Technical Dual Quad Gods please help!! Cant burn rubber!

Sounds like too much in the way of carburetion and not enough motor. I'd slap a 4 barrel on it and I'll bet your problems will be over.

 

Edelbrock makes several different "500 cfm AVS" carburetors with different characteristics, they are not all the same. What is the specific model number of the carburetors you are using and trying to tune? It would be a lot easier to diagnose the problems you are having if we knew exactly what carbs are giving you the problem.

 

Depends on the origin. The 340 version was the smallest ..About 625 CFM, then the one Chevy used, a little bigger, and then the big Mopar one @ 750.
Best case , a bit too much carburetion for a mild 350, in my opinion.

BTW, If you wind the flapper spring too tight, it goes into a bind. Just take the secondary link completely off and try it.

 

I am all for simplifying things with one carb but the style points aren’t there which Is a deal breaker for my pal.

The carbs are 500cfm 1803 thunder avs units.

Good idea on the secondary linkage, will add that to list of things to try next time we tinker.

 

I own one of the shops that had no interest in trying to diagnose and tune the clones. I told you that we just won't discuss them, but having read this thread, and seeing that you are attempting to use AVS carbs, will offer a clue.

Have done well over 100 dual quad 350 Chevy set-ups with happy customers. We would have suggested the following:

Mild to moderate street 350 - 2 Carter 9400 (400 CFM type AFB)
Hobby racer with some street, manual transmission - 2 Carter 4758 (500 CFM type AFB)
Serious racer, trailered racecar - 2 Carter 4759 (625 CFM type AFB)

We always used solid linkage, except for professional racers that wanted 2 750 CFM type AFB.

Note the consistant 3 letters of the carburetor type.

As a general rule, the street (2 - 400 CFM) would run well perfectly stock.
As a general rule, the hobby racer would change step-up springs, and maybe mild jet change
The serious racers and the professional would need modifications.

Hopefully, the above clue will help.

Jon.

 

Excuse me. I mistakenly assumed you were using OEM carburetors.
Still, I agree with The King . Still too big on a mild motor.
If you're looking for the wow factor down at the Tastee freeze cruise night, you want the smallest 2x4 set up you can put together, and very lean.
Neither here nor there though. If you're looking for crisp, tire burning response, I'd use a Q-Jet .
Jon would probably say AFB, but either way, you get my drift.

 

I dyno'd my 350 with a vintage dual quad intake and a Howards cam. I think is was getting around 325 HP. I also had 2 comp series carters (500 cfm) progressive linkage. I ended up installing a old street tunnel ram, a pair of center squirter Holleys (And a clay smith cam - around 290 dur) & straight linkage. HP jumped to 415. I've since sold the holleys & dialed in the Carters. It's around 395 horse now. Those low dual plane intakes are no good.

https://www.jalopyjournal.com/forum/media/dyno1.500552/full

https://www.jalopyjournal.com/forum/media/400-horse-350.514424/full

Also, I bring the RPM over 3000 before I load up the motor.

 

I run two 600's (Carter's) on a mild 350, it's not the size of the carbs that is the problem here.
What kind of choke do the carbs have ? Mine are manual choke which I wired wide open, so no choke effectively.
What kind of air cleaners are you running and did you try any acceleration experiments without the air cleaner or air cleaner spacer if any ?
I run my carbs one to one on linkage, without changes to the secondary linkage, which I don't believe is your issue either.
I don't run much stall converter so I don't think that's your issue.
If your choke, air cleaner, fuel pressure at 5.5 isn't it I would lean towards the ignition since your metering rods, springs and jets showed little effect. I run points and MSD without much difference on the street, really sounds like your advance is lazy. Have you checked ignition voltage from idle to 3k to see if there's a change ?
Here's the mild 350, single points, one to one linkage, as you can see it has no problem spinning the tires.
If you want I can check my notes on the rods and jets.

 

I completely agree, style points are one thing, but if it doesn’t run correctly, what’s the point? I’d put an Edelbrock Performer RPM and a single Holley 3310 on it and be done.


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Taco Bandit:
I ran 2 Carter AFB 500 cfm units on my Chevy 355 in my Coupe for 19 years.... and the 'key' for them being able to work was the 10" TCI torque convertor (3500 RPM) that I used. With a stock convertor it was a pure dog.
For 30+ years I worked for the company that made Carter AFB carbs, and they were "Not" the same as the units made for Edelbrock. Sadly, due to a declining marketplace, Carter exited the carb business several years ago, and finding the 9400 units are ridiculous.

Take the car down the road and hold it a 3000 rpm in 1st gear for a few seconds, then jump on it, click 2nd around 5500 and then look at you readings again. As Mark Yac said (Hi Mark) we didn't have all this Hitech equipment to figure out what was going on, only a tremendous amounts of trial and error. You "may not" be able to achieve what you are looking for in numbers due to the 'design' of the carburetors.
All this is based on all other parameters being correct and operating as they should: Ignition system, correct fuel pressure, no vacuum leaks.

And Jon - Carb King is spot-on (Hi Jon)

 

So you are using AVS's then?
How much is not much stall? Rear end ratio? How much weight?
Please include that info because our o/p friend doesn't know what he has. Thnx

 

OK heres my .02 when running 2x4s many people forget that now you have a 4 barrel feeding 4 cylinders. You need to lean it out a bunch. You should get a reading of 13 to 1 or 13.5 to 1 at idle and low cruise. Your secondary jets should start out at 3-4 sizes lower than stock and work from there. THERE is no MAGIC solution it is all trial and error.
I do know what I am talking about. I run 1200 CFM on a 306 sbf.

 

Taco - one other comment, and others have alluded to possible ignition issues:

A long time ago, we did a dual 400 CFM 350 for a mild street engine, and he had all kinds of issues. I had told him to not use the intake, but he got a great deal on a new one (won't mention the company, but you will know), so went ahead with the deal. The carbs ran rich! I rebuilt those carbs twice more, and finally got him to remove the intake and pressure test it. Found a crack in one of the runners. Had that welded, and replaced the intake. Better, but still no cigar. He was running some wild electronic ignition that I had never heard of, and got him to take it to a shop that understood electronic ignitions. He was running a stock alternator. The ignition shop told him no way would the stock alternator feed that ignition, and sold him a high output alternator.

Problem completely disappeared.

Food for thought.

Jon.

 

Your car sounds great! Would be pumped if this one ran half as good! If you get a chance, would be great to see what you have for carb settings, just for a comparison to see if we are working in the right direction.

I have noticed the Edelbrock Dual quad settings for the 1803/1804 are considerably different than the 1403/1404 carbs.

I forgot how much I dislike trying to decipher someone else’s work. I’m sure the car builders “engine guy” had a plan, just not sure what it was.

 

I would like to rule out ignition. The system has some nice components (not saying fancy works great out of the box, I know it doesn’t). We just pulled the O2 sensor out of the car as he has a car show to hit tomorrow and the O2 sensor was very sooty. Not wet at all, just very black and sooty, like the plugs. I’m sure investigation wouldn’t hurt.

 

You guys work too hard with all that antiquated stuff! I can burn with my modified 263 in a four door sedan. Don't even have to hit WOT.

Ben

 

Ok here's my set up if it helps...When I was building my 29 I wanted an early 60s corvette sbc look, The engine is a 327 ,10 to 1 compression ,30-30 cam . I used a late 50s corvette intake with two Edelbrock 500s, I'm not a big fan of Eddys but wasn't gonna fork out the cash for Vette WCFBs . The chokes are removed , the jetting is stock. The dist. is an old Mallory dual point with a Petronix conversion.16 degrees initial 38 total. Has a muncie 4 spd and 350 gears. From a roll at 3000 it will destroy the tires , granted they are bias plys with about a 7" tread width. The engine pulls hard from 3000 to 6500 .I forgot to say the linkage is repro vette progressive..

 

I've ran 2 different engines with the same intake carbs, the mild 355 in the above video had a stock stall and the other hotter 355 had a 2800 stall from my buddy's big block and it acts like there is no stall at all, pushing through the brakes while waiting on red light.
As far as gears go I've ran both 4.11 and 3.25, car runs the same, so I don't think gear has anything to do with it.

 

AVS's ?
I agree the gear would be secondary to the stall. RPM probably needs to come up some with this particular setup. It would make things a whole lot easier.
Oh yes, the weight of the T- bucket?

 

Are you running a ballast resistor with the msd distributor? If you are, try bypassing it. IIRC, MSD doesn't need one. It's worth a try.
There's something in the back of my head that says this could be an ignition problem.

 

Main jet is .098
Secondary jet is .095
Metering rods are 071 x 047
Lightest spring

Something else to check is your fuel filter and #23 in the exploded view, which are the "fuel strainer screens"

Check to make sure your air filter is not interfering with the choke shaft, that's why I ask if you tried it without air filters or air filter spacers, also check those things I mentioned earlier.

 

Yes check for batt voltage at coil.

 

Interesting. I do wonder how we should relate this to the o/p's friend's situation.
"The engine pulls hard from 3000 to 6500"
We're still waiting to hear if the 37 does the same thing

 

Nope 37 will see at least 7 grand Mark ... Hope to have engine on test stand in a couple of weeks

 

That's why I said to check the voltage earlier, I didn't have a ballast resistor but I had a wire that reduced voltage. When I switched over to a full MSD setup in my 41 it wouldn't start at all due to less than 12 volts trying to fire the MSD box.
A weak connection could also simulate a low voltage drop until you are really spinning the alternator (3k+) which would introduce more voltage. A heated connection point maybe an identifier there, could be a battery cable or a wire in the system, but checking the voltage at idle and at 3k would rule that in or out.

 

Totally agree!!! Good grounds are definitely your friend too!
Engine to chassis, body to chassis.

 

Ok! Here’s the rig in question! No ballast in sight, pretty sure it was a fresh wiring job. Would like to test the coil out put some how....

 

You need to test the coil input, could have resistor wire in there somewhere.
Sweet ride!!! BTW!!!

 

We run a similar set up on my kids 355 SBC, ie edelbrock airgap and avc 500 carbs and progressive linkage. I suggest you do the following:
1. Set the timing curve to 32-35 total all in by 2500. We use a 20 degree bushing to give 12-15 deg initial. Makes it easier to start. MSD will have tables indicating which springs to use.
2. Disconnect the vacuum adv. and set the timing. Reconnect the vacuum hose to the correct port on the carb edel. literature will identify the correct port. Reinstall your O2 sensor. Put in a new set of plugs and go run the car. Check the plugs to see where you are. If you're still having issues it's likely in the carbs.
3. Set the carbs to the original settings or, if possible get edelbrocks dual four settings for 500 cfm carbs as a starting point. Drive the car to see where you are.
4. Start with the primary carb and dial in your cruise. I like to go slow with regard to jet size. We reset the idle air every time we make a change. When your afr at cruise is good then proceed to mid and wot. Remember jet size will determine how much fuel is delivered and the needle and spring when full fuel flow is delivered. This will take time.
It took us about 5 hrs and 2 tuning kits on the dyno to get my kids setup dialed in. It works very well and is crisp.
As an aside I'm planning on running dual fitech fi on my 31 coupe.
I'll let the computer dial it in. I hope this gives you a starting point it's not rocket science just time consuming especially when doing it on the car. There are many ways to approach this. I do it this way because it works for me.