Since there have been so many carb tuning/set up related threads I thought I would share my thoughts after replacing a List 4346 780 model 4150 with a brand new Avenger 670 which is also a model 4150.

Out of the box the 670 floats were not adjusted properly and the electric choke and fast idle were way off. I've seen the same problems on other brands of carbs fresh out of the box, so my recomendation is go through any new carb you get and make sure it's set to factory spec's. My guess is the tech's on the assembly line could use better quality assurance practices...

The choke and fast idle really can't be messed with until you fire it up for the first time, be ready for your rpm to jump to about 2500 - 3000 when you do turn the key. With everything else cheched and reset, things went real well with the exception of the fast idle. You can't get to the adjustment screw with the carb running as you have to move the throttle wide open to reach it. The instructions tell you counter clockwise to reduce the fast idle but they don't tell you that's if you turn the carb upside down.

Once everything was set to factory spec's and I got the choke and fast idle figured out this carb was sweet. Idle mixture screws dialed in 14" - 15" of vac at 5/8 of a turn from seated. It idles steady at 700 rpm and there is no stumble off idle at all. Throttle response is better than the 780 but the 670 has a notable kick when the sec's open so I will be going to a heavier diaphram spring to slow down their opening some.

I noticed something between the 2 carbs that I'm not sure about. On the 780 I could rev the engine by manually cracking the sec throttle blades. The 670 stumbles and then revs when I do the same. The only difference I believe is the 780 has a sec power valve and bigger jets. This could indicate the 780 was a bit rich I believe.

All and all I'm impressed with the performance of the Avenger carb and thou I was worried I would be under carbing my 383 it doens't seem that way. My 383 has a hyd roller 224 @050 and out of the box sportsman 76cc heads with 9.8:1 compression.

So avoid lots of tuning frustrations and check out that new carb before you install it and when ever possible don't fire a new engine with a carb that's never been run. No sense in complicating things if you don't have to...

Dennis, is the Avenger the line with the clear sight plugs for the float level adjustments? How does that work out in practise? It alwsys seemed like a good idea.
-Andy

Hi Andy - Yes, the Avenger has the clear site plugs. I don't like them (ok for initial tuning) as I've seen them break and leak. I tried some in the 780 and they got soft and clowdy and broke when I removed them. I'll be replacing these shortly...

The 4346 has a lot of jet in the sec. for a SB with a dual plane. I just worked one out for a friends L-72 and the stock sec. jet was perfect. The Avenger may have the throttle blades closed so far that the sec idle slots are completely covered. that may cause the difference although I haven't really paid much attention to opening the sec by hand. Remember a while back I said in my opinion a 780 is really too much carb for a street SB? It may even pick up your mileage some. Good luck

My 4053 780 carb was way rich for my 302. I had to go from 76 to 66 on the secondary jetting to get an acceptable WOT air-fuel ratio on the dyno.

Neil what changes did you make on the primary side of things?

Mike with the od trans I managed 23 mpg out of that 780... Might be more carb than a typical sbc that only spins 5500 - 6000 rpm. But big cubes and/or high rpms and the 780 will get a workout. Vacuum sec's make it forgiving, though throttle response might be better with a smaller carb..

DJD,

Prior to the dyno test, I had to jet up from 68 to 70 on the primary side to eliminate a part-throttle lean condition. I also had to change the shooter from a .025 to .031 to fix a tip-in stumble.

On the dyno, we changed the stock pump cam to a green cam in the #2 position to fatten up the mid-range before the secondaries came in (a spring change would have been a better solution, but we chose the easy fix while on the dyno).

I was a bit concerned with the severe jet change. But thinking about it some more, my 70/66 jetting with a secondary PV is pretty similar to the 70/74 jetting with no secondary PV that the 3310 guys seem to run. In my opinion, my 780 required too many changes for my combo. I think a 600 or 650 mechanical secondary carb is a better match for my needs.

Thanks for the info Neil - I never finished tuning the 780 on the 383 but was fairly happy (drivability wise) with 70/78 jets, 8.5 pv's 2 sizes up on the squirters over what ever is stock and the pink cam. Idle mixture was about 1/2 turn out and on a cold CA morning it would idle without the choke after 30-40 sec's. Just didn't have the response I was looking for though and I never have had the chance to do the dyno with it... I've only driven the car once with the 670 and initial feeling is the mill likes it much better...

I got the good mileage with the 68/76 jets and it improved performance a touch going bigger. I did go bigger on the vac sec spring though so the sec's didn come in as soom...

The 4346 has power valves in both ends and comes with 68-76 jets. the pri. side is pretty close for a small block but the 76 sec jet is usually too rich for a small block. I hope people aren't running a 74 sec jet with no power valve in the later 3310s that is really too lean for just about any application.

DJD,

Although the clear site plugs are notorious for becomming discolored and often leak, they are fixed on the Avenger carbs. I know this because I was struggling to set my rear float on mine, so I pulled the rear apart and began to remove the site plug only to find that it crumbled while trying to remove it now I had to tap the bowl assembly to put a brass plug in the thing.

Just a word of caution


Regards,
Tom

Thanks for the word of caution, I saw an avenger the other day that had the big thick (as opposed to the smoothed out factory one) ones you can buy for holleys to replace the brass ones. It raised a flag as they were tight but didn't fight all the way in so the "O" ring made a seal... :eek:

sorry if I'm hijacking here
but what's the concensus from the people who prob'ly know (that's you all) of the demon vs. avenger matchup?
I still can't decide
was thinking 670 now thinking maybe a speed (or road?) demon of the 625-650 persuasion
333 (.040 over 327) 236 @.050 cam, trickflow heads, 2500 stall and 4.10s, rpm intake

thanks as always!

I was running the Avenger 670 and swapped to the Demon 625. All in all, I would have to say my engine ran the same with both carbs, though the idle seemd better with my cam and the 625. I love 4 corner idle screws!