Engine not running for more than a few seconds

[Christian]

Here is where I really need some help:

The car has been running fine for years through regular maintenance.
This is a 1971 LS5 (454) with a Q-jet. Bought it 12 years ago and never had this problem. I drove the car last weekend and didn't notice anything unusual.

The engine would fire up but won't hold idle. Stops immediately. To keep it running, I would have to keep it in the 2000 rpm range or more. Plus it doesn't run smootlhy and is kind of reluctant to rev in the 3500 rpm zone. Like if I had a couple of sparkplugs disconnected (I checked, they are not).
I questioned the fuel supply and looked at the fuel filter. Dirty. Time to replace it which I did. No change, still the same problem although there is no doubt I am getting plenty of fuel there).
When the engine quit, the carburetor "coughs", throwing a small cloud of gas in the air (one time, the cloud caught fire).

Any suggestion on where I should look at is welcome.

 

[Vettehead Mikey]

Start by examining the points for wear or misadjudgment. Set the ignition timing to stock specs. Ensure that the plug wires are not shorting to ground or cross connected.

 

[Christian]

I did check: All spark plugs seem to be firmly connected. I did not unplug them since the last time I drove the car and don't see how they could be cross connected. The routing of the wires look the same as it has always been.
Distributor cap does not seem damaged and is fairly clean inside except for a little bit of red dust which I rmoved. Same with the rotor. Ignition points look clean.
I checked the ignition coil: I get the same voltage between the (+) and the ground than between the center connector to the distributor and the ground. The only weird thing is that I only get 6V.
Timing: I dont know how to do this and I can't see the timing scale on the front of the engine. Can the timing go out of whack so suddenly while garaged?

How about vacuum?

 

[Vettehead Mikey]

You'll need a tach/dwell meter and a timing light to do any sort of troubleshooting and future DIY maintenance.

 

[Christian]

Engine idling only if fuel return line to tank is clamped ?!?!?!

Car is 1971 454 with Q-jet carburetor.

Last weekend, I couldn't drive it: The engine was running very rough, only if I kept it over 2500 rpm or so, and won't idle at all. That was a surprise because I had drove it the week before without noticing anything weird.

I checked the distributor, the ignition points, the coil ..... apparently is is not electric.
I plugged the vacuum line to the carburetor. No change ... apparently it is not a vacuum leak.
I replaced the fuel filter. I check whether the pump was working by connecting the fuel line to a bottle: The gas was gushing abundantly in the bottle when cranking the engine.
Everything was now pointing to the carburetor.

I clamped what I believed was the fuel line TO the pump and started the engine: It had it running and idling fine for several minutes. I removed the clamp with the engine still idling, and it stalled a couple minutes later ?!?!?!

I had a doubt: I was surprised the engine could idle for so long just on the fuel left in the carburetor when the fuel line is clamped. I was wondering whether I had clamped the correct line (the one TO the pump, not the one back to the tank). To make sure, I did the test again, with both lines clamped. I started the engine and it ran out of fuel after a few seconds. I removed the clamp on the line I hadn't clamp the first time and restarted the engine and kept it idling fine for a couple minutes, like the first time. Obviously, this line must be the line TO the pump. Then, with the engine still idling, I removed the clamp on the other line, which I believe now is the return line. After one minute or so, the engine started to idle rough and finally stalled.

Why would the engine run and idle smooth with the return line clamped and not with return line open to the tank?
Does it point to the fuel pump now? (again, when disconnecting the fuel line from the carburetor and plugging it in a bottle, the fuel is gushing abundantly in the bottle when cranking the engine).

Does it make sense to anyone?

 

[tmkassin]

It sounds to me like the Qjet needs a rebuild. The jets are probably gummed up. You could try some carb cleaner in the tank but you have to drive it for that to work.

 

[Christian]

It sounds to me like the Qjet needs a rebuild. The jets are probably gummed up. You could try some carb cleaner in the tank but you have to drive it for that to work.

Thanks. But I just had a 1/2 hr drive with that return line clamped and it runs just fine. If my jets were gummed up: 1) I would think the issue would have appeared progressively and not without warning. I would have noticed something. and 2) why would it runs fine with the return line clamped?

 

[Vettehead Mikey]

Weak fuel pump.

 

[Christian]

Weak fuel pump.

That is the conclusion I am narrowing down to, indeed. Thanks.