[bowtie44s]

Originally Posted by DanielC

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.

[ap72]

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.

[bowtie44s]

Originally Posted by ap72

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.

[ap72]

Originally Posted by bowtie44s

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.

[496CHEVY3100]

Originally Posted by bowtie44s

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,

[snakebit68]

Keep in mind that when under load your carb is dumping alot of fuel through the system, but your RPM's arent going to build fast enough to burn all that fuel when towing, resulting in poor fuel economy. I had a similar issue on one of my work trucks. We switched it over to an Edelbrock 750 with vacuum secondaries and increased our average fuel mileage by 2 MPG and it towed much smoother. Of course, to be honest, we may have gotten the same results by fine tuning the Holley 750 we had on it to begin with.

[spinn]

A towing application , you want velocity. Opening the secondaries up wide from low rpm will kill the velocity and need for fuel. At load the engine is hungry, but that sudden decerease will bog.

[ap72]

Originally Posted by spinn

A towing application , you want velocity. Opening the secondaries up wide from low rpm will kill the velocity and need for fuel. At load the engine is hungry, but that sudden decerease will bog.

That's not at all how it works.

velocity is irrelevant, VOLUME and CONDITION is.

[cdminter59]

I have always ran a single plane intake Victor JR. and 750 DP (383) or 850 DP (454) on my engines. But both of these engines had long duration mech. roller cams, 3500 stall converters, and 4.11 in my 67 Camaro 454 and 4.56 in my 75 Camaro 383. Neither of these cars were daily drivers. I have my truck as my daily driver.

[PatM]

So, I'm no carb expert, but I was thinking along the lines of 496chevy3100. If this was my truck, I'd look to stiffen the secondary diaphragm spring. It's really easy and quick to do, not to mention cheap. A REALLY stiff spring would virtually lock the secondaries out. I think driving on the primaries only might be a pretty good test. If the bog is because too much air is entering at low RPM when the secondaries open, tuning with this simple spring might also offset the rich condition that the OP says he is experiencing.

PatM

[bowtie44s]

I have the bog taken care of, I went from the white to the long yellow. I was just wanting the secondaries to open at a lower rpm and that created the bog. I guess I was thinking about it wrong. The engine can't use more air if it can't burn the fuel.

[snakebit68]

Bingo! Thats what I'm saying. Glad to hear you worked it out.

[496CHEVY3100]

Originally Posted by snakebit68

Bingo! Thats what I'm saying. Glad to hear you worked it out.

Your welcome.

[spinn]

Originally Posted by ap72

That's not at all how it works.

velocity is irrelevant, VOLUME and CONDITION is.

disagree wait.

[vinniekq2]

velocity where?
in the carbs venturi or in the intake runner?