Hot Rod Forum : Hotrodders Bulletin Board

Hot Rod Forum : Hotrodders Bulletin Board (
-   Engine (
-   -   383 driving me crazy, help with tuning issues (Holley Street Avenger) (

JLeatherman 11-26-2009 07:18 PM

383 driving me crazy, help with tuning issues (Holley Street Avenger)
Hey all. I stumbled across your message board while googling Street Avenger 670 problems. Seems like there are some knowledgable people here. I searched the old threads and haven't yet found an answer to my troubles, so I joined up and am posting my own thread.

I built a 383 for my '84 vette. The original 2-bolt block, Eagle externally balanced forged crank, reconditioned stock rods, SpeedPro hyper pistons with coated skirts. 9.8:1 compression with ported stock iron heads (462's) opened to 1.60 exhausts and stock 1.94 intakes. Basically a nice budget 383, aiming for 400 horses. Cam is a custom grind. 224/234 @ .050 .465/.488 lift with 1.5's, 112 or 113 LSA. Engine produces a good solid 15" of vac at idle. Weiand Stealth intake manifold. I think that's about it for the specifics.

Fuel supply is from the original in-tank EFI pump through a Mallory 4307 bypass-style regulator. Very steady 6psi.

So, when I dropped the engine in the first time it was sporting a Barry Grant 650 Speed Demon. That carb had some quality-control issues and hit the can after a couple long weekends of fighting with it. In retrospect, it may not have been the carb's fault, but that'll depend on what I find is causing my new troubles.

The current carb on the motor is a Holley Street Avenger 670. This was built to be a streetable engine, and I decided to run a good street carb on it. Seemed like the perfect fit, but I've had nothing but massive troubles.

At first I thought it was the typical "Street Avenger stumble" that I've read so much about. The car would idle, but would not accelerate from a stop AT ALL. I mean, it took 3k rpms and feathering like mad just to get going on level ground. But the engine ran fine at the higher ranges. From 2500-3000 and up it ran great. Broke the tires free in 3rd, etc, etc. So I set about checking all the usual suspects.

I went through the distributor first. I re-timed it entirely this week. The engine is now set to 14* initial timing. Another 20* from the centrifugal is in by 2500 rpms for 34*. The vac canister is another 16*, for a total of 50*, and ~30* at idle. Timing curve seems smooth, and is atleast a good start for an engine like this.

With that settled I went back to basics on the carb. Fuel level was next. It is right at the bottom of the sight-holes on each bowl. Check. Then I went back to trying to tune in the base-lines. Started the car, got it warmed up, and started in on the idle mixture. The car about choked me out of my garage with unburned fuel fumes. Yes, the choke kicks off entirely. I went back to the manual and it said to start out at 1-turn out on the idle screws and move them in 1/8 turn increments until max vacuum is achieved. With the gauge on the car has 15" at the baseline 1-turn. I start leaning them out and the car maintains 15" until about 5/8 of a turn at which point it stalls out entirely. So I set 'em to 3/4 and fire it back up again. The idle is pretty smooth at 800 rpms, but the car is still rolling dark smoke out the exhaust and the fumes are killing me. At this point it's atleast idling and I wanna get a roadtest with the new timing settings so I get my buddy to tail me in case of the worst and I go for a 5 or 6 mile road test.

No change. The car accelerates like crap, barely leaves a stop, atleast most of the time. Every once in a while I'll come to a stop and roll away again without too much effort. An easy 900rpm pull. Very rarely though.

So we get back to the house and my buddy who was tailing me says that the car was rolling dark exhaust the whole trip. Said the only time it eased up was when getting on it on a hill. So, it's rich at idle and I can't adjust it away, but it's also apparently rich through the whole range unless it's under hard load.

Sounds like a power valve problem to me, but this is out of the box and supposedly wet-tested at the factory. How do I set about checking the PV? And how do I get to it and remove it? With 15" the factory 6.5 PV oughta be fine right?

Can anyone else think of another cause for too much fuel across the board? I seriously doubt that the historically over-lean SA factory settings would cause my 383 to run this rich across the board. And it's definitely not fuel-pressure blowing the needles off the seat because the fuel level is fine. What else could it be?

johnsongrass1 11-26-2009 09:18 PM

Carb Spacer?

I would disconnect the VacCan for testing. I wonder if you have it hooked to something that reducing timing underload. I would run a bit more timing it in too. Add 2 degree's initial.

What rear gear? That cam isn't real good for off line power anyway. You can work it out though. Unless you are way down to 2:73 or something.

Where is the fuel pressure measured at?

You should get 17-18" of vacuum at idle with that cam.

Last it's common to have the secondary's closed to far at idle and the forces you to open the primary's to get a decent idle and that uncovers the idle transfer slots which makes the needles sluggish to respond or otherwise not at all. The carb then idles partially off the main circuit for which there isn't enough vacuum signal to properly atomize fuel. The fix? Open the sec a little so you can close the primary's and regain control of the idle circuits. BG can give you a decent starting place on settings but the final adjustments are up to you. Do not be afraid to get outside of the normal realm as long it works for you.

cobalt327 11-26-2009 09:49 PM

Wet flowed or not, I'd suspect the PV is N/G, loose or gasket torn, etc. A 6.5 should be OK for it.

Do the routine for transfer slot exposure. Should you need a reminder, use the search function.

If that doesn't help, the idle bleeds may need opened up a little. Start w/0.002".

NXS 11-26-2009 10:28 PM

yes, drop the vacuum advance for tuning.
find what the engine like at an idle, and then find what it likes wide open. adjust accordingly..

also, does this have an MSD box on it by chance?

have you took a visual inspection in the carb, looking at the boosters at an idle to see if it's clean or dirty (droplets of fuel) coming out of the boosters?

do the rpms every change greatly or erratically when trying to adjust idle either by the mixture screws or idle speed screw?

DENCOUCH 11-26-2009 10:32 PM

I was kind of wondering if you were running an automatic tranny & if so, how tight is your converter.

What ratio is the rear end gears?

1BAD80 11-27-2009 05:53 AM

There are many threads with the same problem as your's.

I don't know why its always the power valve being bad which may be 1 out of a 1000 being bad, and very easy to tell if it is bad.

If you expect a blown power valve, use this simple test.

TEST: At idle turn your idle mixture screws all the way in.
If your engine dies the power valve is not blown.

95% of the time a bad power valve will cause the carb to flood out and can be seen from the top of the carb, you will know something is wrong thru the whole RPM range.

for a stumble , Jetting & or a larger squirter is called for.
What are the color of the spark plugs ?

Adjust the floats to the very bottom of the sight plug, the clear sight plugs can be deceiving, I like to adjust the floats with the plug out and the gas just barely trickles out when rocking the car.

Sometimes the intake gasket moves a little and leaks to the other chamber from over tightening or using silicone sealer which would cause it to hesitate or backfire.

My .02 cents worth is the float level is high.

JLeatherman 11-27-2009 08:57 AM

Thanks for the responses so far. Lemme answer a few questions.

The rear end is a 3.07:1, stock for the manual-trans car. Dana 36 posi. The trans has I believe a 3.27 first gear, which is actually a bit steeper than the stock Doug Nash 4spd trans it came with, so it certainly shouldn't be too bad. Steel flywheel, somewhere around 24-28 pounds as I recall, and the clutch is a RAM 3200# PP with a regular organic disc.

Fuel pressure is being measured at the gauge port of the regulator.

It's not an MSD box. It's a regular coil-in-cap HEI distributor from Summit.

There isn't any room for a carb spacer unfortunately. I had to run a 1 1/4" drop-base with a 2.5" filter to clear the corvette hood.

The boosters look fine at idle. It doesn't appear to be exposing too much transfer slot, but I'l go back through it again and measure it for sure.

If you crank the idle screws in all the way the car will stall at idle.

The float level looks great. Right at the bottom of the sight-holes. Doesn't fluctuate at all while running, atleast not in the garage.

The spark plugs are actually a healthy dark-ish tan color, which is surprising since the car rolls fuel-smoke out of the exhaust the whole time I'm tuning/driving it.

As for this being the same problem as the other Street Avenger threads, I've read them. I've been searching through them on a lot of forums, and I know about the usual causes for a stumble. Not enough pump shot, too lean jets, squirter cam misaligned, etc. Thing is, this car rolls rich-smoke out through the entire rpm range except when it's under a real hard load. And at idle it smokes like it's rich, but if I lean it out any farther it stalls. And it's not just an initial problem either. It isn't real happy at anything under 2500 rpms.

As for pulling the vac cannister, the car isn't happy at all with just the base timing at idle. Takes atleast 28-30 degrees of advance at idle to make it run smooth. At the base 14* it sounds like a monster cam and tries to stall all the time. At the moment it's got 34* with the vac cannister hooked up, but I can give it a shot without it. I'd have to crank the base up to about 30*, which would give me a total of 46* in by 2500. Might give that a shot later.

But, why is it running rich through the entire rpm range?

NXS 11-27-2009 09:37 AM

and you are sure the cam is degreed correctly?

have you checked the spark to make sure it isn't breaking up at low rpm?

and have you pulled the bowls off the carb yet to make sure everything is OK in there?

B/c one time i bought a carb of ebay, looked brand new (advert said it was new, installed and removed b/c it was too big hehe).
bolted it on and it fired right up but wouldn't run, rolled black smoke and died. I pulled the bowl and there were no jets. put jets in and it ran fine.

as notedthe jets could be gone, the power valve could be torn or even installed without a gasket...

JLeatherman 11-27-2009 11:59 AM

Spark fires off just fine, or atleast the timing light doesn't miss a beat.

I pulled the carb off to check the transfer slots and they were way out of whack. The secondaries were totally shut (shoulda checked that before I put it on). I had the primaries open atleast .125" to compensate. Definitely part of my troubles. So I set the secondaries to ~.020 (maybe a hair higher) and the primaries to .020 as well, reset the idle mixtures to a base of 1-turn and reinstalled everything.

After a tricky startup I got the choke kicked off and went back to setting everything. Trouble is, now it idles too fast. Even with the curb-idle screw out all the way (primaries closed) the idle is up around 1600. I suppose I now have the secondaries open too far? I figure once it cools off again I'll try .010" on the secondaries and start over again?

cobalt327 11-27-2009 12:04 PM


Originally Posted by JLeatherman
Even with the curb-idle screw out all the way (primaries closed) the idle is up around 1600. I suppose I now have the secondaries open too far?

That's likely the problem- IF you're sure that the fast idle cam isn't holding the primaries open, and there are no vacuum leaks.

But sounds like you have the problem on the run- good news, that!

JLeatherman 11-27-2009 12:08 PM

Yeah, I physically pushed the fast idle cam off and made sure it wasn't still touching anything. Also, it dieseled a bit when I tried to kill it, so I figure now the secondaries are open too far and it's getting too much fuel through them. Back to the garage to tear it all off again.

eric32 11-27-2009 04:37 PM

Well brother I feel your pain but mine is a little different. My motor is similar to your's but my motor is a 350 with world product sportsman 2's, edelbrock rpm performer with 9.1 compression and cam is a hydraullic roller wich is 222/230 @ 50 510/530 lift and I get around 14 inches of vacuum at idle. I got a brand new holley 670 aluminum street avenger and I had all kinds of problems with just bolting it on. Engine would just fall flat on its face and go too lean and cause the engine to quit and drop cylinders cause lack of fuel. Thought something else was wrong and spent over two weeks of long weekends working and checking this and that to find out it was the carb.

Well got a new one to replace the faulty new one and it came out ok but the carburetor is way to lean. My plugs are bone white and I have already went from the stock 65 main,68 primary to 69 main and 74 secondary and my plugs are still white with little to no change and I have a off idle lean stumble and it back fires through the carb if you rev it fast. I bought some bigger squirters and will have to go from there. I have my timing at 16 degrees at idle and is 32 all in at 3000 rpm. I hope you can find out whats wrong with it. I checked my transfer slots as well and you won't be able to have the rear ones at .020 or anything close to it. I reclosed my secondary's and opened them up several turns but without the transfer slot showing. I readjusted my front to show about .040 of the transfer slot and it idle's excellent at 800/900 rpm and the idle mixture's screws are about 1 1/8 turn. Hope that might help you out as well. Also might want to try a different pump cam assortment and squirter's if needed and also go down in jet sizes if needed. I would die for tan plugs now. Still trying to get mine a little richer.


ericnova72 11-27-2009 05:12 PM

How close is the air cleaner lid to the tops of the bowl vent tubes and choke tower??. If you don't have enough clearance here (1/2-5/8" minimum) it will cause the problem you are having. Maybe take a short drive without the air cleaner as a test.

JLeatherman 11-28-2009 07:27 AM

ericnova, I've been tuning/driving it without the aircleaner anyway because I can't reach the idle screws very well when it's on (drop base).

I reset the secondaries to .010" transfer slot and put it back together again. This time I managed to get the car down to 1000rpm idle before I had the primaries totally shut and ran out of adjustment. I guess the secondaries are still open too far.

I will second eric32 on not having much, if any, of the transfer slots showing on the secondaries. For some reason you have to open the secondaries pretty far to expose any transfer slot. The slots are pretty high in the secondary bores. It was several turns of the screw just to get them exposed at all. I'm gonna keep shutting them a turn or so at a time until I get enough adjustability out of the primaries again.

Man, I wish there was a way to get to that screw without taking the carb off every time :mad:

dawg 11-28-2009 07:40 AM

[QUOTE=Man, I wish there was a way to get to that screw without taking the carb off every time :mad:[/QUOTE]
there is
take the screw out and replace with an allen set screw.
.05 cent fix

All times are GMT -6. The time now is 07:11 AM.

Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright 1999 - 2012. All Rights Reserved.