bbc454500

Okay I know i am new here but i have read several posts about cranking over an engine with the bolt that secures the harmonic dampner to the crank! if this bolt breaks it could be a very costly repair not to mention its hard to get at. on your starter solinod there are two connections 1- the large red wire + from the battery, 2- a smaller wire going to the terminal on the engine block side. connect these two together and the engine turns over ( if you DO NOT have the key to on the engine will not start, it just turns and turns..... i have a push button switch a piece of wire and two aligator clips i connect to the startes terminals as metioned and push the button and the engine turns as for the back fire it could be a lot of issues, engine timming, vavles out of adjustment, carb out of adjust. or a major vaccum leak. you have use the "process of elimination to determine what your problem is...

68sbcoupe

Vacuum leak. Choking the engine removed the source of air (through the carb). It didn't stall, so its pulling air in somewhere else. Check that all the vacuum hoses are connected on both ends. Them starting with the MC booster, then PCV, then vac cannister, pinch them closed.

Hopefully one will determine where the leak is.

MonteHall

what kind of vacuum leak are we talking?
a hose? the only 3 that come off the carb and go anywhere are the Distributor advance,
secondary vacuum, and brakebooster, all those connections/hoses look good.

or are we talking intake manifold gasket failure?

68sbcoupe

If you pull the spark plugs, there is no air being compressed in the engine. The only torqe required to spin the engine by the crank hubbolt is the friction resultant from the rings in contact with the cylinder walls, and the compression of the valve springs.

So, the likihood of twisting off a 1/2" grade 8 bolt is pretty limited. The tensile strength is over 700 pounds. I dunno, maybe The Rock or Hulk Hogan (or whomever the contemporary big strong guy might be today) might be able to do it.

Use a 6 point socket and a half inch breaker bar.

68sbcoupe

There is probably a vacuum fitting off your intake manifold, behind the carb. Check that too. Doesn't necessarily have to be a line connected to the carb.

As for intake manifold failure; I have never seen one fail. Seen them installed improperly, but not on an engine that had been running for some time. Was the intake manifold removed or replaced recently? If so, try re-torquing (in the right sequence).

MonteHall

I mis-typed.
The one behind the carb goes to the brake booster.
The third line off the carb goes to the PCV valve.

I pulled them and plugged them, one by one, the only one that seemed to make any difference was the brake booster. When I first started the car with that line plugged, I actually idled (at half choke).... for about a minute, then it dies and it was the same old thing. If I let it sit for a few minutes, it would idle again, then start acting up.

edit: for some reason it idled long enough for me to get out of the car and get up by the carb and manually rev it.
There is a loud sucking sound in the intake (down the carb)
that's all it told me. yay.

MonteHall

I thought Engine Timing at first, but since it seemed to "fix" itself when I closed the choke, I can rule out timing.

Valves? not sure about that

Carb- this is very likely, as I've had an issue with it idling since day one, and the DSPO did mention that the carb needed to be tuned. The idle was lean to begin with (It was hot enough I could smell it)

Major Vac leak- plausible, and I experimented with that theory, but couldn't narrow it down.

the odd thing is that the issue (the original issue of backfiring and killing the engine) occurred during a drive, at around 45mph.

now, it does still backfire a bit, but it's very light, almost sounds like something is knocking in the exhaust.
It's also idle for about a minute (at around 1000rpm) and then die again, but that's only on the first start about every half hour or so.

MonteHall

how about the Carb gasket?