AFB Tuning

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eekvonzipper

eekvonzipper

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Is there a Quick Method to Tune the Transition when you Step-On-It?
Runs Great at Idle, Cruise and W-O-T...
but during Cruise it makes a BlehBleh before it Goes when you mash it.
Choke Fully Open and Warmed Up. I wouldn't mash it otherwise.
It's a Carter 500 on a 273 2bbl engine w/ a Halifax Distrib.
Runs Great other than this One thing...
What is it?

1.jpg
 
If those are like the Edelbrock AFBs, the secondary air door has weights on it. They have to be either trimmed and lightened up, or drilled out and lead weights pressed in to make them heavier. Yeah, it's kinda a pain. That's why the AVS and Thermoquad and Street Demons are all a little easier in that department. The secondary air door is adjusted on the outside of all of them.
 
eekvonzipper said:
If it Opens late... Does it need to be Heavier?
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From your description, it sounds like it has a bog. That's normally opening too soon. How have you determined it's late?
 
eekvonzipper said:
I didn't. I don't know.
I would just go out there and fix it tonight if I did. :D
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Can you explain a little better about what it's doing? I don't do well with sound effects. lol
 
I am not sure if the 500 cfm model has velocity valves [ weights ] in the secondaries. That looks like a factory carb.

What model is it?
Four digit # should be stamped on one of the mounting feet.

Does the carb have a large brass screw in between the idle mixture screws?

I can see one problem: it looks like it has a single plane intake.

Is the engine stock?
 
Before re-jetting, make sure you are getting maximum accelerator squirt, enough ignition advance and vacuum advance is working. Then consider enriching the idle circuit or filing the metering rods.
 
Bewy said:
Does the carb have a large brass screw in between the idle mixture screws?

I can see one problem: it looks like it has a single plane intake.

Is the engine stock?
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Yes. It's a 3853s.
Stock 180hp 2bbl engine (Commando Clone)
Has a Stock Commando Iron Intake and Carb and a Racer Brown Commando Cam (.420 lift I think)
Triple R, it just has a slight hiccup or Hesitation when you ask the secondaries to come in.

carb.JPG
 
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Murray said:
Before re-jetting, make sure you are getting maximum accelerator squirt, enough ignition advance and vacuum advance is working. Then consider enriching the idle circuit or filing the metering rods.
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Nah, Runs Great! Pulls Hard all the way to 5k...
just sometimes has a lil stumble when your doing 30 behind some a-hole on the highway and you jump on it to pass.
 
has about 18-20 initial and low 30's revved up. I think it goes into the 50's with the Advance hooked up to the Ported on the carb
 
Add shooter nozzle volume and see if it gets better. In some applications I opened them up to the size of the tube originally feeding the shooter nozzle to cure a bog when you mash it. You need yo make sure there is enough fuel when the air door decides to open.
 
So, no stumble off idle? Stumble only when the secs kick in? Trying to narrow down exactly where/when the stumble occurs.


Have you checked float level?
Does the acc pump squirt fuel?
Do the secs have vel valves [ they move ]?
 
Bewy said:
So, no stumble off idle? Stumble only when the secs kick in? Trying to narrow down exactly where/when the stumble occurs.


Have you checked float level?
Does the acc pump squirt fuel?
Do the secs have vel valves [ they move ]?
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No off-idle stumble.
Yes, only when secs kick in.
Checked Level? No.
Squirts? Yes
I see a set of weights Waaay down in there, each side of sec butterflies.
Really runs fine, just doesn't Instantly react when you floor it while Cruising.
 
Your problem seems to be in the "transition area", when the carb is going from the idle circuit to the high speed circuit. This is a common area to have a problem with. Follow previous recommendations.
 
Murray said:
Your problem seems to be in the "transition area", when the carb is going from the idle circuit to the high speed circuit. This is a common area to have a problem with. Follow previous recommendations.
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Yeah, That was literally the First line in my First question. Thanks.
 
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Cruising at 30 mph, the transfer slots are wide awake, so they should not be your problem.
And in post 14 you already said that you do not have a tip-in stumble.
Thus, IMO, your stumble has nothing to do with the transfers, and nothing to do with the primaries, unless they are lean..
 
AJ/FormS said:
Cruising at 30 mph, the transfer slots are wide awake, so they should not be your problem.
And in post 14 you already said that you do not have a tip-in stumble.
Thus, IMO, your stumble has nothing to do with the transfers, and nothing to do with the primaries, unless they are lean..
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I think we got that part nailed down, buddy. He's said multiple times his stumble is tipping into the secondaries.
 
eekvonzipper said:
Yeah, That was literally the First line in my First question. Thanks.
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It's beginning to sound to me like maybe you need to either live with it, or if you want to go to the trouble, take it apart and add some weight to the secondary counter weights. Without being there and riding with you, I'd be hard pressed to guess "how much".
 
Here's an article I found claiming to have some "tuning tips" but to me leaves more questions than answers regarding the secondary blades and weights. Although it does say the blade "attack angle" is important, it doesn't get specific at all, so that's like zero help.
 
You need to remove the carb top & check float level.
If you still have a stumble, I would increase the sec jet size by 0.006". The jets are the same as Edel jets, which can be used, but they maybe numbered differently. Use a numbered drill set as go/no-go gauge to see what jet size you have.
 
First you need to know if it is stumbling rich or lean. If you don’t know, you don’t know to add or subtract weight from the counter balance. You can try adding front jet, it will make the cruise rich, but it is only a test to see if it helps the opening. If it does, you are probably lean on opening. On some AFB’s there is a secondary enrichment port that the tip of the butterfly swings by. It pulls a bit of fuel to help with the secondary opening transition. I run an AVS and have the same issue, on the data it spikes lean at the hit. I have all the pump shot and drilled the shooter out a bit and I am definitely using the carb WAY out of its comfort zone. I can only tighten the air door spring so much. With an AFB you just might want to live with it. AVS and you can play around with it pretty easy.
 
- Much easier to change jets than add or remove metal from the velocity valves [ VVs ] ...especially if you have removed too much metal & you have to put the metal back....
- a bog/hesitation on opening the throttle is rarely, if ever, a rich bog.
- there is a low speed or 'starter' port on the secondary side of many AFBs that use VVs. I believe that is what post #24 is referring too.
- having worked on AFBs for decades & I came across something I was unaware of. Y'day I installed a 625 Competition Series AFB #4758 on a 360 engine. I noticed that this carb had an additional port above the low speed port mentioned above. Did not have the carb apart, so not sure if this port was drilled through [ active]. Anybody have any info on this?
 
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