Interesting. I may be having the same problem. What bothers me is how did this system work for the first 30 years, only recently appearing to be a problem? Have many others had similar experience? If the problem is modern fuel formulation, I'd expect to be hearing of this problem much more often.

Your change seems to confirm it's heat related, something on the top of the engine being heat sensitive. A posting from Faster Rat also appeared to indicate this was a problem.

You may have something there. I'm just a bit hesitant to change something which used to work without knowing why the change is needed.

 

When I bought my car last year, the previous owner who owned the car for 8 years warned me about this condition. He said it did that ever since he bought the the car from the previous owned who had it for over 10 years. The story was "vapor lock" dew to fuel new style fuel. So nothing was done about it as they thought it was a symptom of the product and not the car.

Took the best Corvette shop in the city a few months to figure it out. When they took off the intake manifold the amount of carbonized and burnt oil stuck to the underside of the intake was dramatic. He took some picks to show me. It looked like a chocolate was stuck to the bottom of the intake. They had to chisel it away before sand blasting it clean. I found a pick on google that looks just like my build up. Its is the smoking gun.

On the way home from the shop yesterday it was 32 degrees and 85% humidity. I was stuck in traffic for almost a hour with out even a slight hick up. 100% problem solved. Check

Those small ports between the large intake ports are the exhaust cross over ports. You can see they cross over right under the carb. Supper heating that carb to high heaven. So hot in fact that is burns up the oil creating this large build up. I mean that's f'n hot right there boys. When you change the intake gasket the cut outs are not there. Or you can actually fill in the ports with a billet slug. The gasket is easier and cheaper. Just like Nike said, Just do it!

 

It looks like the heat shield is missing from the underside of the intake in that area unless it's underneath the carbon. Did you remove the heat riser from the right hand manifold or fix it to stay open permanently?

Still wondering why many others aren't complaining of the same issues unless most others already modified their cars, same change? Could be regional fuel formulations, problem is worse in some areas than others?
I'd like to try some fuel without ethanol, see if that makes any difference. I don't know if such fuel is available in CT.

 

I have a similar problem on a '77 SB I haven't figured out yet. I may have to give this a try as I don't drive it in cold weather.

 

Yeah, someone took the stamped steel heat shield off that was there to prevent oil from getting stuck on the underside and burning. Small block intakes are notorious for cracking under that heat shield so it may have been taken off to Magnaflux it but never put back on.

 

My car was totally stock with the shield in place and still had a huge amount of burnt oil stuck to the bottom of the intake. The pick I posted of the burnt oil is not from my car. It was to demonstrate what I was talking about. It did look about the same though. The intake and the carb were getting reads of 230 to 250 degrees. That's how hot it was getting. While the engine temps were in the 180's.

 

intake gaskets usually come with a pair of little metal blockoff plates that install in those crossover passages. I always use them. you can get solid ones or the ones with just a small hole.

another note: correlating low oil pressure with poor idle needs supporting data, you cannot just draw that conclusion. oil pressure tends to drop when warm, and vacuum leaks often only show up when the engine gets hot.

either way, the advice on checking ignition coil is sound. it's often said most carburetion issues are actually ignition issues, so put it on a scope and verify there is no ignition problems that are contributing to the problem, like hooking the coil up backwards, or bad ignition parts when they get hot, plug wires arcing when hot, that kind of thing...

 

I also wonder if this could be ignition related. Lately I've been running into lots of ignition condenser failures. I don't know what the deal is but the symptom is usually the same, runs fine when first started but as soon the temp rises it goes from a slight intermittent bucking to a misfire to complete loss of spark. OP says he has HEI, I wonder about the quality of aftermarket HEI ignition modules.

 

Delco and DUI units are good. Store brands are crap.

 

They're being made in China so their lifespan is very short as compared to the original AC Delcos of years ago. I have had pretty good luck with Standard Blue Streak as long as you get the better grade (Standard sells them in two grades).

They all come with the special "thermally conductive" goop that is supposed to transfer heat to the distributor housing but I have noticed that goop will quickly harden when exposed to heat then fail to conduct heat. As an experiment several years ago I started using Permatex Ultra Black RTV to transfer the heat and so far its doing a great job because it never hardens.

 

Update!
Measured the voltage at coil and it was 11 volts at idle and dropping to 10 volts when reving...
Suspecting that previous owner did not change original resistor wiring when installing the HEI.
Will rewire this week and see what happens.