Avs2 650 tuning on a 340

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fez440

fez440

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Hi. I am looking for help with my edelbrock avs2 650 carb. The engine is a 340 4 speed mildly modified. Air gap intake,aluminum heads, headers, comp cam 231 duration. 480 lift. Vacuum at idle is 10" cruise 15".

Timing is set at 18 initial and 32 total. I have tried different timing spots From 20 intial to 35 total with no change. No vacuum advance as per my recommendations from ignition box company which is a rev n nator with a firecore distributor and their coil. Has summit plug wires and autolite plugs 3924 .035 gap

I have been through the carb and Manuals and believe i understand this carb well even though I'm new to carbs.
I have a o2 gauge with a vacuum gauge next to it which I can watch will driving.

The issue I have is from when i 1st installed this carb it run very rich. Cruise was 10.8:1 wot about 11:1. I have jetted it down much more then their calibration carts shows. Right now I have 98 jet with 73 37 rod and 95 jet in secondary. Orange spring. Middle pump shot.
Still cruises at about 12.8:1 wot about 12.5 which is good. Idle screws react well. Idle is about 13.8.
Now if I try and lean out cruise any more say to around 13.5-14 I get a very bad lean spot while under load. I still get get this lean spot now but it is not as bad. While accelerating with 15" vacuum down to say 5" (before power mode kicks in) my air fuel will start to lean out almost 3 points. I'll go from 12.8 to 15.0 ( once the power mode kicks in air fuel goes to about 13.5) if I lean out cruise to around 13.5 then when accelerating it goes to almost 17:1.
Engine will hesitate above 15:1.

I have tried to use to stronger spring but with the low idle vacuum it doesnt work right. Will bounce even if I turn up the idle more right now it's at 800rpm. I can make the pink work ok at 1000rpm but still will bounce if leaving from a idle as vacuum will drop a bit.

I have tested for vacuum leaks to the best of my knowledge I can't find any.
Have a psi gauge set to 5.5. Floats set to factory specs 7/16" and 1 "

Is this normal for it to lean out this much while under load? Any way to solve this?

Hoping someone has some ideas that way help me out as I have tried all I can think of.
I am new to carbs and older car but I feel like I know how this carb works but cant seem to figure out this issue. I have had no luck trying to find help online so I thought I would try here
Thanks for your time
 
How clean is the fuel filter? Is this happening between shifts? Have you increased the pump shot? Have you used the Orange spring, it is 5" and you supposed to use a spring that is half your idle vacuum. Remember O2 sensor are a lagging indicator of what has happened and not what is happening.

My combo is similar to yours accept I have a bit smaller cam and a Quick Fuel 650 Mech. Secondary. The issues I had was it was way to rich out of the box and a lean bog between shifts just as you describe, 13:1 to 17:1 to 13:1. I needed to add more fuel and for a longer duration via the pump shot between shifts.

What springs are you running in the FireCore and have you mapped the timing advance?
 
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I am using 2 blue springs in the distributor. Fuel filter is clean. I checked. Issue is not when shifting but at a steady increase in acceleration. I can see the pump shot richen the mixer based on my af gauge. I believe it's not that
It happens when you have a steady increase in acceleration before the power mode kicks in, it will start to lean out. Which I assume might be normal but it leans out to much.
Yes I have used orange spring. Pink spring and silver spring. Orange does work the best at idle but the power mode does not come in fast enough before it gets to lean.
Total timing comes in around 3000 rpm
 
Go bigger on the AVS2!

What rear gear ETC!

Sounds to me you are tying to tune the wrong CFM for your APP! JMO!
 
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2 blue springs means little to many.

18 initial and 35 Total is in the "good" for timing!

Your 340 is likely hungrier than you may think!
 
Rear gear is 3:55. If I bump up the initial timing anymore the starter doesnt seem to like it.
 
fez440 said:
Rear gear is 3:55. If I bump up the initial timing anymore the starter doesnt seem to like it.
Click to expand...

That is in Alignment for most on the timing! You have that part nailed!

You seem to be chasing a CFM issue. Any chance you have a friend nearby with an 800AVS2?

I assume the total timing no vacuum advance and blocked is 35 degrees at preferably 2k RPM.
 
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1969383S said:
That is in Alignment for most on the timing! You have that part nailed!

You seem to be chasing a CFM issue. Any chance you have a friend nearby with an 800AVS2?

I assume the total timing no vacuum advance and blocked is 35 degrees at preferably 2k RPM.
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I do not know anyone else with a carb that I can borrow. Yes I know thet would be a great way to test out stuff. No vaccum advance. Total right now is 32. I have been trying between 32 and 38 total. Total comes in around 2800 I believe. I wouldn't think a 650 is to small. Based on the carb calculator a 600 is all I would need. Plus at wide open throttle the air fuel is good. But I'm still learning could be wrong
 
fez440 said:
I do not know anyone else with a carb that I can borrow. Yes I know thet would be a great way to test out stuff. No vaccum advance. Total right now is 32. I have been trying between 32 and 38 total. Total comes in around 2800 I believe
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My younger brother snatched my 750 Holley DP while my 383 was being rebuilt in the mid 80's.

Cranked his factory 70' 340 up big time!

Once found I took it back and gave him the 650 Holley DP.

Seems most later 340's were running larger TQ carbs. Story is the 340 in the right set-up is very Hungry! JMO!
 
1969383S said:
My younger brother snatched my 750 Holley DP while my 383 was being rebuilt in the mid 80's.

Cranked his factory 70' 340 up big time!

Once found I took it back and gave him the 650 Holley DP.

Seems most later 340's were running larger TQ carbs. Story is the 340 in the right set-up is very Hungary! JMO!
Click to expand...

Thanks for the reply. I will keep it in mind. I am trying to find another carb to try.
 
The problem is NOT the carb. It is the tuning procedure. 10" of idle vacuum is the problem. It needs a LOT more initial timing. The other problem is that the pri throttle blades are probably open too far at idle, exposing too much transfer slot [ TS ]. This in turn gives a domino effect. Idles too rich, off idle flat spot, because TS is wrong.
You first need to get idle ign timing correct.....before touching the carb.
Unless Edel changed the instructions, they say for low vac situations to use vac adv connected to manifold vacuum.
Before doing that, try this simple 5 min test & report back with the result. Warm up engine, in D if auto trans, engine idling, loosen dist clamp & slowly turn the dist CCW. This will advance the timing. Keep going until you get the highest idle rpm/smoothest idle; toggle dist to be sure. Now check what the timing is & report back.
 
Bewy said:
The problem is NOT the carb. It is the tuning procedure. 10" of idle vacuum is the problem. It needs a LOT more initial timing. The other problem is that the pri throttle blades are probably open too far at idle, exposing too much transfer slot [ TS ]. This in turn gives a domino effect. Idles too rich, off idle flat spot, because TS is wrong.
You first need to get idle ign timing correct.....before touching the carb.
Unless Edel changed the instructions, they say for low vac situations to use vac adv connected to manifold vacuum.
Before doing that, try this simple 5 min test & report back with the result. Warm up engine, in D if auto trans, engine idling, loosen dist clamp & slowly turn the dist CCW. This will advance the timing. Keep going until you get the highest idle rpm/smoothest idle; toggle dist to be sure. Now check what the timing is & report back.
Click to expand...
I'll try doing this again this weekend. I am already at 18 initial not sure how much more I can go buy I'll try.Thanks for the tip
 
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So, what I have found pushing the AVS pretty far out of its comfort zone, in a race car. is in some circumstances it likes being fat in front. What I mean is say your total fuel load is now a step rich in the front and in the back. Try going two steps rich in front and the stock setting in the rear. I got to a point where I was 3 steps rich in front and three lean in back. Stands to reason if I put it back to the stock setting it would be fine. Nope. The car hated that. Also you might be fluttering the air door a bit and there is NO transitional enrichment system for the secondaries. Unlike the AFB that has a small port near the air plate. It takes TIME for the fuel to be pulled out of the sec well and through the booster. The secondary door opens much quicker than I would have though. At WOT it takes less than 1/3 sec to go from closed to fully open. BAM! Here’s a bunch of air! As the fuel meanders through the well. I log AFR to RPM and there is a lean peak right before the pump shot hits. 14.5? There is a second lean peak as the air door opens. All happening in less that 1/10th? Point being it takes time for the fuel to start moving. You can’t “feel” either peak. To me it seems you are floating in transition.
 
Bewy said:
The problem is NOT the carb. It is the tuning procedure. 10" of idle vacuum is the problem. It needs a LOT more initial timing. The other problem is that the pri throttle blades are probably open too far at idle, exposing too much transfer slot [ TS ]. This in turn gives a domino effect. Idles too rich, off idle flat spot, because TS is wrong.
You first need to get idle ign timing correct.....before touching the carb.
Unless Edel changed the instructions, they say for low vac situations to use vac adv connected to manifold vacuum.
Before doing that, try this simple 5 min test & report back with the result. Warm up engine, in D if auto trans, engine idling, loosen dist clamp & slowly turn the dist CCW. This will advance the timing. Keep going until you get the highest idle rpm/smoothest idle; toggle dist to be sure. Now check what the timing is & report back.
Click to expand...
I tried what you said today. Warmed it up to 180 played with the timing
Went to 24 intial 35 total. 800 rpm idling at around 13.8:1 afr. Measured transfer slot was just above square at about .037 showing under throttle blade. No gain in vacuum still sitting at 10"
Starter not happy with that much initial hard to start when hot.
No change in afr while under light acceleration goes from 13.5 to 15.5 before power mode kicks in then goes to 13.5. 12.4 wot.
Had to run the orange spring as pink would bounce.
 
The starter not acting good when it's hot could be a hot starter not the timing so much. Usually the first sign of the starter not working well... I too have somewhat a similar setup with maybe just a tick more cam. I am running dual Holly Street demons which are basically across between the Edelbrock and a thermal quad. Also metering rods step up Springs same kind of gamma-rama..
I'm sure in the Edelbrock instructions it talks about long duration cams? And the possible necessity of a weaker step up spring. When you say it's bouncing maybe that's what it wants to do.
The real name of the game here is just patience. Writing down what you've tried and writing down the different combinations. Test and try again always with a warmed up car. You can imagine with two carburetors that were never supposed to be put in a dual quad situation I had to do this dozens of times before it got right..
He might try taking one of those Blue Springs out and putting a light silver spring in. Make the timing come just a little quicker.. beyond moving the arm for the pump squirter you can purchase larger squirters to put in there as well and there's a different pump you can put in as well.. all cheap stuff to buy to give a try. When I read your first post I was pumped shot as well but I think you're getting real close. Also I like putting a new set of plugs in and driving it for a day or two and seeing what they look like. A lot of times the O2 sensor isn't everything but plugs never lie.
I seem to live with high 13 low 14 at idle and mid to low 13s at Cruise and about a 12-3 at wide open throttle sometimes a little lower..
 
I have tried a larger squirter but that did not help. Went to rich. Also have tried the stronger accelerator pump from the 800 carb. Right now with the top shot and .031 squirted it seems ok. I can see the rich blip on the af gauge.
I appreciate your reply. I will look into changing the distributor springs.
 
I run a near identical setup but with an auto . I had the same problem with both a 650 and 800 AVS-2. I would recomend 15-16 intial with the vacuum advance hooked up to ported source. This way it will crank and start fine but idle around 21* ( depending on your vacuum can) and allow the throttle plate to be adjusted properly.

Make sure you get the fuel pressure down around 4lbs or it could push fuel by the needle and seat .

Contrary to popular opinion Eddies are not easy to tune if you are running a cammed engine . And those ******* tiny clips .....urgh !!!
 
fez440 said:
I have tried a larger squirter but that did not help. Went to rich. Also have tried the stronger accelerator pump from the 800 carb. Right now with the top shot and .031 squirted it seems ok. I can see the rich blip on the af gauge.
I appreciate your reply. I will look into changing the distributor springs.
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Lose the O2 for now and make it run. You are chasing a number that may not mean anything. Don’t be married to an O2 reading. Ever.
 
mbaird said:
I run a near identical setup but with an auto . I had the same problem with both a 650 and 800 AVS-2. I would recomend 15-16 intial with the vacuum advance hooked up to ported source. This way it will crank and start fine but idle around 21* ( depending on your vacuum can) and allow the throttle plate to be adjusted properly.

Make sure you get the fuel pressure down around 4lbs or it could push fuel by the needle and seat .

Contrary to popular opinion Eddies are not easy to tune if you are running a cammed engine . And those ******* tiny clips .....urgh !!!
Click to expand...
It's not a vacuum advance distributor he said that a couple times and yes those little effing clips:BangHead:..
I have found some similar ones in the Ace hardware little drawers. They're not exactly the same but they work.. even worse the ones on the Holly Street demons are microscopic c-clips.. worse than those Edelbrock hair pins and you have to take the one on the passenger side off to change the metering rods or Springs, the arm for the choke flap goes right over top...
 
j par said:
It's not a vacuum advance distributor he said that a couple times and yes those little effing clips:BangHead:..
I have found some similar ones in the Ace hardware little drawers. They're not exactly the same but they work.. even worse the ones on the Holly Street demons are microscopic c-clips.. worse than those Edelbrock hair pins and you have to take the one on the passenger side off to change the metering rods or Springs, the arm for the choke flap goes right over top...
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Distributor does have a vacuum can. I dont use per recommendation on ignition company. Fuel psi is at 5.5 at idle
 
I ended up using some metering rods from an old Carter Strip Kit that I have . It was trial and hair pull...lol
Have you plotted out your mechanical advance curve ?
 
fez440 said:
Distributor does have a vacuum can. I dont use per recommendation on ignition company. Fuel psi is at 5.5 at idle
Click to expand...
Oh my bad I thought it was not a vacuum advance and that's what you were saying.. I personally don't use one with a vacuum advance at all...
 
448Scamp said:
I wonder myself. It probably should have a working VA. Did the OP mention who the ignition company is?
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It's a rev n nator
They say
Q: Do you recommend Vacuum Advance?

A: No We do not recommend vacuum advance, but in some applications it's fine. You need to take the time to make adjustments to your distributor, if needed.

The reason we do not like vacuum advance is because our box has a digital type flywheel built in. This helps smooth out the timing, and it does not like all the movement and "chatter" vacuum advance causes. Some cars may seem to run better with their vacuum advance hooked up because they do not have enough initial timing in their car. If you have a 318 or a low performance motor, it can benefit from vacuum advance at part throttle. However, If you set your timing up with enough initial timing and the correct total timing, your car will run great without VA and get the best gas mileage and performance that can be expected out of these older cars.

Example: How we set up our customers' timing: Approx. 18-22* initial timing and most motors like between 32-38 total, depending on your engine (with no vacuum advance). This is a combination that works well for the R/T Garage as well as many engine builders. Our customers are very happy when we perform this type of tune up or they follow these steps.
 
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