Alright, this forum helped me fix my off idle bog problem (YOU GUYS ARE AMAZING! MUCH APPRECIATED!), but now have a new issue that I have been pondering for weeks now.

350 engine. 670 Holley avenger vacuum secondaries.

Engine idles very poorly. RPM is not consistent. manifold vacuum fluctuates. I put my hand over the tail pipes and I'm not getting a steady pulse from the engine. When I put it into drive it get's even worse. Very choppy, almost sounds like it wants to stall. white/grey smoke out back while warming up. Here is what I have tried so far.

Checked and rechecked everything on the carb. Squared the primary throttle plates and closed the secondary throttle plates since I get the best idle that way. Idle mix screws are about 1.5 turns out, if I tighten them any more the RPM slows down and if I loosen it more it's too rich. Set to ported vacuum since manifold vacuum it backfires through carb. getting 20 in of vacuum from manifold.

Timing is set to 11 initial. Any more or less the idle gets worse. The other day I couldnt even keep it running. It would start, but stall right away. Found out it was flooding and replaced the power valve in the holley and it ran again. Plugs looked fouled after that so I replaced them too. I then checked compression. Compression varied from 125 (was the lowest) psi to 150. So nothing was dead.

the only other thing I can think of is that I started putting regular gas in it a few weeks ago when I always put in plus gas. I didnt know how to drain the tank so I tried flooding it with higher octane gas. I thought it sounded better but after a drive it was still running kind of rough. Still no luck.

OH! And the crazy thing is, when I jump on the gas the thing really fly's! Seems amazing when I'm accelerating hard! But once I let off the gas and it slows down it farts out the back and runs bad again.

Should I try flooding the gas with higher octane even more?

I'm out of ideas.

higher octane only makes the fuel harder to ignite.

its most likely either a fuel/air issue or ignition issue

If it actually does run perfect when the revs are up like you say, then I would say it has to do with the carb tuning (when i say carb tuning I am referring to only to idle mixtures and idle speed) or the timing

I'll just get a stupid question out of the way:

did you unplug the vacuum from the distributor when you timed it?

Off idle seems good, so it seems you are running too lean or rich at idle. Do this to check idle lean/rich condition. With engine idling and distributor vacuum advance line disconnected and plugged (like you are setting the timing), cover carb with your hand to reduce air to the engine. If rpm increases and engine idles better, then you are running too lean at idle. If engine tries to die when covering carb, you are probably running too rich at idle. To check idle rich condition, with engine idling as above (without hand on carb), unplug vac advance line to let in more air to engine. If engine rpm increases and runs better, you are running too rich at idle. After these tests, you can determine which way to adjust your idle mixture screws.

Some passages in the idle circuits of the carb may be plugged - I'd take the carb off the car, and REALLY clean it all up. When it's off, readjust the carb to normal settings (accel pump arm, throttle blade opening, idle screws out 1.5 turns or so), and push your timing back where it should be at initial (16-18°).

Start the car up and see how it reacts, adjust idle speed and screws accordingly.

Ok, I tried the hand over the carb test and it didnt change it either way unless I completely choked it and it stalled. Unplugging vac advance line made no difference in RPM either. I tried fine tuning timing, idle mix screws, and idle screws for hours (literally). I also forgot to mention that I tore the carb apart several times and cleaned it all out. Also a complete rebuild of the carb a month or two back. Fuel bowls are set perfectly level. Still giving me a rough idle and even worse in drive. Could there be a bad part in the carb? Try my old idle mix screws? Still good acceleration but I noticed while coasting down a hill the exhaust would make popping sounds.

I also wanted to ask; why does the car run worse as I open the secondary throttle plates? Shouldn't it increase idle speed? Instead it just runs slower, choppy and stalls.

it runs worse when you crack the secondaries at idle becasue you are giving it a bunch more air without giving it more gas at the same time. unless it's got 4 corner idle screws, it has no idle circuits on the secondary side and it takes more airflow than what it sees at idle to start sucking gas out of the venturi.
forget about the secondaries- they seem to be working fine. concentrate on the front half of the carb.
baseline everything- idle screws 1.5 turns out, idle screw set so the transfer slot looks liek a square (back the idle screw off until it isn't hitting the arm, and then turn it back in a turn or so. or just take the carb off and look at the slot). hell, take the carb off and shoot every orifice you can see with a can of carb cleaner.
set the base timing to about 18 without the vacuum advance hooked up, then put the vacuum advance to a port that always has vacuum.
also check all of the intake bolts to make sure they haven't backed off and caused a vacuum leak.
if all this playing around doesn't help, then you've got to look at the engine itself- it's possible your cam decided to go flat or something else mechanical. your bouncing vacuum reading and varying compression numbers kind of point towards something like this, but look at the free and easy stuff before worrying about this.

Ok, thanks for the replies! I took the carb apart, AGAIN, and decided to replace 3 of the 4 blue gaskets (since that's all I had), that separate each section of the carb body. (even though I just rebuilt the whole thing 2 months ago) After putting it all back together and started the car, it idled a lot better. Took it for a spin and idled in drive a lot better too. Perhaps an internal carb leak? Although I'm still concerned since it still runs crappy while warming up, so I'm going to buy all gaskets and see if it clears up any more.

BUT, I'm not calling it fixed since I thought I fixed the problem before and the next day I started it up and it was back. :mad: I will let you know. Thanks again

did you give the carb a good cleaning with carb cleaner while it was apart?

I took it apart 3 times, cleaning everything out with carb cleaner 2 of those times. I paid close attention to every hole I could find.

Has any one heard of using Pam to spray on the blue gaskets so they do not stick or tear when taking apart the carb later?

And I notice when I take my carb apart there is a brown sugar like substance on the inside that seems to be corroding the carb itself. It's hard to describe but it looks like this substance is breaking apart a thin layer on the metering block.

I took it apart 3 times, cleaning everything out with carb cleaner 2 of those times. I paid close attention to every hole I could find.

Has any one heard of using Pam to spray on the blue gaskets so they do not stick or tear when taking apart the carb later?

And I notice when I take my carb apart there is a brown sugar like substance on the inside that seems to be corroding the carb itself. It's hard to describe but it looks like this substance is breaking apart a thin layer on the metering block.

I've heard of this before with a very specific batch of Holley's - might want to contact them with your carb numbers, they'll fix it if it's what I remember. This would explain it going bad continually also.

Hot Damn! Xtreme you were 100% correct. I just got off the phone with Holley tech support. My carb is about 3 or 4 years old and the tech said that during that time they build a batch of holley's with the wrong metal material, causing the metal to break apart. They are sending out a new one and I will let you know if this resolves the problem.

Just for reference here are my Holley numbers.

Holley 670 Street Avenger
80670-1
0916

got the new carb, slapped it on, idles beautifully (so far). Thanks to holley's quick service, no questions asked.

secondaries still not opening, I'll start a new thread.

thanks again!

That's why I love this site. :thumbsup::yes: