Hi Everyone,

I have a stalling issue with my 71 LT1. When the engine is warm the car will cut out when in neutral, traffic lights etc. No issue when the engine is cold. I have done some research and people talk of a fuel vacuum issue. Any ideas?

thx dave H

 

I presume the car is stick, being an LT-1.

What is your normal idle RPM? Is this something where that could be bumped up a small bit?

Do you happen to have a vacuum gauge you could plug into the carb? You'd want to put it on the lowest port which sees the intake vacuum after the throttle plates.

You could also start disconnecting vacuum-operated items to isolate a potential leak. Start with the vacuum advance, then move on to the accessories. There's a whole lotta hose that goes to the headlight and wiper assemblies. Many potential leaks if the hoses, fittings, and baffles are anywhere near original.

 

Another thing I had an issue with in the past is float heights in the bowls of the carb. If your front (primary) bowl is too high, fuel can 'slosh' into the overflow tube and rich-out the engine while braking. My particular problem with the opposite...my secondary bowl was too high and my engine would crap under hard acceleration.

Not sure if that sounds like your problem or not, but I figured I'd toss the thought out there. An item that is sometimes overlooked.

 

Hi Keith,

Idle 600 RPM, I tried bumping it up and its doesn't help. Seems that when the engine is hot the fuel evaporates at idle. Not sure how to resolve. and yes manual box.

 

As posted it might be a problem with the float system causing a very rich situation (stuck needle valve or dirt). But my thought is a vacuum leak. When the choke is set and demanding a rich mixture and the fast idle cam is set, the higher engine RPMs may mask a vacuum leak. At normal operating temps, the choke is off and idle RPMs are low, and any vacuum leaks will affect the engine. When the engine is hot, disconnect a vacuum line to allow an extra air bleed. If the engine smooths out it's rich, if it dies, it's lean.

 

I'm leaning toward vacuum leak for the moment.

Most auto places should still sell rubbers to block off the vacuum lines. Isolate your carb at idle (remove all vacuum lines and block off with rubber stoppers/covers) and I'd be surprised if your car didn't miraculously run better.

I'd also remove and block off the PCV. Those go bad too.

Just get the engine to nothing but carb, engine, and exhaust. For the sake of completeness, check the timing and the plugs. If it is an LT-1, you don't have to worry about points.

An outside possibility would be a weak coil, but I would think that would be an issue when winding the engine up, not when idling.

Then comes the awesome task of finding the leak.

 

OP hasnt been back, I may as well hijack right?

Ok, not really, I'm wondering if I have the same problem. The float height issue seems to be a reasonable explanation for what I'm experiencing, maybe you could enlighten me. If you think it sounds like my issue, maybe you could tell me how to fix it?

1975, L48 4 speed, but with a new Holley 600 on it. HAD been running fine, but I hadn't put many miles on it. After it gets warm, it feels very sluggish after a slow down or stop. Like it's loaded up. Not firing clean, but after a bit of throttle it cleans up. Definite chugging though.

Most recently, once warm it will die as I'm braking. Tach will show it stuttering at about 3-400 rpm as I'm braking, and sometimes die. No start unless I floor it, then it fires right up. If I don't touch the gas pedal it won't fire. It also smells of gas when I park / stop it, after one of these little episodes.

Tonight it let a huge puff of smoke after sitting for a bit after running like a$$ on the way home, not sure if it's related though.

Hopefully it's similar to what the OP is experiencing, and my symptoms/diagnosis can help him as well.

Thanks for the posts so far, I'm interested in the black magic /voodoo thing under the breather!

 

eliminate the possibility of any vacuum leaks, first thing...
Isolate all vacuum accessories. then see how the engine runs.
after that you can start tweaking carburetor.

 

could be choke plate not working well. it might open properly when first started cold then it remains open but maybe the linkage get caught and closes it a bit.

tho vacuum could be the problem. but his problem is only when the car is warm. meaning it runs wells cold. vacuum trouble is consistant.

 

Is the distributor vacuum can connected to 'ported' vacuum (as per factory intent) or 'manifold' vacuum (direct to intake manifold port)?

Try hooking the vacuum can directly to manifold port, if it is not already setup that way.

 

This tells me the engine is flooded. It could the the choke, as was mentioned, or the fuel level in the bowls is too high or you have a carburetor gasket problem. The other descriptors of the problem would lead me to a fuel level problem and I would first pull the sight plugs with the engine off to see if fuel was running out of the bowls. If no fuel comes out with the engine off, put the plugs back in, start the car and carefully remove the plugs one at a time to check the level. I write "carefully" since if the fuel level is too high, you will have fuel dumping out of the sight hole onto a running engine. You have to slowly unscrew the plug and pay attention to seepage around the threads. The seepage will tell you there is fuel behind the plug. Keep some light pressure on the plug as you unscrew it (you don't want it to just fall out of the hole) and once the threads are clear, slowly tilt the plug away from the bowl and be prepared to quickly screw it back in if fuel starts to pour out of the hole. Have a dry rag under the bowl to catch any escaping fuel and a means to extinguish a gasoline fire should things go wrong.

If it is a fuel level problem, you now have to fix what's causing it. If you haven't fiddled with the float level, it's not likely to be an adjustment issue since they don't go out of adjustment. You then have to investigate the float to see if it's sinking or if the needle and seat is faulty or if the o-ring in the seat is bad. It could also be a fuel pressure issue that is highlighting deterioration of the needle and seat assembly. And you might just have some debris trapped in the needle. Hard to know until you start checking things out.

 

Thanks for all the advice guys, much appreciated.

Just to elaborate, I just bought the car, and the previous owner had installed an Edelbrock intake and Holley carb prior to selling it. I do have the originals but am keeping the new parts on it. I'm not sure who installed the new parts, as the cheap bolts on the intake and silicone patching the coolant inlet at the intake manifold shows me a complete in attention to detail.

The problem, is now all time however. Every stop, idle goes down to 3-400, chugging, hold your breath, comes back up and works fine after a second.

I'll start looking into your suggestions, but one thing just occurred to me that I've changed when this started. The PCV into the valve cover was wrapped in tape to seal, and I peeled the tape off just letting it sit. It looked brutal, but maybe it was there for a reason? Is there vacuum there that could cause the issue? Maybe I did it to myself lol... I'll start there!

Really appreciate the detailed replies, sorry I didn't get back to the thread with better details sooner. Our Country was closed for anything and everything yesterday due to the Tragically Hip final concert. Drying our tears and getting back to life today.

 

If you have power brakes, your brake booster might have a vacuum leak in it when applying the brakes. You could test this with a vacuum gauge hooked up to manifold vacuum line. Have vehicle stopped but engine running and see if vacuum level changes as brakes are applied.