probably isn't the ticket.

 

[beanhead]

I would start at the beginning, go through the linkage and verify it's operating smooth and proper..and that your actually getting the carbs all the way open at full throttle.

 

[dan juhasz]

I would start at the beginning, go through the linkage and verify it's operating smooth and proper..and that your actually getting the carbs all the way open at full throttle.

They are definitely opening all the way, certain of that.

 

[dvw]

What type of carbs? 5 psi should be plenty. Maybe the secondaries aren't opening.
Doug

 

[tonysrt]

Do you have 3/8 fuel line running to carbs? I have a 472 hemi with 3/8 fuel line and don't have that problem. I also converted to Fitech dual throttle bodies and went to a tanks inc. setup and still no problem.

 

[lewtot184]

fuel could be pushing thru the needle and seats and run very rich. check the float levels. 8-9psi is too much to cruise at. at WOT the float will drop but may not be able to control the pressure and volume. this will cause the engine to go flat. I don't like large needles and seats if your doing a lot of street driving. i'd take the carbs apart at check them first.

 

[Turboomni]

A wide band O2 sensor would take all the mystery out of it.

 

[dan juhasz]

What type of carbs? 5 psi should be plenty. Maybe the secondaries aren't opening.
Doug

Original Stock afb’s

 

[dan juhasz]

A wide band O2 sensor would take all the mystery out of it.

I was thinking about that as I still have the 02 bung welded into the exhaust on one side

 

[dan juhasz]

Do you have 3/8 fuel line running to carbs? I have a 472 hemi with 3/8 fuel line and don't have that problem. I also converted to Fitech dual throttle bodies and went to a tanks inc. setup and still no problem.

Factory 3/8 line from tank to brass block at carbs where it splits to 5/16 th

 

[dvw]

Original Stock afb’s

6 psi is plenty, 8-9 is to much. Raising the float level to 7/32 will bring the main circuit quicker. But the standard setting should be fine in this case. There is an interlock with the choke linkage. If the choke (or part of the linkage) is in the slightly closed position the secondary lower blades will not open. You have plenty of fuel line size.
Doug

 

[Jackpot50]

Talk to carb guy that rebuilt them!

 

[dan juhasz]

6 psi is plenty, 8-9 is to much. Raising the float level to 7/32 will bring the main circuit quicker. But the standard setting should be fine in this case. There is an interlock with the choke linkage. If the choke (or part of the linkage) is in the slightly closed position the secondary lower blades will not open. You have plenty of fuel line size.
Doug

Spoke to the carb rebuilder today and his feeling is at 8 to 9 psi while I'm cruising the carbs are probably loading up the engine. He says much of the problems today is do to ethanol in the fuels. He suggested to install a pressure regulator or a new pump and still add a regulator. He says once the fuel pressure is in spec if a problem still persists we can address it from there. Does that sound like good advice?

 

[lewtot184]

street hemi's used an 8+lb fuel pump from the factory, BUT they also used a vapor separator with a return line to the tank. there was a .060" orifice in the separator that relieved and stabilized the fuel pressure. this worked pretty good.

 

[rakort]

Spoke to the carb rebuilder today and his feeling is at 8 to 9 psi while I'm cruising the carbs are probably loading up the engine. He says much of the problems today is do to ethanol in the fuels. He suggested to install a pressure regulator or a new pump and still add a regulator. He says once the fuel pressure is in spec if a problem still persists we can address it from there. Does that sound like good advice?

hrmm, cruising isn't the problem as I recall. Seems like a stretch to rework the fuel delivery system to solve the problem and only open more questions and dodges about what the prolem really is after reworking things.

 

[rakort]

what are we calling the pressure spec for the oem system?

 

[dan juhasz]

what are we calling the pressure spec for the oem system?

5.5 to 6 psi

 

[dan juhasz]

street hemi's used an 8+lb fuel pump from the factory, BUT they also used a vapor separator with a return line to the tank. there was a .060" orifice in the separator that relieved and stabilized the fuel pressure. this worked pretty good.

I think that was utilized on 68 and newer?

 

[tonysrt]

The 71 FSM calls for 7-8 1/2 psi for the Hemi.