340 idling rich, RPM Air Gap intake and 1406 carb

[Force Fed Mopar]

Working on BadNewz's '72 Dart, has a 340 in it (not original) w/ a large-ish cam (not for sure what cam, thinking it is a 284/.484 Mopar cam), stock 587 heads, Edelbrock RPM Air Gap intake and 1406 carb (both new), stock manifolds and dual exhaust. Electronic ignition, orange box, Accel wires, RN14YC plugs, new coil.

So it revs great, but idles rich and shakes intermittently at idle. At a steady rpm of around 2k or so (no tach) with no load, it sounds good but has a slight intermittent miss. Doesn't miss a beat when revving it quick though. Holds 14-15" of vacuum according to my Innova gauge.

Moving the initial up from 12* to around 14-15* made it idle faster and better, reset the idle speed down and it was better but still rich and shakes a bit at random.

Had the same issue with the previous Performer intake and Carter AFB, only a bit worse as the old carb had a vacuum leak at the back somewhere (warped or cracked or something). New combo has no vacuum leaks.

Thinking that maybe it is a timing issue rather than a carb mixture issue? At 12* initial, the total was at 34-36. 15* initial seems much better, but that puts the total timing a bit higher than what I have always read to be optimal for a smallblock. So maybe it needs a recurved distributor that will allow it to run more initial timing without getting over 36* total? Or should I try running using the full vacuum port for the vacuum advance instead of the ported one? That would give it more timing at idle but wouldn't (shouldn't?) affect total timing at WOT, correct?

 

[rumblefish360]

Run the vacuum line at the ported tube. You need to re-curve the distributor. The richness is a rod and/or jet change. Run as much initial timing as needed. I do not see the need beyond 15, though more is not always a bad thing. It is just the distributor needs to be limited on how far it can go. See if your vacuum advance can on the side of the distrib. has the abilty to be adjusted with a small Allen wrench.

 

[70aarcuda]

after you get the timing set ..check the step up springs on the metering rods...more then likely the are not being pulled down into the jets cause the vacuum is so low..

best way to check...remove caps from metering rods....start engine..and see if they are being pulled down..if not down while idling change spring to lighter spring or less vacuum....

 

[72bluNblu]

I run my challenger's 318 at about 18* btdc, for some reason that's where it likes to be. And that's with a pretty mild (possibly stock) cam. I reset the distributor to be all in at 36* with mechanical advance and unhooked the vacuum advance. Runs like a top. That's also with a carter AFB, 600 cfm. I did change the rods and springs as well, but the Carter's were set up a little different than the eddys are.

 

[Force Fed Mopar]

I'll check the step-up piston position. Pretty certain the vac advance is not adjustable, it is just a regular parts store replacement dizzy.

Limiting the mechanical advance requires welding and filing the ends of the slots in the governor, correct? Or can you buy a different governor? All my spares are from 70's dizzies and are 14's or 15's.

 

[rumblefish360]

Mech advance limit does require a spot weld and fine tune grind with a dremel. Check the distributor anyway.

 

[Force Fed Mopar]

Step-up pistons are down at idle. Dizzy does not have adj vac adv. Set the timing at 10* BTDC, it seemed to idle happiest there. Might go back to 12 if we can lean out the idle a bit. It definitely idles a lot smoother than it has before.

Go 1 step leaner on the metering rods maybe? It isn't puffing out black smoke or anything, but it burns your eyes slightly if you stand by the pipes.

 

[RLF Cuda]

I have had similar problems with my 340 (Edelbrock performer, AVS, 284/484 cam). Turns out that the original vibration dampener slipped by about 20 degrees. Check your timing mark using a piston stop tool to verify the TDC mark. If the ring slips, the timing will be retarded causing some of the problems you have.

Good Luck,
Bob

 

[Force Fed Mopar]

I have had similar problems with my 340 (Edelbrock performer, AVS, 284/484 cam). Turns out that the original vibration dampener slipped by about 20 degrees. Check your timing mark using a piston stop tool to verify the TDC mark. If the ring slips, the timing will be retarded causing some of the problems you have.

Good Luck,
Bob

Yeah I forgot to check that, thanks for the reminder.

 

[RLF Cuda]

One other issue I had with my Cuda was leaking from the venturies at idle. This I traced to using a HP Carter Mecnaincal pump with 8 PSI pressure. This was too much for the Carter AVS fuel needle and seat. Raising the float level didn't solve it. So I went to a standard 5.5 PSI pump and that solved the issue. I wanted to use a fuel pressure regulator, but they were too much hastle to install, and the cheap in-line adjustable regulator leaked immedicatly (junk Mr. Gasket item), so i returned that one.
Bob

 

[RLF Cuda]

Although the dyno tune showed 330 HP at the wheels, the engine ran poorly at low speed and didn't run well at the track. So, I did a compression test and got 120 PSI in most cylinders, and 107 in #7. This is at 5800 ft elevation and 11.9 PSI atmospheric pressure. Engine has Sealed Power 2332P pistons with ~9.8:1 compression ratio measured.

I made a leak down tester and found most cylinders had very little leakage, except for #7 with ~10%. So, off with the head! Solvent testing the valves showed leakage in 2 valves

 

[Force Fed Mopar]

After running it for a while and adjusting the idle speed and such, it ran good. It was inside the building at first, which made it seem richer because the fumes built up. Getting it outside and driving around made it seem much better. He drove it home and said it worked great and was much more fun on the highway, like a different car. So it seems to work good.