Hot Rods 383 Mopar Help.

I have a 383 in my 63,. It has the purple hydraulic 484 lift 284 duration cam, roller rockers, 10-1 motor , 750 cfm vacuum secondary carb 4 spd. The motor has 7" of vacuum at an idle. Has the petronix ignition also

The motor has been running great. But late last week when the motor got bogged down a bit in RPM's it gave one pop out the carb and that's it.
I have checked for vacuum leaks, checked for a wiped cam lobe, checked the valve clearence, checked for broken valve springs,changed the power valve in the carb down to 3.5, set the carb with the vacuum guage, changed the coil,new plugs, new wires, set the timing, This has me totally stumped. Any ideas what else it could be?. Sometimes it won't do it all day and other days it will do it once or twice.

 

7 inches of vacuum sounds low what do you have the timing set at?

 

34 total. It runs great, starts great, doesn't run on, it pulls like crazy, just when the rpm gets down a bit the odd time it will pop through the carb

 

How long has this combo been together?
Does it do it when you just ease into it or when you floor it?

 

It has been together for around 3500
trouble free miles, It just does it when you ease into it. Seems to be around 1500-2000 RPM's

 

Vacuum leak? Did it blow an intake gasket when it backfired?

 

When they pop through the carb it always make me think lean condition.
Maybe fuel do you run ethanol blend fuel Has the weather changed it got cool here for a few days.
How did the plugs look that you took out I think I would be trying more timing if you can get good fuel and maybe more jet if the plugs looked like it wanted more.

 

I guess the first question should have been did it always have 7 in of vacuum.

 

With that cam, 7-9 in is about right. Check the firewall harness connector and the amp gauge circuit; intermittent problems occur where the main battery feed goes through that dang connector. Also check the vertical play in your distributor. In a B/RB the pump drive tries to push the distributor up out of the block. That can drastically affect timing. you can fix that with a short length of hose on the lower dist shaft. Hope this helps

 

The plugs were a tad carboned up. The distributor shaft seems to have quite a bit of play up and down. No vacuum leaks at all that I can find, its always had 7" of vacuum

 

How much up and down play should be on the distributor in these dodge's ? I have a little over 3/16 of and inch which seems like alot

 

Not sure what the spec is but if you have the plastic sleeve with the roll pin through it on the shaft it could have broken.

 

Is the distributor recurved or as produced. The curve should be shortened with a cam even with the purp shaft. Intial should be 12 to 15 and your total is fine as is. Doing this puts a lot of umph into the bottom end performance. In general terms I shorten the curve by 1/3. Then when you wack it at a slightly lower rpm instead of protesting it will stand on it tail and be gone!!!!!!!!!!! Really wakes them up!
don

 

Are you running a Holley 750 dual feed 4bbl carb?

 

In case you are not familiar with this old time hot rod trick.... to shorten the advance you need to take the distributor out, braze up the slots at the end where the pin hits, and file it out little by little to get the exact amount of advance you need. 34 - 12 = 22. A distributor machine makes it easier but you can do it with a timing light.

 

Dolmetsch is setting you on the right track here. It's a bit of a balancing act but it's worth the effort for the improved driveability.

Try to keep your total advance about right where it is but work towards getting some more initial timing in it. As already mentioned, you may have to restrict the amount of mechanical advance that's in your distributor presently. But more initial timing will bring your idle vacuum up and help keep the idle speed under control.

Having a working vacuum advance on the distributor could help too. Make sure it's connected to a source of full manifold vacuum, not ported or timed vacuum.

You might also gain some by experimenting with the opening rate of the vacuum secondaries on your carb. They may be opening a little slowly, which would be more suitable for an automatic trans car. The old rule of thumb used to be vacuum secondaries for automatics, mechanical secondaries for 4 speed cars.

 

Well I tried the hose trick that cptn60 said. Worked like a charm. Stopped it all together. Timing is steady now. I ripped it apart took the hose off and put a steel locking collar on it. Put lighter springs in the distributor for the curve and it runs like a champ now.

 

Thats good to hear.
Thought it ran like a champ before???
But its good your out burning a tire again.

 

I know that I'm way late in this but back in the day a bud of mine had a new 69 Plymouth 440 and wanted to improve it's performance. He talked to a well known circle track racer known as the KING and agreed to send his engine to his shop for a "blueprint". The agreed upon price was about 1/4 of the price of a new car(about $7,000 now). The 5 or 6 of us who hung out at a Sinclair station decided to disassemble the engine methodically and reassemble it and then send it to the KING.
When it was returned we found that there were no changes other than a piece of rubber tubing on the distributer shaft and some solder in the distributer advance slots!
Kind of sounds familiar.