[big gear head]

I'm having problems with a customer's '72 Chevy truck. The engine is a stock 350 bottom end with a high performance street hydraulic cam, 461 heads, dual plane Weiand intake, 650 Quick Fuel carb with vacuum secondairys, HEI distributer with MSD conversion and 6A box, Headman headders. The guy who put the cam in said that he checked it and it was one degree off. I put a Moroso curve kit in the distributer and set it up for about 18 degrees advance. I used my piston stop to mark TDC on the ballancer and marked off 36 degrees. I set the timing for 18 degrees and 36 total.

The engine would pop back through the carb when I jabbed the throtle hard. I checked everything I could. I went over the valves, put in another distributer with a complete different ignition system, put my Camaro carb on it, checked all of the plug wires and made sure that it was wired right, checked the compression on each cylinder (150 to 170) and went back and checked TDC again with the piston stop. Nothing that I did would fix the problem with it popping though the carb. When I drive the truck and step on the gas the engine pops back through the carb and then it sounds like the mufflers are full of pop corn. It won't pick up any speed. If I get into it easy it will go ahead and pick up speed.

I got tired of messing with it and grabbed the distributor and turned it hard. I advanced the timing way past what it should be. It runs much better. It stopped poppling and it picks up pretty well when you stand on it. The timing is set at about 40 degrees and the total timing is somewhere around 60. I've never seen an engine that would even start with the timing set this high. It doesn't rattle when you get it under a load. I can't figure out why it has to have the timing set so high to run right, and how it even runs like this. Anybody got any ideas?

 

[Jerry70]

Considering that the problem didn't go away when you swapped out the carb and distributor, the only thing that I can think of is a mismatch between the balancer and the tab.

 

[jthomp1256]

sounds like balancer spun,,exspecially when you bumped timing and it help,,, jeff

 

[zeke67]

Either, or both, of the above are the first things that I thought of too.

 

[FRYNTYR]

My opinion is the Cam timing is off. Lined up the wrong tooth. If the balancer is correct and it takes a ton of timing to get it to run, it's normally the cam timing off a few teeth. Is the valve train noisy? If it is, it's another sign of the cam timing off (at least when it's 3 teeth)

Also, could be a balancer like what they call a van balancer. The TDC is almost straight up and down and uses a cover with a tab there as well. Normally would be found on a 305. Thing is you checked for TDC and marked the balancer so this kind of rules this out. I'd look back at the cam timing. Bet its 2-3 teeth off.

 

[FRYNTYR]

Here's a link to Chevytalk from several years ago that has some good reading on the possibility of the cam timing being off.

http://www.chevytalk.org/fusionbb/s.../311232/hl/Valve+problem/fromsearch/1/#311232

 

[big gear head]

I checked TDC 2 times with a piston stop, so I'm sure that the ballancer and pointer are not off. The valve train is quiet. Cam timing is all I could come up with too, so I'm wondering if the guy who put the cam in knows how to degree a cam. Could the cam be ground wrong? The reason that I thought of the cam timing is that when a timing chain jumps a tooth you can advance the timing to get the car running again. But it still doesn't make sence that the timing could be advanced this far and not cause problems when trying to start it or with spark knock. When I get time to work on it agian I think I'll put my degree wheel on it and check the cam timing myself. Is there anything else that could cause a problem like this?

With a standard differential, 3.73 gears and a 350 this truck should make tire smoke until you couldn't see it anymore. It won't spin the tire from a stop, so I know that something is wrong.

 

[pnugene]

Freddie, I'd pull the timing cover on it, it's just about got to be cam timing. Have you pulled the valve covers to make sure this truck doesn't have a dead cam lobe? Probably a good idea to adjust the rockers too.

 

[big gear head]

Yea, I checked all of the rocker arms to see if they were moving the same amount. They all looked like they were doing their job. I went over the valves 2 times. The first time I found about 3 that were too tight and I thought I had found the problem, but it still ran the same.

 

[mr 4 speed]

use Harold's "quick and dirty method" of checking the cam timing:
http://www.chevelles.com/forums/showthread.php?t=24805&highlight=Harolds+quick+dirty+method

I found this in the performance forum doing a search...

 

[big gear head]

Thanks, I read it and I'll look into the cam timing when I get the truck back to my shop. I don't know what brand the cam is and I don't know any specs on it. The cranking compression seems pretty good at 150 to 170.

 

[77 cruiser]

MSD ign. Are you using a dial back light?

 

[t10648]

you must be off on the original stabbing of the distr. ( one tooth or two) that car would never start with 40 degree advanced timing

 

[big gear head]

No, I have a standard Craftsman light that I've been using with MSD ignitions for years. I borrowed another light and tried it to be sure that my light wasn't the problem and the other light checked exactly the same as mine.

I didn't think it would start with 40 degrees advance either, but it does.