Engine not running more than a few seconds
Thread Starter
Instructor
Joined: Feb 2003
Posts: 164
Likes: 1
From: Houston Texas
Engine not running more than a few seconds
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.
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.
Last edited by ATHANASE; Sep 14, 2014 at 11:34 AM.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
You could be dealing with an ignition problem (points closing up, condenser going bad) or a bad timing chain. Just two possibilities of where to look based upon the description and the assumption the engine is as the factory built it.
Intermediate
Joined: Mar 2006
Posts: 37
Likes: 1
From: Massachusetts
My '71 (base 350) had about the same problems. Check the exhaust butterfly valve (not sure if the LS5 has one?) to see if it's sticking closed. And perhaps you'll need to rebuild the carb - my RQJ was 'professionally' rebuilt, but the floats were off and jets were clogged and the wrong size.... You say you're getting plenty of fuel to the carb, but is the carb delivering the fuel to the intake the way it should?
Thread Starter
Instructor
Joined: Feb 2003
Posts: 164
Likes: 1
From: Houston Texas
Iginition coil gives me the same # volts at the (+) and at the center connector to the distributor (6 volts ?!?!).
A bad timing chain is probably way beyond my skills to assess but could it go bad while sitting in a garage for a week and not giving any warning sign the weekend before?
Thread Starter
Instructor
Joined: Feb 2003
Posts: 164
Likes: 1
From: Houston Texas
My '71 (base 350) had about the same problems. Check the exhaust butterfly valve (not sure if the LS5 has one?) to see if it's sticking closed. And perhaps you'll need to rebuild the carb - my RQJ was 'professionally' rebuilt, but the floats were off and jets were clogged and the wrong size.... You say you're getting plenty of fuel to the carb, but is the carb delivering the fuel to the intake the way it should?
I don't know if the carb is doing his job or if the problem comes from elsewhere. I wish I could have someone help me go through the diagnosis process.
Carb rebuild: Does is typically involve replacing parts or just cleaning parts ?
Racer
Joined: Feb 2013
Posts: 364
Likes: 39
From: Sebastian FL
Exh. Valve
I am not sure what you call the "exhaust butterfly valve". Can you elaborate?
I don't know if the carb is doing his job or if the problem comes from elsewhere. I wish I could have someone help me go through the diagnosis process.
Carb rebuild: Does is typically involve replacing parts or just cleaning parts ?
I don't know if the carb is doing his job or if the problem comes from elsewhere. I wish I could have someone help me go through the diagnosis process.
Carb rebuild: Does is typically involve replacing parts or just cleaning parts ?
Le Mans Master
Joined: Sep 2003
Posts: 8,812
Likes: 2,029
From: eastern NC
I am not sure what you call the "exhaust butterfly valve". Can you elaborate?
I don't know if the carb is doing his job or if the problem comes from elsewhere. I wish I could have someone help me go through the diagnosis process.
Carb rebuild: Does is typically involve replacing parts or just cleaning parts ?
I don't know if the carb is doing his job or if the problem comes from elsewhere. I wish I could have someone help me go through the diagnosis process.
Carb rebuild: Does is typically involve replacing parts or just cleaning parts ?
https://www.paragoncorvette.com/p-35...alve-rh-2.aspxGood luck... GUSTO
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Burning Brakes
Joined: Dec 2003
Posts: 1,150
Likes: 154
did you check the dwell on the points?should be 28to 32 degrees(30 degrees always worked best for me when I had points) If I remember correctly a coil with points should have about 6.72 volts to 8.6 volts to run points,yours seems low.A bad condenser and it won't start at all.
Thread Starter
Instructor
Joined: Feb 2003
Posts: 164
Likes: 1
From: Houston Texas
Passenger side exhaust manifold, between the manifold and the header pipe. With the engine cold (cool to the touch) grab the weight on the side of the valve and flip it back and forth. It should move freely. If it doesn't move freely, try to move it back and forth to free it up. Spray some penetrating oil on it as well. If there is no weight, the valve has likely been replaced with a plain spacer, in which case... never-mind!
Good luck... GUSTO
Good luck... GUSTO
Thread Starter
Instructor
Joined: Feb 2003
Posts: 164
Likes: 1
From: Houston Texas
did you check the dwell on the points?should be 28to 32 degrees(30 degrees always worked best for me when I had points) If I remember correctly a coil with points should have about 6.72 volts to 8.6 volts to run points,yours seems low.A bad condenser and it won't start at all.
Melting Slicks
Joined: Jul 2009
Posts: 2,091
Likes: 40
Something sounds clogged to me in fuel delivery. Like there is trash in the lines, filter , carb jets . When you rev the engine , the pressure shifts the trash from letting fuel pass.
Could be the excuse to overhaul the carb you've been putting off too long.
Is there any knocking or tapping noise from the valve train not normal to your ear.
Could be the excuse to overhaul the carb you've been putting off too long.
Is there any knocking or tapping noise from the valve train not normal to your ear.
Last edited by LS4 PILOT; Sep 15, 2014 at 09:23 AM.
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
Typically, when the vapor/fuel in the area if the carb air horn (top of the carb) catches fire, a "spark" is needed to kick it off. If you're not lighting it with a match, the spark is coming from inside the engine. In a correctly functioning engine, the valves are closed when the ignition sparks occur. So either the ignition system is providing spark at the wrong time, or the engine isn't closing the valves when it should be. This points to a timing issue (points, distributor housing rotation, plug wire crossfire, plug wire mix up, etc) or mechanical engine failure- valve sticking, timing chain/gears etc.
Test 1
Check the distributor to see if it rotates by hand easily. It should be locked down & not rotate. If you have a manual trans, put it in neutral.
Test 2
Remove the distributor cap and rotate your engine by hand until the distributor points are fully open and verify the correct point gap.
Test 3
While the dist cap is still off, rotate the engine clockwise/counter clockwise while observing the rotor movement. A large "dead spot" where the rotor doesn't move when the engine is rotating back & forth indicates a severely worn timing chain & gears.
Test 4
A compression test might help identifying a sticky valve.
Test 1
Check the distributor to see if it rotates by hand easily. It should be locked down & not rotate. If you have a manual trans, put it in neutral.
Test 2
Remove the distributor cap and rotate your engine by hand until the distributor points are fully open and verify the correct point gap.
Test 3
While the dist cap is still off, rotate the engine clockwise/counter clockwise while observing the rotor movement. A large "dead spot" where the rotor doesn't move when the engine is rotating back & forth indicates a severely worn timing chain & gears.
Test 4
A compression test might help identifying a sticky valve.
Thread Starter
Instructor
Joined: Feb 2003
Posts: 164
Likes: 1
From: Houston Texas
So, based on all your suggestions, here are the two areas I am going to focus on:
1) Timing: Probably more the timing belt than the distributor / ignition points which look fine, but I will conduct the tests 1 to 3.
2) Carburetor: Consider a rebuilt.
Thank y'all. I'll keep you posted.
1) Timing: Probably more the timing belt than the distributor / ignition points which look fine, but I will conduct the tests 1 to 3.
2) Carburetor: Consider a rebuilt.
Thank y'all. I'll keep you posted.
Last edited by ATHANASE; Sep 15, 2014 at 10:40 AM.
Le Mans Master
Joined: Sep 2003
Posts: 8,812
Likes: 2,029
From: eastern NC
So, based on all your suggestions, here are the two areas I am going to focus on:
1) Timing: Probably more the timing belt than the distributor / ignition points which look fine, but I will conduct the tests 1 to 3.
2) Carburetor: Consider a rebuild... not a rebuilt.
Thank y'all. I'll keep you posted.
1) Timing: Probably more the timing belt than the distributor / ignition points which look fine, but I will conduct the tests 1 to 3.
2) Carburetor: Consider a rebuild... not a rebuilt.
Thank y'all. I'll keep you posted.
Most on here would recommend against purchasing a rebuilt Quadrajet from a typical parts house. Because GM in particular actually sold so many varieties of the Quadrajet, every one different in someway from another version, many rebuilt Quadrajets end up with a Hodge-podge of parts in them. Few will work as well as an original that has simply been rebuilt by a competent technician. I have rebuilt a number of them over the years with a quality rebuild kit, and never been disappointed. They can always be rebuilt better by someone such as "Lars" or "Cliff Ruggles", but even I can do better than most autoparts houses seem capable of.
If you do feel inclined to tackle rebuilding your Quadrajet, do yourself a favor and pick up a copy of Cliff Ruggles book, "How to Rebuild and Modify Rochester Quadrajet Carburetors". If nothing else, you will end up amazed at just what these carburetors are capable of.
http://www.cliffshighperformance.com/buy_book_2.html
Good luck... GUSTO