I've been trying to get my '70 383 Road Runner running better, while learning carb tech at the same time. Lot of searching, reading, etc.

Car : 1970 Road Runner 383, 727, 3:55 rear, 27" tires. I believe a factory stall TC.

Engine: Original '70 383 block/heads. Heads have been milled, not sure how far. Had to have the intake below milled 30thou to get the bolts to go in the heads.
Compression: ? Was told basic rebuild by previous owner. Have never had the heads off to check deck height or pistons.
Cam: Mopar Puple shaft 284/484. (Assumed part of the issues due to low stall TC)
Intake: Edelbrock Performer RPM
Carb: Mighty Demon 750 annular booster vacuum secondary (no choke).
Distributor : has been recurved (FBO ignition) and set to 16 initial and 34-35 at 3000. Also running FBO electronic box and coil.

Onto my issues. 6-7" of vacuum in park, yuk. Rich/rough idle, tip in stumble while cruising, and lean surging. To try to help things, i installed #35 accelerator discharge nozzles (31s were in there), and installed a 3.5 power valve, thinking this would help lean the idle and richen tip in. I also reset the throttle blades to show the correct mount of transfer slot, and made sure the 4 idle mix screws were at 1.5 turns out. Car started good, and seemed to idle a bit smother and less rich. Car is idling at 1000rpm, and drops to 500ish when in drive (no doubt converter has somoe influence here). BUT, I now had hesitation and a backfire at part throttle while on my short drive. Must still be lean.

So... I pulled the carb the other night, installed 2 size larger primary jets(now #69s), sprayed carb cleaner though the holes, blew everything out, put in new bowl/block gaskets, and buttoned it all back up. While trying to prime the primary bowl through the vent tube (as I had done the other night without issue), I noticed fuel spilling out from the main boosters. Crap! Took it apart, checked, reassembled, and same damn thing. Frustrating. Apart again. I can't tell if the power valve is shot (maybe from the backfire, or me blowing it out), or if I'm missing something else. Ugh.

Anyone have any thoughts on why the boosters would be flowing with the engine off while priming the bowl???

Floats not floating is usually the culprit. but I have only seen that at idle, but then again I never fill my bowls via the vent tube, nor do I run Holleys, so YMMV. I have messed with them, but I do not run them.

Your 500 rpm drop between park and drive is usually an indication your idle mixture is off. You should use a vacuum gauge to set the idle mixture and with four corner idle you really need to make small changes and mimic the change at all four corners. You want all the mixture screws to be as close to the same all around. Idle mixture will affect transition and cruise, not so much WOT.

Try to make one change at a time, your post is unclear that you have done that or that you've done a bunch of changes all at once. It's hard to narrow down what's working or not when you do more than one thing at a time.

I think you need more than 16 at idle. I would do the "vac gauge method" to find what initial it wants & if more than 16 then likely you'd need to shorten the dist slots. Is the 16 at 500 or 1000 RPM. To answer your Q when filling the bowl thru the vent you ain't even interacting with the float system & you are filling the bowl cavity directly like pouring water into a drinking glass in your kitchen so yes if you pour in too much it will overflow. I put in just enough so the accell pump kicks out a healthy squirt then I know I'm ready.

how much initial timing do you have?

not enough initial timing will cause the engine to drop the rpm like yours is....

Timing is set at 16 initial, as stated above.


First, thanks for the comments/replies.


I've got the carb back together with a new 3.5 power valve, and no booster leaking - yay. I also found the intake manifold bolts on the pass side weren't tight. Oops. Started the car and let it warm up, have a bit more vacuum, 8-8.5". However...

New issue... now I can't get the idle down from 1400-1500. Throttle blades are set correctly on primary, and now closed on secondary (had them right at the bottom of the transition slots when installed. I've farted around with moving the idle screws in/out, and it only makes minor changes to idle speed. Idle air bleeds?

Throttle blades on the primary should be closed. The idle speed screw is what sets the opening for idle.

Ok, I'm a numb nut. Left the brake booster line off. Duh. Idle can now be set down lower, but still wants to fall way low and die when put in gear. ...and back to 6.5-7" of vacuum at 1100 rpm (not in gear). Ugh. Even moved the timing up to 18 degrees, no change in vacuum.


Anyone know of a good mopar/carb shop in the northern suburbs of Detroit? I'm thinking of a torque converter and/or cam swap.


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