Hemi AFB accelerator pump issue - initial bog on opening throttle

[AR67GTX]

The manifold vacuum advance does increase total timing at idle and idle speed. But I can’t drive (or probably start) the engine with 25 degrees base timing. My total base and mech advance would be 43 degrees. I would have to disassemble the distributor and carb again to do this and maybe find something more than the 91 non-ethanol premium in it now. For whatever reason it seems to want more pump stroke, at least on the back carb.

I plan to experiment a little more with the rod springs and see if it will take a bit softer one OK. Since I only had one set of each I had to order another kit. It has the stiffest ones in it now that which makes it transition to the power range quicker. So if I go to the next softer spring it will go rich a bit later.

 

[Geoff 2]

AR,
That is why I suggested a test drive, not permanent. Adding 10* at idle by this method would simulate what adding 10* with MVA would do. You only have to punch the throttle to see if this mod works, not take the engine to high rpms.
This is definitely a lean-ness problem, for whatever reason. I think softer springs would make it worse.

 

[Dragon Slayer]

From my perspective you rolling along at 10, so your on the transition and major contributor is idle circuit. The power valve (meter rod) are not in play. There is plenty of fuel for the idle circuit and primary main when activated.

Your timing doesn't sound too bad other than maybe a little slow to get to peak mechanical.

So either your lean in idle, or main primary doesn't start soon enough (not really sure about this).

An air fuel meter might really help pinpoint this. Be interesting to know.

66-67 carbs had the richest idle jets. In 68 with cap rear idle jet went smaller. Shooter house and pump never changed other than 68 up rear going larger housing as discussed.

Your way over on the acceleration circuit richness. So this all points back to the economizer circuit being clogged enough to hurt the transisiton mixture? Or ignition off timing at this transition only.

 

[AR67GTX]

At this point I plan to do a couple more adjustments but pretty much stick with what seems to be working. I’m going to leave the rod in the inner AP hole but try to go back to the .033 shooter as the fuel coming out of the .043 is a big stream but kind of lazy. Then I’ll pull my distributor and change it back to 16 degrees mech advance and move base timing up to 17 degrees. If it’s a little soft on opening the throttle, I’ll try putting the rod in the inner hole on the front carb.

I got a pin gage set with a .049” pin so I’ll double check the economizer path but all of that has been probed and cleaned throughly so I’m not expecting to find anything there.

thanks

 

[Dragon Slayer]

I saw the measuremeant of your accelerator pump overall lenght. What is the collapse length if you have it? Curious.

 

[AR67GTX]

I have a spare and I’ll measure it this morning.

George - the spare measures 2.25” expanded and 1.977” collapsed. I measured one of the installed ones a couple weeks ago and got 2.28” overall extended. I don’t have a delay spring installed on the spare so I may have squeezed the length slightly with my caliper in the extended length measurement.

 

[Geoff 2]

Any progress?

 

[AR67GTX]

Any progress?

Well, I’m not sure. I finally got the economizer passages opened back up to .049” per Dragon Slayer and I put the .033” AP shooter back in the rear carb and set the pump height for the rod on the inside hole and put everything back together. I noticed then that my secondary linkage on the rear carb was out of adjustment and not fully opening so I fixed that - the front carb was fine. I also noticed when putting the carbs back on the engine, that after about a half dozen removals/replacements, my carb progressive linkage had gotten way out of adjustment and the front carb was being delayed/not fully opening. I’ve adjusted it in the past when installing the carbs but one end free swivels (no left hand thread lock nut) and I guess it unwound a couple times handling it. I also dialed back the centrifugal advance in the distributor by 2 degrees and set my initial timing up slightly to 16 degrees.

Anyway, we’ve had bad weather and then I threw my back out for the first time in a year, etc. I finally drove it between rain showers a couple days ago and it seemed to be back to a little brief hesitation or bog on opening the throttle from 10 mph w/o the big .043 squirter - kind of like kicking down an auto at low speed, into first gear - a brief hesitation while the transmission bands and clutches are doing their thing. But once it recovered it did seem to pull harder than ever and broke the tires loose until I let off so the adjustments may have helped a little. The streets looked pretty dry after overnight showers but probably had some moisture in the surface. Doing the same in 2nd gear from about 1200 rpm it seemed to respond pretty well.

So, I’m not sure. I need to drive it a little more to really see how it’s doing. I have a have time believing I need to run a .043 AP squirter in it. I may try putting the linkage on the front carb in the inner hole also and see what it does. It’s not bad, the car is definitely driveable and pulls like a jet as the revs climb. I’m not a drag racer anyway and other than this testing I’ll probably never do a banzai launch in it on the street. Most of my playing around involves an occasional 2nd and/or 3rd gear pull on an open road.

 

[Dragon Slayer]

Interesting enough while looking for info on an unrelated topic I ran into a 1963 article on avanti with carter AFB. They had an issue with the secondary air valve opening too soon above 1500rpm when throttles wide open. Caused a lean flat spot. The secondary venturi have Secondary discharge port under the main venturi in the body. These act like transition ports to add fuel as the airvalve opens until the main secondary circuit is on line. I guess the avanti had undersized ports and wrong air valve.

So making sure the secondary air port is open and bleed too, plus do you have the right air valve in the carbs. The front has blades stamped 198 and a U shaped cutout under the venturi strut. The rear has flat blade no U and stamped 222. One venturi in the rear carb will not have the discharge port. Distribution reasons I believe. Maybe you have air valves in wrong spot and the rear opens too soon.

 

[AR67GTX]

I’ve kind of wondered about that. Are those part numbers accessible on the assembled

Interesting enough while looking for info on an unrelated topic I ran into a 1963 article on avanti with carter AFB. They had an issue with the secondary air valve opening too soon above 1500rpm when throttles wide open. Caused a lean flat spot. The secondary venturi have Secondary discharge port under the main venturi in the body. These act like transition ports to add fuel as the airvalve opens until the main secondary circuit is on line. I guess the avanti had undersized ports and wrong air valve.

So making sure the secondary air port is open and bleed too, plus do you have the right air valve in the carbs. The front has blades stamped 198 and a U shaped cutout under the venturi strut. The rear has flat blade no U and stamped 222. One venturi in the rear carb will not have the discharge port. Distribution reasons I believe. Maybe you have air valves in wrong spot and the rear opens too soon.

I have wondered about that but didn’t have any part numbers to check. Are the numbers only visible with a tear down? The front and rear counterweights are different shaped and I’ve been careful to keep them with their respective carb. But someone in the past could have got them mixed up.

 

[Dragon Slayer]

Yes should be able to see. Here is front air valve with U slots on blades.

 

[Geoff 2]

Jon Hardgrove of the Carb Shop in Eldon Missouri will probably have the #s for the correct velocity valves. He bought all the old Carter parts & documentation. There were a few different VVs, might be listed on his weebsite.

 

[AR67GTX]

Yes should be able to see. Here is front air valve with U slots on blades.

View attachment 1438155

George - mine match up to that. I have a 222 in the rear with no cutouts and a 198 in the front with u cutouts. I haven’t noticed anything on them in the past to indicate anyone might have tried to modify them.

thanks

 

[Dragon Slayer]

Jon Hardgrove of the Carb Shop in Eldon Missouri will probably have the #s for the correct velocity valves. He bought all the old Carter parts & documentation. There were a few different VVs, might be listed on his weebsite.

I know what they are didn't you read the post?? Hardgrove doesn't answer questions or respond to calls from what I can see. Just wants to sell kit.

 

[RJRENTON]

IMO...the rearward oriented slots in the secondary velocity valves shown are clearance slots for the fuel feed tube located on the secondary booster venturii. These auxiliary fuel feed tube act like an accelerator fuel feed, that when exposed to manifold vacuum feed a metered amount or fuel while the air flow thru the secondary circuit is just starting to operate. As the air flow increases and the velocity valve start to open, the calibrated auxiliary fuel feed tube add fuel to the air stream (secondary butterfly valves open, velocity valves starting to open), As the velocitiy valves approach full open, the main secondary booster venturi are feeding fuel and the auxiliary fuel feed tube will act like air bleeds to the main fuel flow in the secondary booster venturi, due to differential pressure. The fundamental way to adjust the opening characteristics of the velocity valves is to drill the counterweight to lighten them, making them lighter for faster response: conversely, adding wt will delay the opening of the velocity valves....subtle tuning techniques...
BOB RENTON

 

[Dragon Slayer]

Bob they are clearance for the strut. The Secondary Discharge are offset on the side. The rear carb AV does not have the slot, just flat cut. Basically the valve shape, size and weights are different depending on the carb. It also controls distribution in my opinion. No need to mess with them in a stock hemi with stock manifold. Just make sure the right ones installed.

Your description is a little off as the SDC is not part of the main. Only the carbs with AV have this, early AFB with side mount secondaries did not. The AV is blocking/disrupting flow for the main circuit, so you need this feed to prevent going lean. Just read what I said a few post up. The anti perc bleed and the High Speed bleed are the air source for secondary main.

 

[RJRENTON]

Bob they are clearance for the strut. The Secondary Discharge are offset on the side. The rear carb AV does not have the slot, just flat cut. Basically the valve shape, size and weights are different depending on the carb. It also controls distribution in my opinion. No need to mess with them in a stock hemi with stock manifold. Just make sure the right ones installed.

Your description is a little off as the SDC is not part of the main. Only the carbs with AV have this, early AFB with side mount secondaries did not. The AV is blocking/disrupting flow for the main circuit, so you need this feed to prevent going lean. Just read what I said a few post up. The anti perc bleed and the High Speed bleed are the air source for secondary main.

I disagree....in an AFB CARB (subject of the discussion), the slots in secondary velocity valve assembly is for the clearance for the calibrated, pressed in fuel delivery tubes, attached to the bottom of the individual secondary booster venturii assemblies (bottom side of the strut), which l refered to as "secondary acceleration fuel delivery system". IF, in your pix of the disassembled AFB carb, you would turn over the secondary booster venturii assemblies, the small calibrated fuel feed tubes would be evident. This is the reason for the clearance slots in the velocity valve assembly. The secondary velocity valves are to control the air flow thru the secondary throttle bores and, likely, help with the overall air/fuel distribution pattern, when the counter weighted velocity valves and secondary throttle plates are open.
YES...the velocity valve assembly is unique to the carb and engine....most applications use the velocity valve, some engines do not.
IF, the secondary throttle plates are opened, WITHOUT the velocity valve present, a large bog or flat spot in the acceleration curve would result, b4 the secondary booster venturii assemblies start feeding fuel, similar to an inoperative or weak primary accelerator pump system. The secondary fuel delivery system is elegantly simple and is static, not including the counter weighted velocity valves, unlike the primary fuel delivery system which uses a "mechanical" pump in the carb. The secondary "accelerating fuel feed system" works on a combination of velocity pressure (air flow) and differential pressure (fuel flow) thru the secondary booster venturii assemblies. GM has successfully used this principle for years on the Q-Jet carb, as a pull-over fuel feed delivery system on the secondary circuit of the carb for when the air valve starts to open as well as controlled air valve opening rate.
I've "tuned" the AFB carb for years (Hemi as well as other Mopar applications and others) by varying the velocity valve counter weights,when present (both by lightening and adding weight) and mods to the secondary booster venturii assemblies fuel delivery orifices, to achieve the end result desired. I understand very well how the AFB carb operates. Just my opinion of course....
BOB RENTON

 

[AR67GTX]

At this point. - this is a completely stock application. The engine is a bit of a mix of various year parts but as far as I can determine it’s all factory street Hemi. The cam is possibly not Mopar but my guess is it’s the factory mechanical cam - idles down, a little lope at idle but not much, pulls away smooth in upper gears from 1000 to 1100 rpm, has about 14 - 15 inches vacuum at idle. I’m not wanting to do any mods to the carbs - just get them performing as well as possible. Possibly I need to pull the secondary Venturi boosters again and do a more aggressive cleaning out of the bleeds and fuel passages. I ran bits or pins into the passages and flushed them out but didn’t try to gage them tightly. They are open but might be constricted with some hard deposits.

 

[Dragon Slayer]

Ok Bob your wrong. First, the subject is not AFB but Hemi AFB specifically 66-67. There is no secondary discharge pickup tube. Just a hole in venturi and it is offset to the side. NO WHERE near the center strut, and the strut does not have a strut bleed. Only carbs I have seen with that were Cadilliac AFB. Secondly, the rear carb does not have the cut out on AV. Besides any tubes like that are in the fuel well of the body. So it is not a cutout for the Secondary discharge. It is centered under main strut for main feed. I am sure it allow main fuel to have a clear path into the air stream, but not all AV had it.

Yes the air valve is like vacuum secondary on a holley in that it does not open until demand from the air flow and DP. Difference is it is another throttle valve in the flow. With an offset angle blade.

Here is a hemi secondary venturi. Just the main pickup tube. Notice the discharge port on the wall of the venturi way off center. With the large pickup hole on the opposite side. The smaller hole is actually covered by a lack of transfer hole in body of the carb. Also as I stated earlier it is a separate circuit from the main, so it does not act as an air bleed one main is on. They are not connected. Plus only one venturi on the rear hemi has the discharge. They other does not. Solid. Again distribution issues. The front does have it on both.

I have made it a hobby to study the AFB especially Hemi. So I am not speculating. You may have tuned but did you test on a dyno and measure AF ratio cylinder to cylinder. Chrsyler did. It was all about distribution improvements as they moved from 66 to 71. Also, 68-71 are different. They are AFBs that actually have a secondary idle circuit. Rear carb only.

 

[dvw]

I am amazed what knowledge is being shared here.
Doug