Car is a 68 Charger, 383, low miles since rebuild (two or three thousand) 10 years ago, stock cam, stock Eddy heads, MP duel plane, stock exhaust manifolds, MP electronic ignition with orange box and Holley Street Avenger Carb. Timing is about 34 total at about 2500 RPM

Car has been in storage for last four years and I am bringing it out. Pulled fuel tank, cleaned all lines, etc. Pulled the plugs yesterday and they were extremely carbon fouled. Plugs are Champion RC12YC and gapped at .035.

This car always had an off idle stumble and when driving straight and level it would surge. Other than that the car ran very well.

I bought the carb new and this is the first time it has been apart. Jets were 66 primary and 69 secondary. Power valve is 6.5, and squitter is 35. The mixture screws were almost exactly ½ out on both sides.

My plan was to increase jet size and to try to resolve the surging issue but looking at the plugs it seems the car jets may be to large; I would think 66 and 69 would be a little lean for this engine.

Looking for some advise on what the correct jet size should be and correct power valve should be for this particular application. Why are these plugs so bad?

I'd start with the simple stuff: clean it out thoroughly/float levels/ confirm immediate tip in when throttle is barely moved/replace or glassbead the plugs/jetting up would be easy to do & see how it acts/confirm no vac leak anywhere in intake tract. EDIT excessive choke and or pedal pumping to start it could have fouled em & confuse the diagnosing cuz its acting lean but the plugs show rich

What do you think about a hotter plug? FYI the stock jet size is 65/68. Holly recommends going jetting down because of the altitude here in Tucson; about 2000 ASL

not a heat range issue. I'd get some clean plugs in there/make sure it ain't an excessive choke on problem, then we can get a baseline to work with. keep posted

I know you don't want to hear this but replace the carb. That carb is based on an emissions version of a CA carb and no matter how you jet, change squirters etc. it's never going to run like a Holley 3310 series or other older Holley design.

i agree an old Holley would probably have been better out of the box, but if you want to work on this one, its possible to do.

A lot of good information on http://racingfuelsystems.myfunforum.org/ as well as the archived innovate and MSV forums.

There's some things that are important to understand so you figure out what is the logical items to test and adjust. The main jets have little to no effect on idle, off-idle, and light steady throttle. The 'idle circuit' provides the fuel mix during those conditions as well as deceleration.
However the idle circuit responds slowly to change in throttle and load, and that's where the accelerator pump comes in.

If the engine is surging at interstate speeds, say 60 - 65 mph and up, then the main jets are the primary fuel restriction and and a little richer jetting is needed in that situation. The main jets plus the power valve restrict the fuel during wide open throttle - and under full load being too lean can cause damage. So on the drag strip, I prefer to work in from the rich side as long as the plugs aren't getting carbon fouled!

I completely agree that cleaning or replacing these should be done first. I'd probably clean them if they're not too bad. Then once idle and very gentle off-idle acceleration are working and not fouling the plugs, put in a fresh set.

IMO the best explaination and diagrams of how a Holley works are in Urich and Fisher's Holley Carburetors and Manifolds and the second best is Urich's 4150/4160 book. Both published by HP Books. You don't need the latest versions, a used copy will do fine. Vizzard's new book is more thorough in some ways, but in doing so, I think its not as good an introduction to the basic concepts. Also if you follow Vizzard too closely sometimes you end up going down wrong paths. Been there. Some of that is IMO the format of SA Design books.

Problem: WHICH Street Avenger?

There are four different sizes.

The biggest problem seems to be with the 670. My 570 worked very well and was better than the List 1850 600cfm Holley it replaced.

The 670 is supposed to be pretty lean and swapping a 750 metering block into it should help.

Those plugs don't look like you have a lean condition to me, but surging at steady highway speeds sounds like it's lean there. There are a lot of questions. For example, idle mixture screws usually are set up to baseline at 1 1/2 turns out from lightly seated. As you get higher in altitude, they tend to go in a little bit, like maybe 1 1/4 turns out from lightly seated. But, being able to run at 1/2 turn tells me that you're getting fuel from somewhere else.

This is going to be a long process and you need to get a good book, as mentioned above, then tackle one problem at a time. I'm with RR on the first thing to do being buy a real Holley carb kit, take the thing apart and make sure everything is clean and sparkly. Idle and high-speed air bleeds need to be open, float levels must be set properly, power valve checked for integrity (even though all Street Avengers have PV protection).
There are probably a number of things that are contributing to the problem.

It'd be nice to know what your cruise vacuum reading is.

And what is the List number of your carb?

R.

The list number is 80670. The carb is about 10 years old. I don't really remember. The carb has been rebuilt and it's clean. he plugs were pretty fouled; I'm surprised they even worked. I'm going to put the carb back to stock and go from there