Have a 74 big block - have had no previous problems, but while cruising along at 30 mph yesterday the engine just abruptly died. Pulled over and tried to restart, and it took for ever to fire. Revved it up and held it at 2500 rpm for about 30 seconds to clear it out, but when I slowly allow the rpm to drop anywhere below 2000, it coughs and dies. Then hard to restar again, then same routine. At 2500 rpm or greater, runs fine, so does not appear to be a fuel supply issue. Drop it to 2000, and it slowly sputters, then dies.

Note: ignition was serviced about 4 months ago - new points, condenser, distributor cap, and new shielded wires. No problem since then - until this occurred.

Ideas?

 

Possibly weak coil. When the RPM's are high it will fire, but low RPM's won't cut it. The other option is clogged idle circuit. At high RPM you will be running the primary carb circiut, at low RPM, the idle circuit takes over.

 

check fuel filters had same thing would run a short time and die, what happens is when running it sucks the dirt to the filters and kill the engine and when it is off the dirt will drift or get back washed away then you can starte and run again till the dirt gets back to the filters

 

Had the same thought about coil - had not thought of the carb primary. Is there a simple way to check this without pulling the carb apart?

 

Sounds like carb problem. Sounds like its flooding out at lower rpms.

Symptoms - wet sooty plugs, gas dripping in primaries of carb at low rpms.

Crank engine and look closely in carb as its running, looking for the primaries dripping gas. At high rpms the engine can burn the excess gas but not at low rpms.

Be careful not to get your head directly over the carb while its running in case it backfires. Saw my dad get his face burned that way when I was a kid.

Roger

 

Suck, squeeze, bang, blow.

One (or more) of those is not right. Without more clues, everyone is just shooting in the dark and hoping to hit something. For instance, when it does fire after a stall, does it puke black, stinky smoke out the pipes? If it does then that could point to an over-fueling condition that is loading up the plugs and killing the engine. For this you'd want to make sure the idle air bleed is clear since it is easy to get some carb cleaner with the straw and force some solvent into the emulsion to clean it out. If it was the filter, higher-speed or high-load failure would prevail. Underfueling (clogged filter and such) rarely shows up a low speed, low load such as at idle. But it could if the idle fuel restrictor or the idle channel was dirty and it's not getting enough fuel in the idle circuit. You'd also have to look at a severe vacuum leak since this can greatly upset the idle -particularly if it is on the lean side.

For the ignition, you'd want to make sure your points have not closed up. Poor low speed behavior is common with excessive dwell.

The squeeze and blow parts are harder to diagnose. If the cam timing has retarded due to a loose chain, it will cause low cylinder pressure at low engine speed. You'd have to do a compression test to see if you have appropriate compression. There are other issues here, but that's just one of the more common ones.

As far as the blow, that would be a restriction in the exhaust (had wasps build a nice dam in my exhaust once) but that would be a high-engine speed issue.

 

I had almost exactly the same thing happen to my 70 big block a few weeks ago. Limped home keep the RPMs in a very narrow range.

It didn't seem to be flooding in my case, no strong gas odor or black smoke. Acted like it was starved, possibly the idle circuit clogged as someone mentioned. My filter is clean and the carb was completely rebuilt, parts plated, new components where needed, etc.

I thought maybe some water in the tank. Car was sitting for several months, some very hot humid weather. Thought maybe with gas sloshing around in the tank it may have sucked up some water.
Tossed a can of dry gas in the tank.

I also discovered I had the wrong coil installed. One for transistor ignition, so I replaced it with the proper one. Not sure which of these may have made a difference. Haven't had the problem recur so far.

If you find another cause I'd like to know. I'm not completely convinced all is well with mine, not having hit on any one fix which definitely fixed it. So far OK, waiting to see if the problem has truly been solved.

 

Ill shoot in the dark also, 4 mounts ago points serviced ? did you do it or a garage, also was the cam lobe on the dist. lubed. This at the moment sounds like a points problem, reset your points or at least check them. If that's not it go from there. Knowing that the points were replaced or adjusted 4 months ago, gives the points lobe enough time to wear down until the points are almost closed or are burning up. 2 cents. Good luck.

 

My thoughts exactly! Check proper gap/dwell ... if Good ... then look at fuel issue.

I know what fv69 means ... but it really ain't the points lobe that noticeably wears ... instead, it's the fiber or plastic follower riveted to breaker arm that wears.

 

Without the electronic diagnostics used on new cars, "shooting in the dark" is what we do. It's part of the fun of working on our old rigs.

More "shooting",..I'd look first at the ignition system. A cracked distributor cap can cause the symptoms you describe. Also, make sure the big lead from the coil to cap hasn't worked it's way a bit loose. Also, it's my experience that coils either work or they don't. I've not found them to 'sort of work', but I've been fooled before. Check for good stable voltage to the coil.

If the ignition checks out, take a look at the fuel system; clogged filter, crimped line, bum fuel pump. Although, with fuel supply problems, it's usually higher RPM's that suffer; not lower.

Good luck with your trouble "shooting".

 

Wow, I am having same issue with my LT1. Wondering about coil. I didnt use the lil single wire cap or what ever it is on my full petronix setup. Seems I still have fuel as I can see it squirting when we hit the peddle. seems several are having this issue.

 

On points equipped cars, the pigtail between the coil and distributor can give up too- it moves EVERY time the vacuum advance moves the breaker plate in the distributor. And pretty often, it's the last thing anyone looks at.

 

i checked float bowls on mine... front dry and rear was full.. Pluged carb filter i suspect.