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I've had this 73 F100 390 for about 2 months. I live in Houston and its been in the high 90s or low 100s seems like everyday. The previous owner only had trouble with it in the winter months, also this was a truck he kept at his bay house and only drove it a few times a month, and just a couple miles at a time. I don't know if that information is important but I included it because it means the truck didn't really see any use the last few years, but the last two months I've put a couple hundred miles on it and things are starting to break.
I've been having some cooling problems over the last few weeks starting with overheating one hot afternoon on the freeway. Water pump went out. I had to stop three times to top off the radiator on the way to the house, but made it home without overheating again. It ran hot but only up to about 3/4 of the temp gauge, I don't think I did any permanent damage to anything.
The following week I replaced the water pump and leaking upper radiator hose, and flushed the cooling system several times. After that it was cooling fine, but was having trouble restarting a hot engine. Usually I could get it to start but it ran like crap. Once it starts doing that hot restart problem, it idles poorly and even stalled a few times stopping in gear. Got to the point where I'd have to shift into neutral at stop lights.
I've made several adjustments to the carb and replaced the choke heat tubes and fuel filter. Also, though this may be unrelated, I couldn't start it at all for two days. The carb would flood and dump gas out of the vent holes onto the intake. I took the top off the carb to check the float and jets and everything looked good. When I put it back together it started right up and hasn't failed to start since. I think maybe the float was stuck? And poking at it with my finger got it moving again?
Anyway, the last few days it's been starting up fine, but still idling poorly after it's warm. It has been staying at normal operating temps when running but really spiking after I shut it down. The old thermostat opened at about 195°-200°, based on checking it in hot water on the stove. I replaced it with a new 180° thermostat this morning. It's running temp is a bit lower, but it still spikes after I shut it down. I know that heat soak happens on all old trucks but I don't know if the spike I'm seeing is in the normal range. It nearly pegs the gauge a few minutes after shutting down. With the old thermostat it did peg the gauge. I'm beginning to think my starting problems are caused by this, and maybe its also affecting idle? Doesn't seem like it should be because once I restart the engine it cools back down in 10-15 seconds. Here's what the temps look like running, and after I shut it down. Normal?
The heat spike after shutdown is normal for an iron headed engine. As long as it cools down after a minute or two of running, no problem.
The hard start after heat soak is probably the result of fuel boiling out of the carb. It is common to start a little hard after heat soak... I usually have to hold the throttle open a bit until it starts then fast idle it for 5 seconds or so. After that it should smooth out.
Your isn't smoothing out and that sounds like a problem. When mine acted like that it was the gummed up 35 YO carb. A reman carb fixed it. A tune up including new wires could only help also.
Yeah, I hate these "my factory gauge is overheating" posts.
get an aftermarket water temp gauge (mechanical) and see what it actually looks like.
In regards to hot start, things I have to do here in georgia to keep the engine easy to start:
-Block heat crossover in the intake
-route all fuel lines away from heat sources
-carb spacer/heat insulating gasket for carburetor
Sounds like you have a variety of minor problems making a major problem. Get the timing perfect, get the idle screws and float level setup perfectly, insulate the carburetor from heat, and then see where you are at. I'd wager most of the issues will be gone after that and you'll be able to work out what is left.
All of my hard starting / trouble restarting issues have been traced to exhaust leaks. The starter gets baked when the doughnut seals on the manifolds fail. Ended up having to replace the gaskets and starters in three of my dents.
I've had multiple bad float check valves in new rebuild kits too. The float is set right but the check valve leaks, causing overflow.
Anymore, I don't replace the check valves when rebuilding the carb, just leave the old one in there.
I think I'm closing in on the primary problem with hot idle.
I checked all the plug wires for spark, I don't have a multimeter so I can't check resistance, but they are all sparking. And the plugs look decent, all about the same.
Yesterday it was having a hell of a time warming up, I took a look at the choke valve and it was all the way open when cold. So I adjusted it until it was just about to close. That made it idle smoothly when cold, but once it warmed up it started idling poorly again. Disgusted, I took a break and waited until this morning to revisit the issue.
Started up this morning great and idling smooth, but started getting rough after 5 minutes. I took of the air cleaner and saw that the choke was wide open, which by this temperature seemed right, and when I pushed it about 3/4 of the way closed the idle smoothed out. Of course when I let go of the butterfly it opens up again and starts idling poorly. So, I made adjustments to the fuel mixture screws, opening them up until it smoothed out. But I think it's way too much. Both screws are out about 5 full turns.
At that point I started spraying throttle body cleaner around the intake and carb. There's a slight increase in rpm when I soak the base of the carb on either side, so I'm assuming this is the primary issue, or at least an issue. I couldn't find any other areas where the spray had any discernible affect (effect?). I think I'm on the right track. Seems like if I have to partially close the choke and open up the fuel mixture screws to get it to idle well, it must be sucking in too much air somewhere. Interestingly, spraying carb cleaner has little to no effect when the engine is cold.
One last thing, even with the rich mixture to compensate for an apparent vacuum leak, and though this smooths out the idle, it still seems to rev up and down a bit. I don't have a tach so I'm not sure exactly how much, but it sounds like a couple hundred RPM.
I tightened the carb bolts a bit. None were really loose but I was easily able to get about 2 turns out of each one. That was with a short 5" wrench so I don't think I over-tightened them. I also noticed a small vacuum leak where the PCV line enters the back of the carb, so I replaced that hose. Now neither the PCV line or the carb base are leaking, and the idle has smoothed out considerably. However, there is now hesitation on acceleration.
If you do pull the carb for rebuild or replacement with reman - which sounds necessary - agree with Deeproots adding a thick insulating gasket can only help keep the heat away.
If you do pull the carb for rebuild or replacement with reman - which sounds necessary - agree with Deeproots adding a thick insulating gasket can only help keep the heat away.
Yeah I've been looking at spacers. I spoke with the previous owner's mechanic last month, and he said he remembered rebuilding the carb a few years ago. I might call him again and see if he can be more specific. But, I've already resigned myself to rebuilding it in the near future.
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