JLeatherman

Having some trouble with my new 383 engine. It's in a C4, but since I carbed it I figure this is the better area to go for tuning advice. Engine is a relatively mild 383. 9.8:1 compression, ported stock 462 iron heads, cam is .465/.488 with 1.5's and 224/234 @ .050. Distributor is a vacuum advance HEI MSD from Summit (just a basic distributor, no MSD box or anything like that). Base timing is at 10*.

So here's the deal. I had a Barry Grant carb on this engine and had nothing but problems with it, so I changed to the Holley StreetAvenger 670. Vac secondaries, with the heavier springs in the package (the yellows instead of the blacks). Got the engine barely broken in at this point, maybe 500 miles. It fires right up no problem. Choke kicks off and idle settles in at around 850. Seems just a tad rich at idle right now, but I'm not done fiddling with that yet. The engine produces good vacuum and all that at idle. But the preformance between idle and ~2000 rpms is terrible.

When leaving a stop the engine is very sluggish to respond at lower rpms, and you really have to feather the clutch and be quick with the gas pedal to keep it from stalling. As it accelerated from off-idle to 2000 rpms the engine stutters and bucks a bit, almost like it's too lean, but it doesn't backfire. At around 2000 rpms the engine clears up and runs great from there to 6000.

Once above 2000 rpms it cruises fine too, with very light throttle the engine maintains 2000 no problem and runs nice and smooth. From idle-2000 you gotta give it a lot of gas and it still bogs terribly. If you try to cruise at 1400-1800 rpms is when the stuttering is worse. It gets a bit better under heavy throttle at low rpms, but it's still pretty rough until 2000 rpms.

So, I'm wondering where to head first here. I'm thinking either the distributor isn't advancing to where it needs to be until the higher revs, or the carb isn't opening the secondaries early enough. Like I said, it feels lean based on the stuttering, but it doesn't backfire. Fuel pressure is a rock-solid 6 pounds because it's an electric pump with a return-style mallory regulator and a gauge. Anyone got a suggestion? The car is down at the moment to have the trans rebuilt, so I can't hit the road again for a little while. Anyone had a similar problem? The Barry Grant had a similar problem of very poor performance down low, so I'm thinking it might be the distributor curve? Possibly the vac advance mechanism? It is adjustable, but I'm not sure how to set it statically since it's based mroe on engine load. I guess I could unhook it and see if the car behaves the same way?

1982CorvetteDude

Not sure if you got the dvd with the carburator or if it's the same...but holley puts out a really nice and detailed tuning dvd that would be worth investing in. It could answer a lot of your questions.

It's called "Holley installation and Tuning DVD" and I got mine from amazon.

Petes73

One suggestion from earlier posts is to move the back jets to the front and then put #76? jets in the back. I did this and then followed the setting instructions that came with the Avenger. I did not have any issues getting it set up and running correctly.

JLeatherman

Yeah, it did come with a disc. Guess I should go watch it. At the time my shop was real far from the house (like 2 miles) and I never remembered to bring it home where there's a TV. I've since moved and the shop is a whopping 6 feet from the house, so it'll be easier this time

1982CorvetteDude

If it's the same disc I have you will learn a lot, it's interactive with exploded views and stuff.

temvette72

Are you sure you are getting the shot from the accel pump when you punch it? I remember a Lars post from a while back that said from the factory the little arm that rides on cam on the throttle linkage is sometimes mis-aligned. The accel pump won't squirt, and you get a bad bog when you punch it. Simple fix if that's it.

Temvette72

JLeatherman

Temvette, I came across that post as well. I'll check, but what I'm getting isn't a bog when you punch it. It's really more of a bog until you get above 2000 rpms. Like, if I'm setting still, and I rev the engine up to 1800 and hold it there and start easing the clutch out the engine bogs terribly, and sometimes stalls. That, atleast, shouldn't be accel pump, although that's not to say mine's working properly. I think this idea about the secondary jets being too lean might be hitting the nail on the head. Theoretically, when I'm leaving a stop (when this bog is the worst) would be when I hitting the secondaries hard. Too light of a spring and too lean of a secondary jet might very well be the issue here.

By all means, though, please keep the ideas coming and I'll take a crack at them asap. I'm gonna get some jets to try later in the week.

Ganey

You need to be sure idle mixture is right as it affects off idle transition. Primary jets need to go up at least 2 sizes. Secondary jets are not the problem here!

JLeatherman

I guess I could have mentioned that it's on a Weiand Stealth dual-plane manifold. I built the engine with some pretty good low-end torque in mind because I still have the tall 3.07 stock gears, and a T5 with what I believe is a 3.27 1st. That's why I kept the cam fairly mild, with a LSA of around 113, and I'm also running SLP Tri-Y headers. It really shouldn't be acting like an over-carb'd Victor Jr

One more piece I'll throw in here, since I missed it before in my narrative, is the odd behavior about returning to idle. If you ease the engine to a stop it idles fine. By ease to a stop I mean you downshift and brake slowly so the engine goes from 2500-idle gradually. It will sit at 850-900 and purr quite smoothly. BUT, if you come up on a stop sign at 2500 and just push the clutch in and throw it in neutral the engine will stumble and try to stall, idling at 500 or so. If you crack the throttle a couple times to clear it out it will eventually return to a decent idle. Very strange behavior that makes me feel like it's loading up with fuel, which is odd since it sure acts lean in the idle-2000 range.

Matt Gruber

Stealth is a great choice
i always use a 4 hole gasket, but that's me.
Sounds like it has a few things wrong.
I'm sure u will get it sorted out eventually, everybody does.
I'm not familiar w/Avengers. I won't use any vacuum secondary on anything except a truck or motorhome, and then only if my mom will drive it.
Since c4's have o2 sensors, u should be checking the A/F ratio, WHY GUESS? a waste of time.
Good luck!

LFZ

this is very common with Street Avengers...and the best cure is to get your total advance in around 36 before 3000rpms...actually, just read the Lars papers. Street Avengers are pretty good out of the box though.

SteveG75

90% of so-called "carb" problems are really timing issues.

Get a good advance curve in the distributor. Shoot for 36 degrees total mechanical all in by 2800 rpm. Then add another 16 with a vacuum can.

After that, diagnose the carb. BTW, IMHO a 670 cfm carb is pushing the small side for that combo.

FKING1

This is good for fixing the stumble that is inherent in the Holley 670, but , may not apply to this problem. would not hurt to try though.
Check the acceleration cam to make sure it is riding on the lever.
I put the secondary 75 jets to the primary and 83's in the secondaries.
Cured my stumble.

JLeatherman

Um, what are the Lars papers, and where might I read them?

redstingray74

Look at beguining of the forum here for the sticky on timing long post;
Lars has web site to go to thru there. Excellent info.

Phil Zell

I agree.

I have close to the same setup but use a 770.

Xakk

This might help some. I've been hanging on to this article for a while now.

http://forums.corvetteforum.com/c3-t...solutions.html

MelWff

Assuming it's not a timing problem the first thing on the Holley you should check are the float levels before messing around with jet sizes.

cherrybombc3

Well you have a wealth of info here but I have to one thing. STOP!!
& Start with the basics. That 670 S/A should be pretty good for your set up right out of the box without messing with jets, just the fine tuning below!!

1) Check your timing first & set curb idle.
2) Check float level per your Holley manual.
3) set your A/F mixture screws per your Holley manual.
4) set your curb idle again, cuz it will change.
5) re-adjust your accelerator pump. When ever you change curb idle on the Holley you must go back and re-adjust your accelarator pump. They are directly related.


You'll get it, but make sure your timing is right first.

69vettester

I have a 383 and Need to use the 770SA In my case I use the Speed Demon 750.
I think your 670 would be Ideal and right in your range.
Before changing jets Just check or adjust your Dist. advance , all in @36degrees is a good number for performance sb chevy engines.
Play with the idle speed according to instructions, Check for That immediate squirt that needs to happen as soon as you touch your accelerator pedal. Your problem is common. youll have it dialed in in no time