I am noticing a slight missfire or exhaust popping sound on the gradual transition from steady cruise at say 45-50 mph or any other steady higher speed.If you are aggessive in the transition then it is not noticeable. The carb is an Avenger 870 with a 4.5 power valve in the primary.Could this be the problem in that the carb is going lean on the transition from a high vacuum cruise point until the power valve opens ? The jetting on the primary side is #78's.Any suggestions?

OK, I'm not any kind of carb guru but I met one once. He probably would've recommended that you get a long piece of vacuum hose and hook up a vacuum gauge to manifold vacuum and route the hose and gauge into the passenger compartment. Then he would probably tell you to HAVE SOMEONE ELSE sit in the passenger seat and watch the gauge while you drive. You tell the passenger reading the gauge when it starts to happen and he/she notes the gauge reading. This carb guru guy would then probably tell you that if the vacuum is higher (when it happens) than the power valve opening rating (4.5" in your case) then you need to change to a power valve that is rated at about 1" higher than the vacuum reading your passenger saw. If this doesn't make any difference, check your float level and be sure it's set correctly. It should be at the bottom of the sight plugs with the engine idling ON LEVEL GROUND. That same carb guy would also say that if the float level was OK, then you probably need to change the primary jets two(2) numbers higher. In your case that would mean #80's.
But then, that's what the carb guru guy would say, not me.
Louie Hammel

Louie I would like to meet that carb guru friend of yours someday. http://www.chevelles.com/forum/biggrin.gif

Alright Louie DOH!
I mean, carb guru.

Officially, all guru's were ordained during the sixties. If you didn't get your application in before 1970, you missed your chance. Since I didn't even graduate from high school until '76, it ain't me. It's been a while since I've seen that guy, I think he's somebody else now.
Louie Hammel

While I don't consider myself any kind of a guru (plus, I mised the application deadline due to not being born yet!) it sounds like you are on the right track. I have been messing around with the transition circuits alot lately so it sounds like you are doing the correct procedures. I have been cruising around with a vacuum gauge in the car just as Louie says and was surprised to see some different things. In the situation you are describing, I have been seeing vacuum readings about 15" high! I definitely didn't think it would be that high. I actually emailed Holley about it and they said to refer back to the instructions for picking a PV. Hook uo a vacuum gauge, then record the lowest vacuum reading with the car in gear at idle. Then pick a PV about 1.5 or 2" below that number. I was still having lots of problems, so asked about drilling out the idle air bleeds... here is what they told me:

"Drilling the idle feed restrictions will help this problem, when you take
the jetting down too far this will give an off idle stumble that a larger
squirter and cams won't take out. The idle feed restrictions in this carb
should be in the 26 thousandths range and drilling them to 28 or 29
thousands should help the transition. Do not drill them larger than that
or you will lose control of the idle circuit. Thanks."

I found out that I had drilled mine out WAY too big and Yes, had jacked it up pretty good!

Of course, with a 870 Avenger, 78 pri jets, and 4.5 PV, I'd say you had a pretty stout motor or it is way overcarbed!

My 467 BBC still runs a tad rich with 68 jets primary and 72 secondaries. Of course, I live at 6,200 feet elevation also!

Hope that helps,
Bill C.


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Bill C.
'71 SS (now with 467BBC)
'70 Nova 350/700r4
ACES #2780
Colo Spgs, CO

Wow, I am taking notes, this seems to be up my alley....
I have a BnB 350, sportsman 2 heads, Comp .50 cam, tunnel ram w matching twin -450 Holley carbs, headers w 2.5 inch exhaust to turbo mufflers, beefed up 350 tranny with 3500 stall, new driveshaft, posi rear end (orig). Trouble is in dialing it in so it transitions well. All carbs, manifolds, heads, shortblock, dist are new. Seems like the idle circuit on the carbs is fine, revs in park ok, pulls 10in vac at 850-900 revs. As I drive in first, I accelerate slowly to 1400 or so and then it starts "2-cycles" "stumbles", like the choke is stuck.. lol ( no choke, manual secondarys). About the same time, is when the vac drops, so I adjusted powervalves, etc. Checked float bowl positions, Timing set to 35-40 total. The engine has never been really dailed in right, or dyno'd, But it has run better. It did have an electrical short, alt wire melted part of wiring harness, etc. and I am tring to get it back to some form of running. Thats why I thought maybe coil shot. Thought maybe I flattened a cam or something... <scheese>, but they adjust well, vaccuum is steady at idle, got hyds, 1/4 turn from zero.
Anything I can try and /or check would be greatly appreciated.
Thanks, Scott