Holley Carb Tuning Question

I am in the process of fine tuning a Holley 670 Street Avenger Carb (electric choke and vacuum secondary) on my 1965 Mustang with a mildly worked 351W and 4 speed toploader in it. Here are some details and what I've done so far. Through some trial and error I have progressed considerably.

-Timing has been set (16 at idle and all in 37 at 2500 rpm)
-6 psi coming from mechanical fuel pump
-Bowls set correctly
-Idle mixture screws set from highest vacuum reading (750 rpm idle)
-I was getting a rich bog at WOT so I put a stiffer spring in the vacuum secondary and that has been fixed
-I had a slight surge at steady cruising speed in 4th gear so I bumped the primary main jets up a few and fixed it
-Accelerator pump arm and cam have been adjusted and work well

2 Questions:

The ONLY noticeable issue I see at this point is when I'm in 3rd cruising at a higher rpm (mostly because a corner might be coming up and I don't want to shift into 4th to seconds later shift back into 3rd) the car has a surge. It feels exactly like what I was experiencing with the lean surge in 4th. I upped the main jets a few and that fixed the 4th gear cruising surge. Would you assume that I need to increase the main jet size even more to get this lean surge out of 3rd gear, assuming that's what it is? It seems like the logical next step but I have to buy every part to experiment so I wanted to see if I could get any opinions before I potentially waste money experimenting.

If I can get this 3rd gear surge fixed and I cannot notice any issues at all, how do you go about getting very detailed with the fine tuning? IF I can find a buddy with some Holley parts kits, most things are easy to put in and undo so it wouldn't be a big deal to try and make it even better and if nothing works go back to what I originally have. Is it going to the drag strip and test and tune? I just wonder if this is the route to go because I'm not experienced and I think my launches are going to be different each time due to wheel spin, shifting, etc so the results might not be accurate. Anything else to give me data to fine tune?

its going to be hard to diagnose without driving but I'm leaning towards float level its to low. you said you adjusted it, at idle with the plugs out of the float bowl did it dribble out the hole? or are you just running out of gas to the carb plugged fuel filter or some restriction along the way as far as jets what do the plugs look like after hard pull the engine doesn't care what gear its in if its good to go in 3out of4 gears there's something else going on, I would look to your timing 16 at idle seems a lot recheck it and make sure your curve is coming in as you want and the total is correct does it ping at all if you lug it? it might need more timing just my 2 cents

Well many years back in the FUN DAYS i worked on many performance Fords . Now now knowing everything about yours it would be all guess work here . (1) i never set curb timing at 16 degrees usually around 8-10 degrees and on a four speed car 38 degrees coming in at 2000 ,BUT there was a curve that i would build into the dist. for the mechanical side and then add in what the vac would also pull on . But after 45 years i can't pull that out of the OLD MEMORY DRIVE . Carb jetting for our area of this country would be some place between 54's to maybe 62's in the primary and on the secondary side 68-78's . Depending on cam's the power valves could range from a 9-10.5 . Staggered jetting would also come into play for fine tuning . Having all the toys o play with was a plus . As the dealership i was working at we had a chassis dyno latest Sun and allied rotunda test equipment . I made the mistake of letting a friend use my note book that i had all this info wrote down in when he was building his 65 Mustang and he and two other guys went out drinking and he got killed in a car wreck , i never saw my note book again . It contained all the info i wrote down on everything for every engine we worked on all the timing curves and what worked and what did not , all carb spec.'s , info from factory engineers and factory test data , part Numbers . i was involved in ford Drag racing since 1964 .

what Power valve ? Throttle and load in 3rd may be putting it on the bubble. Id try PV if jetting is okay in 4th , Might install o2 sensor in exhaust, give you better idea of rich/lean under load.

(quoted from post at 07:46:22 08/27/18) its going to be hard to diagnose without driving but I'm leaning towards float level its to low. you said you adjusted it, at idle with the plugs out of the float bowl did it dribble out the hole? or are you just running out of gas to the carb plugged fuel filter or some restriction along the way as far as jets what do the plugs look like after hard pull the engine doesn't care what gear its in if its good to go in 3out of4 gears there's something else going on, I would look to your timing 16 at idle seems a lot recheck it and make sure your curve is coming in as you want and the total is correct does it ping at all if you lug it? it might need more timing just my 2 cents

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Float levels are right at bottom of sight hole. They are correct. Fuel filter was just changed a couple hundred miles ago. I changed plugs a couple weeks ago and they looked very good. Jets were just installed so I don't think they would be plugged. I thought maybe 3rd gear was putting more of a load on since higher rpm and the jets weren't enough to keep up. It does not ping. The car likes 16. It pings at 18.

(quoted from post at 07:47:25 08/27/18) Well many years back in the FUN DAYS i worked on many performance Fords . Now now knowing everything about yours it would be all guess work here . (1) i never set curb timing at 16 degrees usually around 8-10 degrees and on a four speed car 38 degrees coming in at 2000 ,BUT there was a curve that i would build into the dist. for the mechanical side and then add in what the vac would also pull on . But after 45 years i can't pull that out of the OLD MEMORY DRIVE . Carb jetting for our area of this country would be some place between 54's to maybe 62's in the primary and on the secondary side 68-78's . Depending on cam's the power valves could range from a 9-10.5 . Staggered jetting would also come into play for fine tuning . Having all the toys o play with was a plus . As the dealership i was working at we had a chassis dyno latest Sun and allied rotunda test equipment . I made the mistake of letting a friend use my note book that i had all this info wrote down in when he was building his 65 Mustang and he and two other guys went out drinking and he got killed in a car wreck , i never saw my note book again . It contained all the info i wrote down on everything for every engine we worked on all the timing curves and what worked and what did not , all carb spec.'s , info from factory engineers and factory test data , part Numbers . i was involved in ford Drag racing since 1964 .

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Your info seem to be more in line with a stock engine ??? This car is a hot rod and my adjustments seem to be more in line with what I've read for hot rods.

(quoted from post at 07:57:23 08/27/18) what Power valve ? Throttle and load in 3rd may be putting it on the bubble. Id try PV if jetting is okay in 4th , Might install o2 sensor in exhaust, give you better idea of rich/lean under load.

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I was actually thinking of the PV as well. Before I noticed this I had a vacuum gauge hooked up while I was driving but I didn't see what it was reading when cruising in 3rd gear at a higher rpm. I can check that. I'm only running a 6.5 so I think I should be good though.

I would think a 6.5 p.v. would be ok I run a 600 cfm Holley on a boat with a 2.5 at 1600ft it stumbles a little cold but the back of my boat stays clean since I have through transom exhaust I still think there might be a timing issue could the vacuum advance be bad and is flutting .16 degrees at start seems high I doubt it would start and not kick back how are you checking it vac. advance unplugged ? well good luck let us know what it runs , my son runs a 78 Malibu small block on nitrous runs 9.50 132mph

(quoted from post at 09:15:58 08/27/18) I would think a 6.5 p.v. would be ok I run a 600 cfm Holley on a boat with a 2.5 at 1600ft it stumbles a little cold but the back of my boat stays clean since I have through transom exhaust I still think there might be a timing issue could the vacuum advance be bad and is flutting .16 degrees at start seems high I doubt it would start and not kick back how are you checking it vac. advance unplugged ? well good luck let us know what it runs , my son runs a 78 Malibu small block on nitrous runs 9.50 132mph

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It does not have vacuum advance. Mechanical only.

then maybe there's the problem when and how much timing is coming in. might try lighter springs or just remove one for a test the advance might be bumping back and fourth right at cruse speed

(quoted from post at 09:41:12 08/27/18) then maybe there's the problem when and how much timing is coming in. might try lighter springs or just remove one for a test the advance might be bumping back and fourth right at cruse speed

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ok. I will check this out. Probably won't get a chance to play again until Sunday.

My suggestion would also be late timing. I like to set timing at 36 degrees maximum advance at 1200 rpm. Doesn't matter if you have vacuum or centrifugal advance. I would also can the Holley carburetor, but that is just me.

Are you running the correct distributor?
Is it using venturi vacuum.
Does it even have a vacuum advance system?

(quoted from post at 10:52:28 08/27/18) Are you running the correct distributor?
Is it using venturi vacuum.
Does it even have a vacuum advance system?

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This car was built 10 years ago by someone else, so I don't know. It's been running for 10 years with it though. The distributor does not have vacuum advance.

center or side hung floats in the carburetor? does your issue happen while cornering or after you exit the corner? do you have jet extensions on secondary side?

(quoted from post at 16:13:27 08/27/18)

(quoted from post at 07:57:23 08/27/18) what Power valve ? Throttle and load in 3rd may be putting it on the bubble. Id try PV if jetting is okay in 4th , Might install o2 sensor in exhaust, give you better idea of rich/lean under load.

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I was actually thinking of the PV as well. Before I noticed this I had a vacuum gauge hooked up while I was driving but I didn't see what it was reading when cruising in 3rd gear at a higher rpm. I can check that. I'm only running a 6.5 so I think I should be good though.

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You could easily drop the power valve down to a 45 with no negatives.

(quoted from post at 13:24:35 08/27/18) center or side hung floats in the carburetor? does your issue happen while cornering or after you exit the corner? do you have jet extensions on secondary side?

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Side Hung. Straight driving, no corners. No jet extensions.

You could easily drop the power valve down to a 45 with no negatives.

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I do have a 2.5 PV that I could try just to see if that changes anything. That would rule out PV. Then buy something like a 4.5 if it did fix it.

how many inches of vacuum are you pulling in 3rd gear at that rpm? you may need to go to 85 power valve to richen the circuit a bit. it will come in quicker than a 45.

(quoted from post at 13:43:05 08/27/18) how many inches of vacuum are you pulling in 3rd gear at that rpm? you may need to go to 85 power valve to richen the circuit a bit. it will come in quicker than a 45.

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I will have to hook the vacuum gauge up and drive it. I won't get to do that until Sunday most likely. I would think if I'm not accelerating that I shouldn't need the PV to be open though. Don't you think cruising whether it be in 4th at lower rpm or in 3rd at higher rpm, they would both rely on primary main jets? That's why my next move was going to be increasing the main jets.

if you increase the jet size, the carb will be richer from idle on up. the power valve supplements the fuel curve when extra fuel is needed, and the jets are sized correctly. if you remove the power valve, you need to go up 10 jet sizes for starting point. you can always try that. works good on race cars, but not too practical on street car. I built 331 that dyno'd 525 hp at 6700 rpm. I have a carb for limited street use, and a carb for when I road race. they both work well for intended purpose, but different as night and day. race carb has no power valves.

watch your cruise vacuum. divide it by 2 and subtract 1. try that size power valve. buy genuine holley valves. generic ones do not always open where they should.

what I told you below about sizing power valve was correct. however it is to done at idle, not at cruise. sorry old age brain fart. good luck