Ive gotten my 70 442 up and drive able, but i have got some concerns.
I dont like the really choppy idle. I dont like having very little brake assist. I dont like the mileage or how rich it runs, the power however I am totally fine with



The motor is .030 over 455, with edelbrock heads, a rpm air gap intake, MSD hei ignition (no box), and a Holley hp 850 (4150). It has the mondello JM-3-4 cam . I am told it is around 9.75 to 1 compression, and that it has aftermarket rocker arms. It has full length headers, a crossover pipe and two flowmaster 40s. It has a th400 from B and M, but i haven't determined anything about the stall. It has a 3.42 gear.

I can get the car to idle at around 850 but once i start driving the idle will move around from 500 to 1200. It is also running super rich. I am getting between 5 and 6 mpg on the highway at 2800 rpm. The car has no problem smoking the tires at 40 even without the downshift hooked up. I get between 5-10" of vacuum at idle, depending on rpm.

I dont know much about carburetors so i have watched a bunch of videos and thought i had it set pretty well. It starts right up, but i have to keep it revved up for a few moments until it starts to get warm. A friend came by to look and we figured out that the primary mix screw on the drivers side has zero affect on the idle of the motor. You can turn it all the way in or out and nothing happens.

Doesn't this indicate that there is a problem in this carb?

I have read that the edelbrocks are more friendly for street driving.

I would like to change the carb out first and see if that solves some of the issues, but i really think that I will still have an issue with the choppy idle of the cam.

Is there a good order of operations for changing parts out? I am worried that if i change the carb now (I was thinking about the edelbrock thunder 800 with vacuum secondaries) , and in a few months change the cam to something less radical then I will have to change the carb out again to a smaller CFM.

I have done several calculations online to determine the right size for the carb. Depending on the website I get answers between 600 and 850 CFM.

The holley i have is made by sean murhpy induction, and i have one of their Q-jet carbs that came with the car as well.

Thanks for any input

 

What power valve are you running in the primary side?

 

I have no clue. I think the jet sizes are written on the top in sharpie. I cant even determine the part number of the carb, it just says 4150 on the bowl. If i take the carb off how do I determine which power valve is in it? I did change out all of the plugs and they were fouled, but not the worst I have ever seen.

 

Look on the airhorn for a number stamped into it. An 850 DP will be 4781-X or 0-4781-X or something similar to that (4781 is the key number you are looking for).
They come out of the box with a 6.5 powervalve and if your vacuum is that low then you should be needing to drop it to something like a 4.5 or less.
What Holley recommends is to start with a power valve that is half of your vacuum reading at idle and go from there.
If you can't get the idle nailed down with the valve in you might want to try with a block off to rule out the valve causing an enrichment issue at idle and then go from there.
It's been a while since I messed with a Holley carb, but I used to sell them 20 years ago sooo.....take it with a grain of salt. Lol.

 

Is this where I should see a number?



The jets say 72 in front and 81 in rear.

I took the bowl and metering block off. The only number I see on the power valve is b156. But the b is 9 o'clock the 1 is 6 o'clock the 5 is three o'clock and the 6 is at 12. That doesn't seem to match anything I see online

 

That cam shouldn't be too bad in your combo and the jet sizes don't sound that far off either, you have other issues. What are your timing figures, at idle and wot?
The dp obviously doesn't have an air horn, you'll need to contact Holley for further info.
But my experience tells me it's an older DP. If so they're junk for your application. Most likely your transfer slots are open and the idle circuit needs calibrating, which on that carb is hard to do because of its limited adjustability.
You still need to check what power valve is in there, and I'd start with a 4.5 two hole. Then go from there. And don't buy an Edelbrock carb, they're junk too. Get either a Quick Fuel or Proform, theyre easy to adjust and a great value.
Good luck.

 

matt, your combination is designed for drag strip use and not for the street, you can't have both. the only possible chance to get any driveablility out of your combo, is maybe find a competent chassis dyno shop and work out the carb, timing and air fuel ratios when the car is operational. good luck

 

I agree with a chassis tune. Best money you could spend. I would put an AED 850 HO on it first. Ditch that old Holley.

I disagree with this combo being drag race use only. The 3-4 cam is plenty streetable. You can have both. I had a similar set up with a JM 4-5 cam and know of several who are still running the 3-4 with a decent converter and 3.42 to 3.73 gears.

Good luck with the tuning.

 

I agree with you on this not being a drag race only combo!I also think it may be timing and a carb issue.This motor came factory with a 750 cfm and no reason to go less cfm period.I would in the long run agree with mark or anyone else to change the carb. Not sure you really need a 850 cfm carb. I do know you don't want any bigger. If i had to choose a carb it would be a demon or a quickfuel. I like the mighty demon. I have one and it works good on street or strip.

 

Thanks to everyone for all the replies. Right now I'm just trying to enjoy driving it until winter gets here. I've been calling around looking for decent shops.

Everyone here in Memphis that I've found only does LS and efi tuning.

Does anyone know of a decent shop around Memphis ?

Thanks again

 

Matt the reason you are revving the motor up till it is warm is because it has no automatic choke! That is common for carbs without a automatic choke.I think they still make a speed demon with a automatic choke.