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Hello, I have a 1971 GMC 950, I am storing firewood in the manure box that's on it now and have been unloading some into the woodshed as needed. I'm not sure what engine is in it but I believe it's a small block. It has a 2 barrel Rochester Carb. It has been running near perfect and today I moved it across the yard from the garage where it's parked to the woodshed and let it run while unloading wood and could hear it loading up. When I went to park it it wouldn't hardly rev up, backfiring through the exhaust and carb and blowing lots of blue smoke when given throttle. I thought maybe the plugs were fouling so I wire brushed them and burnt the carbon off with a propane torch and there's no change. The vacuum advance is unhooked and has been I'm not sure when that was done but it was running fine up until today, nothing in the engine has been touched in the last couple years. It idles just fine but when given a little throttle it starts to stumble and when increasing throttle it begins backfiring and a couple times shot flames out the carb. 

Any ideas what's causing my issue.   

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Where to start.


Well backfiring out of the intake can be caused by too much fuel.


Blue smoke is caused by burning oil.


Cleaning plugs with fire and a wire bush is not nearly as effective as spraying them with brake cleaner.


Vacuum advance wont effect your issues. Might as well hook it up anyway though. Easy to do.


I would drop some new plugs in, inspect the distributor rotor and cap and replace as needed, disable the PCV by blocking it, test or replace the spark plug wires, Check the points (if it has them) make sure the choke is not stuck closed, replace the fuel system with a 5 gallon gas can and a rubber hose, check the timing, and .... what else. Oh ya! BEER !!!!






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The cap had a little corrosion on a few terminals, I cleaned it off with my knife but maybe replace it? Rotor looks fine. Choke isn't stuck closed. PCV is already unhooked, it has chrome valve covers with breather caps in each. It still has points they are a little corroded around the edges, I will try cleaning them up and re gapping. 

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I put the points, condenser and plugs in. That fixed it. The exhaust smells a bit gassy and the new plugs are a bit black like it's running rich. The only mixture screws on the carb appear to be for the idle circuit? It's also a little hesitant sometimes when I punch the throttle to wide open, it could have been like that before. 

Edited by Aaron Wilson
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