[71nova]

i had a quadrajet on my 383 engine and it pulled like hell. i than got a 3310 holley 780 carburetor with 72 and 76 jets. it did not pull hard at all compared to the quadrajet. i than upped the jets to 76 primary and 82 secondary and no black smoke or anything it feels better but still doesnt feel nearly like the quadrajet did. also i no for a fact the secondarys are fully opening on the holley. what do you guys think?

 

[chevy72nova]

its possible you are too rich. can you smell any fuel on hard acceleration? if not i would tune by trying some different pump cams and squirters and maybe even different secondary springs. does it have a stumble or just doesnt pull as hard all around? what do the plugs look like?

 

[71nova]

its does not smoke at all or stumble. the plugs look perfect. i dont know if it also matter but the distributor in the car only advances to a max of 29 degrees.

 

[DriveWFO]

its does not smoke at all or stumble. the plugs look perfect. i dont know if it also matter but the distributor in the car only advances to a max of 29 degrees.

You can't analyze your jetting on your plugs with the naked eye. You need to cut the threads off so you can see the very bottom of the porcelain.



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[Mike Goble]

i had a quadrajet on my 383 engine and it pulled like hell. i than got a 3310 holley 780 carburetor with 72 and 76 jets. it did not pull hard at all compared to the quadrajet. i than upped the jets to 76 primary and 82 secondary and no black smoke or anything it feels better but still doesnt feel nearly like the quadrajet did. also i no for a fact the secondarys are fully opening on the holley. what do you guys think?

If carburetor 'Q' pulls like hell and carburetor 'H' runs like crap, why isn't carburetor 'Q' on the engine?

 

[fxrs1hd]

Jets for Holly

Most people tend to over jet this carb. Bigger is not always better, I run 68 Primary and 72 Secondary. Change springs and weights in Distributor to get full advance at 2200 RPM's. This is with the GM L-79 350 Hp cam Engine pulls strong from 2200 to 6800 RPM's.

 

[chevy72nova]

You can't analyze your jetting on your plugs with the naked eye. You need to cut the threads off so you can see the very bottom of the porcelain.

when my holley ran too rich the whole tip of the electrode and insulator were completely black and sooty. i couldnt see smoke out the exhaust but this still indicated being way too rich. by downsizing my jets two sizes at a time and playing with the accelerator pump circuit i managed to tune it perfect so that the plugs cleaned up and only had slight discoloration at the tip. i could stand on the gas from a stop and pull hard all the way up through third. this was with a 450 Holley on a 305 with a th350 and a 3.08 gear! now im going through the same process with the 600 holley i just built for my 350 in the same car.

i agree that with the 780 you should try downsizing the jets and see what happens. i forget what total advance i had with the 305 but i had a little lighter springs to let the advance in a little sooner than stock so that all the advance came in as quick as possible without pinging.

 

[Stan D.]

29 degrees of total timing is not even close to enough. You need to get it up to the 34-38 range and it shoud all be in by 2.5k to 3k in rpm.


also do you have a power valve in the back of the carb, if so you don't need to stagger the jets, but you could be leaning out the secondaries due to fuel flowing away from the jets and power valve, which would also drastically affect the way it pulls.

If so you could get rid of the power valve then stagger the jets and also put jet extensions on the back.
Just some thoughts.

 

[DriveWFO]

by downsizing my jets two sizes at a time and playing with the accelerator pump circuit i managed to tune it perfect so that the plugs cleaned up and only had slight discoloration at the tip.

You can't read jetting on the tip. You can read idle mixture and cruising mixture up there though, but not WOT.



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[nova10sec]

Quad-carbs kick in like no Holley ever would. That is not to say that a Holley won't ET as good if not better. The primaries on a quad are so small, when you kick the secondaries in you are adding probably 75% more CFM. On Holley, since the primary and secondaries are the same size, you are adding 50% more cfm. So the seat of the pants comparison will feel a great deal different. But a track comparison should demonstrate that they are very comparable.

 

[chevy72nova]

ok i can see that. i never knew you were supposed to cut the threads away to read them for WOT.

 

[71nova]

the secondaries have jet extensions and a power valve already. when i did increase the jet sizes though it actually did pull a lot better and i just got my distributor back from being recurved on a distributor machine maybe that will help. the reason i took off my quadrajet was the primarys couldnt get the car going at all. but a little off of idle it would pull very hard the whole time. i'll be at irwindale thursday with it so maybe i can get it all dialed in

 

[DriveWFO]

ok i can see that. i never knew you were supposed to cut the threads away to read them for WOT.

You can do that or buy one of those tools that doctors use to look in your eyes/ears:

http://store.summitracing.com/partdetail.asp?autofilter=1&part=MOR-62345&N=310108+115&autoview=sku

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[Stan D.]

One problem to think about is that jet extensions don't do any good if you still have a power valve in the back because fuel will go away from it and you negate the point of having it even with the jet extensions. Just a point to make.

 

[JoeII]

Stan, I'm not convinced the jet extensions have anything to do with the power valve. The jet extensions are there to keep the jets in the fuel while launching the car. The power valve works relating to the drop in vacuum. Unless I'm in the weeds here... Anybody?

JoeII

 

[DriveWFO]

Stan, I'm not convinced the jet extensions have anything to do with the power valve. The jet extensions are there to keep the jets in the fuel while launching the car. The power valve works relating to the drop in vacuum. Unless I'm in the weeds here... Anybody?

JoeII

If the fuel is forced to the back of the secondary bowl, the power valve would be uncovered.



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[Stan D.]

Yes that is the problem when fuel flows away from the power valve it cannot pull fuel, thus negating the point of having it.

 

[JoeII]

OK. Fuel doesn't flow through a power valve does it? I thought the power valve movement opens up passages in the metering block to enrich flow. If it were true that the power valve being uncovered negated the point of having it, you would never see one in drag racing. I think a lot of people use power valves. I used a 10 jet size spread on my 750 (ie. 72/82). That was recommended by several Holley "gurus" that I trust. If you block off the power valve opening with a plug, you will need to have another 6-10 sizes bigger on the secondaries. Also that will make part throttle very rich. I'm no expert, but I've always used power valves without a problem. I read the holley tuning tips at www.mortec.com and other sites and find them helpful.
JoeII

 

[DriveWFO]

OK. Fuel doesn't flow through a power valve does it? I thought the power valve movement opens up passages in the metering block to enrich flow. If it were true that the power valve being uncovered negated the point of having it, you would never see one in drag racing. I think a lot of people use power valves. I used a 10 jet size spread on my 750 (ie. 72/82). That was recommended by several Holley "gurus" that I trust. If you block off the power valve opening with a plug, you will need to have another 6-10 sizes bigger on the secondaries. Also that will make part throttle very rich. I'm no expert, but I've always used power valves without a problem. I read the holley tuning tips at www.mortec.com and other sites and find them helpful.
JoeII

Most double-pumper carbs don't have a power valve in the secondary metering block, just the primary metering block. If you were to remove the primary power valve and plug it, you're right, you would need to go up about 8 jet sizes on the primaries.



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[chevy72nova]

ive setup a carb like that with a pv blocker. put much bigger jets in to compensate. ran like crap down the bottom. worked really good at higher rpm but without something to split the jets you get a poor transition from the idle circuit to the main metering circuit because of the massive increase of fuel that come through the bigger jets right away.

a carb with a complete absence of a power valve is usually only used on a race motor that is only gonna see high rpm use.