M211 - Too Slow

[Oldfart]

Number 5 is most likely valves or a burned piston. Upper cylinder cleaner is most effective for gummed up rings, but can help with valves if they are just carbonized up and not seating all the way. Given the higher compressions at each end of the block, (the end cylinders tend to run a little leaner than those closer to the carb) it might be old leaded fuel deposits gumming things up a bit. Did you run your compression test dry only? If you did, you might try oiling each hole before you test it. The oil will seal up things like valves not seating completely or stuck rings. You should see an improvement between dry and wet readings (except on number 5). How much of an improvement will give you a clue as to what might be dropping the compressions. Another thought on number 5 is the possibility of a broken valve spring allowing the valve to hang open. If its an intake valve, you would hear burping in the carb with the air cleaner disconnected. If the exhaust valve, then it should flutter a piece of paper held across the tail pipe (usually can hear it as well.) If a burned piston, there should be blow by and oil in the exhaust. While readings in the 60's are somewhat low, they are runnable. 90psi is good and 105 is excellent for a low compression military engine at your altitude. If the middle cylinders are low due to ring wear, a slightly higher viscosity oil will help some. As to number 5, it really needs to get fixed. Having said that, I have seen guys extend the use of an engine with a bad hole by removing the spark wire and in one case, the spark plug was removed as well. The latter did not make sense to me as there was all sorts of whooshing and depending on which valve was open or burned, exhaust or intake gasses could escape the plug hole.

Bob

 

[delos]

I took off the valve cover and i could see right away that the pushrod on #5 intake was not hooked up like the rest. it was bent and laying in there crooked. I straightened it, ordered a new one and put it back together. wiped the gunk out of the valve cover, which was probably from water vapor (head gasket going out). Cylinder # 5 now has 65 psi of compression. I did a valve clearance adjustment on all the valves too. Went for a test drive and came up to 30 easy and continued up to 40 in a short distance after. Looks like a head gasket is still in the works, but i think we have the main issue corrected.

Thanks for all the help, looks like she's back on the road!

 

[williamh]

i like those rims !!

 

[overlordofmars]

Now that's skill, Congratulations. The GMC truck engine was a tough animal, but as most are over 50 years old things are going to let loose. There is one on the web all hot rodded out with three carbs and a cam.

 

[jollyroger]

I've got a 261 Chev stovebolt with 2 single Rochester BC carbs on a 216 manifold, Fenton Cast Iron dual exhaust and a 261 with 3 single BC carbs and Fentons. They were good except for the full flow oil system and the oil passage through the head bolt to the rocker arms.

Good job with the GMC. Glad you found the basic problem. Really they are pretty simple machines.

 

[Gunfreak25]

That is the 3rd 302 I have seen lately with a bent or broken pushrod. I would agree, most of these engines are probably over 40 and some even closer to 60 years of age. Still though, darn tough considering what they had to push. I still like my 455 though.

 

[Oldfart]

Bent push rods are nominally due to over revving the engine or a sudden change in RPM's. Given the governors in these trucks, I expect the bending of push rods occurs when the mighty Dual Range GMC Hydromatic finally decides to shift after lumbering up that hill and crowning over the top.

What Jollyroger did not mention was the unique sound of the stove bolt 6 with split exhaust (and how his parents could likely hear him coming home from more than 2 miles away.)

 

[Mastertech]

What Oldfart said, and also dont discount the idea that the push rod jumped out sometime on cold start up if the valve was stuck open from sitting in that position. I would also have all the valves done when you pull the head for the gasket job and dont forget to have he head presure checked, checked for flat and or surfaced. I would also do the compression check the way Oldfart said to do with engine cold, and warm, wet, and dry. and When I have to pull the Head on Mine I will have it milled down more than just to flatten it but also enough to bring the compression ratio up to a more liveable point and up the power output greatly. So let us know how it goes.
Byron

 

[Mastertech]

BTW Nice Truck

 

[Gunfreak25]

I am the type of person who would rather properly repair something than go with a quick fix, but have you considered trying that "Restore Engine Lubricant" in your truck? A lot of the older mechanics I have talked to swear by the stuff. I wonder how much pep it would bring back to your old 302?

 

[rosco]

I can't believe this.... The absolute first thing your do in trouble shooting an engine, is to set the valves!!!! You can't do anything else, and expect meaningful results, until you set the valves in an engine with mechanical lifters. You don't tune up an engine, like this, unless the valves are set. Tight valves can cause leakage. Just pulling the valve cover, often tells you all you need to know. Like right here.

I drove one of these all over the North, from Prudhoe to southern Alberta. Its not a fast truck. Its powered by not much more then an oversized pickup engine - you need all cylinders to woking. The valves need to be checked often!

 

[Oldfart]

I can't believe this.... The absolute first thing your do in trouble shooting an engine, is to set the valves!!!! You can't do anything else, and expect meaningful results, until you set the valves in an engine with mechanical lifters. You don't tune up an engine, like this, unless the valves are set. Tight valves can cause leakage. Just pulling the valve cover, often tells you all you need to know. Like right here.

I drove one of these all over the North, from Prudhoe to southern Alberta. Its not a fast truck. Its powered by not much more then an oversized pickup engine - you need all cylinders to woking. The valves need to be checked often!

Sorry to disagree, but the first thing in old engine diagnostics is to listen to it run and accelerate up from idle in a no load condition. Then it is easier to do a quick vacuum check followed by a compression check. This will usually tell you where to look further. My experience is the first thing that usually needs attention are the ignition components starting with distributor points.

 

[overlordofmars]

I have to agree with oldfart, an engine can talk to a trained mechanic. When you know your stuff, and engine can give you hints to where its having trouble.

 

[76Z5M]

Looking for a muffler for an M221

Sorry to break into this thread with a parts request, but I'm too new to the site to really understand how to post a "wanted" item in the classifieds. Does anyone have a source for mufflers for the M135, M221 series of trucks?

Thanks,

John

 

[delos]

I might have to try that Restore Engine stuff, probably try it in my diesel pickup first and see if it works. Checking the valves first is not how I learned diagnostics. There are so many things to consider when determining the health of an engine, and you have to start somewhere. For me, doing a compression test gave me a baseline on which i could gauge corrective measures (fixing my pushrod and adjusting the valves.)

 

[Atomic]

Any updates on the truck? Good looking GMC.

 

[mobileauto2000]

Put a vacuum gauge on the truck. If the needle is steady the engine has no major mechanical problems. Low compression if even on all cyinders will still have a steady needle. That is always the first test. Steady needle move on to timing, spark , fuel or low compression. Flucating needle is a machanical fault.

I can tell you you engine needs rebuilt by your compression readings. Oil seals rings not valves. that is why when you have a low compression cylinder you squirt a small amount of oil in it. if compression comes up its rings if it stays the same its valves. If squrting oil in the cylinders makes it start its rings. Fixing the push rod and getting the other cylinder working may be enough to get it to start without oiling cylinders. But it still needs to be rebuilt.


restore dose work. It takes about 100 miles for it to have effect and will work for about 2000 miles before tapering off in performance. Try it it could put the rebuild of for a while. I would strongly recommend not using it in a vehicle that is not almost completely worn out.

 

[robert c neth]

my distributor was loose on mine and moved, the truck was slow once timing was reset it runs fine.also you didnt say if the tank was empty or full 8000 lbs of water will slow it down a bit.