Thread: Holley Jets, WHAT the heck???

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....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.

Originally Posted by 48fordnut

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

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

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

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

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

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"?

Originally Posted by NTFDAY

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

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?