The Stovebolt Site Forums The Shop Area Engine Shop GMC 270 misses unless under load

Howdy - I have a 54 GMC 2-ton dump flatbed w/ 270. Overhauled it last year with rings & bearings, replaced the old cracked head with a used one that was milled and had new exhaust valves fabricated, milled the mated manifolds and installed a Holley/Weber 2 barrel carb. Valves are adjusted; good compression 150-180 in all cylinders. At idle, the engine misses on cylinder #4, but under a load (less than 10 inches vacuum) it pulls with all six as smooth as can be. It DOES have a vacuum leak (I can hear the whistle) that I've not yet been able to localize. My plan is to shoot some starting fluid against the block at idle to see if the vac leak is at the middle intake port, what else should I be looking for? Where else should I look for the vacuum leak? (A note: this truck has a two speed rear axle that shifts by vacuum.)

It sounds like a vacuum leak problem. Plug off the vacuum connections temporaily like the 2 speed & vacuum wipers & then check it. Recheck the valve adjustment on #4 cyl. Pull #4 plug & see if it is carboned up.

This is a little info of bad luck, I found out on my 270 GMC during the motor rebuild.

1. Head has a crack in #4 cylinder, machinest did not find it until the head was rebuilt, lucky me.
2. Bad News, Cylinder #4 was Hydro Lock. Sounds Expensive, The cylinder block stretched to allow water to pass by the piston. THis required required not one but 2 sleeves, we should sleeve # 3 cylinder because of the groove caused by the broken Rings.

3. Last week I found out from another reader when the block is getting a new Thermostat, the air needs to be purged from the head by cracking the Temp gage. This bleads the air out fo the block, drill a 3/16 hole in the Thermostat housing allowing water to pass by the thermostat collar preventing an air lock in the head.

Somthing to check out.

Try wd40 to see if your rpms pick up. I just think it`s safer. @ cents worth. Hope it`s just a vaccumn leak.

An unlit propain tourch work great for finding leaks. If you have an old one you don't care about cut the end off and put a piece of 3/8" hose on it. This help get in thoughs tite spots.

OK, found it: The back intake port (closest to the firewall) leaks. Engine picks up great w/ starting fluid sprayed onto it. Problem now is that studs cannot be further tightened, and I don't wnat to break one in the block. I have already milled the mating surface of the manifolds. Do I need to do the same to the head, or is there a meatier gasket that I can get?

Ranger,

Not being familiar with the GMC, I will assume it is similar to the Chevy six. It sounds like your manifolds were not surfaced or assembled properly.

Both manifolds should have been machined together, as an assembled unit, then installed loosely bolted together before a final tightening.

If lots of material was removed during surfacing, the alignment rings might now be too wide. This could hold the intake off the head.

A freshly machined combination intake/exhaust with properly fitting alignment rings should fit perfectly flat on the head when installed correctly.

I don't think a head could ever warp on the axis that would effect sealing of the intake manifold and still have the engine run.

Stuart

Stuart, the mated manifolds were machined as a paired unit, but they did have a fairly significant warp before being machined, and I believe that a fair amount of material was taken off. It's entirely possible that the alignment rings are preventing good coaptation of the intake ports to the manifold. I'll try disassembling, sanding down the profile of the alignment rings about 20% and putting on a new gasket. Also will take the opportunity to put on new studs - truck has the originals.

I agree with your comment about the head, I think it's fine. I ground the intake valves at overhaul and had some hardened exhaust valves fabricated for it.

Thanks for your thoughts.

R

Ranger,

What a fascinating word... coaptation ! It seems to have a medical origin....are you a DOC? Poor old 'spell check' doesn't like that word.

Stuart

"Doc" - Guilty as charged. It's humbling to diagnose this amazing relic of the Eisenhower administration, though -

*Sigh* - Well, Loadmaster may be right. I found a trickle of anti-freeze coming from the #3-4 exhaust port. Whether the block or the head, this would be the third time in three years I've torn down the engine. So my question is this: I have a friend with a Chev 292 in excellent condiiton, now sitting in a '65 pickup which is otherwise rusted out. Will it be more of a hassle to make the modifications necessary to drop in the 292, which he has offered to me for free, or should I go through the rigamaroll of tearing down and sleeving (or replacing the head) of the 270? Which do y'all think is the better engine? One guy I know thinks that the 292 has better pressure lubrication. Tom Langdon has a blurb on his site stating that engine swaps between GMC & Chev are not all that tough.

Ranger, You got mail.

OK, update: turns out that, unbeknownst to me, that the 270 head comes in small port and large port versions. Mine is a small port, and the engine had a large port gasket that leaked like a sieve. I had to hunt a little for a gasket specific to small intake ports, and found them available from Larrow & Sons in Sandy, OR. I also bought some of their manifold studs, which use a finer machine thread than the stick studs, and the manifold is now tight - engine runs smoothly.

Thanks!