Replace the HD plugs with quality NGK plugs first of all.

Since it's getting worse, I'm thinking it's an electrical connection that is getting looser and looser.
Do the easy stuff first.

 

You're probably right. I've pretty much gone over all the electrical connections and replaced the electronics except the CPS but it could be something simple as that and I just haven't looked in the right place.

 

Awesome! Lots of things to check so it looks like I have a busy day tomorrow.

Thanks guys for these good common sense suggestions...sometimes I need a kick in the another direction when I get stuck in one place.

 

Have you replaced the lifters?

 

Sounds like it may by a cam lobe issue, would explain the gradual lack of power.

 

I keep kicking myself because I just didn't think to replace the lifters when I replaced the tensioners and that has been nagging at me especially now with this elusive problem popping up out of nowhere. It is possible I might have a bad lifter.

I am leaning toward the big-bore modification if I have to go back into the valve train again. That will replace everything, cams, lifters, pushrods, heads, pistons, cylinders, and firing system...

 

I am back on the road...

It was the ignition module and the exhaust leak all along. I first noticed the lean symptom of the leaking exhaust and was tweaking the mixture before the decel with all the backfiring started up.

I still had the problem after I rebuilt the carb and replaced the intake and exhaust gaskets so I ordered a new performance module and coil kit. I should have stopped there and waited for the parts but I broke the cardinal rule of wrenching...continuing to troubleshoot instead of waiting for the parts and going from there.

I just could not let go that the problem was the carb/vacuum so I just had to pull it all apart again and check one more time before the ignition parts arrived and that is where I really screwed up. When I went back through the carb, I got the needle jet reversed...and to make it worse, I assumed to believe that I could not have put it together wrong because I have done it so many times so I never pulled it out and reinstall it to make sure, just glanced down the throat and verified that I could see it but not really looking at it. I did this not once, but twice. And to top it off, one of the suggestions I got was to pay careful attention to how the needle jet is installed. So, when I put in the new ignition module and coil, it really did run worse and it was all my doing. I had it right the first time. Going back into the carb without verifying the ignition system was good was unforgivable, stupid, careless, complacent...all the above.

Anyway, I got the carb back together right and tuned, put the old module and coil back in and positively verified that I had a bad ignition module causing all the backfiring.

I have been wrenching on small motors and machines since I was 11 fixing lawnmowers around the neighborhood. I spent over 20 years as an avionics professional working on all kinds of airplanes and I KNOW to never assume, always check and recheck...and then check one more time. My failure to follow this simple common sense standard practice caused me a lot of unnecessary problems.

Now I am back on the road deciding whether or not to do a basic top-end/cylinder rework on the 88 because of the low compression (120 each jug) or put this SE big bore kit in and punch it on out...decisions, decisions, decisions.

Thanks for all the suggestions and advice. It really helps me to sound out things and listen to what others have to say.

 

Is this 120PSI a wet or dry reading?

 

all cold readings

My manual lists 120psi as the minimum for cold reading. The front cylinder is barely making that and the rear is around 125. I'm not smoking so might be able to do a basic top-end job with new rings and ride another 50K

 

Glad you got it all sorted out.