Been scratching my head and need some advice

[mr402]

The basic problem is when under a medium load my truck will pop through the carb. It rarely happens in 1st gear. Never seems to happen above 2500 RPM. The pop happens during normal acceleration or when going up hill at lower speeds (under 2500 RPM). I can accelerate lightly and it usually won't pop. If I hit the gas hard it won't pop either. It idles fine and will cruise very nicely at 65 (2500) RPM.

The carb was an Edelbrock so I tried replacing springs etc. but finally gave up and put my old Holley back on (6210 spread bore and installed a Holley renew kit) . No change. As an aside note it had been running with the Edlebrock for a few years.

It does have an adjustable vacuum advance so I put it a little more advance which seemed to make it worse. Then tried disconnecting the vacuum advance altogether. Nope, still popped. While still chasing the advance theory I put the stock weights and springs back in the HEI distributor. No Change.

I've used carb spray to check for vacuum leaks. Everything seems to come up OK.

A few other basics.
The engine is a 402 that was rebuilt about 80,000 miles ago.
Modifications include an Edelbrock performer intake, HEI ignition, the previously mentioned carbs, and advance mechanism changes (which are now back to stock). The exhaust is basically stock. All modifications were made some years back and it ran OK. The cam was basically referred to as an RV2. Overall pretty mild and kept power in the lower ranges for towing etc.

So after his long winded explanation what do I check next? I'm pretty much at a loss as to where to go from here.

To make matters worse I'm moving and really need to get my GMC running right so I can tow my other projects to their new home.

Your advice is appreciated.

[Cannonball1]

If it's popping back through the carb, the plugs are firing while the intake vavles are still open or wayyyy too early (advanced). Your statement about adding more vacuum advance helps support that idea. You didn't mention what your static timing is set for. You might try backing it off a few degrees to see if that helps. But it really sounds like the advancing system (vac or mech) is working too quickly and once you get up past a certain RPM the engine "catches up" to the spark timing.

Was anything changed just before the problem started?

[GASoline71]

Sounds like a timing issue or you have a couple of plug wires crossed.

Gary

[ole dollor]

i would start with checking the plug wires and dist cap and rotor. you could have a cracked cap or rust trail inside the cap causing your fire to cross over, your rotor could be burning through the middle....there is a lot of things it could be....wont cost much to check the cap.....and you do have 80,000 miles on the motor....that's a place to start....

[mr402]

Timing is the first thing I checked. Initial with the vacuum advance disconnected is 10 deg. It's been running at that setting ever since I installed the HEI. Over time I also installed heaver weights and lighter springs in the centrifugal advance and installed an adjustable vacuum advance. While troubleshooting I reinstalled the stock weights and springs and even disconnected the vacuum advance altogether.

Plug wires are in the correct positions. Nothing here was changed from when it ran great. It idles and cruises at 65 very nicely. Just takes while to get there if I try to keep it from popping.

While changing the centrifugal advance back to stock I took a good look at the cap and rotor. No trails, no burn holes. They were replaced at the last tune up about 2 years ago. This is not a daily driver anymore so they only have about 8 - 10,000 miles on them.

The only change made before this problem started was to put a renew kit in the Edelbrock carb. That's why I was working so hard on that angle, changed springs etc. but it never helped the pop under a medium load (thought the accelerator pump was lean). After a while I gave up and put my tried and true Holley back on even though the Edlebrock had been run for a few years before hand.

The last thing I forgot to mention is it seems to pop less when cold (or colder outside temps) than on a hot day. Now you see why this has me puzzled.

[ole dollor]

well then back to the carb. check for vacuum leaks bottom gasket , check all rubber lines to carb and intake, booster.....you said that the carb. would not adjust . try cupping your hands over the throat of the carb like your cutting all the air from going in and see if it runs different., if the carb wont adjust it may need to come back off and look inside of it again....that is what i would do next.
does it seem to be using a lot of gas lately....look to see if your back butter flies are closed all the way.... good luck...

[PHAT TONY]

I hate to say it, but I'd pull the valve covers off and make sure all the rockers are moving the same amount; seems like you're describing a cam with bad lobes.

[ole dollor]

phat tony: that's a great idea.....or one rocker just came loose....seen that before....

[mr402]

Ole Dollor, the carb does adjust. The only vacuum line not disconnected was to the brake booster. It's possible I have a vacuum leak there but I've sprayed carb cleaner all around it and didn't detect a leak.

Phat Tony, unfortunately that's where my head is going too. I've looked just about everywhere external to the block. Looks like I better take get all my tools to the new place and start digging in. I would have thought I'd hear some noises from the valve train if it was that worn but it does have hydraulic lifters so who knows. If I do need to replace the cam this time it's going to be a mild roller.

Thanks guys, if anyone else has good ideas please comment.

[msgdsrf]

Just sorted a similar problem, worn exhaust lobe. Pull off the covers and check out movement.

[elf w/capital T]

i had the same thing, hopefully your not running off 7 cylinders like i was and didnt know. ha good luck!!