| [GMCnet] Engine running rough [message #66373] |
Wed, 02 December 2009 15:08  |
Don Baker
Messages: 19
Registered: October 2006
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For the first time since owning my '77 I am experiencing a rough running engine. It has been idle for several months except for the occasional warm-up. We went to Navarro Mills Lake in the Liberty Hills camp for a week for our Thanksgiving with all the kids(about 90 miles each way). On the way I began to think I had lost some rubber off a tire because of the vibration. Can a rough engine give that sort of vibration? I pulled over to inspect the tires, but found no problem with the tires. However the engine was missing very badly. Can a rough engine give that sort of vibration? Also smelled of unburned gasoline. When I continued I reved the engine thru the gears a couple of times and it seemed to smooth out somewhat. But if I let it idle for very long the roughness returns. On the way home I believe it was a little smoother. I also think the gas guage went down too fast. I'm of the impression the carb is the problem. Any ideas?
Don Baker, Burleson, Texas '77 Kingsley twin bed
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| Re: [GMCnet] Engine running rough [message #66384 is a reply to message #66373] |
Wed, 02 December 2009 16:27  |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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""Can a rough engine give that sort of vibration?""
Yes, by all means an engine misfire can generate that kind of vibration. It sounds like you have a rich or ignition misfire or both. I'd clean or replace the plugs and get that old fuel out of there. It could very wellbe the carb, but I'd put in some fresh fuel and give it a good hard run!
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] Engine running rough [message #66386 is a reply to message #66373] |
Wed, 02 December 2009 16:38 |
Rick Denney
Messages: 430
Registered: January 2004
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Senior Member |
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Don Baker writes...
> On the way I began to think I had lost some
> rubber off a tire because of the vibration. Can a rough engine give
> that sort of vibration? I pulled over to inspect the tires, but
> found no problem with the tires. However the engine was missing very
> badly. Can a rough engine give that sort of vibration? Also smelled
> of unburned gasoline. When I continued I reved the engine thru the
> gears a couple of times and it seemed to smooth out somewhat. But if
> I let it idle for very long the roughness returns. On the way home I
> believe it was a little smoother. I also think the gas guage went
> down too fast. I'm of the impression the carb is the problem. Any ideas?
Your symptoms sound to me like a textbook case of the float needle
valve in the carburetor not sealing well. At idle and other low fuel
demand conditions, this will cause the float bowl to overflow and
flood the engine--creating rough idle, black smoke, and high
consumption. When you gun it, you'll empty the float bowl a bit and it
will run better until the float bowl again fills up and overflows. The
needle valve is suppose to shut off fuel flow when the float bowl is
full, but if the seat for that valve is dirty it won't seal. It's a
common problem, but also dangeous because it can overflow fuel on the
top of the engine.
If you open the hatch and remove the air cleaner housing, then you'll
see the fuel overflowing while the engine is idling. You'll see liquid
dribbling into the throttle bores, and probably also see it spilling
over onto the top of the engine.
Rick "been there, done that" Denney
'73 230 Ex-Glacier "Jaws"
Northern Virginia
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| Re: [GMCnet] Engine running rough [message #66389 is a reply to message #66373] |
Wed, 02 December 2009 17:05 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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| Don Baker wrote on Wed, 02 December 2009 16:08 |
For the first time since owning my '77 I am experiencing a rough running engine. It has been idle for several months except for the occasional warm-up.
We went to Navarro Mills Lake in the Liberty Hills camp for a week for our Thanksgiving with all the kids(about 90 miles each way). On the way I began to think I had lost some rubber off a tire because of the vibration.
Can a rough engine give that sort of vibration?
I pulled over to inspect the tires, but found no problem with the tires. However the engine was missing very badly. Can a rough engine give that sort of vibration? Also smelled of unburned gasoline. When I continued I reved the engine thru the gears a couple of times and it seemed to smooth out somewhat.
But if I let it idle for very long the roughness returns. On the way home I believe it was a little smoother. I also think the gas gauge went down too fast. I'm of the impression the carb is the problem. Any ideas?
Don Baker, Burleson, Texas '77 Kingsley twin bed
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Don,
Yes, a bad miss can cause that much vibration. A bad miss can be caused by many things.
I'm with the group that says check the plugs, but you say the coach has been idle a while "except for the occasional warm-up". This may be a lot of your problem.
- You may have too much carbon on some valves. (the highway run should have fixed that.)
- You may have a bad float needle (ala RickD).
- You may have a choke that is not opening correctly. (A sticking choke may cause just about the same things.)
Rick is right that your symptoms are a pretty good match for a needle sticking. I'm going to guess you ran I-35 most of the way. If the vibration didn't get a bunch better at highway speed, it probably is not the float needle. You still need to do more diagnosis before anything much should happen.
Check the plugs first. If they are black but all the same, then open the air filter housing and watch the choke open as the engine warms up. If you see fuel pouring in (may need a flashlight), Rick was right - shut it off and count on changing the oil SOON. Needles and seats go bad all the time. Chokes stick all the time. If you still have the OE choke, this is almost a sucker bet.
Let us know what you find.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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| Re: [GMCnet] Engine running rough [message #66390 is a reply to message #66373] |
Wed, 02 December 2009 17:25 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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Remember to change the oil and filter as soon as you fix the misfire problem. Oil is now most likely fouled with gas and byproducts of running (or not running in the case of some cyls) poorly. There are 2 mostly paths to trace---ignition fault or fuel delivery fault. Those who posted already have described pretty well what to do. Real test scope will point to the problem directly or use the shadetree mechanic methods of logical elimination. I'm guessing a wire has become chaffed or oxidized at the connection. Sparks at night observation and spark jumping snaps are also a clue. You might need a new cap and rotor too as a bad wire will cause the path of least resistance to waste the cap. The distributor, wires and plugs all work together and if any one is bad it can foul or shorten the life of the other components in the chain.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] Engine running rough [message #66395 is a reply to message #66394] |
Wed, 02 December 2009 18:10 |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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Senior Member |
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| Don Baker wrote on Wed, 02 December 2009 18:54 |
Thanks fellows, I think I have plenty of things to check out. Of course you might know, I'm coming down with something and will have to get to feeling better before I get out in the weather. I think I will start with the plugs. thanks again, Don Baker, Burleson, Texas '77 Kingsley
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Don: First you might want to do a visual inspection. Look at the plug wires, vacuum lines, remove the air filter and check for obvious signs of leaks, also look at those 2 little rusty tubes coming out of the intake to the carb. If they are not intact the choke will not open all the way. Then I would follow the excellent advise you have and start with the plugs.
Good Luck 
C. Boyd
76 Crestmont
East Tennessee
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