toobroketoretire

People tend to believe the so-called "double pumper" carburetors must be really terrific carburetors because they read and hear about them all the time. Yeah, they ARE terrific carburetors when they're used for what they are designed for; drag and track racing where engine speeds seldom drop below 4000+ rpm and the engine can tolerate having the secondaries open. But for street use the only carburetors that are suitable are the AVS (air valve secondary) and the vacuum secondary carburetors that keep the secondaries closed until the engine can swallow the additional air. To put a double pumper/mechanical secondary carburetor on a street engine is a sure way to get horrendous bogging problems.

toobroketoretire

When I was young and stupid I wired the air valve of my QuadraJet open so the secondaries would open like they were uh "intended to". Well, I found out REAL quick the air valve is designed to "throttle" the secondaries because I ended up with a horrendous bogging problem. Same goes with the double pumper/mechanical secondaries type of carburetors as they too will cause horrendous bogging problems when used on any street engine and is why none of the factory performance engines ever came with them. If you're ever in doubt about which type or size of carburetor to use look at what the factory used.

MotorHead

The carb will only use what the engine needs whether it is mechanical or your "AVS" type. The engine will not bog with with a properly set up carb be it vacuum or mechanical secondaries.
If you have have had problems with a double pumper the carb wasn't tuned properly . You can get better mileage than an "AVS" carb if you disconnect the secondaries on a mechanical secondary carb.

427Hotrod

Uh,,,,yeah...Ok......right......

If a double pumper is bogging....someone needs to set it up properly and tune the thing.

Of course it's the same deal if a vacuum secondary carb opens too late. Time to adjust things......


OEM's use what is "idiot proof"...(take no offense from that term). It has to work under ALL conditions no matter what. If someone specs 3.08 gears and a close ratio...they didn't change the carb tune..or timing. They aren't worried about if it runs a few tenths quicker in the 1/4 mile...it has to run "OK" when grandma gets in it as well as a gear jammer.

Vacuum carbs and or AVS carbs can work very well....I love Q-Jets BTW....but to say double pumpers are never a good street choice is just causing people to never engoy the benefits of a well setup combination with a double pumper. I'll take a DP any day of the week. I prefer to control power with my right foot vs. a vacuum operated deal especially in limited traction situations.



JIM

7t9l82

I have a 750 double pumper on my street 350 and the average person would guess it is fuel injected.because I didn't just take it out of the box and expect it to work.oh and I even put bigger jets in it plus a bigger shooter.
That size g guide holley came out with about 40 years ago is almost total b.S , unless you drive a station wagon with a 11/2 single exhaust.

MotorHead

That carb size formula is total garbage yet some still use it

Les

Jim, thanks for saving me the effort.

Alan 71

Hi,
I put a Holley Spreadbore double pump, single feed, mechanical secondaries, 4 barrel on my 71 base motor a few weeks after I bought it.
It was a very popular change back then.
I liked it enough to put it back on when I restored the car. When I decided to have the car Flight Judged I put the q-jet back on.
I even ran the Holley with the CEC system in place for a while before I changed back to the q-jet.
It seems like once you get a feel for the mechanical secondaries they can be quite a bit of fun on the street.
Regards,
Alan

cooper9811

I ran a 750 DP with my 383 and 700r4 - against the advise of a few people (they told me not to run it with an auto trans). I wanted to try it and once it was set up, had no problem. It just took a little time and research (since I had no previous experience with one). It's not really the setup a DP was intended for, but it can (and did) work pretty well.

However, I do think a vacuum secondary is more of a "plug and play" option that most folks would consider first.

By the way - not to throw fuel on the fire... technically, isn't the Quadrajet a mechanical secondary carb. Or is the OPs post intended to be more specific to the double pumper (vs mechanical secondaries in general)?

Street Rat

I simply love Holley carbs.

I have been installing Holleys since I was 16. They are so easy to tune. Within about 10 minutes I can change my jets, power valve, acc pump nozzle, etc. They have so many aspects of tunability and infinite combinations.

If someone knows little about the workings of a Holley then you can easily mess things up. Getting a larger carb than needed is a common problem too.

My .02

650 DP on a street only 355 that redlines at 5200 and has NO bog.

LT-1 kid

my 69-427 loves the 750 DP, would't be the same car without it.

Chinaski

Running a Mechanical Secondaries 650cfm chokeless AED DP on my new Vortecpro 350 build. Still got some tuning to do, but so far so good.

ykf7b0

My 69 Plymouth Road Runner with a 440 engine loves it's 850 Holley Double Pumper with mechanical secondaries. It has great manners on the street and the track.

69427

I'm a bit perplexed why the OP keeps insisting on being the forum carburetor "expert".

Les

Trust me, there's at least one other person who has wondered the same thing.

TedH

I ran a 600cfm Holley DF/DP on a hopped up 283 with Streetmaster manifold, General Kinetics cam, headers, duals and 3.54 12-bolt back around 1980. Only problem I had was cold starting/choke. Otherwise, it idled down to 500rpm and, when I dropped it into 2nd (I didn't know a TH350 had a 'kick-down' cable), HOLD ON!!!

My current ride produces more HP/TQ but is dialed in with a Q-jet. My reading indicates that the mech carbs are intended for engines that cannot produce sufficient vacuum (to actuate vac secondaries). They can be set up to work with engines that produce sufficient vacuum but it comes down to cost and practicality. Unless ideally suited to the power combo (or just very lucky in the draw), mech carbs are better for racing app's.

I ultimately swapped the Holley/Edelbrock for a q-jet with cast iron intake on the 283... improved cold starting but it never equaled the WOT performance of the Holley

74modified

That may be YOUR experience, but my experience is a little different. I have run nearly every type of carb over the years, and have used many Holley double pumpers. Most problems come from people not matching engine components to work together, and/or not tuning everything properly. If you don't know what you are doing, then yes leaving it stock may be better. But, I try not to use terms like "To put a double pumper/mechanical secondary carburetor on a street engine is a sure way to get horrendous bogging problems."

commander_47

Double pumpers are awesome and can produce truly impressive mid range torque numbers. They need extra care in matching the carb to the motor.

Most street guys prefer vacuum secondaries because they are much more forgiving of being mismatched to an engine.

marc1973

I have a holley 800 dp spread bore(mechianical secondaries) on my mildly built 350 and it drives perfect on the street

EASYGEAR

I once had a very aggressively cammed 355 SB, Vacuum issues by nature.
Actually more of a old-school racing mouse, but you could use it on the street if you kept the nerves to use it in-town with it`s non-existing low end range.
Engine stall occured if you wouldn`t keep it alive, especially when going downhill combined with turns - needed to learn driving with the right foot positioned twisted on both pedals for those situations (meaning both right hand pedals, car was a 4spd.) .

The shop builded the engine for me built street and race engines, they allowed me to try any common carb for the SB you can imagine as they had them around plenty.
Different manufacturers, sizes, they were drilling holes in Holleys and i don`t know what they tricked here and there, installing calibration kits and i was trying around for two years with various carbs.

The ONLY solution that worked for my engine was a double pumper.
Did away with the engine stall and the vacuum issues were no problem anymore.
Lot`s of power and yes, less mileage.

I had the holley 750dp and then they let me try the 650 dp, which was way more responsive down low and way nicer to drive in town on the lower revs and off idle.
But the first time on Autobahn i got into it it turned out the 650 was to small in the upper end which the engine was built for, so i turned back to the 750 because i could have built a way cheaper engine for the peak-hp i achieved with the 650, so it made no sense for me to use it with the 650.


If i would use a dp again today ?
No - because mileage counts in with gaz prices on my side of the pond and because i would never again order an engine that agressive thinking i have good times with it using it in real world.
But it`s good i had the experience and others may want such a beast for their street machine, i believe for those drivers a dp is not a bad option.

In my case - i was bogged with vacuum units until i tried the dp.