[Tom Vaught]

Originally Posted by 25stevem

Why, one AFB on a single quad intake would get lonely and refuse to run right? I don’t follow!

You asked for help understanding carburetors and posted the comment above.
I recommended talking to Jon Hargrove and others with knowledge of dual AFBs on Pontiac Intakes.

So now you are coming after me again (even though you have minimal knowledge of carburetors and how they work).

You are just "Just stirring the Chit". Please go back to grinding holes in people's intake runners on their cast iron heads.
And leave the carb questions to people who know about carburetors.

Just a suggestion, nothing more. This thread is about carburetors.

Tom V.

[25stevem]

Ha ha!

[61-63]

Originally Posted by 25stevem

Why, one AFB on a single quad intake would get lonely and refuse to run right?

I don’t follow!

I like AFBs, dual or single, because they are simple carbs to calibrate and tune unlike Holleys and Holley knockoffs. The go back and forth in this entire thread has been repeated I don't know how many times by folks having their problems getting a Holley dialed in. I'm not knocking a Holley carb. Is it a great carb and can make an engine run like heck, but it can be a real pita to get it there. AFBs not so much.

25stevenm my comment was not in response to your post that preceded mine but a tongue in cheek response to this entire thread.

["QUICK-SILVER"]

Seen several Holley carbs that the throttle lever was loose on the throttle shaft. They would have an off idle bog from the spray pump shot landing on nearly closed throttle blades.

If you ain't looking for it, you may not notice it.

Clay

[Tom Vaught]

Originally Posted by 61-63

I like AFBs, dual or single, because they are simple carbs to calibrate and tune unlike Holleys and Holley knockoffs. The go back and forth in this entire thread has been repeated I don't know how many times by folks having their problems getting a Holley dialed in. I'm not knocking a Holley carb. Is it a great carb and can make an engine run like heck, but it can be a real pita to get it there. AFBs not so much.

25stevenm my comment was not in response to your post that preceded mine but a tongue in cheek response to this entire thread.

I ran Rochester "Tri-Power" carbs from March 1965 on.
I ran Carter AFBs in College. A lot more of them out there in the early 70s and if someone stole the carb / carbs off the engine they were easy to replace.

I had little experience with Holley carbs before the mid 1070s.
I was never a fan of the spreadbore carbs BUT with the right tune-up they worked fine.
Then I was hired by Holley Engineering. Amazing what you can learn when you work with the things daily
and after reading Urich's books and having access to the Holley Carb MASTER BUILD SHEETS.

I provided the MASTER CARB BUILD SHEETS for the Pontiac Tri-Power carbs to the people on the board when RP went under.

Am I a Holley Carb "Lover"? Not really. But some like to "Label" people when they do not actually know the people at all.
I just worked there for a period of time.

And some on here like to stir the chit so people will think they know stuff about engines and carbs.

The key is actually do the job for a period of time and then go from there.

People who grind holes in people's cylinder heads like Tony Sizemore and the Flooper and a current member really do not know when to stop with the grinder and call it good.
A member just had to keep grinding to get 2 more cfm and ruined a good set of heads a friend of mine owned.

Don't let some people LEARN on your Stuff. Whether it be carb tuning, head porting, or engine modifications.

Tom V.

[Holeshot71]

Originally Posted by Tom Vaught

Either is a part of the same enrichment circuit. Could be either or both.
I would try a Swap of the 30cc front pump & parts with the rear 50cc pump & parts.
If it had TWO 30cc pumps on your carb, would try opening up the Pump Squirter holes by .004 (each hole).
If you have one 30cc pump and a 50cc pump I would do the .004 (each hole) on the Primary first and then
if it needed more pump shot, swap the front pump 30cc parts with the rear 50cc pump parts and try it again.

Tom V.

The pump number is based on 10 strokes so a 30cc pump actually sends 3 ccs of fuel per pump shot and the
50cc pump actually sends 5 ccs of fuel per pump shot

Have to agree with Tom here. I have a 750 DP that I run on my 400 from time to time. Had the same issue with a flat spot just off idle when you take off. I tried Larger squirters all the way up to a 40 with a hollow screw. Tried a couple different pump cams, made sure the pump was adjusted with no play/not bottoming out etc. Also tried changing the PV to a 10.5 so the power enrichment would come on quicker. Richened up the idle etc. All of these helped but still had a hesitation when I jumped on it aggressively. Swapped in a 50cc pump on the primaries and the flat spot was gone. Went back to the 6.5 PV and a 31 squirter and it runs great. I may even try a 28 squirter the next time I run it. My guess is these Pontiacs have really good low lift flow and when you crack the throttle quickly you need to supply enough fuel for the huge volume of air.

[Tom Vaught]

Careful Holeshot71, several of the young and not so young "Carb Experts" on the board
will blast you with a Thunderbolt for agreeing with me on carb tuning.

Tom V.

[West Coast GTO]

Did you adjust the secondaries to square up the transition slot on your carb and if so, how did you do it? Adjustment screw or bending the linkage rod?

Originally Posted by chiphead

I had gotten the plugs to clean up fairly well. I'm running 16* base timing with a slow curve to 35* at 3500 RPM. Another 10* of vacuum advance. The behavior during accel or cruise seems pretty good. Keep in mind I've hardly gotten into the secondaries. Even though 2.56 gears causes the converter to flash pretty hard, I don't want to lug the motor at WOT. (Yes, a gear swap is pending)

I pulled the carb off tonight and flipped it over. Looks like I had the throttle plate settings messed up. The primaries were not showing any transition slot, I had the primaries closed too far. The secondaries were about to expose their transition slot. So I had it idling almost totally on the secondaries.

I opened the primaries until the transition slot is "square" when on the curb idle screw. Look at the pics, is this how the primary should be set at curb idle? I took these pics after cranking open the curb idle screw about one full turn.

How far should I close the secondaries?

[Tom Vaught]

No need to square up the secondary throttle blades.
Just don't have them touch the rear carb walls. (.005/.010 air gap on the secondaries)
typically gives you a nice idle on a street car.

Tom V.

[West Coast GTO]

I didn't mean to highjack this thread so also started this question on my thread.
Tom V: Thank you for the response, this is what I will do.

[West Coast GTO]

If you adjust the primaries via idle adjustment screw, how do you adjust idle down if engine is running above 1100 rpm at idle? With engine warmed, adjusted four corner idle screws for best vacuum, then have to unscrew main idle to get below 1000 rpm. That pretty much covers the transfer slot.

Again, sorry to highjack this post.

Originally Posted by Tom Vaught

No need to square up the secondary throttle blades.
Just don't have them touch the rear carb walls. (.005/.010 air gap on the secondaries)
typically gives you a nice idle on a street car.

Tom V.

[TCSGTO]

Originally Posted by West Coast GTO

If you adjust the primaries via idle adjustment screw, how do you adjust idle down if engine is running above 1100 rpm at idle? With engine warmed, adjusted four corner idle screws for best vacuum, then have to unscrew main idle to get below 1000 rpm. That pretty much covers the transfer slot.

Again, sorry to highjack this post.

Not being able to get the idle speed down when warm could be 1 of 3 things
1-Secondary throttle plates open too far
2-Initial timing too far advanced or too much vac advance if hooked directly to manifold vacuum.
3-Weak advance springs in the distributor causing centrifugal advance to come in too soon.

To add 1 more thing, having the secondary plates open too far can also cause an off idle stumble. Shouldn’t need to set them more than around 1/4 turn from closed.

[West Coast GTO]

Thank you,
Secondary plates set at .005 open and transfer slot covered.
2. Initial timing at 13 degrees BTDC ( a great improvement over the previous 18-20 and mechanical advance only.
Will try stronger advance springs if not already installed.
I am going to replace the MSD race dizzy with a Davis HEI.

Originally Posted by TCSGTO

Not being able to get the idle speed down when warm could be 1 of 3 things
1-Secondary throttle plates open too far
2-Initial timing too far advanced or too much vac advance if hooked directly to manifold vacuum.
3-Weak advance springs in the distributor causing centrifugal advance to come in too soon.

To add 1 more thing, having the secondary plates open too far can also cause an off idle stumble. Shouldn’t need to set them more than around 1/4 turn from closed.

[Tom Vaught]

Sounds like it is all coming together for you. Good Job.

Tom V.

[chiphead]

I installed one of the adjustable secondary pump cam brackets.

https://www.jegs.com/i/Quick+Fuel/79...SABEgLfGvD_BwE

That lets you adjust the secondary throttle plate opening without having to get on that upside-down throttle stop screw. The tall screw hits the choke housing, you'll have to cut the screw shorter. Square up the primary transition slot, set the metering screws, then dial in your hot curb idle on the secondaries.

Here's my latest: I went with the 218 white pump cam that gives a big shot right off idle. I enlarged the primary shooter from .031 to .033". Finally got to test drive it, after fixing a few other things. Now I think it's getting slight too much fuel when heat soaked. Sometimes the response is perfect, sometimes it blubbers when I step on it. I've got a set of .025" and .028" shooters I can try with the same larger pump cam, to see if that clears it up when the engine is heat soaked.

[Tom Vaught]

Looks like it will work well.

I just replace the "flat blade screw" with a "Allen Head" screw you can adjust from the top side of the installed carb. Whatever works best for you.

Tom V.

[chuckies76ta]

Originally Posted by chiphead

I had gotten the plugs to clean up fairly well. I'm running 16* base timing with a slow curve to 35* at 3500 RPM. Another 10* of vacuum advance. The behavior during accel or cruise seems pretty good. Keep in mind I've hardly gotten into the secondaries. Even though 2.56 gears causes the converter to flash pretty hard, I don't want to lug the motor at WOT. (Yes, a gear swap is pending)

I pulled the carb off tonight and flipped it over. Looks like I had the throttle plate settings messed up. The primaries were not showing any transition slot, I had the primaries closed too far. The secondaries were about to expose their transition slot. So I had it idling almost totally on the secondaries.

I opened the primaries until the transition slot is "square" when on the curb idle screw. Look at the pics, is this how the primary should be set at curb idle? I took these pics after cranking open the curb idle screw about one full turn.

How far should I close the secondaries?

I'm just wondering about timing as was mentioned by TA Man. Post #34

Is all in by 3500 rpm kinda high? Also curious about eliminating vacuum advance for testing purpose.

[Tom Vaught]

Vacuum Advance, as explained to me, can be a benefit for Fuel Economy.

If you are running a good Mechanical Advance curve in your distributor and not really
concerned about that extra fuel mileage, do not see the need to have it installed.

Course the mechanical advance should be correct for your engine timing.

Tom V.

[chiphead]

I slowed the curve down to add less timing below 3000 RPM. The current rear gears are 2.56, so it tends to lug around town at 1800 RPM. Didn't want to be loading it heavy at low RPMs with a lot of timing. Once my 3.25s go in, I figure I can quicken the curve and get a better tune on the carb.
I pulled one plug tonight and it's still too dark. It's running good but the porcelain is only white near the end. Might need to quicken the curve to clean that up?? Right now the timing is 16* base, 19* of mech timing by 3500 and 12* of vacuum advance.

[ta man]

As you are tuning are you doing minor adjustments on the idle mixture? Finding the sweet spot for idle mixture will help keep the plugs clean..once you go too lean you will feel a lean surge.