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| | | | Joined: May 2009 Posts: 5 New Guy | | New Guy Joined: May 2009 Posts: 5 | I have a 41 GMC 1/2 ton. Hasn't run in many years but is a low mile truck that was put away running. I have replaced wires, plugs points etc. Turns easily but backfires like a cannon. The grease cup on the dist is right up agaist the side cover, it rocks easily, haven't touched the vacuum advance. It looks like there's a screw on a collar on the back of the dist shaft, is that to adjust timing? Any tips would be appreciated. thanks, Neil | | | | | Joined: Feb 2005 Posts: 469 Shop Shark | | Shop Shark Joined: Feb 2005 Posts: 469 | Check the valve train sounds like some valves might be stuck. A compression test will tell you the tail should have 100 or so per cylinder. | | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 |
Did you remove and replace the distributor?
Do you have a GMC Maintenance Manual?
| | | | | Joined: May 2009 Posts: 5 New Guy | | New Guy Joined: May 2009 Posts: 5 | Pulled my valve cover and the plugs, good commpresion on all, no sticking valves. I have a manual but can't find it. Besides the thumb screw adjustements at the diaphram are there any timing marks? Distributor has never been out. Help!!!!!
Last edited by Neil Gerrard; 07/11/2010 12:55 AM.
| | | | | Joined: Dec 2004 Posts: 1,897 Shop Shark | | Shop Shark Joined: Dec 2004 Posts: 1,897 | pull the battery cable
pull the # 1 spark plug
spin the motor by hand till your air squirts out of the # 1 spark plug hole. You are now in the compression stroke. Place the spark plug wires back on the engine in the correct firing order... 1-5-3-6-2-4. Make sure you have the wires in the correct order... which should be clockwise.
There is a timing mark on the flywheel. It is a steel ball that is pressed into the flywheel, and is visible with the pointer and window above the starter. The ball will line up with the pointer on the compression stroke for either cylinder 1 or cylinder 6, so it is important to check with the spark plug trick mentioned above.
if the engine is still backfiring, advance the timing a little bit. | | | | | Joined: Apr 2005 Posts: 1,107 'Bolter | | 'Bolter Joined: Apr 2005 Posts: 1,107 | I have a 41 GMC 1/2 ton. Turns easily but backfires like a cannon. The grease cup on the dist is right up agaist the side cover, it rocks easily, haven't touched the vacuum advance. It looks like there's a screw on a collar on the back of the dist shaft, is that to adjust timing? Any tips would be appreciated. thanks, Neil does it back fire up thru the carb or out the exhaust? when ever it backfired thru the carb for me on my 40 228 it always ended up being a vacuum leak somewhere. once i was driving along enjoying the ride when all of a sudden the engine shut down with no warning. after close inspection under the hood with no tools i noticed that the grease fitting for the dist was up against the side cover which i knew that wasnt the proper position it should have been. i guess i didnt have it tight enough and it spun until the cup stopped it but that was after a 50 mile ride. it would turn but no fire or sputter until i turned it back then it i got it to fire and set the timing using the vacuum gage i have on the intake manifold. depending how your dist and cap were orientated it may or maynot be correct but up against the side cover leaves no room for timing adjustment. i would set it up static with TDC on #1 cylinder and with the flywheel and your rotor should be pointing to #1 plug wire. should run but with no popping unless you have a vacuum leak somewhere. | | | | | Joined: May 2009 Posts: 5 New Guy | | New Guy Joined: May 2009 Posts: 5 | I have a 41 GMC 1/2 ton. Turns easily but backfires like a cannon. The grease cup on the dist is right up agaist the side cover, it rocks easily, haven't touched the vacuum advance. It looks like there's a screw on a collar on the back of the dist shaft, is that to adjust timing? Any tips would be appreciated. thanks, Neil does it back fire up thru the carb or out the exhaust? when ever it backfired thru the carb for me on my 40 228 it always ended up being a vacuum leak somewhere. once i was driving along enjoying the ride when all of a sudden the engine shut down with no warning. after close inspection under the hood with no tools i noticed that the grease fitting for the dist was up against the side cover which i knew that wasnt the proper position it should have been. i guess i didnt have it tight enough and it spun until the cup stopped it but that was after a 50 mile ride. it would turn but no fire or sputter until i turned it back then it i got it to fire and set the timing using the vacuum gage i have on the intake manifold. depending how your dist and cap were orientated it may or maynot be correct but up against the side cover leaves no room for timing adjustment. i would set it up static with TDC on #1 cylinder and with the flywheel and your rotor should be pointing to #1 plug wire. should run but with no popping unless you have a vacuum leak somewhere. Now it's making sense, I checked number one and the flywheel, looks good. It backfires through the exhaust. It sounds like it's just in my dist. How do you loosen it to reset? The bolt to the right does not do it and the bolt on the left doesn't either. Is there a collar with a screw? Also where do you put the vacuum gauge and what should it be set at? Thanks for all your help, I see light at the end of the wind tunnel.
Last edited by Neil Gerrard; 07/11/2010 7:27 AM.
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