I started out in 1977 running Holley vac sec. carbs,cause that's what everyone said to run on the street....DP carbs were too "jerky/finicky" for street use according to the experts.......and gave terrible fuel mileage.....
I've used 600's/650's/700's/725's and 750's on small block ford engines, they ran good once tuned to the engine............

Then in the late 90's I was at a buddies shop,helping him work on his 1967 Mystang Fastback,289 engine,he took the Holley 600 DP off and threw it in the garbage can,said it was no good, always ran too rich,he put on one of those shiny Holley 600's vac sec. carbs and pronounced immediately that it ran much better and it might have......

So,when I left, I asked him if I could have the old Holley 600 DP in the garbage can for parts and he said yes. Took it home,left it on a dusty shelf for a year or two, then one cold winter day in the shop I decided to take it apart and check it out, it was fairly clean inside, but I soaked it in carb cleaner overnight anyway, then washed it clean and blew it out....I'm sure it was running way rich on his 289 as it had jets in it about 10 sizes too big for his application!!!!!!!
I re-jetted the carb to what I thought it should be,installed it on my 350hp,351-W, set the floats,set the idle mixture screws and the idle and have not touched it since.......
Without a doubt,it out preforms any of the 4 Holley vac sec carbs I've used on the same engine in the prior 8 years!!!!!! and my gas mileage is no different......
For my street and occasion track driving,I'll take a double pumper carb over a vac sec carb any day.....and that's with 35 years expirence with Holley carbs......

David

I didn't trust my memory, so I did some research. The Carter Thermo-Quad was a mechanical secondary carb. It was mostly used by Mopar, but was also used by GM and Ford.

My first car was a 1966 Mercury Comet Cyclone GT with 390 HiPo. It came from the factory with a 780 CFM vac 2ndary Holley. At least the original purchaser told me it did. It stumbled when the 2ndarys kicked in and it also stumbled when the auto shifted from 1st to 2nd. Original owner claimed it did it from day one and Ford dealer could not fix it. I rebuilt it, tried several power valves, and practically every jet combination in the kit. After many months of frustration, I put a new Holley 650 CFM spread bore (adapter plate) with mechanical 2ndarys. I think it was a double pumper, but I am not certain any more. Anyway it solved the stumble and fuel mileage went from 8-10 to 12-13. Seat of the pants it made more power too. Drive-ability was perfect.

I know vac 2ndary allows you to get away with too big a carb. Too many people are believers they are great for me to say they are not. However I do believe mechanical 2ndarys get a bad rap. They do not forgive being sized too big, but when properly sized and set up they too will run very well. People who cannot get them to run well often times have too much carb for the engine and will not admit their mistake, rather they blame the carb. Just one opinion.

I have mechanical secondaries and advance. I get too much bucking at low speeds. What I'm hearing from this debate is the mechanical secondaries are probably ok for me on the street, but I could improve the low speed driveability with a vacuum advance set up. Would I need a completely new distributor to convert to a vac advance? I can probably get some kind of conversion kit from MSD. I have a Bigs Stage 5 modified Holley DP. Sure knows how to suck gas! I'm a dummy so go easy on me....

After spending hours upon end with double pump & vac sec carbs on my small ford its not hard to say which is better for street use. I have multiple carbs in dble & single pump in holley. The main problem with stumble in vac carbs is tuning. Most tuners dont spend enough time with the idle circuts. Although I have had good success with dbl pumps the vac carb still comes out on top for street use for me. I am still spending a lot of time with timing. To me the jury is still out on vac timing.

I thought this tread was about distributor vacuum advance.

Ward, I think your low speed bucking is probably a lean idle circuit condition. Rather than swapping out any parts, try turning out the four idle mixture adjusting screws another half turn. If it runs better, then slowly turn the screws back in until it doesn't. Then back them out an eighth of a turn.

I have the same engine you do with a ProSystems 850 DP. No bucking or hesitation from 1500 rpm on up.

An easy way to tune a holley weather it is vac sec or double pumper is:
get the engine fully warmed up and put a vacum gauge on it and begin turing your idle mixture screws in or out and watch the vacum gauge, your looking of for the highest reading possible, do one screw at a time, when you get the highest reading, move to the next one, you'll have to re-adjust your idle speed and you'll have to go around the carb 2 or 3 times to get it to the highest point, then take it out on the road for a test run.........this is also assuming your timing is correctly set from the get go.......
Not sure if you can add vacum advance to a mechanical distributor using an MSD distibutor, you could call the MSD tech line and ask.....

David

Timing and carb tuning go hand in hand. Just wondering how many of you use a air fuel meters to tune your engines. After reading the very well written article on vac advance It seems as though the cruise timing is about the same as my mech. The difference seems to be in the idle. Here in utah I have to pass an emissions test. If my idle timing is beyond about 8 degrees it wont pass.

Thanks! I'll give your suggestions a try.

And then with the engine off, wind all the screws in until seated counting the turns as you go.
Back them all out to an average of all the screws and tweak each one for highest vacuum at idle, making a mental note of where all the screws are relative to seated position.

I have two Wideband O2 data loggers that I use on my cars.

My question is do you take data from one side at a time on dual exhaust cars or do you have some sort of system to connect both sides. Or do you use two meters.