Hot running help...Again!

[marcsherriff]

Hi there potential helpers!
I have a problem with the car running when restarted once hot. It starts, and drives, albeit poorly.
When the car is cold, it runs beautifully, once run for about 3 minutes from full cold.
After about 20 minutes of driving, or thereabouts, it starts to bang and pop when leaving in gear and foot is off accellerator, essentially during engine braking.
If the car is then turned off, and restarted before getting fully cold again, it basically runs like it is on 3 cylinders up to about 3000rpm, after which it clearly has the missing cylinder come on, as it feels like an early 930, once that happens! Driving it hot is very reminiscent of the car having fouled plugs in the old days. The back firing during engine braking when restarted after hot is off the scale, very anti social.
The details that might assist in trouble shooting, or providing relevant advice.
BT5 normal coupe.
6 month old full engine rebuild.
912 NOS heads, with new valves, springs, and retainers.
1720 big bore kit
lightweight flywheel
Scat crank
Carillo rods
123 Ignition distributor, with vacuum advance, and supplied coil.
New plugs
Full flow oil filter
Weber IDF 40 carbs (which provides the vacuum advance)
Neutek SX-2 camshaft
Sebring style exhaust, with bolt in baffle

During rebuild, if something could or should have been replaced, it was.
The car is still a 6v car, and when running well, pulls very strong.

I'd really appreciate any advice that does not include why change what the factory built. I have changed it, and will persist in making it work as good as it should be, with the parts it has been built with. I have not used anything that is not fairly common amongst the faithful, so am hoping for an easy answer?

[Bruce Smith]

If the 123 is using a vacuum retard, you may want to try plugging the vacuum at the Webers and the distributor and run mechanical advance only. Sounds like either a vacuum or idle mixture issue.

[David Jones]

Air leak somewhere, I would suspect a manifold leak caused by expansion when warm. Check with the usual methods.

[Phil Planck]

If running beautifully when cold, maybe too rich when hot. More Weber info? Accel pump output and jet sizes.

[Don Gale]

I'm thinking it's too lean, which would explain both running hot and the popping, and perhaps an air leak as David suggested. Get it warmed up and drive it hard above 3k and shut it off. Let it cool for say 15 minutes and remove the plugs and "read" them. If bright white and clean, then definitely too lean. You can check for air leaks with quick start -- be careful, could precipitate an engine fire. You can also use an unlit propane torch to probe for manifold leaks. An exhaust flange leak and/or a free flow exhaust could also be suspect. Sounds like a trial and error approach with larger main jets or enrich the idle jet adjustment.

[Ed Pimm]

If it runs well when cold it is definitely not a lean condition. The backfiring makes me think an electrical issue. I once had a similar problem with one of my cars, it turned out to be a bad condenser that started to fail once the engine got warm.

[David Aronson]

So far you have suggestions of mixture lean, mixture rich, vacuum leak, intake leak, fouled plugs, timing advance/ retard problem with distributor, electrical problem. I love it. If confused, you are following the correct lead. One more, bad spark plug wire? Had to throw that in as it had not been offered. Good luck...what a hoby!

[Dick Weiss]

I don't use (or ever needed) a 123 distributor, but yours has vacuum advance? (Never serviced a car w/one)
Weber carb vacuum ports should be closed off.

[Keith Lee]

Are you sure you have adequate fuel delivery? Could be non vented gas tank and you're pulling a vacuum on the tank after a while causing a lean condition . (or restriction in the fuel line??)
Easy to check and eliminate as a cause at least.

[Jay Darlington]

So far you have suggestions of mixture lean, mixture rich, vacuum leak, intake leak, fouled plugs, timing advance/ retard problem with distributor, electrical problem. I love it. If confused, you are following the correct lead. One more, bad spark plug wire? Had to throw that in as it had not been offered. Good luck...what a hoby!

also possible hair line crack in the cap.
Jay D.

[Paul Ahnell]

+1 on Ed Pimm's comment re: bad condenser. I've had two of them, which when hot, caused the problem you are having. I'd get ~4-5 miles from home and would have to pull over (coast over), open the deck, and let it cool down before I could start and drive back to the garage, Turkey-man condensers.

[marcsherriff]

Thanks everyone. Long list of items to address!

[Al Zim]

Are you taking your vacuum advance from above or below the throttle plate? What is the venturi size on the carburetors? What is your hot fuel pressure? If you have the early fuel pump (up to 1963) what is your fuel pump stroke and the condition of the seats that the phonelic valve sits against? If you have the 1963 on fuel pump you can adjust the pressure. Vacuum advance distributors are designed for economy and to eliminate pre ignition when the throttle is opened under full engine load in top gear. I would be most interested in hearing from you on your decision to build your engine the way you have done. Thanks al@allzim.com

[marcsherriff]

Al, thanks for the response. Am away for work not for 3 weeks, so will get back into the garage on return, and come back with some answers to your questions. As to why the motor was built the way I have done. It is sort of an evolution. The exhaust was fitted a long time ago, for sound, and aesthetic. The webers and manifolds came new in a box with the car, and when my Zeniths needed rebuilding, the mechanic, bolted them on, and the car improved significantly. Possibly as much because I went from carbs needing rebuild to new carbs, as any reason.
The Vacuum advance looks like it is coming from below the throttle plate, just from me physically looking at it, and only from the left carb. The 123 was installed after throwing bad money after worse on so-called rebuilt distributors from so-called reputable distributor rebuilders, listed on the registry. Three times shipping and paying to USA for this, for them to return with play in the shaft was enough to cure me, so I tried the 123, and it honestly made the car much more drivable. This was pre rebuild. Since rebuild, which involved top and bottom end, from local reputable builder, motor is very good to drive, but for problems originally posted.

[Craig Richter]

Marc, your motor is full of some very high-level parts, and if your builder is also as high-level as you say, there should be nothing mechanically wrong here. The 123 distributor is only a little suspect to me because I have no experience with them (yet, one's coming just for learning this Winter). Several of my friends do like them, and I know they are really good tuners. So for now, unhook the vacuum advance, you can play with that after we get the motor running better. After you check for all the things already advised here, it all comes down to the Webers. I do think Webers are a much better carb than what came on 356's; but that's also the problem, getting them tuned to your motor takes some experience. You need to post ALL the jet sizes you're using, and while you're in there use a calibrated drill set to check that all 4 of each type jet are actually the same size and nothing is blocked. Then we can try some better informed long distance tuning if it's still running badly.