On Sun, 07 Apr 2013 15:12:37 -0700, wrote:

Well, Case backhoe troubles actually. I think it's a CK 74. Anyway,
here are the symptoms: The 1970s engine (gasoline) starts right up,
idles well, and runs OK when it's not too warm. After fifteen or
twenty minutes it starts running poorly. It misses badly.

Can you pull individual plug wires when it's running to see if it's
only missing on a certain cylinder or two? (Carefully, Eric. It's
quite shocking.


If I let it
die it is very hard to start until the engine has cooled some. What I
have done: replaced the plugs, rotor, cap, coil, and wires. At first I
thought it was the coil. I had another laying around so I swapped
coils. This seemed, at first, to solve the problem. When I replaced
the plugs the only brand available on the island that were the proper
type were Champion plugs. I have been told by several mechanics to
avoid Champion plugs. The old plugs were Champions and they looked
like they were burning correctly when I pulled them. Now maybe the
replacement coil I had kicking around is junk and I need to get
another one, brand new. The ignition system uses a ballast resistor
and the coil is just warm to the touch, not hot like it would be if
the coil was wired wrong and getting the full 12 volts all the time. I
checked that too. The ballast resistor is only bypassed when the
starter motor is energized. The engine is not getting so hot that it
boils over, in fact, sticking my finger in the radiator neck to feel
the coolant temp tells me that the engine really isn't running too
hot. Steam and bubbles don't come out of the open radiator neck when
the engine is running so I don't think I have a bad water pump that is
letting the engine cook while the coolant in the radiator stays only
warm. I do have to run the engine with a little choke for it to run
half way well. I have adjusted the carb, which is a very old design
similar if not exactly the same as the one on my '39 Ford 9N,
according to the directions for this type of carb. That seemed to help
some, but even if I adjust the carb so that it runs rich I still get
the missing problem after the engine has run for 15 or 20 minutes. I
am going to buy a carb rebuild kit but I don't expect it will fix the
problem. So, any ideas? I'm stumped and I know there several folks
here who know about old engines.

One really common problem for an engine to run OK when cold and then
crappy (or quit altogether) when hot is a plugged gas tank vent. Take
the gas cap off when it starts running crappy. If it immediately runs
better, that's your problem.

Since you've played with the manual choke, we can somewhat rule out
the carb. AAMOF, that seems to somewhat confirm the plugged tank
theory. Doublecheck all the rubber portions of the fuel lines so you
can rule out air leaks, too.

Misadjusted points usually make it run crappy even when cold, but do a
quick doublecheck that they're still set right. I've had new sets of
points close up on me due to swarf on the end of the rubbing block.
Man, that was a whole lifetime or two ago, I think. I don't miss
carbs or point sets at all. Vive la EFI and ECMs!

--
A sound mind in a sound body is a short but full description
of a happy state in this world.
-- John Locke