It wouldn't hurt to check the mechanical advance. With the timing light, observe that it advances when giving it throttle and that it returns to the base timing. If it is consistent, it's most likely alright.
Before swapping the power valve, put a vacuum gauge in the cab so you can see what's going on. It can be a regular vacuum gauge used to diagnose problems on a long piece of tubing. Nothing fancy. This will allow you to see what the vacuum does in varying conditions. You can use that info to ensure that the power valve "fits" the engine.

 

I will second that.

That is realy the only way to select the power valve correctly. Just make sure it is hooked to full vaccum not ported.

 

OK I'll try that. Since my vacuum line is metal I'll have to get a threaded nipple and a tee for a rubber line, but it can be done.

 

Change #2. As you guy's know I've been dealing with the miss and backfire while under load. Well this morning it did it while going DOWN hill. As soon as I got to a traffic light and stopped it smoothed out like nothing ever happened. And I thought I was confused before.

 

Blown power vavle. That is my guess. I bet you backfired it once and now you have that problom.

 

OK, I put in a new 9.5 power valve. No effect one way or the other. What now coach?

 

Did you check your vacuum so that you know without a doubt that the 9.5 power valve should work?

 

No I didnt. FLIPKLOS had mentioned that he put a 9.5 in his, and I found the new one in my box of extra part's, so I thought I'd try it.

 

In the interest of thinking outside the box. Is it possible that these problem's could be the result of a defective plug wire or microscopic crack in the distributer cap?

 

I also got a 4 barrel carb and probobly a hotter ignition. Should have verified that that was the correct PV for your application.

Even with the mention of probabale carb issues I thought it was a spark issue. Verify those issues first.

 

OK guy's update time. After a whole lot of effort and expense, to include having my carb proffesionally rebuilt, a new distributer and tune up part's I think I got it. Keep in mind the affore mentioned repair or replacement's had no effect on the miss or backfiring. As it turnes out the brand new Borg Warner coil that I put on less than 6 months ago was defective. I had it tested which showed an "intermitten drop in output when under load". So the parts store gave me a new one under warrenty. It runs so well now, I think I could climb a tree at 90mph and not have a single miss. Thanks for all your help.

 

Glad to hear it. Make sure you let a few chebby boys hear that nice y block burble.

 

Thought I had the mis figured out, but no such luck. In fact the problem now seem's to be getting more prevalent. In fact it not only misses and backfires in third gear under load but second gear at higher rpm's as well. I'm really stumped. There are a couple of other issues that might be relevant. Even though I re ringed the engine I'm still getting oil into the #2 cylinder. I'm not sure though if it's coming from an oil ring that didnt seat or from a bad valve seal. Also I didnt bother to replace the cam, so I'm wondering if it could be worn so bad that it would cause this condition.

 

I would think that that is a strong possibility. I would check your lift asap and consider investing in a new bump stick.

 

Thanks flipklos. Now please help walk me through this. My obviously mechanicaly inept brain cant grasp how the engine could idle perfectly and even excelerate smoothly and with good power, but then fall on it's face under load due to the cam lobes being worn. I would think that it would run badly and mis at any speed. Or am I over simplifying my grasp of it?