chris383

Hello,

I've rebuilt the carb on my LT-1 mounted when I bought the cat : a Holley 4777 650 cfm double pumper.
Unfortunately, the engine runs very poorly with a lot of backfire. The carb is quite old (made in 1980) and I plan to buy a new one.
I don't want to buy an original Holley R6239A because they are really too expensive and not avaiable new, only rebuilt (it is true?).
I think of a 3310 C but I'm wondering if this type of carb (750 cfm) will not be too large for a 350 cid engine ?
And what must I do with the choke ? I don't want to add a manual choke. Maybe an electric choke kit will be the solution or to adapt the original choke system on the Holley 3310 ?
Thank for your help.

Chris

PS I want to stay with Holley carbs, because I know them and these are the original brand mounted on the LT-1 !

Gordonm

The original carbs were a 780 vacuum Holley. So a 750 would not be to big. Since it is a vacuum carb it will only open as much as needed so it is tough to over carb with a vacuum secondary carb. As for a choke, buy the electric conversion and never worry about it again. That carb should work fine for your motor if it is jetted and setup correctly.

chris383

Ok, fine !
Do you have some advices for the jetting ?

I had a 750 vac sec on my previous Dodge Challenger 383 magnum (wtih Edelbrock alu heads) and it's true : the car ran fine and with no bigger milage than the original 600 cfm.

Chris

Gordonm

I have played around with jets for years and finally bought a O2 sensor for tuning. What a big difference. You can actually see what the motor is doing. Any other way is just educated guessing. You can get pretty close this way but until you can see what is going on with the air fuel ratio you will have a tough time being spot on. I would try it out of the box as is and it will probably be fairly close. I rarely do carbs anymore. Switched to fuel injection a few years ago and never looked back. Both the LS3 in my 72 Vette and Cobra are running FI.

Snoopysvet

Consider a Holley 1850, with smaller primaries you will have better throttle response for daily driving. A 600 cfm is plenty for your motor, and they are cheaper.

If you want an electric choke a 80457S

Seriously consider this Holley tool to select your size, they would know...

http://www.holleycarbs.com.au/chooseCarb.htm

MelWff

You have a choice of two 770CFM vacuum secondary with secondary metering blocks. Both of them have jetting close to the original 1970 LT1 which would be 70 primary, 76 secondary with 8.5 power valves

https://www.holley.com/products/fuel.../parts/0-83770

https://www.holley.com/products/fuel.../parts/0-80770

Snoopysvet

I will bet it will be faster with a smaller carb. If you rebuilt that 4777 it would run nice, it might guzzle gas though.

chris383

Thank you for your advices.

The street Avengers carbs are certainly very good carburetors, but too far (in look) from the original one : I prefer the 3310 C with dichromate finish !
I didn't drive the car with the 4777 carb, but this could be rougher than a vac. sec : right ?
Fuel injection is not an option for now : it is of course too far from original and seems to be a bit complicated for me...
A smaller carb is also a interesting choice. My experience with the 383 Mopar engine (I'd tested 600 and 750 single entry vac sec carbs) was clearly for the bigger one...
I've seen today an original Holley R6239 rebuild carb on Ebay for $ 800 : it is also a tentation !
Chris

MelWff

a page full of 3310-1 in the finish you want

http://www.ebay.com/bhp/holley-carburetor-780

chris383

I've found this one :

http://www.ebay.com/itm/1972-6239-HO...dWjtjr&vxp=mtr

TheSkunkWorks

Anything smaller than a 750 on a genuine LT-1 is a crime. JMHO

chris383

I think it is right : a Corvette specialist here in Switzerland said today the same thing. He tested himself some 600 cfm carb and 750 dfm and found out that the 750 worked better. He also told me taht a remanufactured carb is NOT a good idea because of the age of the carb...

chris383

I think it is right : a Corvette specialist here in Switzerland said today the same thing. He tested himself some 600 cfm carb and 750 dfm and found out that the 750 worked better. He also told me that a remanufactured carb is NOT a good idea because of the age of the carb...He think the carb is completely worn after all these years and probably several rebuilts...

mikem350

I have been trying out different carbs on my hotter than stock 350 and would say that the 650 DP would be the perfect size...if it worked properly!!!

This also agrees with Holley site for a typical LT1

Snoopysvet

Rebuild that 4777, if set up right you will not be disappointed

TheSkunkWorks

For some background as to why my earlier statement...

I too having been down the 650 path on the 350 SBC, IMHO Holley's formula for calculating optimum carb CFM has proven somewhat conservative when it comes to genuinely Hi-Perf applications, particularly those that make real power in the upper regions of the tachometer. There, the increase vacuum necessary to draft A/F mix thru a smaller carb (read, "greater restriction") at higher RPMs will necessarily cost peak HP. This fact has proven IMOE to make a noticeable difference even on the mildly worked over L82 (SFT cam w/1.6, headers, re-curve...) formerly in my now 427 BBC shark (on which I've run as much as a 1050 Dom with success on the street). At the other end of the tach, where the 650 is certainly more forgiving, good tuning can make a 750 every bit as user friendly. JMHO, YMMV





.

cv67

Edit: A modern day Holley HP or similar will make worlds of difference same size will surprise you how well they work
gave up on messing with older carbs and getting them to work right

If youre stuck on the original send it Jeff at AED he will have it working perfectly dont pay that kind of money (in ebay link) youre throwing it away.
You wont get better customer service period and it doesnt cost a fortune

chris383

Well, I've the choice of contiuning rebuilding that 650 DP : I'm sure this is a good carburator.
I'm pretty sure I have cracked the base during installation....but this base was already weak, I'm quite sure because I didn't tighten the nut very strong....or maybe just too strong ?

Or I can buy an original 6239 780 cfm rebuilt like this one :
http://www.ebay.ch/itm/262229174028?ul_noapp=true
(new carbs are not avaiable today...)

The base cost about $ 220 and the 6239 $800....

chris383

Hi,
Today, I fixed my engine problem and this thing had fired :-)
I'll keep my 650 DP. I've some doubts about the throttle plate (I see a crack on a corner...), so I plan to change the throttle plate and add an electric choke kit.
Maybe later I'll see for an original or modern carb, but for now I'll run this summer with the 650 DP and see what appening.
I've made a few miles today and averything seems good.

chris383

Hi everybody,
After a long time, I'm back here ;-)
Regarding my carb quest, I had spoken with a Crovette guy here in Switzerland (for 41 year, he has made only Corvettes!)
It seems that a sec vac is the best choice to drive the car on the roads (I never do racing...).
I have offers to take a 770 cfm Street avenger and to add a gold dichromate finish on it. I know that is not correct with originality, but the carb is new and not rebuilt.
An another proposal is to take a 3310 (already gold) and add a second metering block to make a 4150 carb...

What do you think about this ?

Chris