Hello everyone,

I bought a 78 f150 recently and I am having some trouble getting it running right. When I first bought the truck it was hard to start and ran like crap so I replaced spark plugs/wires, rotor and cap looked fine, I rebuilt the carb and it fired right up as soon as i turned the key.

However after running about 10-15 minutes it would idle rough. I assumed an overheating issue but the gauge wasn't working so I replaced the temp send switch. Its "normal" operating temp was at the high end of normal, on the line. If I'm forced to sit in traffic the needle will rest on the H on the gauge. So I flushed the system, replaced the thermostat and rad. cap which brought the temp down slightly. but the idle remains rough and temp seems high though I have read that the temp gauges on these older trucks are not accurate.

I don't have tach on the truck so I used my dads old tach. the idle is right around 650 according to it but the engine surges up to about 1000 then will drop down again but it is never real steady. timing is best as I can set it but with idle speed never steady I can't be 100% certain

I was thinking it might be a vacuum leak but I hadn't been able to find one so I'm thinking maybe the vacuum routing is wrong but I can find no diagrams to verify I have correct routing and have not been able to find them so far on the forums.

The truck starts everytime as soon as I turn the key, idles a little rough for about 30 seconds and then it smooths out and runs how it should, but then starts to idle rough when warmed up.

What should be the next thing to test and/or check?

1978 f150 Ranger 400. Carb is motorcraft 2150. The steel tube going from intake manifold to bimetallic thermostat is broken, have not been able to find a replacement so I just have rubber tubing slipped over each end. Also noticed that my main jets are 57f (not sure what the "F" denotes) and from what I can tell 56 is stock. I don't know if this would cause these issues?

 

I'm going through the same thing and I think it's a head gasket. Don't know if that applies to you or not

 

I don't know about that I'm not having any other signs of a bad head gasket. No coolant or oil loss. No white or black exhaust. I usually I do a compression test but I couldn't find my gauge and now that I found it it's too dark to see outside so I'll have to do that tomorrow morning.

 

It sounds like you still have carb issues. I would check compression, verify the firing order is correct and set the timing. Then adjust the mixture screws. If they do not have any affect on idle speed/smoothness, rebuild the carb again. Also, make sure the choke is operating properly and the throttle shaft is not binding and opens and closes fully.

 

I'll check compression tomorrow morning when it's light out. I have already verified that the firing order is correct and timing is set as best as I can set it without having a steady idle. I had found a detailed thread on these forums on how to adjust the Motorcraft 2150, so I am confident that it is adjusted properly. However, as I mentioned before, the main jets do no appear to be stock size. Perhaps this is the source of my issues?

Also, the hot air choke tube is broken and I cannot find a replacement. Surely someone else has faced this issue and either found someone who sells the part or has a workaround?

It looks like I can buy a choke tube kit from an autoparts store and just bend it to what I need. Ill pick that up in the morning and see if I can't find some #56 jets and see what happens. Will update tomorrow evening assuming I get it all done before work.

 

Ok so I fixed the choke tube and found that one of my vacuum switches was broken and replaced that. I haven't had a chance to verify that all vacuum routing is correct but it starts up smooth now, however, it still has the rough idle issue once it is warm and it is running hot. Could my radiator be bad?

None of the carb shops seem to be open on weekends around here so I haven't been able to get any stock size jets yet.
What should be the next step in solving this issue?

 

It sure sounds like a vacuum leak or carb issue. When cold, the choke richens up the mixture and it idles well. When hot, not so much.

Also sounds like the cooling system is marginal but that shouldn't directly case the issue. I'd look for vacuum leaks first. The stock setup uses a vacuum switch to advance timing at idle by providing full manifold vacuum to the distributor at idle when coolant temps are high. This advances timing to help cool things off.

Suggest you check all vacuum lines - especially those connected to vacuum switches - for good connection and condition... also re-adjust carb mixture screws when hot.

 

OK so none of the vacuum diagrams I can find make any sense to me so I made a rough sketch of engine components and their vacuum locations to try and sort out this routing. Hopefully someone can fill in the blanks as far as hose routing goes.