AFB's: Stutters on acceleration

[Topcat]

Took both my Hemi carbs apart today. Flushed everywhere with carb spray cleaner, then compressed air.
Nice and clean inside and out.

Then I took all 8 plugs out. Inspected them.
One was a bit fouled. Cleaned them and light sand paper on the electrodes making sure they'd all fire well again.

She fired right up and idles fine and then took her out for a drive.
Right when I easy get on it, it has a stutter then it clears right up once I get steady speed going.

Sounds like a circuit issue?

Do I need to change the accelerator pump rods to a different position?
They're in the middle right now of the three holes available.

[cudabob496]

well, if there were no problems before the cleaning, then
I would not change a thing, and see if cleaning caused any
new problem.

assuming its not a plug wire is not on all the way. ie, its not an
electrical problem?

[Topcat]

well, if there were no problems before the cleaning, then
I would not change a thing, and see if cleaning caused any
new problem.

assuming its not a plug wire is not on all the way. ie, its not an
electrical problem?

Like I said, she idles fine. Went to the back and listened carefully.
She is hitting on all 8.

Now before anybody goes saying go get different carbs, it's not all that easy.

It would run me well over 2 grand to convert everything to the marine intake, carbs, air cleaner base plate, linkage etc.

[Topcat]

Running a brand new re curved to engine specs FBO distibutor and ignition system with firecore 80 wires.
Can't get any better than that.

[tman]

Would not hurt adjusting pump rod.  One or the other positions will improve or worsen condition.  I would suspect the accel pump would need to be changed out based on your symptoms if the adjustment does not work.

[Bullitt-]

There should be a measurement of how far above the carb top the accelerator pump extends, changing the hole the pump linkage is in will effect this necessitating bending the linkage to get the measurement correct again. If you don't you chance shortening the travel of the pump reducing the fuel shot.   1/2" according to http://www.carburetor-blog.com/manuals/carter_afb_rebuild_manual.pdf
New Viton pump might be the ticket http://www.ebay.com/itm/Edelbrock-Carter-Weber-AFB-Carburetor-High-Performance-Accelerator-Pump-1470-/191524140629?hash=item2c97ba7655&vxp=mtr

Might look at your transition slot exposure measurement,no more than 0.040 inch, not covered in the rebuild sheets I've seen. If the idle adjustment opens the primaries to far it it exposes to much of transition slots creating issues getting off the idle circuit.  covered in this article http://www.superchevy.com/how-to/45638/ 
I found a slight adjustment in timing allowed for closing the primaries to get correct. 

View from the bottom

[Topcat]

Wade,

I think you hit the nail on the head.
This is got to be my problem(s) why it stumbles/stutters.

Thanks.   

[Bullitt-]

Wade,

I think you hit the nail on the head.
This is got to be my problem(s) why it stumbles/stutters.

Thanks.   

  I hope it's that simple.... Let us know what you find. 

[Topcat]

I picked up a set of the ethanol resistant accelorator pumps for the AFB's.

I'm throwing in the towel on trying to get the dual carbs synced and running flawlessly.
My last drive I started fouling plugs 25 miles into driving.

I was given a referral for a shop near me to get them jetted and needles set up correct.
Place called RJT automotive in Fremont. Roger has a good reputation on getting the carbs running right.

[Chryco Psycho]

You have check balls in place under the squirters I assume , it seems the pump shot is not working with minimal throttle movement , you need to have an immediate shot of fuel even with minimal throttle shaft movement , with Holley type carbs you can change the pump cam so the accelerator pump moves more relative to throttle shaft movement , this is not possible with AFB/AVS carbs

[Topcat]

What I need to learn is jet sizes and needle sizes and what go where and for what.
Add to that an exhaust sniffer or O2 sensor to see results when you've made changes.

Basically, I'm blind.   

Tuning dual 4 barrels to aftermarket cam is a bit more complicated than a single 4 barrel.
When, what, and how does the primary carb begin and then transition to the next secondary carb for best power band is what I am struggling with.

I bought new ethanol resistant plungers.

4 jets and 4 needles to get figured out front and back.
Idle, throttle position...it goes on and on.

A single 4 barrel is way more simple to do.

[Chryco Psycho]

It sounds like jetting is close if the plugs are looking good & staying clean but ethanol fuels complicate the readings .
 O2 sensors are definatly a better / more accurate way to be sure .
 Basically the jetting is a smaller  step while rods make larger changes changes , there should be charts showing the step by step changes , this is again why I like Holley better , 1 number is the next step not a combination of changing both jets & rods everytime you want to make one step .
 The float level makes a big difference as well , the factory spec may not work , extra fuel pressure raise the level , low pressure reduces the level , if the level is low this will cause lazy reaction from the accelerator pump so simply raising the float levels 1/8 " may make a big difference .
 the other issue is the bleeds , if the bleeds are too large responce becomes lazy , Holley allows change , AFBs do not , using a longer duration cam reduces vacuum so the so the bleeds need to be smaller , you can accomplish this by inserting a small wire into the bleed & twist the wire so the wire cannot fall out so I would try this before you go to a "Pro" , most pros will miss this .

[Topcat]

It sounds like jetting is close if the plugs are looking good & staying clean but ethanol fuels complicate the readings .
 O2 sensors are definatly a better / more accurate way to be sure .
 Basically the jetting is a smaller  step while rods make larger changes changes , there should be charts showing the step by step changes , this is again why I like Holley better , 1 number is the next step not a combination of changing both jets & rods everytime you want to make one step .
 The float level makes a big difference as well , the factory spec may not work , extra fuel pressure raise the level , low pressure reduces the level , if the level is low this will cause lazy reaction from the accelerator pump so simply raising the float levels 1/8 " may make a big difference .
 the other issue is the bleeds , if the bleeds are too large responce becomes lazy , Holley allows change , AFBs do not , using a longer duration cam reduces vacuum so the so the bleeds need to be smaller , you can accomplish this by inserting a small wire into the bleed & twist the wire so the wire cannot fall out so I would try this before you go to a "Pro" , most pros will miss this .

It fires right up, then after about a half hour or more later cruising around, the plugs begin fouling.
34 degrees total with a FBO re curved dist. 10-12 degrees initial.

I'll let him know about the bleeds.
This guy is supposed to be really good and I just don't have the time for trial and error as I wish I could deal with.

I don't believe the cam is that radical for a stroked Hemi.

[Chryco Psycho]

I don't have the factory street hemi grind specs handy but I am sure I can find them on the internet so I will look but I would think that factory duration @ .050 will be more like 230* + in 71 they used a mild hyd cam , not even a solid grind anymore so those carbs will be tweaked for a very different manifold vacuum than what you have .
 As far as driving with the 2 carbs you want both to be fully open & WOT but you want the second carb to come in as late as possible so you should  have 25-30% rear carb opening before the front carb does anything more than contribute to the idle circuit , this is all done with the different leverage points of the linkage on the throttle shaft levers .
 I feel you could use more initial timing closer to 16* with 34 total as well , so if the plugs are fouling you need either a hotter plug or less fuel to clean it up

[Chryco Psycho]

.467" Int, .473" Exh (1966-1969)
.490" Int, .481" Exh (1970-1971)

276 deg (1966-1967)
284 deg (1968-1971)
 these may or may not be correct & the duration is advertised measured @ .006 lift not .050 lift

 Larry Shepards info is different
.467 /.473 lift with 276* ad  duration  for 66-69 solid lifter being you cam is solid lifter this is a closer comparison but I would bet the duration @.050 is around 230 not 250 that you have .
.484 /.475 lift with 284* ad  duration  for 70-71 hyd lifter
 
So you have significantly more lift & duration as well , the lift realy affects nothing but adding 20* duration & increased overlap with significantly change manifold vacuum , the carbs are designed to work with say 16" of vacuum possibly more at idle & now you  have maybe 12"so thereis less vacuum to pull in fuel from the carb especially as you transition from idle to fuel feed through the boosters , this is where the bleeds are important allowing lower vacuum pull to still draw in fuel so by making the bleeds smaller less vacuum has more effect drawing in fuel .

Too bad I am not closer