Swapped carbs, big improvement
 

I installed a deluxe GM performance crate engine (SP350/357 HP) earlier this year. It came pretty much complete, as far as fuel and ignition. 670 Holley carb sweet avenger with vac secondaries and HEI with a GM dual plane manifold. I have a 2 1/2" dual exhaust with stock exhaust manifolds. TH2004r trans stock converter and 4.10 gears. To me it seems like it's never run quite. Loads of power, but drivability lacked. Very uneven idle, Lugs at low RPM's really bad. I have tried tuning the carb, have had it dyne tuned still not all that great. Just swiped the carb with one a friend had laying around. 750 Holley classic style with vac sec. So far it is a 100% improvement. Seems to be a much better idle with mixture screws out 2 turns. with the 670 I had them (and the dyno tune was the same) 3 1/2 turns out. very smelly exhaust backing in the garage. Now its much better. I will need to give the carb back and am thinking of EFI since I now know it was the new carb that came with the engine that made it run crappy.

Re: Swapped carbs, big improvement
 

Sounds like you need a quadrajet......

Re: Swapped carbs, big improvement
 

I would just keep an eye out for a used 4160 750 holley. I see them for $50 in decent shape atleast 3 times a year.

Re: Swapped carbs, big improvement
 

I was curious about the carb. 2nd hit on my search found this:
https://forums.holley.com/showthread...venger-Problem

Sounds very similar to OP's issue.

Wondering if they are tuned out of the box for max power at higher rpms and don't work well at low speeds?

Re: Swapped carbs, big improvement
 

I don't think you can do better than a Quadrajet.

Re: Swapped carbs, big improvement
 

I'm thinking the same thing. Maybe the idle and transfer circuits are not well matched to something over base stock motors.

Re: Swapped carbs, big improvement
 

Just spoke to my friend and he is open to selling me his carb. I'm going to run it for a week or so and make sure I'm just the improvement is just not in my head.

Yes a Quadrajet is a great carb, but I'm not ready to swap manifolds to accommodate one and tuning for a non stock engine is not the easiest thing to do with these.

Re: Swapped carbs, big improvement
 

Did you have to modify the new used Holley linkage much for your 200 4R?

Re: Swapped carbs, big improvement
 

No it’s a bolt on attachment for the trans cable. I used the Holley attachments and bracket. Works great

Re: Swapped carbs, big improvement
 

Did you change jets and adjust the 670 beyond just turning screws?
Otherwise this could just be a fluke because the newer carb has the right jets to meter correctly.

Re: Swapped carbs, big improvement
 

Actually I did. Had it tuned on a chassis dyno. They rejetted the carb and commented it really likes to run rich. Worst $550 I spent. I told them I was more concerned about drivability than max power, but didn't turn out well. Since the out of the box jetted borrowed carb works so much better I'm still thinking EFI might be a step up.

Re: Swapped carbs, big improvement
 

Before you jump right in with a EFI system, check with a couple manufacturers and see if their system will work with that cam having a 108 LSA. The factory EFI's typically don't like tighter than 113 or 112.

Re: Swapped carbs, big improvement
 

Good thought, I will make a couple of inquiries. I do read about some that have installed the EFI units with much bigger cams than mine. It really is all about the amount of overlap, I have a tight LSA, but fairly low duration at 215/223 @ .050.

Re: Swapped carbs, big improvement
 

If you are looking at EFI systems, I'd suggest looking at Edelbrock's Pro Flo 4. It's really nice. I run it on this engine, https://blueprintengines.com/product...etor-bp4002ct1 . It took a little help/advice, but it runs great now :)