427Hotrod

The double pumper can run fine. Make sure timing is correct first before you touch carb. Work on distributor as needed to get 18-22* initial- at least. Limit total to 36-38* to start with (no vacuum).

Then watch the accerator pump system. I've done it before by removing the linkage from the secondary side and operating on just the primaries. Get it to take full throttle instantly...and then work on secondary side. Makes it a little easier if you're just beginning. There are multiple positions on the pump cams...I usually start with them rotated to the #2 spot.

JIM

454Luvr

If there's no vacuum, there's no pressure differential, thus no velocity. It is exactly the loss of velocity that causes A) the carb to meter poorly and B) fuel to drop out of the mixture within the intake system. I don't agree that a DP can't be used on the street, but I think 750 CFM is way too big for most small blocks. A higher capacity accelerator pump might relieve the symptoms, but a smaller carb or vacuum secondaries would be a much better choice for street use.

MotorHead

A quick and inexpensive fix is to increase the primary squirter size up by 2 and maybe 4 sizes as it is probably not getting enough primary shot. Carb is correct for your application

I run 2, 50cc pumps, primary and secondary and have a 45 shooter in the primary and 40 in the secondary. The carb flows 975cfm and I can hit the throttle at 2000RPM's in any gear and it just goes like a freight train, no stumble or hesitation

Please don't listen to the forums new expert as he obviously has no experience with double pumper mechanical secondary carburetors and is unfortunately giving forum members bad advice

454Luvr

I was probably building motors before you could tie your shoes. The only thing wrong with my advice is that the OP is stuck with the wrong carb, so he probably won't take it. Let's hope your band aids at least solve his immediate problem. Then maybe he can think about the right way to get this done.

SH-60B

The motor is getting plenty of air at WOT with little to no vacuum. With zero vacuum the motor has 14.7 PSI of atmospheric pressure, at sea level, pushing air into the chambers. Vacuum indicates a restriction in the intake. The throttle blades closed are a restriction. The venturi effect shears fuel into the mixture, not vacuum. I think the accelerator pump needs adjustment. You didn't see his build? 750 CFM is actually the minimum carb I'd run on it. Look at all the smallblocks chevy built with Rochester 750s, millions probably. The beauty of a Holley is all of the adjustments that can be made for tuning, it's also what confounds the tuners that fail to grasp that. Too bad you built all those motors before the rest of us were tying our shoes and you didn't even understand them.

homestead

That is impossible sir. As long as the crankshaft is turning there is air velocity (movement of air). Manifold vacuum (or negative pressure) may go to zero but it will in most applications. I have used double pumpers on most of my street cars and never had a problem. It all comes down to tuning. Get a book on tuning Holleys. I would venture to say that its in the arm adjustment for the pumps. Get a book on tuning Holley carbs. There are many to chose from. Study it, learn it. It`ll come in handy......

454Luvr

Right, but when the carb first opens, the fall in velocity kills the venturi effect. Almost no fuel is pulled out of the ports, and pump shot is needed to prevent a lean misfire (bog).

Those weren't double pumpers. They also had dime-sized primaries so metering would be more accurate at lower flow (rpm). Look, I don't disagree that more pump shot will probably stop the misfire. Nevertheless, a 750 CFM square-bore carb won't meter as well as a smaller carb at lower flows. The motor might perform slightly better at high RPM, but torque and efficiency will be degraded over the wider range of engine speeds required for the street. The car will be slower and use more fuel doing it. It's a shame all the racing hype keeps more people from figuring this out.

SH-60B

A 383 with 195 heads, a 274 cam, needs minimum of 750 cfm regardless of which carb. There is barely any air entering the carb at idle, there isn't any "velocity", there's just barely enough air going in to keep the motor from stalling, lol. Opening the throttle increases both volume and speed, it needs matching fuel. A 600 cfm carb on his combination will be a restriction, easy to spot too. Just read manifold vacuum at WOT. Anything more than 1", there's a restriction

454Luvr

That's incorrect. The small area present when the butterflies are closed creates a good velocity at the idle ports/slots. It's enough to extract emulsified fuel.

No, sudden opening of the throttle reduces velocity in the throttle bores due to the increase in throttle area. Velocity in the throttle bore won't increase until motor RPM increases. Air/fuel mix during the interim period is prevented from going lean by pump shot. Once motor RPM and velocity increase, off-idle metering becomes effective.

That's the hype that falls onto the street from the track. One of the fastest street cars I ever drove was a 454 Vette with a 650 CFM double pumper. It could pull the front wheels off the ground in 1st with a 3.73 and stock TH400. An 850 CFM DP on the same car required modification of both pumps and didn't come close to the mid-range performance of the smaller carb.

I'm not sure who suggested reading a book on Holleys, but I second it. I think HP used to publish a good one.

MotorHead

Sorry your not the one I was referring to. However whenever someone has to boast how old they are and how many engines they built and blah blah blah you know they are a little insecure.

There was one "expert" who found out a long time ago that the CF members are a lot smarter than your average back yard mechanic. He had to tell us in almost every post how many engines he built. He started at 200 and it was only a few months before it was 300

I don't do that and you won't find many here that know what they are doing have to tell everyone how great they are and how many engines they built which by the way, unless you keep up with times the engines built in the 60's and 70's are no where near the kind of engines built today with the same blocks. However since you brought it up, I was racing that '70 Cuda at the dragstrip when it was brand new.

454Luvr

But in this case it was a response to your "no experience" crack. No one contributing here deserves to be insulted.

That's very cool. I never had the money when I was younger to buy any of my cars brand new. My first hot rod was a cross-rammed 327 transplanted into a 15-year-old '57 T-bird. Not the fastest car I've had, but more fun than most.

SH-60B

[QUOTE=454Luvr;1591017311]That's incorrect. The small area present when the butterflies are closed creates a good velocity at the idle ports/slots. It's enough to extract emulsified fuel.

No, sudden opening of the throttle reduces velocity in the throttle bores due to the increase in throttle area. Velocity in the throttle bore won't increase until motor RPM increases. Air/fuel mix during the interim period is prevented from going lean by pump shot. Once motor RPM and velocity increase, off-idle metering becomes effective.

That's just flat out wrong. There is hardly any mixture in the intake at idle, and whats there isn't moving at any great speed. Snap open the throttle, now you have lots of mixture moving into the cylinders (and as long as its matched with fuel throttle response will be instant), as much as the motor will take, and the OPs combination will take a lot more CFM than a 600 CFM carb can deliver. Cliff notes: Closed throttle = no "velocity"

74modified

If you have read Tech/Performance posts lately, you would know he was not refering to you. And as far as "no experience" being an insult...wow, we must have tender sensibilities. The "expert" brags about his ability, then about 80% of his problem solving ideas are BS. He is not always wrong, but has led too many people down the wrong path to let it go without calling BS.

toobroketoretire

Gee, I wonder why the big three never used any mechanical secondary 4-barrel carburetors. Maybe the stupid engineers just didn't know how well they would work on street cars with high rear gears and automatic transmissions.

74modified

Well, a smart production engineer knows that the consumer product usually has to be a compromise to meet a wide range of conditions. They don't always get to use what they want. Ever wonder why those cars never came with long tube headers - ya think they could be an improvement in certain situations?

diehrd

You post to much nonsense , Not sure why but I think you do it for 2 reasons

One your clueless and
Two you just want attention

454Luvr

In the intake? Who said anything about the intake? I was referring to velocity at the metering ports/slots in the throttle bore. You realize that when the motor is idling, velocity in the intake isn't the same as in the throttle bores, right? The velocity around the butterflies is much higher. That's what causes emulsified fuel to be pulled out of the idle ports. It's also the reason you hear rushing air when you pull the air filter. That noise isn't coming from the plenum.

Wrong. No velocity = dead motor.

454Luvr

I think they did, actually. The horsepower wars got so hot for awhile, manufacturers and customers were both willing to sacrifice streetability for high-RPM numbers. People bought those cars to race, and they didn't care if they drove poorly on the way to the starting line (substitute traffic light, parking lot, entrance ramp, etc.). In the right combination of drivetrain and vehicle weight, nothing does a full-throttle quarter quite as well as a double pumper.

diehrd

Can we get back to helping the OP ..

Jeff_Keryk

Q-Jets are mechanical secondary carbs. Just sayin'...