[66GTO Jim]

In the book, ROCHESTER CARBURETORS by Doug Roe, it has a warning not to mix jet types between carbs on page 116. It also has cutaway drawings of the different types of jets on page 117.

Here's the quote in a warning box at the bottom of page 116:

DON'T MIX JETS
"If you tune several cars with different types of Rochester carburetors, remember that different styles of jets are used. Monojet and Quadrajet carburetors have jets with a squared shoulder under the head of the jet. Jets for the B, 2G, 4G and H have a tapered shoulder under the head of the jet."

So the Tri-Power (2G carbs) would use the tapered shoulder and the Q-Jet would use the flat shoulder. You can easily see the difference in jets once you know what to look for by looking down through the top of the jet into the fuel passage.

Do any of the carb experts know why the difference and what if any affect it would have on either carb types performance by swapping in the incorrect jets? It sure seems like a very small difference but Doug Roe took the time to put the warning and pictures in his book so there must be a reason.

By the way, I only use the tapered jets in my Tri-Power due to the book. I did buy several sets of "Tri-Power" jets from a reputable vendor (not our sponsor) . All packages were marked "Tri-Power" with the jet size. All were flat shoulder jets for a Q-Jet not the tapered jets for a Tri-Power carb.

Did a search of the Forums and did not find anything on this. Thought this might be an interesting topic to kick around.

Jim

[steve25]

The tapered entry jet would flow flow slightly more fuel, but I think the location of the orfis in reguards to top to bottom needs to checked as when used in a Q-jet that will ecffect metering rod to orfis height?

[Tim john]

Cliff only warned me to not use used qudrajet carburetor jets for they could have internal wear and the sizing may not be correct anymore. Cliff sells brand new ones so I purchased mine form him. I run in a 1966 Tri Power 64 in the center carburetor and 70's in the outboards with real nice results and excellent street manners.

Tim john---

["QUICK-SILVER"]

They're just plain made different.
The taper in a quads jet works with the needle. On the 2G it's just a hole, like a holley, not made for a needle to mesh with.

I can see using quad jets in a 2G not being any different than modifying an OE jet for better high RPM flow. BUT it wont work the other way, a 2G jet in a quad.

I think what the author was trying to convey is that it's just easier if you don't get them mixed up. Less chance of error if you're not familiar with how the parts all fit together and work.

[DiamondJim]

I agree, it is just a hole for fuel dump. My 2G ends have been tapped for Holley jets, FWIW. -Jim

[Tripower455]

All 3 of mine have Holleys in them!

Jim, what size jets are you using in your ends? Also, are you using original tripower bowls, or the modded 2G bowls? I am having major rich issues with my CVM modded 2G secondaries that cannot be controlled by jetting. I think one or both might be leaking through the epoxy sealed power valve passage. I've gone down from 74s incrementally down to 67s in the ends, and still running high 11 to low 12 AFR and blowing a ton of black smoke at WOT.

I'm gonna pull them next week and put my stock tripower bowls back on and see what happens.

[goatman65]

I ran q-jet jets in mine for years,no problems.

[Cliff R]

I just rebuilt a 2GC Marine unit yesterday, 1983 vintage and it had a type of jet in it which I have never seen. It had a wide-gentle taper from the outside area all the way to the hole.

I use tapered entrance jets in all the 2GC's and Q-jets we set-up here, no issues anyplace. I have seen deviations in the actual distance down from the flat top surface in jets supplied by various manufacturer's, which isn't a problem if the builder is checking the position of the metering rods in the jets during the build (which they should be anyhow).

NEVER assume that the jets are even the same distance from the top of the casting right to start with. I've seen quite a few that were set deeper into the casting during the machining operation, and on the Edelbrock q-jets it's quite common to find then at different depths!

Since at least 90 percent of the power piston arms are bent up or down on carbs I get in here for rebuilding, we ALWAYS equalize the arms and set them for perfect positioning of the metering rods in the jets during the rebuild, something most builders never even think about.....FWIW.....Cliff

[66GTO Jim]

Originally Posted by Tripower455

All 3 of mine have Holleys in them!

Jim, what size jets are you using in your ends? Also, are you using original tripower bowls, or the modded 2G bowls? I am having major rich issues with my CVM modded 2G secondaries that cannot be controlled by jetting. I think one or both might be leaking through the epoxy sealed power valve passage. I've gone down from 74s incrementally down to 67s in the ends, and still running high 11 to low 12 AFR and blowing a ton of black smoke at WOT.

I'm gonna pull them next week and put my stock tripower bowls back on and see what happens.

I am running 68s in the center carb and 73s in the end carbs on 447 cu in (stroked 389). Also stock 2G carbs and bowls. I used to have constant boilover especially in the front carb but that went away permanently when I added the phenolic spacers.

This setup runs very good for me, probably too good as I can easily overpower the rear tires with all the torque I am running.

Reason for this post was to get some feedback on the various style jets Rochester used. Great info so far, hope some other carb experts can find the time to chime in. Jim

[slowbird]

What is the biggest q-jet jets you can get?

[Kenth]

ACDelco 7031979, stamped #79 measuring .082".

[slowbird]

we need 86, we have been drilling them, i guess that is what we will have to continue doing