I’m looking for a little advice. I have a 73 that I rebuilt the TH400 and dropped in a crate motor that I bought from a friend about 12 years ago. Spec’s on the crate motor are as follow:
• 355/380hp
• LT1 GM Aluminum Heads with Stainless Steel Swirl Valves (GMPP 12464298)
• Bored .030 Over
• 9.8:1 Compression Hyperutectic Pistons
• File Fit Rings
• Nodular Crank
• 2-Bolt Main
• Comp Cams Extreme Energy 477/480 Hydraulic Cam (12-242-2)
• Comp Cams Spring Kit
• Degreed Double Row Timing Chain
• Roller Tip Rocker Arms
• Fully Balanced and Blueprinted to 2 Grams
• Barry Grant Road Demon 625cfm (4282010VE)
I rebuilt the distributor and put a Pertronix Igniter II kit and Flower Thrower II coil in it. We got it running okay but found that it seemed to be running pretty rich (a lot of smoke) and I felt it didn’t pull from a start all that great. Also, after the motor was hot, I had trouble restarting it. I did a rebuild kit on the Road Demon and at the same time redid the jets and went from #70 Primary to #66 and a #78 Secondary to a #70, I went to a lighter secondary spring, I adjusted the timing to 36 degrees total and it is currently all in before 2,000 rpm. It runs better but the hot restart issue isn’t going away. I’m looking for some wisdom in two areas. Firstly, I concerned that even if I get the Road Demon to lean out, the hot restart issue will probably persist (aluminum intake and aluminum carb with big honking fuel bowls. So I’m considering changing the carb and selling the BG. Since my intake has the dual pattern, I could go to a Quadrajet or a Holley. This car is just for fun on the street so I don’t see me doing a whole lot more upgrades. Are the rebuilt Quadrajets on Summit (SUM-210216) worth considering or should I try to find a core and have it rebuilt? Secondly, should I mess further with the distributor or leave it be?

 

Full mechanical advance occurring at or before 2000 rpm is way too soon. I'm surprised you are getting detonation if you give it much throttle. Do you have a vacuum advance can on your distributor? Is it hooked-up? Does it actually work?

You need the vacuum advance system to be functional so that your idle quality, starting, and fuel economy at low throttle are reasonably good.

 

I do have a hooked up vacuum advance (it is a #B25) and it does change the idle when I disconnect it to do the timing.

 

Does the carburetor use power valves and if so what number and what is the vacuum you generate at idle? I agree you timing is coming in too soon, should be closer to 3K. Those jet changes are rather large and uneven, if you do have the wrong power valves you should go back to the original jet sizes.

 

With that cam, I get about 17 at idle. I changed the power valve to the new one in the rebuild kit which was the same size as the original at 6.5. I know I probably still need to jet down more. Even if I recurve the distributor and rejet, the whole hot restart 'Fuel Boil" issue still concerns me though.

 

Have you checked your float levels ? Can you borrow a known/good carb to see if that will make a differance at all ?

 

When I was running my 4150 I had the fuel boiling issues - talked to Lars, he recommended 2 things:

1- run a fuel return line from the carb back to the tank ( you can either use the original return line by the fuel feed near the pump; or in my case I used the return line that my missing charcoal canister used to hook up to).

2 - a heat shield to deflect radiant heat away from the fuel bowls.

That combination completely eliminated the problem for me.

 

try the carb jetting at 68 front, 76 back.install the heaviest secondary spring to start with. install a new 6.5 power valve.get a distributor with adjustable vacuum advance.set the idle screws 1 turn out to start with .what vacuum reading do you get with it in gear ?

 

I've set and rest the float levels several times. It has not eliminated the problems.

 

I had read about both of those things too. It's nice to hear from someone that definitively said it works. Thanks.

 

Thanks Gary. You really think I have it jetted backwards? I'll have to get back to you on the vacuum reading when I put it gear.

 

Should have also added that if you use the return for the old charcoal canister, you need a vented gas cap, to ensure you relieve any possible pressure differential in the system (again, Lars advice, I simply followed it and it worked well).

 

Thanks Coop! Where would we be without people like Lars. I'm curious though, dispite all this do you guys still feel it's work sticking with the Demon Carb?

 

Maybe not, it seems that your car has enough underhood heat to really bother a Demon.
But you'll probably still need these heat-soak fixes with the next carb too just for safe measures, may as well try the Demon again afterward - before moving on to a good Qjet.

 

Yeah, I see what you're saying. The headers keep everything under there nice and toasty although the car never overheats. I'll tel you it sucks that every time the wife and I go for a ride and stop somewhere, I have to go back out to the car 5 minutes before she does just the get the car started and running well to return. It always runs good once I get past the vapor problem.

 

What exactly is the 'Hot Start' issue?
Is it fuel or ignition related?
Is it hard to crank when hot or does it crank fine and just isn't firing?
What's your initial advance set to?
36 degrees before 2000 RPM is a bit too aggressive as other have already mentioned.

 

It's fuel. It cranks just fine, its just that I have to floor it get it to catch and when it does it run s like crap for 3 to 4 minutes and then just smooths right out. Starts fine when it's cold. Initial advance is 16 degrees in order to get 36 total.

 

Yes, guys like Lars. 63Mako, Billa and others here are worth thier weight in gold - anytime I read thier posts, I try to learn as much as possible from it - even when they don't always agree. I'm sure those guys hav forgotten more than I'll even know.

As for sticking with your Demon carb - I found that my Holley was much more susceoptible to the fuel boiling/hot start problem than my QJet.

I eventually went back to my QJet - more because I felt as if I got better part throttle response and a little better fuel efficiency than any other reason - but that is just me.

I retained the fuel return and heat shield (cheap insurance).

 

Do NOT get a Summit Quadrajet.

Do find a core and have it rebuilt.

 

Thanks.