AFB carb problem

I got this modified GMC 302 inline engine,I had a Holley 390 cfm 4 bbl on it.ran quite nicely,no bogs or flat spots.Well,I loaned the Holley to a friend to try on his engine,he liked it and bought if.I wanted to try a different carb anyways,got my hands on a 400 cfm AFB 9400 series 4bbl .I figured it would need some jetting changes,disconected the secondary linkage for testing.Got the primary jetted up real nice,good idle,great throttle response,engine pulls cleanly from idle to redline,impressive for 200 or so CFM.This engine has a water heated intake.
So I reconnect the linkage for the secondaries to sort out the jetting.Hmm,no so ****in good,the typical big bog as ya floor the pedal at about 2500 rpms.Then the engine catches and takes of real strong.I jetted up the secondary about .007,a little better.Then I pulled the counter weighted secondary air flap,welded up metal to make it heavier ans sloer to open so to speak.Again a small improvement,but still the stumble when the throttle is opened suddenly.If you sneak up on full throttle it's almost tolerable.I checked the carb parts numbers of the venturi clusters,air bleeds and so on,all are correct,appear to be unaltered.In fact this carb is almost new actually.
I even tried primary metering rods with way richer power step,no improvemt at full throttle and for sure made the primary too rich when pulling hard at part throttle.
I'm no carburation expert but I've never had problems with getting jetting close enough.It's hard to believe this smallest of AFB's acts like it too large,but the 390 Holley was fine.Maybe it's just the Holley vacuum secondaries just work better? I suppose I can jet the AFB secondaries even richer but experience say if you need to rejet more than 6 or 7 sizes the carb is wrong for the application.Again,400 cfm should be spot on for a 302 GMC with cam ,header,and all that stuff.

 

I know this is a different road, but alot of times a little more timing can help a stumble/flatspot.
Or a vacuum leak you didn't have before?

Frank

 

I checked for leaks and since the engine idles ok and responds to turning the idle mixture screws,must be ok.
The timing curve is fairly aggressive,12 degrees initial,34 total at about 2800 rpm,HEI distributor with a vacuum advance.

 

I got a 235, with alot of speed stuff. When I went to the pair of B rochesters, to get the plugs to look a little colored after a pull, I went up in jets to 72's where it started out w/ 56's.
I was running crazy timing and it wouldn't ping, and now have it at more normal like you have. 16 and 38, with weak springs to get it in low. I also have an HEI w/ vacuum adv.

So maybe you do need richer, but why would you think it's OK on the primary? Maybe the vac signal drops off real low and retards the timing when you floor it, whereas when you ease in, the vac signal keeps the advance a little higher. Sometimes even 2 or 3 degrees more will help, or partially block the vac adv arm to not retard so much when you lose vac signal.

Frank

 

Increase the shooter size, either by drilling or get the edelbrock shooter kit. The correct way to slow the air door opening is by drilling the counterweights and using allen set screws of differing size and weight. The shooter will probably help most.

 

springs...check the springs...may not be letting enough fuel in...creating the lean condition...

 

I have three 302 GMC engines and all use a larger carb than 400cfm .
I use a 650 cfm holley on my model A with a 322 ( over .125 gmc) and it runs very well.
My ha/gr 302 uses two afb 500 cfm four barrells.
The little 500 CFM caddy afb wouldnt ever work on my model a either.
Have you tried different lift springs under the power pistons??

 

One of the best AFB guys on the Planet is a good friend of mine. His custom carbs are on several of the fastest NSS drag cars out there.

He did a white paper for setting up AFB's and put it up on his site a while back. It works like a champ. Read it and if you have any questions shoot him an e-mail. Here is a link to it: http://www.fugly-racing.com/index.php?pid=2 He also did some other papers and they are on his site too.

-Dave

 

GTS Dave.........you know that Damon knows nothing about carbs.....lol

Did you see the comment John Grinwald posted about the carbs that Damon built for the Asphalt Elephant?..........and that wheel stand all the way to half track.........looks like they are working...

I posted up a bit to check on the springs...someone else mentioned them also...most likely that is this guy's problem...seems to fix most Edelbrocks or Carters that we have run across with the same symptons.

Mark

 

Mark,

Saw the comments on the Asphault Elephant! One wild ride for sure. I picked up the Cuda's engine yesterday. I also bought a pair of Damons famous 800's for the Cuda a few months ago. They are right here next to my computer as I type this. The man knows his stuff! There seems to be a lot of that in the circles you run in. hehehehe.

I am thinking this one is a spring problem too.

-Dave

 

Thanks for the suggestions. I did mess with the metering rod springs,I'm using the springs rated for 6 inches of vacuum.I think a larger accelerator pump nozzle will exhaust the pump supply quicker?

The engine for sure seems like it goes lean for a moment when the secondary is opened.But it "sounds" different than other 4 bbl lean outs I 've experienced.it's a studder sound,then the engine grabs hold,almost like a brief sparkplug misfire than the typical "flat" sound of a leanout.My plugs are gapped for .045 with the HEI.This gap worked ok with the Holley.For kicks,I'll gap the plugs back to .035.The ignition seems fine,spiral wound low resistence wires,quality coil,non resistor plugs.The cam I'm using builds a lot of cylinder pressure at mid speeds,low manifold airflow and poor vaporization when the secondaries open,and 9.5 actual compression might be a big load for the ignition to fire?
It runs so strong on the primaries I might be tempted to leave the secondaries disconnected and forget all this tuning headache
Some guys don't like Holleys,I'm thinking the Holley is more forgiving of engine tuning than an AFB type carb.

 

So,I just regapped the plugs closer,took a ride,another small improvement.Then I stuck a thin wire in the secondary venuri cluster air bleed hoping it will create a stronger vac*** signal to the booster and pull fuel earlier.Another small improvement.But the bog is still there to some degree,not too bad in lower gears,but worse in high gear on the hiway.
I have a brand new Holley 500 cfm 2 barrel in the box,maybe I'll swap on the big two barrel and be done with it.It's looking more like this Carter 400 cfm 4 bbl just don't like my engine.............