The Stovebolt Site Forums The Parking Lot Big Bolts 51 GMC 270 engine trouble

My old truck ran (rough) and drove until I had the carb rebuilt. I put it on and was trying to adjust it when all of a sudden I lost all ability to turn the engine. I went and had my starter rebuilt and still no turnin over. I believe the bendix is engaging the flywheel, just not turning over the motor. New battery, same results.... I finally got under it and put the screwdriver to the flywheel. It turns freely counterclockwise but hits a stopping point going clockwise. Is that real bad news ?

Troy

How long has it been since it last ran?

Is you truck stored inside?

How long was the carb off?

Did you have the carb opening taped over to prevent anything from getting into the intake while the carb was off?

Mike B

I would try pulling all the spark plugs out and seeing if you can turn it over by hand, just in case you somehow ended up with fluid in a cylinder.

Mike, The truck sat I guess for a few weeks or maybe a month or so while I had the carburetor rebuilt then after I got it rebuilt I put it back on and I drove the truck across the road into a field. It was still running rough so I stopped and was trying to tune the carburetor when it sort of locked up on me. This old truck has had a history of the starter not engaging the flywheel sometimes and so that's why I had the starter rebuild done. I did have the intake manifold covered but the vehicle has been stored outside.

Good idea on pulling the plugs to check it that way I will see what happens and get back to y'all

Thanks for the response guys !

Troy

I hope you can get it figured out quick. I've heard of stuck carb needles causing a cylinder to fill up with gas, or a head gasket allowing the cylinder to fill up with coolant. Hopefully your starter isn't strong enough to bend a rod. That's what happened on my 2004 Silverado. That cost me over $70 to fix. At least Chevy parts are pretty cheap.

Saw a guy rebuild a carb on an old ford tractor and then had this same problem. A screw backed out of the throttle plate and hung an intake valve open enough to stop the engine from turning past a certain point. Removed the screw and it still runs to this day.

I got the valve cover off today and I had a shattered valve spring. The valve appeared to be undamaged and straight. I haven't pulled the plugs and tried to turn the flywheel yet with no plugs tho. Could the busted valve spring have affected the turning over of the engine ? Or, could the continued trying to start it have caused the busted spring ?

IF it were me, I would be pulling the head to see what is going on inside the engine. How can you tell if the valve is bent or not if you don't have the head off? You need to lay the valve on something flat like a pair of V-blocks and spin it to see what the valve face is doing (large part of the valve). It is possible a collapsed spring could affect it turning over but, I wold not think so.

If you want to mess with it more, without pulling the head, make sure the valve stays open and only turn the engine by hand.

Thanks Mike, that's what I'll probably do next. I'll get to it and let you all know.

Troy

I dropped the oil pan and turned the flywheel by hand but it's still got a stopping point. I couldn't see anything from underneath that just looked crazy bad or bent up. All the piston rods seem to be ok but I'm no expert and don't know everything to look for. There was zero metal shavings or junk in the oil or pan, just for what that's worth. What's next for me to do ya think, pull the head off?

Thanks, Troy

Could there possibly be something stuck in a timing gear? That's easier than pulling off the head.

I agree with Mike though, a busted valve spring is not a good sign. And with that, there's no guarantee that the valve stayed closed properly, or that it isn't bent. I would try things that are easier than pulling the head first if possible, but it may be inevitable. It couldn't hurt to yank out the distributor and oil pump, just to make sure they aren't jamming somehow.

I'd be pulling the head no matter what. The valve could have dropped and the piston hit it. The only way to know for sure if there is any other damage is to pull the head.

My .02 cents!

Mike B

Thanks guys for the 2 cents worth ! I will get up under it one more time and inspect then go from there. I will get back ASAP.

Thanks again, Troy

Well, I haven't taken the head off yet but I did see where I lost a valve thru the valve guide down into the cylinder somewhere. Everything, including all the pistons, from what I can see from the bottom looks good... All the cylinders look real clean and no damage to any of them that I can see. I hope to pull the head tomorrow and we should know more. I will get back ASAP.

Troy

Troy,

Take some pictures and post them here, so we can share your amazement and misery.

Good luck,

Will get some loaded soon as I can. It's been nothing but misery after misery on this old 51. I go from one crisis to another and I've driven it less than 1 mile in 2-3 years since I got it I'm way over my head I guess and I'm ready to shove it off the cliff. It went from just needing a carb rebuild to a starter to a motor issue now. Anybody want it ? I'll let it go REAL cheap

Troy

Got a friend over finally who qualifies as a true gear-head. We got the head off after working around some terrible screw placement by gmc engineers and found things to look much better than it could have. One valve was busted but very little damage to piston and all cylinder walls are clean. My friend can clean up the piston relatively easy and I guess I will have the head machined and get a valve job and get her put back together. After that I'm hoping I will have the worst behind me and I'll get closer to driving it for more than 3 minutes

Thanks to all who have offered advice and support

Troy

Make sure you install hardened exhaust seats,to compensate for ethanol fuel! Now's the time!