360 Plug help

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Skrawny

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Working on the new to me Dart with a 360, what I have for receipts amounts to a 4876907 short block.
Edelbrock RPM air gap manifold, Performer RPM aluminum heads, TTI headers, and a demon 750 vacuum secondary carb.

Looking at the plugs they appear to be either too cold of a plug or running to rich.

The carb is apart working on rebuilding, and has # 70 main jets and #78 on the secondaries.

What are your thought on the looks of these plugs???

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You need to know your compression ratio before you can choose the right heat range plug. Those plugs look rich to me.
 
Looking thru the books its looking like 9 to 9.5 to 1. The short block is the lower compression one but I am not positive what all has been done thru the years also.
 
Those plugs are equivalent to a NGK 5 heat range, which is correct. I would use NGK 4s, one step hotter.
 
The plugs do show rich, although not what I'd call pig rich, but they are reading rich. What does the carburetor have for PVRs and IFRs? Also for primary and secondary idle and high speed bleeds?
 
I wouldn’t change the heat range just yet. It looks close. Those plugs are pig rich everywhere. Idle, cruise and wot. You’ve got some carb work to do. Then get a new set of plugs and retest. What’s the timing set at?
 

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The carb doesn't appear to have adjustable air bleeds, and the power valve in it looks like a 56 or 65 depending on how you read the numbers. The 65 power valve is the replacement in the kit.


Would i be better off with a different carb??
 
For about 3 decades or more, Holley carbs did NOT have replaceable air bleeds. Carter & Rochester 4bbls NEVER had them them.

And they worked just fine.....
 
So, are those bleed holes threaded? What is the history of the carb? I had a mighty demon 825 that worked well after pulling it apart and cleaning all the metal slivers out of it. It was a new carb. Quality control sucked at demon for a good bit. All my screw in air bleed slots were stripped also. You really need to know what the vacuum reading is to determine what power valve to start with.
 
Looking at pictures for a speed demon 750 online. It looks to have screw in air bleeds that are not present in your picture. If you have them, put them back in as they were, you have to be pretty far down the tuning trail before you start playing with air bleeds. I would put the carb back to baseline settings, connect a vacuum gauge to it and start working on getting idle squared away with the vacuum gauge. On Holley based carbs, with it idling, you can slightly tap the accelerator pump lever. This will dribble a very small amount of fuel. If the idle picks up, you know the idle is slightly lean. There is a good tuning reference book you can get a FBO ignition. I think they used to blueprint demon carbs way back in the day.
 
Carb is going back together today.
The air bleeds are not threaded and yes the quality control is terrible.
found metal chips and burrs hang all over inside along with the easy idle not even drilled out.
Not sure on the history of the carb since I just got the car and am going over it now.
 
What carb is it exactly, part number. So you can see if it supposed to have screw in bleeds. The quality control was so bad, getting one where they skipped installing the air bleeds isn’t out of the question. Good design, but put together by a blind ham fisted monkey! When I was playing with mine, the FBO tuning book was worth the $15. Very methodical. I have only been playing with an AVS for 10 years, can’t remember much with a Holley based carb.
 
I don’t know if current Speed Demon carbs have screw in air bleeds, but they didn’t in the beginning.
I found early Demon carbs to not have very good street calibrations.
(I used to actually thread the holes and install changeable air bleeds in the more problematic ones)

If you’re determined to keep that carb, I’d suggest installing a wide band O2 A/F gauge in the car.
That way you can get a visual aid in seeing what the carb is doing under various driving conditions.

You didn’t mention what camshaft is in the engine, but if it’s nothing too wild, and you’d rather just swap the carb, I’d probably lean towards an Ede 650AVS.
 
I’m looking at pics on my phone, so the scaling isn’t great, but the top view of the carb looks like it might actually be a 650.
The venturi diameter will tell the tale.
The 650 are 1.28” diameter
The 750 are 1.40” diameter

Also, 70/78 jetting is stock for the 650.
 
PRH said:
I’m looking at pics on my phone, so the scaling isn’t great, but the top view of the carb looks like it might actually be a 650.
The venturi diameter will tell the tale.
The 650 are 1.28” diameter
The 750 are 1.40” diameter

Also, 70/78 jetting is stock for the 650.
Click to expand...
Measuring the top venturi looks like they measure the 1.28"

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It’s a 650.
I didn’t notice it was a VS model.
Ages ago we were a BG/Demon dealer, but I don’t think I’ve ever run one of those before.
 
Skrawny said:
Carb is going back together today.
The air bleeds are not threaded and yes the quality control is terrible.
found metal chips and burrs hang all over inside along with the easy idle not even drilled out.
Not sure on the history of the carb since I just got the car and am going over it now.
Click to expand...
Yeah, those are pressed in bleeds. They CAN be changed, but it's pretty labor intensive. They have to be pulled, then the holes tapped for screw in bleeds. If you really want that adjustability, I'd just get a Quick Fuel carburetor and be done with it.
 
Yeah, those are pressed in bleeds. They CAN be changed, but it's pretty labor intensive.
Click to expand...

Actually, the Speed Demon’s I’ve played with just had holes drilled in the casting as air bleeds.
They would come with a small zip lock bag of 4 press in bleeds that could be pushed into the idle bleed holes to help richen up the idle, if you thought it needed that.
 
Skrawny said:
It runs fairly well other than being rich, idle is really bad burns your eyes out in a matter of minutes even with the garage doors open.

Hopefully just a good cleaning and readjustment helps it out.
Click to expand...
Was the cam degreed in?
 
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