Cold Blooded 383

[EngineerDoug]

Hello all,

I have an idle adjustment question - hope you can give me some guidance. I have a freshly rebuilt 383 now installed in my '68 Coronet. Here are the particulars:

CR = 8.6:1
Cam = Summit 6400 (214/224 degrees, with 112 degrees LSA)
Carb = Holley 750 vacuum secondary, with electric choke
Intake = Edelbrock Performer dual plane
Trans = Torqueflite with 2000 rpm converter
Initial timing = 12 degrees
Vacuum advance connected to manifold vacuum

The problem is, this thing wants to die when I put in gear - unless it is COMPLETELY warmed up.

Given the engine specs, which are very mild, I am surprised it is so darn finicky. I am guessing that I should run more initial timing, but I'd like to hear your thoughts on how to tame this issue a bit.

Mebbe I have just forgotten how cold-blooded a carbureted big block can be? Note I am here on the California coast, where the temp is around 60-65 degrees in the morning.

Thanks!

 

[Lefty71]

Have you been able to observe the action of the electric choke?? Maybe pulling off too quickly. Whereabouts are the fast and warm idle settings? Does it run really well when warm, or maybe a little lopey like an air leak is at hand??

 

[steve340]

You could try moving the vacuum advance to ported source. In to the manifold the engine will be getting full vacuum advance at idle.
May not help but something easy to try. And set the timing with the vacuum advance disconnected and blocked so you know your initial timing is correct.

 

[moparjohnny]

You could try moving the vacuum advance to ported source. In to the manifold the engine will be getting full vacuum advance at idle.
May not help but something easy to try. And set the timing with the vacuum advance disconnected and blocked so you know your initial timing is correct.

is that enough timing 12 degree for your set up? just throwing out there.

 

[EngineerDoug]

So I went ahead and fiddled with it today. I set the initial timing at 16 degrees (no vacuum advance). Big improvement. Now I have 17 inches of vacuum @ 1000 rpm idle (no load). Drop it into gear and it does not stall.

I still have to see how it does when I cold start it after these adjustments. But I think with enough choke it will behave better.

 

[steve340]

With a 8.6 CR it probably will not detonate - but it could, so keep watch for that.
That CR is pretty low so it obviously likes a bit more advance.
You will need to work out what the total timing is with the initial set at 16 degrees.
The jetting on the carb will also need to be dialled in.

 

[1 Wild R/T]

Try adding a resistor to the choke feed... Slow down the choke...

Since you've added base timing you may need to limit the total advance... Big Block Mopar's generally like 34-37 total...

FWIW my 383 was so bad cold with the garbage we call gas here in California I finally threw in the towel & went EFI.... Stone cold or hot makes no difference, I can reach in twist the key & the engine lights & settles to a nice choppy idle..

 

[AJ/FormS]

So I went ahead and fiddled with it today. I set the initial timing at 16 degrees (no vacuum advance). Big improvement. Now I have 17 inches of vacuum @ 1000 rpm idle (no load). Drop it into gear and it does not stall.

I still have to see how it does when I cold start it after these adjustments. But I think with enough choke it will behave better.

So when you did this, did you also change the curb-idle screw setting?

 

[beanhead]

Agree with the guys above give it more initial then re-adjust the carb to get your idle speed back. If the total (mechanical only) advance is then too high you'll have to re-curve, really easy on anything other than the older stock units where you have to weld the slots.

 

[1 Wild R/T]

Agree with the guys above give it more initial then re-adjust the carb to get your idle speed back. If the total (mechanical only) advance is then too high you'll have to re-curve, really easy on anything other than the older stock units where you have to weld the slots.

I liked welding the slots...

 

[beanhead]

I liked welding the slots...

Oh it had it's benefits...my wife doesn't bother me in the garage when there's metal burning

 

[miller]

Agree with the guys above give it more initial then re-adjust the carb to get your idle speed back.

I agree with the guys, too.
I'll toss in, RPMs dropping, when you put it in gear, 'can' also be a sign of carb mixture being off. Normally, if mixture is right, you only get a slight drop in RPMs.

 

[Beach Goat]

Just curious, why did you go with such a low compression ratio?

 

[1 Wild R/T]

Just curious, why did you go with such a low compression ratio?

Piston choices for the 383 are atrocious... I haven't seen EngineerDoug's build, haven't measured or done the math but if he's running 906 heads that number may well be optimistic...

 

[EngineerDoug]

Thanks for the tips on this. I did indeed reset my idle speed and tweak the mixture screws. As for limiting the total advance, I already have a limiter plate in the distributor - I will have to reset it for less mechanical advance.

This motor was built "on the cheap", as I intend to build a 440-based stroker for the car. It came with the 383, so I thought I would get it running with that for a while. The piston choices for the 383 are very limited, and these were budget-level hypereutetics. I did cc the '906 heads, so the compression ratio really is 8.6:1.

In the long run I am on the fence about whether I want to stay with carburetion or go to fuel injection. I'll just have to drive it and see how it behaves.

 

[1 Wild R/T]

FWIW FI has been one of the best upgrades I've made...
When I was younger I loved tinkering on carbs but anymore the thought of pulling a bowl off a Holley over a hot engine, worrying about a fire & hating to smell like gasoline for the rest of the day...

I want to change jetting, I pick up my hand held programmer... Idle Speed? Programmer!

A few weeks ago I was on a cruise, the ambient temperature was 104 & the engine was running a little warm, not hot but warmer then I like (197 degrees) ... Oh, my hand held has a "Large Gauges" mode which I program to display Engine RPM, Engine Temp, AFR & Battery Voltage so I know pretty accurately whats going on...

I had the AFR at 14.4... Readjusted it to 13.8... In the next ten minutes the engine temp dropped to 182... Right where the thermostat keeps it....

I would never have made that change on the road on a hot day over a hot engine when cruising with 25+ cars... So being able to make the change And seeing the immediate improvement was excellent...

 

[wyrmrider]

Just what Lindbergh told the WWII pilots to do to increase their range
lean it out till it warmed up a bit then back off some
Mechanics bitched that they would melt down engines
they didn't
Japs had already figured this out
they could get an 800 mile range from a Zero- twice ours
Lindbergh evened the playing field and saved millions of gallons of avgas which the Japs did not have access to
My Uncle was a WWII pilot

 

[66Satellite47]

The choke adjustment can be very finiky. Also adjustment of secondary throttle blades can make a difference in the symptoms you described. IMHO