Hi, I’m Gene. I have a 1970 Duster w/ Factory 340. I’ve switched to DC electronic ignition years ago. I’m having trouble when Im opening the 4 bbl it seems like it’s loading up or not burning the fuel properly. It idles fine and sounds fine in the primary’s until I open the 4 bbl. It just sounds like is holding back or not burning the fuel properly. I have new plugs , wires, cap and rotor with a fresh Holley 750 dp w/ manual secondarys. It’s all been set properly. I have done a coil test of the primary and secondary windings. They are within tolerance. Could the ignition module or voltage regulator cause this issue? I’ve had this car for 43 years and it’s always ran very well until now. It has been sitting for about 10 to 15 years in my garage. I need help please. I’d appreciate any feedback. Thanks
Engine misfire or loading up when opening the 4 barrel.
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TT5.9mag
Two atmospheres are better than one
I would look at the plug gap first, the carburetor tune up second, the fuel pump third, and last but definitely not least (since it’s got a a 292-.510 cam, I read that on your welcome thread) the valve springs. I would not suspect the ignition system to work perfectly at all other times and NOT work when the secondaries are open.
Check all the grounds . The ignition box, engine to body ground, Voltage regulator, battery to body, Distributor pickup , also check the air gap on the reluctor wheel. . Did you swapped the carb with one you know works?
Maybe something in the new ignition parts? When under load, ignition parts, if not up to snuff, can show weaknesses. New or otherwise. I had a distributor cap that did that, years ago. In that case, it ran fine until I put the blower belt back on the Paxton. (87 5.0 Mustang) The extra cylinder pressure from the boost was trying to blow the spark out.
Tell us more about the combo, as TT5.9mag references.
Sounds like a nice car.
Tell us more about the combo, as TT5.9mag references.
Sounds like a nice car.
Last edited:
Builderguy
Well-Known Member
Just an idea, more of an educated guess. I am not that familiar with the Holley 750 DP but IIRC it has duel manually operated accelerator pumps.
I would look into these for the issue. From someone much smarter than me: "The main metering circuit does not deliver fuel on an instantaneous basis. If the throttle is slammed open, it takes a moment or two for the main metering circuit to respond to this change in throttle position. In these moments, the engine will experience a lean air-fuel ratio condition that will create a hesitation or even an intake backfire. The accelerator pump circuit addresses this condition by adding the additional fuel to prevent that momentary lean air-fuel ratio.."
This is explains how to tune these.
https://www.holley.com/blog/post/ac...ors/?msockid=1d392d77879a62e91c9c38f5863063a6
Best of luck to you, sir.
I would look into these for the issue. From someone much smarter than me: "The main metering circuit does not deliver fuel on an instantaneous basis. If the throttle is slammed open, it takes a moment or two for the main metering circuit to respond to this change in throttle position. In these moments, the engine will experience a lean air-fuel ratio condition that will create a hesitation or even an intake backfire. The accelerator pump circuit addresses this condition by adding the additional fuel to prevent that momentary lean air-fuel ratio.."
This is explains how to tune these.
https://www.holley.com/blog/post/ac...ors/?msockid=1d392d77879a62e91c9c38f5863063a6
Best of luck to you, sir.
Ok thanks I’ll check that
I have the ignition box on the fender and the voltage regulator on the firewall. But I don’t have dedicated grounds off the bolts. Yes the carb is a good one
I have already replaced the fuel pump and I did check the plug gap. They are all set at .35
I did check the gap on all my plugs again to make sure they were at 35 which they were but I noticed number six cylinder ceramic was still a little white compared to the others so took the valve cover off to make sure the valves were moving no bent push rods they were good. I did check the plug and I only had an orange spark so I was wondering, maybe if it was a bad plug cause the plugs are new
Make sure the mechanical advance in the distributor is working. Easy check take the cap off, turn the rotor it should snap back if not it is hung up, pretty common issue.
Thanks I’ll read that.
I did Check that and it does snap back
Also did you put a timing light on it? Not a dial back and see the timing mark move also? Just asking.
Super-Cuda
MOPAR .. Move Over, Plymouth Approaching Rapidly!
Fuel pressure problem??
You mentioned the car sat for a while.
Deteriorated fuel inlet sock or pieces of it partially clogging up the inlet in the tank under hard acceleration ?
You mentioned the car sat for a while.
Deteriorated fuel inlet sock or pieces of it partially clogging up the inlet in the tank under hard acceleration ?
TT5.9mag
Two atmospheres are better than one
Remember new doesn’t mean good. Check new parts as they go on the car. Especially fuel delivery parts. A bad fuel pump commonly causes the issues you’re having.
How would you check the fuel pump? It came from O’Reilly. I did change the dual line feed to one of the ones that has like a JIC type fitting that comes out of the carburetor to a braided line with jic fittings as well. It has a gauge in line but it doesn’t read. I figured there’s just not enough pressure to show I mean, they only usually push about 7 pounds
7 is about right what is the scale on the gauge?
TT5.9mag
Two atmospheres are better than one
That O’Reily stock replacement pump may not be enough for your warmed up 340. It should make “some” pressure on a gauge though. 3-6 psi I’d think. You can test it by free flowing it into a bucket for 20 seconds or so and comparing that to published data for that specific pump.
0to 100 I believe
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