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I've been having some drivability issues this season. Some exhaust popping, poor throttle response, harder to start than usual. Now that she's garaged up for the winter, I did a compression test. Cylinder #4 leaks, I see compression rising on the meter when cranking, but it drops off immediately. The other 5 are good, getting 100-110 psi there. I tried adding a bit of oil to cylinder #4 and that helps, getting 100 psi like the others. Would this point to a valve or compression ring problem ? There's a rebuild plate on the engine dating 1951, I'd hate to think that it would need surgery already again

I think you are on the right track, but before you do anything else, re-check the compression on number 3 to make sure it isn't the head gasket blown between 3 and 4. If 3 is OK, next thing to rule out is the valve clearances on 4, just check them cold and compare with the other cylinders.

If all of that is OK then you are back to rings and valves. You'd better start off with a table with a head gasket, sump gasket, set of rings and preferably a set of valves and guides on it. Pull the head and have a good look at the cylinder walls on 4 and the tops of the valves before you decide what to do. If the cylinder wall is badly scored by a broken ring, or a valve is cracked, you may need to do more than just replace parts.

I was taught that if compression increased after adding oil, then the problem was the rings or cylinder walls.

Good tips, thank you both of you. I'll check the valve clearances, once I get the inner fender and some other bits out of the way. Will have to check clearances cold, as the carburetor and ignition system is off as well for inspection. If I recall correctly the cold values should be .35mm (.014''), does that sound about right ?

I have no idea - if it was running well just compare it with the other five.

I included the valves because with oil down the cylinder bore, some of it can be thrown over and temporarily seal a leaking valve, or valve guide, when you crank it.

You can pinpoint the problem (though on an old engine it could be a combination of rings and valves)

Aircraft engineers have a leak-down kit which is only an air hose with a gauge and all piston aircraft have this check every 100 hours.

If you can rig up an air hose to the plug hole you can tell where the blow by is quite easily. Make sure the engine is in the valve closed position and turn on the air.

1. If you can hear air leaking by putting your ear to the exhaust pipe it is valves.
2. If you can hear air leaking by putting your ear to the oil breather it is rings/bore/piston.
3. If you can see bubbles in the radiator it is a crack or head gasket.
4. If you can hear air through the plug hole of the adjoining cylinder it is a head gasket.

Lang

Hi,
as a general rule, oil down the bore showing an improvement in compression does point to a piston ring issue. However when one takes in the exhaust popping, poor starting and poor performance it does point toward insufficient tappet clearance. If the tappet on that cylinder is too tight the compression will bleed down very quickly. The popping exhaust is also an indication of a valve (ergo tappet clearance) not sealing correctly.

Settings are 0.008" inlet and 0.010" Hot, I strongly advise that you set at 0.010 and 0.012 with the engine cold. The simplest method is to remove spark plugs then hand crank the engine while watching the distributor rotor, as soon as the points open, set/check the valves on the cylinder that the rotor/plug wire points to

All of Langs suggestions are right on the money!

Cheers

Ian

I got started on this today and decided to remove the manifolds to get access to the valve covers and see what I am doing when doing adjustments. I went this route as I need to have a look at the rusted up manifold butterfly valve anyway, and I figured this was easier than removing inner wing and wiring. Went smoothly except I'd forgotten that 2 of the manifold bolts enter the water jacket....Got a fair bit of water into the exhaust and some more found it's way in behind the valve covers gaskets. Need to drain the oil as well then I guess. Ran out of time, hope to get some time next weekend to check the valve timings.

If you separate the manifolds to inspect the heat control valve, be very careful removing the four bolts that hold them together. They often break off during removal. This is so common that my engine re-builder includes the cost of fixing the broken bolts in his price for separating and resurfacing the manifolds.

I heated and let cool (for several cycles) the inlet manifold where the threaded bolt holes are, and the bolts came out without breaking.

maigaard wrote: ↑Sun Feb 03, 2019 5:28 pm I got started on this today and decided to remove the manifolds to get access to the valve covers and see what I am doing when doing adjustments. I went this route as I need to have a look at the rusted up manifold butterfly valve anyway, and I figured this was easier than removing inner wing and wiring. Went smoothly except I'd forgotten that 2 of the manifold bolts enter the water jacket....Got a fair bit of water into the exhaust and some more found it's way in behind the valve covers gaskets. Need to drain the oil as well then I guess. Ran out of time, hope to get some time next weekend to check the valve timings.

so now you figured out removing the fender is easier? lol

Well, I finally got around to looking at this again. I found that on one valve someone had fitted the wrong retainer locks, so I replaced those with correct ones. That was not on the cylinder that showed no compression initially though. However, after fixing this and a valve adjustment all round, I get good compression on all cylinders now. I started it up and let it run for about 20 minutes, it runs fine Now I have some valve ticking though. I set all gaps to 0.014'' cold as by the book. Some gaps were a lot tighter than that before I started, probably too tight. So, should I run it like this for a while and see what it's like when properly hot, or dig into it and try and re-set the gaps hot ? Not really looking forward to that though.
Also, after 20 mins the temp gauge was only just starting to rise, and the radiator hoses were warm, appr 50 deg. C. I assume this means that the thermostat is sticking, now wondering if I should drain the coolant again and check that out...there's always something to do

Never run it with the gaps too tight, asking for trouble. Ease them all off to maximum and then adjust up as necessary after a few runs.

I wouldn't bother about the thermostat until you had run it under load for a while, it may well just be lazy and a few heat cycles will bring it back. That said, you should always have a thermostat and gasket on the shelf as spares.

thats good news it was just a valve adjustment issue. .014 is on the loose side. so many books give different specs. I run them a bit tighter cold with an ambient temp of at least 60 F. flathead gaps dont shrink as much as OHV engines do

The cooling system on the trucks is massive. it is unlikely to get to operating temp sitting in the driveway. There is 3 times the coolant in these systems compared to a modern v6 truck. one of the reasons they perform so well in many climates. i have been out on the trail with multiple brands of other trucks like toyotas, GM, ford, susuki, land cruisers and rovers and been the only one not running hot or over heating in rock crawling slow mountain trail going at 95 degrees F.

More progress the past few weeks. I drained the coolant and had a look again at the thermostat, tested it in hot water, and both the thermostat and the water temp gauge are up to spec. It was then that I noticed that the upper thermostat gasket was missing. Someone didnt put it in I guess. When a new gasket arrived, I fitted it and put everything together. Magic happened and the engine got up to temp nicely, idling at a steady 80 degrees C after about 15 mins
The engine was still running rough though, and a vacuum gauge showed engine vacuum fluctuating quite a bit around 18 mmHg. I had another carb lying around, a Rochester single barrel from a 50's chevy which bolts straight on. With that, vacuum is stabile at 20mmHg and it idles smoothly. I'll be doing some test driving with that and then decide what to do carb-wise...Probably a vacuum leak in the Carter carb causing the trouble ?

The Carter carbs have that reputation, they can work really well or be a complete dog. Every one I've had has worked well so I haven't messed with it. I'd think you would really need someone with lots of practical Carter "hands on" experience