Holley pump cam help
- Thread starter Bonnewagon
- Start date
Jim, that is the best video I have seen yet. He explains it all in real world terms. Thanks!!!
I stand corrected. You were right, as explained in Jim's video. We are at 650 rpm idle so it will be the #1 hole for our car. I'm learning!
Stock out-of-the-box I assume the 30cc pump. Right now power valves and jets are above my pay grade. But as my buddy said, 'You may suck at Holley's now, but you will be an expert by the time we are through!'.
Holly carbs are so tunable, even if you get yourself in trouble you can always get back to the begining and try again.
The 10.5 is the "RV" power valve. It's designed to open about 10.5 inch of vacuum, where the normal power valve doesn't open until much later.
The 10.5 is the "RV" power valve. It's designed to open about 10.5 inch of vacuum, where the normal power valve doesn't open until much later.
The pump cam controls the profile. So you need to know if you need a full shot coming on quick. OR. A full shot lasting over a longer period of time.
Think of your Bi-Polar ex-girlfriend in a period of mania, or you current girlfriend that is more steady and stable.
Think of your Bi-Polar ex-girlfriend in a period of mania, or you current girlfriend that is more steady and stable.
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Its been a awhile since since i've tuned/adjusted carburetors, i had a 1975 chevelle i sold in early 2009 till i got my 1979 malibu in late 2021, it came with a ZZ4 crate engine topped of with a holley street avenger 770 carburetor.
I rebuilt it like new and It tuned ok, but i felt the engine probably would have ran a little better with a smaller carburetor, so i go ahold of a used, but clean holley street avenger 670 carburetor, rebuilt it and tuned it to the best of my ability.
Its been said that the carburetor issues you face is mostly ignition related, if you dont have that set right first, you will always blame the carburetor and waste time and money tinkering and buying parts for it you dont need, but as for that mostly stock 350 engine, you're right on par with the holley 600vacsec carburetor.
When it comes to the vacuum advance on the distributor, do not use the ported vacuum port on the carburetor, that is what the OEMs did back in the day, late 60s to early 70s and beyond when emmision restrictions came along, the typical low timing(4btdc) combined with the ported vacuum source, it increased the heat in the cylinder to burn off most of the CO2/NOx before it left the tailpipe, of course this was long before catalytic convertors came along as standard equiptment.
Best advice to give is now that we have it available, install a WB02 sensor/guage set, if not, seat of pants feel and spark plug checking will have to do, but the funny thing is about carburetors. you can have them tunned just right one day, then the weather changes the next day, it throws a monkey wrench into your work, its just nature of the beast im affraid, but as we all know, you wont suffer through that with EFI systems. Even though i have a WB02/guage set(AEM), to this day, i still havent installed it, and i bought it 2yrs ago, being lazy i guess.
I've got my setup tuned as good as its going to get, im currently running my holley 670 with 70(front)/76(rear) jets, the factory orange accelerator pump cam set at the #2 position and a .028 accelerator pump nozzle, turned out each idle mixture screws to 1 1/2 turns out from fully seated, i set the primary throttle plate opening to show .020 in relation to the transfer slot, on the throttle base plate, that transfer slot exposure is very important. With the completely closed secondary throttle plate, i turned the secondary throttle stop screw inwards 1/8" from just closed throttle position to slightly open it, its set like that from holley if you bought it brand new, readjust it to double check if you bought the carburetor used, then i readjusted the accelerator pump lever to take any slack out to the pump lever, you'll want just a pinch of preload, then open the primary full throttle to make sure the accelerator pump lever doesnt bottom out in the pump, you want that accelerator pump to squirt fuel the instant you touch the throttle, if not, thats where the typical hesitation and/or bog occurs because more air than fuel rushes in the engine.
When it comes to ignition timing, OEM/aftermarket distributors typically come with a 20" vacuum advance can, this will result is way too much timing(base+advance) at idle if you connect it directly to manifold(full time) vacuum, ported vacuum will be best in this situation, unless you install a fixed @ 10" vacuum can, an adjustable vacuum can on the stock distributor or the aftermarket distributor comes with a adjustable vacuum can, or find a way to limit the advance inside the distributor at idle, too much will cause very hard starts when engine is hot. Thankfully i ditched my aftermarket distributor that came with the ZZ4, the original that came with the engine went bad or something and got switched out with the last owner of the car, what im running now is a fully electronic one made by progression ignition, a company based in florida, after installing it, all i had to do it manually time my engine @ 10 BTDC and thats it, software thats loaded in a smart phone or tablet via bluetooth that controls the timing on the fly while driving, no more mechanical weghts/springs/vacuum can to mess with, the software has other useful functions as well.
Even stock SBC engines run better with advanced ignition timing, again, most are set @ 4-6 BTDC from factory, best to start off with 10 BTDC to 15 BTDC depending on setup, just keep in mind, when you advance your iginition at idle, the idle rpm will go up, so you will have to readjust it back down, each engine runs differently, no matter how alike, so set it accordingly, with what it wants, not what other engines likes.
Good Read - https://www.tccoa.com/d2/members/392bird/tuning.htm
I rebuilt it like new and It tuned ok, but i felt the engine probably would have ran a little better with a smaller carburetor, so i go ahold of a used, but clean holley street avenger 670 carburetor, rebuilt it and tuned it to the best of my ability.
Its been said that the carburetor issues you face is mostly ignition related, if you dont have that set right first, you will always blame the carburetor and waste time and money tinkering and buying parts for it you dont need, but as for that mostly stock 350 engine, you're right on par with the holley 600vacsec carburetor.
When it comes to the vacuum advance on the distributor, do not use the ported vacuum port on the carburetor, that is what the OEMs did back in the day, late 60s to early 70s and beyond when emmision restrictions came along, the typical low timing(4btdc) combined with the ported vacuum source, it increased the heat in the cylinder to burn off most of the CO2/NOx before it left the tailpipe, of course this was long before catalytic convertors came along as standard equiptment.
Best advice to give is now that we have it available, install a WB02 sensor/guage set, if not, seat of pants feel and spark plug checking will have to do, but the funny thing is about carburetors. you can have them tunned just right one day, then the weather changes the next day, it throws a monkey wrench into your work, its just nature of the beast im affraid, but as we all know, you wont suffer through that with EFI systems. Even though i have a WB02/guage set(AEM), to this day, i still havent installed it, and i bought it 2yrs ago, being lazy i guess.
I've got my setup tuned as good as its going to get, im currently running my holley 670 with 70(front)/76(rear) jets, the factory orange accelerator pump cam set at the #2 position and a .028 accelerator pump nozzle, turned out each idle mixture screws to 1 1/2 turns out from fully seated, i set the primary throttle plate opening to show .020 in relation to the transfer slot, on the throttle base plate, that transfer slot exposure is very important. With the completely closed secondary throttle plate, i turned the secondary throttle stop screw inwards 1/8" from just closed throttle position to slightly open it, its set like that from holley if you bought it brand new, readjust it to double check if you bought the carburetor used, then i readjusted the accelerator pump lever to take any slack out to the pump lever, you'll want just a pinch of preload, then open the primary full throttle to make sure the accelerator pump lever doesnt bottom out in the pump, you want that accelerator pump to squirt fuel the instant you touch the throttle, if not, thats where the typical hesitation and/or bog occurs because more air than fuel rushes in the engine.
When it comes to ignition timing, OEM/aftermarket distributors typically come with a 20" vacuum advance can, this will result is way too much timing(base+advance) at idle if you connect it directly to manifold(full time) vacuum, ported vacuum will be best in this situation, unless you install a fixed @ 10" vacuum can, an adjustable vacuum can on the stock distributor or the aftermarket distributor comes with a adjustable vacuum can, or find a way to limit the advance inside the distributor at idle, too much will cause very hard starts when engine is hot. Thankfully i ditched my aftermarket distributor that came with the ZZ4, the original that came with the engine went bad or something and got switched out with the last owner of the car, what im running now is a fully electronic one made by progression ignition, a company based in florida, after installing it, all i had to do it manually time my engine @ 10 BTDC and thats it, software thats loaded in a smart phone or tablet via bluetooth that controls the timing on the fly while driving, no more mechanical weghts/springs/vacuum can to mess with, the software has other useful functions as well.
Even stock SBC engines run better with advanced ignition timing, again, most are set @ 4-6 BTDC from factory, best to start off with 10 BTDC to 15 BTDC depending on setup, just keep in mind, when you advance your iginition at idle, the idle rpm will go up, so you will have to readjust it back down, each engine runs differently, no matter how alike, so set it accordingly, with what it wants, not what other engines likes.
Good Read - https://www.tccoa.com/d2/members/392bird/tuning.htm
Good info Ernest. Thanks!
The 6° BTDC timing was set by the shop old timer that installed the engine. So was the Uni-Lite distributor. This car always ran great for many years. No complaints. Then it sat for a long time. Now that it is back on the road, the owner tossed the carb that was on there for all these years. I don't even remember what it was. Probably a small Holley, as that is what they knew. I would have just rebuilt it, but that ship has sailed. This is why I am loath to start changing everything since the way it was set up worked so well for so long. The only thing that was changed was the carb. So it is suspect #1. I am always interested in learning new stuff, but I have a grenade handy, just in case.
The 6° BTDC timing was set by the shop old timer that installed the engine. So was the Uni-Lite distributor. This car always ran great for many years. No complaints. Then it sat for a long time. Now that it is back on the road, the owner tossed the carb that was on there for all these years. I don't even remember what it was. Probably a small Holley, as that is what they knew. I would have just rebuilt it, but that ship has sailed. This is why I am loath to start changing everything since the way it was set up worked so well for so long. The only thing that was changed was the carb. So it is suspect #1. I am always interested in learning new stuff, but I have a grenade handy, just in case.
it's a pain chasing your tail,i have no doubt you wouldn't try more than one thing at a time but it bares repeating.the majority of these carbs come with 30cc pumps and its more than addiquate than for most set ups.so infact that you'd have to step to a MUCH larger discharge nozzle before the 30cc doesn't hold enough fuel.rings true in a lot in double pumper applications.the ported/manfold vac source disagrement has gone on forever so lets say there's no right or wrong answer for now.if anything take the vac advance outta the diagnostic equasion for now as it's only for part throttle economy.it was said to jump to a larger power valve that's true the math behind this is divide your idle vac reading in half and there's your answer.again that's further down the line driving stuff.fyi most of these holleys come with a 6.5 PV it's a general rating on their part.
you mentioned the timing and that's a valid point a lot of people mistake a carb issue for timing issue and vise versa but you're a person that knows their way around a light so..again the discharge nozzle size is a general rating on their part and more times than not going to a larger(not too big)has helped me with an off idle stumble.
you mentioned the timing and that's a valid point a lot of people mistake a carb issue for timing issue and vise versa but you're a person that knows their way around a light so..again the discharge nozzle size is a general rating on their part and more times than not going to a larger(not too big)has helped me with an off idle stumble.
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