The Stovebolt Site Forums The Shop Area Engine Shop need help with 55 gmc with a 270

Hello everyone,
I recently purchased a 55 gmc 100. I purchased this truck as a backup work truck and as a fun toy. It fired right up after I first bought it and I quickly rebuilt the entire brake system and hit the road.
it ran perfectly for the first 4 days and then on the 5th day, it refuse to start, if I floor it it will run on what seems to be two cylinders. it will only stay running if I hold the pedal to the floor, if I open up the spark plugs, they are black and sooty. dark smoke comes out of the spark plug holes if I open them up after running it on two cylinders. is this an ignition problem or fuel? I'm not sure.
the carb seems fine and squirts gas out of the usual spot normally if I activate the throttle. The filter is clean, the tank seemed a little dirty but I took it out, cleaned it and put it back in. it's pretty rust free (thank you Arizona) I hope you guys can help me I really want to install some more appliances with this bad boy because I tend to get tipped more when driving it! sounds crazy but I noticed a difference in just 10 installs with it.
any help is greatly appreciated.
Toby in Flagstaff AZ

Hy tob4000, It sounds to me like your needle and seat are not closing, and your carb is flooded all the time. Take the top of the carb off and check your needle and seat to make sure there is nothing in the seat area that could stop the needle from sealing, also make sure your float is not full of gas, hope that helps.

POSSIBILITIES:

Bad fuel

Bent push rods due to bad fuel

Cracked distributer cap

Bad plug wires, bad plugs or miss-wired plugs

Distributer moved/bad timing

Vacuum leak

Cracked coil

Check carburetor for debris

Check all of the above and check your points/dwell too.

I like the needle and seat idea. seems that would explain the flooding. I'll check the other ideas as well. Thanks for the quick responses. if anyone else has an opinion please keep'em coming. I love this truck! I've been a car guy all of my life and I've never liked any vehicle I've owned as much as I like this one. lots of character. the four speed shifts amazingly well. brakes are better than my toyota 4x4 pickup. just feels kool. and the girls! oh my. I own a jaguar xjs as well. and the girls don't give that a second glance, but they just keep staring at this truck as I drive by. crazy

My first guess is the points have closed up due to wearing of the cam follower due to lack of lubrication.This occours slowly usually but at a certain point the effects on starting/running are very dramatic.The clue is the "running on two cylinders" you describe--the gap is no longer sufficient on the others to trigger the coil.

Points, now this would explain why the fuel looks so good and the carb seems normal. also after sitting for what I determined to be 12 years. it could have easily needed some lube on the points cam. I didn't lube it when I started to drive the truck. can I just reset the point gap and lube or are they destroyed now?

The contact surface should not have deteriorated because the gap was going away so if the rubbing block is not too shortened just regapping will bring her back alive.
A peak having developed on one contact may make it impossible to accurately use a feeler but in this case a good guesstimate will let you get by until new ones are on hand.
Look at that cam surface for light rust buildup too--that makes the cam follower go away in a hurry.
I have seen new points close the gap in a couple thousand miles when they weren't properly lubricated.
Has anybody ever heard a stovebolt develop a strange hi pitched squeak at idle and cured it by a drop of oil on the breaker cam or new point set?
That once was a common thing when the cam followers were made of a fiber filled material that have mostly been replaced by plastic.

ya know I never touched the points
when I was getting it ready for the street
I'm gonna try it this weekend.

Thanks for the info on cam lube. I got some at my FLAPS, and will lube that cam for the first time in 20,000 miles, except for the little felt oiler on the points. '56 gmc 270 ci six.

checked out the points this weekend. they were a little tight using my .016" feeler but I re-adjusted and then checked each plug for spark. they all had it. got shocked on one. felt strong to me. Tried to start the engine, same response: runs on two cylinders. so now on to the holley 1904 (R913a)
I ran outta time to do anything else so I have to wait for this Sunday to move on to the carburetor. I also checked the fuel pump by cranking the engine with the fuel line off. pump a ton of fuel out, that seems fine. will keep y'all updated as I figure this out.

if it runs well for a little while sputters and wants to cut off and runs again try replacing your condenser

Just got my 59 going. Also has 270L6. Would suggest a complete overhaul of carb. There are some very small air passages in these units. Be careful to carefully clean the entire carb. Install a new kit and besure to set float height. Start with idle screw at about 1-1/4 turns open, then adjust idle after warmed up. Also check heat riser butterfly (exhaust manifold)to be sure it is opening after warn-up. Good Luck