Thread: carburator upgrade Question

Tweet

Hi there Hot rod guys,

I got a general question about a carburator upgrade on my Chevy 1986 350M engine being used in a Motorhome GM's VAN G30 dually application with a Turbo 400 tranny and dual exhaust.s All air pumps and emission garbage have been removed.

Currently, it has GM electric choked Quadrajunk factory original carb (750cfm I think) on it now but I want to go with a new Modern carb and according to my numerical calculations and my normal cruise RPMS at 3000, could I put an Edelbrock 1406 or a Holly 4160 600 cfm carb with electric choke on it? My top RPMs would never be over 4000 and I cruise mostly at 3000 RPMS give or take. What new carb size would you replace the Quadrajunk with?

This should be an easy question for a motorhead.


Jeff

350 cubic inches at 4,000 rpm's will flow 364 cfm at 90% efficiency, which is probably a stretch for a stock 350. It's probably more like 80% efficiency and 324 cfm, so a 600 carb is way overkill. If it were mine, I'd get some books on the Quadrajet and rebuild it. It's probably the finest 4-bbl carb ever built and the tiny little primaries will give better mileage than the larger primaries on an E-brock carb. That's just my opinion, what do I know?

techinspector,

I understand, but why did GM put such a Huge Quadrajet with such small primaries such as the one I have on now? Why not get a Edelbock and put in small primaries?

Jeff

the primary venturis in the E-brock are larger than the ones in the Q-jet and cannot be changed. The whole idea of the Q-jet was to have small primary venturis for cruise and huge secondary venturis for power when needed, since realistically, you're probably cruising 99% of the time and need power 1% of the time.

Call these guys and talk with them if you don't want to rebuild yours....
http://www.i-5automotive.com/page5.html

Tech,

I got you now, I understand completely and I will just have my brother rebuild the Quadrajet. Now what do you know about a cracked flexplates on a Chevy 4.3 with 4 speed auto. Making an hellva cracking sound at idle and under acceleration. If you back off the gas no cracking sound. RPMS are normal and sfift is normal. Torque converter is new and so is the tranny that shifts PERFECT.

Jeff

what can I say?? Replace the flexplate.

LOL, I knew you would say that. I checked out the primary metering jets on the edelbrock 600 and the Edelbrock 795 Quadrajet and there is a HUGe difference like .073 to .098. That is HUGE.


Jeff

Originally posted by reedfish1954
LOL, I knew you would say that. I checked out the primary metering jets on the edelbrock 600 and the Edelbrock 795 Quadrajet and there is a HUGe difference like .073 to .098. That is HUGE.


Jeff

the size of the metering jets in those carb. dont mean much if you dont know what size metering rods you have.

The jets in a Quadrajet are .073 and the metering rods I have seen in rebuilts are 43M, 50M, and 52M. So if I get this Quadrajet rebuilt what metering rods do I get put into the .073 jets for good fuel effieciency. OR do the rods even have to be replaced? I'm thinking both the jets and rods are replaced in rebuilt carbs? Edlebrock's Rochester Quadrajet 1904 has the 50m rods in them.


Jeff

if you rebuild your carb. use what ever came in it. there are all kinds of dif. rods you can buy for dif. set ups. you should be in the ball park.