Topic: Engine fires up but won't keep running
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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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.
|UPDATED|9/14/2014 8:35:25 AM (AZT)|/UPDATED|
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Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Check for a vacuum leak...it would be a fairly large leak for it to not idle. It would cause it to run rough like that, as well as spit back thru the carb.
Does it have any black smoke from the tailpipes?
Do you smell raw/excessive fuel?
It's possible the float is stuck in the carb, and the engine is flooding with too much fuel. Tapping on the fuel inlet on the carb will usually un-stick a float, depending on what might be causing it to hang.
Also possible the points are burnt, or out of adjustment. Cap/rotor problems.
So many thing could cause it. Just gotta start with the basics. Vacuum leak? Too much fuel? Spark issue?
Eliminate one at a time.
Does it have any black smoke from the tailpipes?
Do you smell raw/excessive fuel?
It's possible the float is stuck in the carb, and the engine is flooding with too much fuel. Tapping on the fuel inlet on the carb will usually un-stick a float, depending on what might be causing it to hang.
Also possible the points are burnt, or out of adjustment. Cap/rotor problems.
So many thing could cause it. Just gotta start with the basics. Vacuum leak? Too much fuel? Spark issue?
Eliminate one at a time.
Joel Adams
C3VR Lifetime Member #56 
My Link 
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Joel,
I did check the distributor: Cap, rotor and ignition points look fine and clean (except for a little bit of red dust in the cap and the rotor. No grease, no carbon trace. I also checked the ignition coil: I get 6V at the (+) and at the center connector tha goes to the distributor. 6V is kind of a surprise but I get the same in and out.
The vacuum: Where should I start to check a leack? I do not understand the role of the vacuum in the carburation. Obviously there is as I got an excess of gasoline driping on the ground just aft of the front left wheel. I believe it came from the canister.
The vacuum system usually needs several minutes to build up vacuum enough to lift the headlights. I wish I could keep the engine running long enough to see that. Not sure where to start from to find a leak (I didn't here any particular noise of this kind).
Christian
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Christian said:... an excess of gasoline dripping on the ground just aft of the front left wheel. I believe it came from the canister.
This points to the carb flooding. It sounds like fuel is overflowing into the vent tube on the front of the carb, and running down into the canister. See if you can pinch off the fuel line going TO the fuel pump, effectively shutting off the fuel supply to the carb. Start it up, and once running again, it will use the excess fuel, and then run ok for a few minutes, until it runs out of fuel. Sometimes doing this will also flush out any debris that might be stuck in the needle/seat of the carb, if that is what is causing the flooding.
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Hope it's something simple.
Years ago, when I was taking the '74 to it's 1st NCRS Judging in Waco, we were to meet up with some other folks on the road. I was driving the car down(for driving points), and trailering it back home.We got to the meeting spot, and I realized I had forgotten to bring the lug nuts for the steel rims, which I was going to change to from the aluminuminum rims once we got there. I hop in the car and haul butt back to the house for the lug nuts. As I exited the hiway, the car wouldn't stay running...it was flooding out big-time. I made it to the house, got the nuts, and limped back to the meeting spot. Once there, I had to take the top off the carb, took the float out, and had someone crank the engine(with the coil wire off), and flushed some crap out of the needle/seat. Took about 10 minutes....car has run fine ever since. Was running fine before, too...just something made it past the filters, and lodged in the needle/seat. Ya never know when it could happen, but you can bet it will usually be at the worst possible time...
Years ago, when I was taking the '74 to it's 1st NCRS Judging in Waco, we were to meet up with some other folks on the road. I was driving the car down(for driving points), and trailering it back home.We got to the meeting spot, and I realized I had forgotten to bring the lug nuts for the steel rims, which I was going to change to from the aluminuminum rims once we got there. I hop in the car and haul butt back to the house for the lug nuts. As I exited the hiway, the car wouldn't stay running...it was flooding out big-time. I made it to the house, got the nuts, and limped back to the meeting spot. Once there, I had to take the top off the carb, took the float out, and had someone crank the engine(with the coil wire off), and flushed some crap out of the needle/seat. Took about 10 minutes....car has run fine ever since. Was running fine before, too...just something made it past the filters, and lodged in the needle/seat. Ya never know when it could happen, but you can bet it will usually be at the worst possible time...
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Adams' Apple said:
... pinch off the fuel line going TO the fuel pump, effectively shutting off the fuel supply to the carb. Start it up, and once running again, it will use the excess fuel, and then run ok for a few minutes, until it runs out of fuel.
Christian said:... an excess of gasoline dripping on the ground just aft of the front left wheel. I believe it came from the canister.
... pinch off the fuel line going TO the fuel pump, effectively shutting off the fuel supply to the carb. Start it up, and once running again, it will use the excess fuel, and then run ok for a few minutes, until it runs out of fuel.
Joel,
Following your advice, I clamped the fuel line and, indeed, I got the engine running and idling fine. Happy as I was, I removed the clamp with the intention to go for a test ride. The time to turn around and get my wallet, the engine stalled. I am back to square one with the engine running rough and not idling anymore.
I understand the carburetor is definitely the problem and is good for a rebuild. Am I correct?
I tried to take the top off. I am struggling with the pump rod linkage: How to disconnect this one without damaging anything?
|UPDATED|9/20/2014 11:56:06 AM (AZT)|/UPDATED|
Now I am even more puzzled!!!!!
|UPDATED|9/20/2014 11:55:21 AM (AZT)|/UPDATED|
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 I kept it idling for a couple minutes. Obviously, this must be the line TO the pump. Then, with the engine still idling, I removed the other clamp, which I believe now was on 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? (Last weekend, I had disconnected the fuel line from the carburetor and plugged it on a bottle: The fuel was gushing abundantly in the bottle when cranking the engine).
Does it make sense to anyone?
|UPDATED|9/20/2014 11:55:21 AM (AZT)|/UPDATED|
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
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