Holley Jets, WHAT the heck???

....Hello Everyone! I got my Holley jets, gasgets, jet removing tool & Holley Carb book yesterday. I had planned on putting in 70/82 jets in my carb because according to the plugs it's running rich. It now has 71/83 jets in it & when it was on the dyno it didn't put out anymore HP & TQ with the 71/83 jets than it did with the 70/82 jets. BUT the Holley book said that there is a 3% +/- tollerence on jet sizes & that the next size smaller might not even be any smaller!!! {If the 70's are on on the + size of the tollerence & the 71's are on the - size}.....GREAT!!! Any way I can measure the jets I have in their now to compare them to the new ones???? THANKS & have a fantastic 4th!!! Bill

Measuring Holley jets doesn't really tell you anything, because two different #'s might have the same bore diameter. The change comes in how the entrance is radiussed as well as just drill diameter. I can recall stories of guys drilling jets slightly bigger, and actually leaning the engine out more, due to imperfections slowing the flow.
Slightly rich mix won't lose any power anyway, just burn more gas.

I have a hilborn f/i on a sbc and the jets are very critical. as was said before its in the radius in the opening of the f/i jet. drill sizes are what jets are measured by,for carburaetors, i think.

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Originally Posted by 48fordnut

drill sizes are what jets are measured by,for carburaetors, i think.

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That is true for Eddy's and some others, in Holley's case it's flow rate and once you take a drill to a Holley jet you might as well throw it in the trash bin.

NOTE: Holly makes the jets, then flow tests them,THEN stamps the sizes on them. Dont alter them, just change them out.

...Ok, Thanks everyone, so I'll just switch to one size smaller.... Bill

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Originally Posted by billlsbird

....Hello Everyone! I got my Holley jets, gasgets, jet removing tool & Holley Carb book yesterday. I had planned on putting in 70/82 jets in my carb because according to the plugs it's running rich. It now has 71/83 jets in it & when it was on the dyno it didn't put out anymore HP & TQ with the 71/83 jets than it did with the 70/82 jets. BUT the Holley book said that there is a 3% +/- tollerence on jet sizes & that the next size smaller might not even be any smaller!!! {If the 70's are on on the + size of the tollerence & the 71's are on the - size}.....GREAT!!! Any way I can measure the jets I have in their now to compare them to the new ones???? THANKS & have a fantastic 4th!!! Bill

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This might be a little off track but 9 out of 10 Holley carbs are set to rich.
I am not sure why you would run that big a spread front to rear un-less you have a manifold distribution problem.
With this large a spread you will tend to have very strange A/F readings.
You also won't be able to even "read" the plugs until you have done a min. of 20 hard pulls on the engine.
I would try and look at the A/F readings and also take a look at the EGT's.

A spread of 71 front/82 rear is normal for vacuum secondary carbs. You are looking at essentually two-two barrel carbs fixed in tandem....

The front bores being smaller use smaller jet size, the rears are MUCH larger, hence the larger jet size.

Mechanical secondary carbs usually run 4-barrels the same size, with close to the same jets. As you noted: stagger jetting in this case is a result of manifold/distribution differences.

When doing jetting changes, reading the sparkplugs is complicated by the lack of lead in the fuel. That was what colored the porcelain so well. True: you must do at least 20 runs to get some type of color to show.

But, When doing seat-of-the-pants tuning, Disconnect the secondary link so the secondaries dont work. Warm up engine and do your 1-2 runs. Rich? (rich is easy to tell due to the black plug color and sluggish acceleration). Drop 2 jets and run again. Better, but rich still, drop 2. Keep doing until the plugs clean up.

Reconnect the link and start all over on the secondaries. Measure your idling vacuum. install a power valve 3 sizes under what your vacuum reading was.

I could go on for hours on carb tuning. Keep us informed as to your progress. MIZ

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Originally Posted by mizlplix

A spread of 71 front/82 rear is normal for vacuum secondary carbs. You are looking at essentually two-two barrel carbs fixed in tandem....

The front bores being smaller use smaller jet size, the rears are MUCH larger, hence the larger jet size.

Mechanical secondary carbs usually run 4-barrels the same size, with close to the same jets. As you noted: stagger jetting in this case is a result of manifold/distribution differences.

When doing jetting changes, reading the sparkplugs is complicated by the lack of lead in the fuel. That was what colored the porcelain so well. True: you must do at least 20 runs to get some type of color to show.

But, When doing seat-of-the-pants tuning, Disconnect the secondary link so the secondaries dont work. Warm up engine and do your 1-2 runs. Rich? (rich is easy to tell due to the black plug color and sluggish acceleration). Drop 2 jets and run again. Better, but rich still, drop 2. Keep doing until the plugs clean up.

Reconnect the link and start all over on the secondaries. Measure your idling vacuum. install a power valve 3 sizes under what your vacuum reading was.

I could go on for hours on carb tuning. Keep us informed as to your progress. MIZ

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What??,a vacuum secondary carb does not use a metering block or rear jets un-less he has put on a conversion kit on it.
Holley use to have a carb much like the 3310 that had a secondary metering block along with the rear jets.
I think it flowed in the 780 area.
If this is a double pump carb I would first start by "squaring up" the jets.

Erik, what do you mean by "squaring up the jets"?

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Originally Posted by NTFDAY

Erik, what do you mean by "squaring up the jets"?

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I mean start with the same size jet in the front as you have in the back.

Thanks for the explanation, I don't believe I'd heard that before.

Bill, throw us a bone here. What's the carb number? A lot can be achieved if we had a number to go with the calibration for the carb. We'ed love to give you some info that's more accurate.

Holley 750 double pumper according to an earlier post . . . provided he hasn't swapped it out.

Earlier post.
http://www.clubhotrod.com/forums/showthread.php?t=31545

Venturies, booster drops, booster length, and metering inside the darn thing along with emulsion jets change what the carb was designed for. Nevermind, Bill, I guess you should keep playing with it. Sorry guys, a 750 is not always a 750 as "we" all assume. I'd like a base point in which to start and if it's been "monkied" with, wouldn't it be nice to have a start point?

THANKS Erik, Mizlplix, Denny, C9x & Nitrowarrior, Sorry I didn't check this earilier to re-post. It's a Holley #4779 double pumper that was gone through by the Carb Shop in Ontario to 'match' my engine {they didn't tune it on the motor, just built it to match my motor}. I gave the place all the spec's on my motor as per my engine builder. I don't have the paper work from them so I don't have that much info {it got lost somewhere between the engine builder & the car builder}. I do know they cut off the choke horn. The plugs show it running rich {tons of black soot on them} so I thought I'd put in smaller jets. I showed up an hour or so into the dyno tuning of my motor so I'm not sure if the Carb Shop or the dyno operator put the 70/82 jets in. But it didn't put out anymore HP or TQ with the 71/83 jets than it did with the 70/82's so I thought I'd go back to the 70/82's.
My dyno sheet has Zero's under the A/F ratio :(. But it does show FHp Hp, ME%, FB 1b/fr, BSFC 1b/Hphr, CAT & Wat. Will any of these figures help??? Oh Eric, what does EGT's mean??? Mizlplix & Denny, I'll get a vacuum gauge & check the vacuum but is there a way I can measure what power jet I have in there now??? I'll look up in my Holley book r/e where is the power jet! THANKS All, Bill

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Originally Posted by billlsbird

THANKS Erik, Mizlplix, Denny, C9x & Nitrowarrior, Sorry I didn't check this earilier to re-post. It's a Holley #4779 double pumper that was gone through by the Carb Shop in Ontario to 'match' my engine {they didn't tune it on the motor, just built it to match my motor}. I gave the place all the spec's on my motor as per my engine builder. I don't have the paper work from them so I don't have that much info {it got lost somewhere between the engine builder & the car builder}. I do know they cut off the choke horn. The plugs show it running rich {tons of black soot on them} so I thought I'd put in smaller jets. I showed up an hour or so into the dyno tuning of my motor so I'm not sure if the Carb Shop or the dyno operator put the 70/82 jets in. But it didn't put out anymore HP or TQ with the 71/83 jets than it did with the 70/82's so I thought I'd go back to the 70/82's.
My dyno sheet has Zero's under the A/F ratio :(. But it does show FHp Hp, ME%, FB 1b/fr, BSFC 1b/Hphr, CAT & Wat. Will any of these figures help??? Oh Eric, what does EGT's mean??? Mizlplix & Denny, I'll get a vacuum gauge & check the vacuum but is there a way I can measure what power jet I have in there now??? I'll look up in my Holley book r/e where is the power jet! THANKS All, Bill

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Twenty years ago or more all we ever went off of was EGT'S or exhaust gas temp.
We use use 1,350 F for good pistons in a drag application and 1,275 F for circle track application.
We would lean the carb. down until we got those numbers or very close to those numbers in each hole.

So what you're saying is not all 750's are not the same?.....wow, what a concept.....who'd a thunk it!?!?:rolleyes: **) I'm sorry but it takes a bit more to help if some of the "better" info is included.:)

Here's the pic again for jets and PV location. The power valve is on the reverse side of the block that contains the jets.

.....Wow, got the carb somewhere around 15 years ago so I have no idea which one it is. I'm going out to start changing jets in a few minutes & I'll look for numbers. If I remember right there were some numbers on the choke horn that the Carb Shop cut off :(.... Denny so it looks like the jets I have in 71/83 could be close to what the factory put in some of the 750 Holley's??? THANKS, Bill

I have the holley recommendations for you here. I too have some 4779's and some 3310's around so I thought you might need this. As you can see the jetting can be stock set at 75 primaries and 76 secondaries or the next two 4779 sets would be 70 pri and 80 sec. Have you measured the venturies and throttle plate bore? It would be helpful to know those. That would differentiate the two styles of the 4779.

Cool Denny, you can upstage me when ever you wish......I care about the info for good results.

Yah, I always change out the secondary metering plate for a block, just so I have adjustability. BTW: at this stage of life, if I need a carb, I just pour out my 4 buckets of Holley parts and assemble one to suit my application.

One thing good about having a "Carb Shop" carb, is that they usually convert them to screw-in air corrector restriction jets. These make removing an off idle stumble a snap.

ALSO: Look on the side of the base section. They sometimes have the carb list and model stamped there too. (For when the air horn is cut off.)

....Ok up-date, I installed my MSD cut to fit plug wires last night & the car doesn't seem to stumble at all. A couple of my old plug wires were cut up. MUCH better throddle response! Then tonight I put in jets that were one size smaller {70/83}. And while I was at it put in a new needle & seat {the Vitro tipped one} & the two washers that go with it {my old one stuck & I washed it out with water a while back. I also had two home made gasgets on the needle seat assembly so I also put new washers on. Then I re-adjusted the fuel level in the secondaries. So tomorrow I will test drive it & if it seems to runs ok then I'll put the new plugs in & see if it's running a little leaner. Oh I also noticed a vacuum plug on my carb that was rotten away so it had to be leaking. I pluged that up also.... Oh to seal the fuel lines I used that white plumbing tape. Is this ok??? I looked over the entire carb & didn't see any numbers. If my car doesn't run right tomorrow I'll try to measure the ventures & throttle plate bore to figure out what carb it is. What's a screw in air correction restrictor jet??? Yeah, it was actually fun changing jets! Never done it before! I even identified all my carb parts by the pictures that were posted!!! THANKS again all, Bill

I too am curious why such a spread? Is a spredbore double pumper? I remember these having sim. #s on the jets. A rich setting is better than lean from a longevity point. With unleaded fuels, a wideband A/F is critical to get it right on the edge. EGT's are too slow but will get you there. As other have said, plug color is hard to use unless it's spotting -which is piston matter from detonation.

Bill, keep truckin'. Go for everything you have read here. Falcon....no......Spreadbores have a huge difference between primary and secondary metering. I only wish to see all that follow this to understand A/F mixtures and that tuning is not that hard, but there is some patience involved.