Intermittent Exhaust Fumes in Cabin - '66 SWB

[304093]

Hey guys,
Over approximately the last 2 summers' worth of driving, I've had occasional exhaust fumes (black smoke, etc.) coming thru the floor vents into the cabin.
It's an intermittent problem, and when I see it, I open the windows to air the car out and the problem "goes away".

Visual inspection yields nothing.

Engine completely rebuilt/restored 2014-2015, returned to me 2016.
Heat exchangers & hoses are brand new (2015).
Heater valves were blasted & painted, and the heat works fine.
Original Leistritz Muffler blasted, recoated.

I've driven the car approx. 6,000 miles or so since 2016. I've only driven it about 250 miles this year, in part because of this problem.

Could there be a blockage of some kind in my muffler?

Could there be a bad weld inside the heat exchangers - but it would have to be both, since the smoke is visible on both sides of the car, passenger & driver.

Do I remove the heat exchangers to inspect?

I'm really stumped here, and don't know where to start - but I know it's a serious problem and needs attention.

Thanks all for any help.

[edmayo]

And the engine has NO oil leaks??

[jim amato]

I have a 67, this is common at first start up after sitting due to oil seepage onto heat exchangers, I have never seen a 100% leak free 2.0, best thing to do is drive the car up to temp to burn off oil, fumes will subside

[304093]

Thanks for responding guys.

No, my engine is leak free. The oil tank to cooler line leaks at the cooler fitting, so some drops of oil have burned on the fitting. After a hard run, there’s usually a drop of oil hanging there.
The problem occurs when the engine is warm, and although I said fumes are present from both vents, it seems worse on the passenger side at times.
Would a leaking exhaust gasket cause this (something that can’t readily be seen by looking at the under side)?
Should I check the muffler for a blockage of some sort?
Is it worth removing the exchangers and visually checking them? Maybe a leak test with air pressure?

[NZVW]

I agree with what jim amato has said.
In all honesty 250 miles per year is a ridiculously low annual mileage.
Fill the car up with gas and go for a good spirited drive, when the petrol tank is at a 1/4 fill it up again and drive home.
Easy Peezy way to have fun and I am sure it will resolve your issue.

[1969911T]

Very interesting!
Intermittent, then goes away
When this happens, does black smoke come out the tail pipe?
Is the heater valve on?

My guess is both the carbs have issues to make black smoke, there is a leak in the headers inside each of the heat exchangers, and the heat is on.
Steve