Runs rough when hot

[DarkBlack]

I’m not intimately familiar with this engine, and I know there was different valve train versions of that engine made.
That said, hardened valve seats, in general, prevent valve depression over the lifespan of an engine. It’s not a significant factor in short term temperature related valve train clearances. That’s determined by other thermal expansion factors like the metallurgical properties of the camshaft, lifters, cylinder head , push rods( if they exist), valve stems, etc..

 

[kirkawilson]

Make sure the distributor cap isn't cracked. A hairline crack can cause some of these synptoms. They are hard to find too.

 

[Enrico8n47]

UPDATE: After reading contributions in this forum and others, I'm thinking this may be a heat-dissipation-related problem, so I'm going to 1) clean the radiator fins with compressed air and water from the hose (from the back of the radiator); 2) drain and flush the cooling system, replace 50/50 coolant; 3) remove and test the thermostat; 4) remove the lower radiator hose & check for sediment in the bottom of the radiator. If these do not work, it's possible the radiator core may have some clogs.

 

[Enrico8n47]

UPDATE: I replaced the points and condenser, reset the timing. It's a PITA due to the distributor's location, but worth the trouble to have a good running engine. I'll do this every other year instead of every 3.5-4 years. Also since I was unable to mow due to rain, I changed the engine oil and filter, and drained/flushed the radiator, poured in some nice fresh coolant. It fired right up, and is running well (for the time being). Examining the old points, they were pitted beyond what a basic cleaning could accomplish. Thank God, not the valves! I did use some Marvel Mystery Oil, that certainly won't hurt and probably helped a bit. Thank you to all who contributed.

 

[Enrico8n47]

POSTSCRIPT: started backfiring and losing power, again. I did a compression test, #2 is much lower (45-50) than #1 (80), #3 (70), #4 (70). Spark plugs, which were worn but OK 2 weeks ago are now dark, #2 being the worst. Something else is going on here. More diagnostics to come. If anyone knows a possible solution short of removing the intake/exhaust manifold and resetting valve lash, I'm open to suggestions. I've also heard that sometimes a valve gets stuck open, but it's impossible to see the valve with the cowl and fuel tank in place. So I might remove them to check for a stuck valve as well.

 

[John0829]

Low compression on #2 could be valves, burnt or sticking but your compression is low on all cylinders. I believe a complete R&R of your engine is in your future.

 

[Enrico8n47]

Low compression on #2 could be valves, burnt or sticking but your compression is low on all cylinders. I believe a complete R&R of your engine is in your future.

You're probably right.

 

[careyo63]

You have a Compression problem no doubt. My gas 2000 110PSI Cyl. pressure when I bought it. While you can still get it to run I would put a pressure gage on the radiator and see how high the water PSI goes, Your checking the Head and Gasket Etc. My guess is you should be around 15psi running. You going to be pulling the head and it will give you a good idea the condition it's in. Warped or cracks will show up with a high pressure so your pretty much know there is a problem. Cyl. may just need new rings?