Frickin carb (maybe) problems, Help!

[USSkoval]

I have an issue with the 350 in my '77. Until yesterday, it had a holley 670 truck avenger carb on it that the PO installed on it when the motor was rebuilt and had troubles with it since day one. When I bought it, it would pop rapidly out the exhaust and lose power, but only under heavy load and in the upper RPM range. On a cold start, it would feel like it was flooding out during very light throttle, but if you revved it once, it would clear up.
It wasn't a very big deal and was plenty drivable, but I messed with it anyway After adjusting the idle mixture, float level and accelerator pump, the problems were about 75% gone, but only for about a week. Then they started getting worse again and stayed that way for a couple days, then over the course of a day, the truck became very hard to accelerate. If I gave it too much throttle, it would pop/backfire VERY rapidly through the carb and make NO power.
Convinced there was something major wrong with the carb, I removed it and installed a used 670 Street Avenger that a buddy used to run on his Trans Am (after taking it apart, inspecting it, replacing some seals and fixing a float spring). This carb is doing basically the same thing. It idles a little rough, and pops rapidly through the carb when I rev it. I'm afraid to drive it to see what happens.
Fuel pressure is between 4 and 6 PSI, and vacuum is around 15-16 during idle. There is no black smoke from the exhaust. The plugs looked OK, maybe a little glazed, but I replaced them. Also, a lifter started ticking right before the drivability issues got really bad. What the heck is going on here? Please help!

 

[MTChevy]

have you checked your timing at all?

 

[topless jackass]

I smell a cam gone flat, or cam timing issue. Maybe bent valves.

 

[topless jackass]

wait, I just reread your post and saw you have a lifter ticking...check valve lash adjustment and while yer there make sure you don't have any bent pushrods. If you are running roller rockers you need to be sure that the tops of your rocker arm studs have been ground flat or the cool locknuts with the set screw will back off. I've experienced this and remedied it with stud girdles. Flat cams on early 80's gm's would pop rapidly thru the carb. I know you said the PO rebuilt the engine but who knows if he broke the cam in properly.

 

[USSkoval]

I had checked the timing a couple weeks ago, and at that time it was OK. The lifter tick developed going about 65 mph and was loud at first, but has quieted down some. It is definitely just one lifter ticking. The cam is a Melling and has around 8K on it at this point. The PO is a member here and I sort of remember him mentioning breaking in the cam in a thread.
I hope the cam isn't the problem

 

[topless jackass]

Not ignition timing, your cam timing. Did they line up the dots correctly when they installed the cam. the reason I question this is because you said the problems have been there since day one. I would do a compression test, too. This will ensure you don't have bent valves. The popping thru the carb still makes me lean towards a cam or valve adjustment issue. If the cam was improperly timed the pistons may have bent every valve the first time it was rolled over. I'm not dissing the PO by any means, I'm just thinking back to things i've seen.

 

[USSkoval]

Ignition timing was in response to the other poster. I'm not so sure that these set of problems have been there since day one, just problems in general with the carb. The minor drivability issues that were present when I took ownership certainly felt like a carb issue, and the problem wasn't always there when the pedal was mashed, just usually. Then the rapid popping through the carb started, so I switched carbs and nothing seems to have changed. If the cam lost a lobe, it should be obvious when adjusting the valves, right?

Also, after taking apart the 670 truck avenger carb, nothing really pops out at me as being wrong. Just a milky/oily substance in the cavity behind the power valve, and the main jets are #68, and the secondaries are #89. Shouldn't the main jets be the larger ones?

 

[USSkoval]

Also, I don't recall losing much vacuum while reving the motor and the popping occurring. Shouldn't that cause a major loss of vacuum?

 

[badmix]

Simplest solution would be to get a cheapo vacuum gauge and install it in the engine bay, just T off one of the carb vacuum lines. Get as much hg as possible. should be between 15-20. keep tuning til it goes up as high and steady as possible, once it starts dropping off, turn it back. Youll have to adjust the A/F mix and the idle as well.

The popping sounds like air is getting in somewhere it shouldnt and/or timing issue. There are various points of timing, from the front gears to the dizzy. also did you disco the vacuum advance to set timing?

 

[USSkoval]

Time for engine repair. The #1 exh lifter collapsed and wiped the lobe on the cam. The pushrod on that lobe only moves less than half the distance of the others. The #7 exh lifter is also collapsed Must be that cheap overseas crap.

 

[topless jackass]

Bummer about your cam. I'm not a fan of Melling stuff anyway. Don't forget to clean out your oil pan, all that metal had to go somewhere.

 

[USSkoval]

On the bright side, at least I get to pick out a new cam and switch out the single-plane torker intake for something more truck friendly. Thanks for the heads up too, it helped ease the pain when I sort of expected it.