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Discussion Starter · #1 ·
Hey guys, I'm sure you've answered this 100 times. I know this isn't a hot rod question so I appoligize. I have a mildly built 454, I'd guess around 400hp. It is in a heavy 1 ton truck with a 5 speed manual. I use this truck to tow a good bit and just cruise around. I have a 750 holley vacuum secondary on it now. It does pretty good for cruising but when loaded I have to get the rpms up enough for the secondaries to open. I'd rather have a douple pumper so if I push the throttle enough I can open the secondaries at a lower rpm. I tried a really light spring in the secondaries but without the pump shot it makes it bog when they open. Every place I read says you don't want a double pumper for towing but no one says why. Would it be a bad idea to switch to a dp? Also would it kill my fuel mileage? I'm getting about 11mpg empty with the vs... pretty good i think for a 6600lb truck.
 

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Discussion Starter · #3 ·
I had tbi and it ran terrible. I had the chip made for the cam and heads. I switched to holley vs and it made it feel like a whole new engine. Why do you recommend the q jet? I'm sure the dp will use more fuel if i don't keep my foot out of it, but yea, that will be hard to do.
 

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Q-jet has very small primaries,is very good at getting mileage and it flows 700 CFM or more.The bb q-jet needs a little tlc to work best. intake manifolds make a difference
 

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Most likely if you switch to a double pumper while towing if you don't have the rpms up it will probably bog. Why don't you just try going up a couple sizes on the jets on your 750 carb. Go from 72 to 74 on the front and on the rear go from 76 to 78. Keep an eye on the plugs to see if running too rich.
 

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DP's are better off with manual trans so for your application with 5 speed that is one good thing, personally for a rig like that I would want more control over the secondaries and the DP will do that, add to that the right gear selection.

MPG > guess you could sqeeze the same out the same milage as a vacuum secondary if you keep you foot out of it.

Going out and dropping >400 bucks for a new one may not be worth the $$$ though ??? your call.
 

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I am going to suggest vacuum is a better way to go. With vacuum secondaries, the engine opens the secondaries when the engine need the extra air. If you have a double pumper, the engine may not need the air, it is at too low RPM. Then air velocity falls too slow through both the primaries and the secondaries, and the carburetor loses its ability to accurately control the fuel air ratio.

The idea with a double pumper carb is the secondary accelerator pump squirts a shot of gas to cover up the opening throttle bog, and by the time that gas is burned, the engine has increased RPM to be able to use the air coming through all those wide open holes, and then can control the mixture.
With a heavy truck, towing something, the rpm will not have had enough time to increase pulling the load, and the engine may just briefly run off the accel pump shot, and then bog down, being too lean.
 

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no way in hell would I put a DP carb on that. Nor would I bother wasting money on a quadrajet. Your current carb will work perfectly- if it is cleaned up and tuned properly.

BTW, why do you want your secondaries to open sooner? That would be the first question I would want to answer. Next, if you need to change your pump cam, do it. You can also change your squirters if that is the problem. And as suggested I would convert to 4150 immediately.

You have an untuned carb, and any untuned carb is NOT going to perform well. Changing to a different untuned carb still will not fix your problem.
 

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If your looking to buy a new carb I would sugest a Eldebrock 750 for this application due to you mostly are using the truck for towing. my dad had a 750 vacume secondary holley on his 454 in a 78 dually and you could not keep power valves in it. He switched to the Eldelbrock and truck ran well. In my opinion a Holley carb is intended for performance applications. I think you will be pleased if you decide to go with the Eldebrock.
 

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Bowtie, For your application you really need a 650 vacuum secondary. You need torque not RPM to hall all that heavy cargo. 750 is to big to do the job.
 

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Bowtie, For your application you really need a 650 vacuum secondary. You need torque not RPM to hall all that heavy cargo. 750 is to big to do the job.
a 750 with mechanical secondaries is too big, with vacuum secondaries it really isn't an issue as they only open with enough air flow, and not a second sooner no matter how hard you stomp on the throttle.
 

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Discussion Starter · #16 ·
I am going to suggest vacuum is a better way to go. With vacuum secondaries, the engine opens the secondaries when the engine need the extra air. If you have a double pumper, the engine may not need the air, it is at too low RPM. Then air velocity falls too slow through both the primaries and the secondaries, and the carburetor loses its ability to accurately control the fuel air ratio.

The idea with a double pumper carb is the secondary accelerator pump squirts a shot of gas to cover up the opening throttle bog, and by the time that gas is burned, the engine has increased RPM to be able to use the air coming through all those wide open holes, and then can control the mixture.
With a heavy truck, towing something, the rpm will not have had enough time to increase pulling the load, and the engine may just briefly run off the accel pump shot, and then bog down, being too lean.
Thanks, that makes perfect sense now. I never thought of burning the pump shot and intake velocity too low to work the main circuit. I had a new carb in the summit shopping cart but didn't click check out until I heard some more replies. I think I'll keep the vac secondary now. I am using the blue cam and a 28 nozzle with 72 main jets, (they are a little rich) Converting to a 4150 makes sense as I can richen up the secondaries.

The reason I wanted the secondaries opened sooner was, I figured more air and more fuel would be better. I pull 12,000lb a lot so I'm around 18,000-19,000 total. It pulls pretty good, as good as a stock 5.9 cummins. I was just thinking more air and fuel would do better on these WV hills.

I am a member of several forums and you guys are buy far the most knowledagable. I appreciate your help and am glad I found this forum.
 

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them opening sooner really isn't going to change a lot, it'll help some, but it'll be very slight. I would also look into properly tuning your PV circuit, its the most overlooked and one of the most important. your high load tuning should be done with this circuit NOT the jets as is so often done.
 

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Discussion Starter · #18 ·
them opening sooner really isn't going to change a lot, it'll help some, but it'll be very slight. I would also look into properly tuning your PV circuit, its the most overlooked and one of the most important. your high load tuning should be done with this circuit NOT the jets as is so often done.

I have a 6.5 power valve. I am pulling right at 20" at idle (yea, mild cam.) I have been watching the vacuum while driving. Cruising in OD i'm around 12-15". When I punch it, it quickly drops below 5 so I haven't noticed any problem with the power valve. I haven't watched vacuum with a load, only empty. I know quick fuel makes a 4 door high flow power valve. I'm not sure what I have, I never paid attention. I just checked it for tears and figured it was good. Is there another way to get more volume of fuel other than a "high flow" power valve? How would I know if I need more fuel on the power valve circuit with out an afr gauge? It doesn't feel or sound lean under load.
 

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I have a 6.5 power valve. I am pulling right at 20" at idle (yea, mild cam.) I have been watching the vacuum while driving. Cruising in OD i'm around 12-15". When I punch it, it quickly drops below 5 so I haven't noticed any problem with the power valve. I haven't watched vacuum with a load, only empty. I know quick fuel makes a 4 door high flow power valve. I'm not sure what I have, I never paid attention. I just checked it for tears and figured it was good. Is there another way to get more volume of fuel other than a "high flow" power valve? How would I know if I need more fuel on the power valve circuit with out an afr gauge? It doesn't feel or sound lean under load.
you don't need a high flow PV and switching to one wont change a damn thing, what needs to be adjusted is your PVCR's (power valve channel restrictors), on a stock metering block they're just a drilled orifice, on a HP metering block they are tapped and threaded for brass inserts that can be drilled and adjusted the same as jets or air bleeds.

how you can tell if you need one is that you should have about a 15:1 AFR when cruising, and when under high load about a 12:1 AFR. your jets will dictate your cruise AFR and your PVCRs will kick in to add the additional fuel needed for high load. Of course air bleeds and transition metering and even the idle circuit also all factor in- which is why you pay a good carb guy the money for making it all work together correctly.
 

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Hey guys, I'm sure you've answered this 100 times. I know this isn't a hot rod question so I appoligize. I have a mildly built 454, I'd guess around 400hp. It is in a heavy 1 ton truck with a 5 speed manual. I use this truck to tow a good bit and just cruise around. I have a 750 holley vacuum secondary on it now. It does pretty good for cruising but when loaded I have to get the rpms up enough for the secondaries to open. I'd rather have a douple pumper so if I push the throttle enough I can open the secondaries at a lower rpm. I tried a really light spring in the secondaries but without the pump shot it makes it bog when they open. Every place I read says you don't want a double pumper for towing but no one says why. Would it be a bad idea to switch to a dp? Also would it kill my fuel mileage? I'm getting about 11mpg empty with the vs... pretty good i think for a 6600lb truck.
put a firmer spring back in start with yellow then adjust firmer until bog goes away ,,secondarys opening to soon causing Bog ,probably end up with brown,
 
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