In article ,
Larry Jaques wrote:
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!
I'm not a tractor guy... never worked on one. However, from the little
I've see of them, they look to have gravity fuel systems... and I agree
with Larry about checking the fuel cap vent. Someone could have even
replaced the cap with a modern non vented one.
I've also seen an ignition capacitor or two that act up when warm.
(BTW, fuel tank vents are a required pre flight inspection item on most,
if not all light aircraft.)
The fuel cap reminds me of a story... if I've told it here before, just
stop me.
Worked a place in the 70's, and they hired a new guy who was a biker
type wanna be. He looked all tough, stopped shaving, had the shiny oil
soaked pants, boots, vest, and even had a chain on his wallet! But he
didn't have a Harley yet... or even know how to ride.
Anyway he found a bike, and as he knew little if anything about them
asked me to look at it with him. I knew little about bikes in
particular, but agreed.
We get there... IIRC they said it was a 40 something Panhead. The thing
had chrome everywhere, 19' forks, 5' sissy bar, suicide shift, multiple
chrome skulls, long dangling leather tassels hanging from stupid high
handle bars, and all the other intellectual accoutrements. A true
beginners bike if there ever was!
Anyway the owner dude said he'd had it for ages, but hard as he tried,
could never run more than 5 or 7 minutes. The first thing that popped in
my mind was the vent. So in looking it over, I took the cap off, and
sure enough, a long dead spider had it plugging.
They deal, they bicker, wannabe forks over cash... they do paperwork,
and wannabe gets a quick riding lesson. I followed him home (scary to
watch), and soon as we were out of ear shot (quite a ways actually), I
flagged him down and cleaned out the vent with a twig. It never stalled
again.
But sadly wannabe immediately had another issue... mainly he lacked the
275lb beer gut necessary to start the damed thing. But he worked hard
acquired one in short order.
Guess you had to be there.
Erik