Aloha everyone from a newbie in Hawaii! I just bought this truck 2 weeks ago. It runs excellent, except very rough for the first 3-5 minutes til it warms up. It was an old farm truck in the upcountry here. It does this whether at sea level or up at 4000 ft where I live.

I have run it to Hilo and back (50 miles r/t) about 6 times and it runs great except for this cold start problem. Once warms up, idles fine, and is very strong at all speeds (haven't taken it over 60mph).

I have read alot of possibilities, but wonder if anyone here has seen this situation before with old F150s. I hate to open a can of worms if not needed since it is operating so well, but of course would prefer to fix it. I am looking forward to going on the back roads here - it's 4WD (automatic).

THANKS for any help.

 

Which engine does it have?

I'm assuming an 81 is gonna have a carb, no FI. I'd do this the old-fashioned way - get some spray carb cleaner and give the carb a good healthy spraydown with it running, and toss some good fuel system cleaner in the tank - such as Techron. That's going to be minimal expense and it may well help.

I have a feeling it may be running too lean on warmup due to ethanol in the fuel. Not much you can do about that without rejetting the carb.

 

Thanks GLC. I'll give that a try. It's a 351 v8.

 

I'd be more apt to watch what the choke is doing if the electric is still hooked up, or has it been converted to a manual setup?

Adrianspeeder

 

Didn't the 81 351 have that junk variable venturi carb?

Yeah, it could very well be a choke issue.

 

Thank you both. Electric is still hooked up, and on the diagram it says choke is not variable.

If I hold the gas down to medium-high level, it warms up in about 2-3 minutes, and runs strong within 3-4 minutes. If I don't do that it stalls, and runs pitifully til warmed up.

What would I do to fix the choke situation?

 

With a totally cold engine, take the top off the air cleaner so you can see what's going on. Depress the gas pedal halfway and release - the choke should close completely. Start the engine (if it will start) without touching the pedal. The choke should crack part way open and the engine should run at a somewhat fast idle. As it warms up, the choke should slowly open all the way. Then, when you tap the gas pedal, the idle should drop to normal.

This is how traditional automatic chokes worked - I don't know if that's exactly how they still worked in 81 due to emissions regulations.

The choke linkage interacts with a fast idle cam which props the throttle plate open past the stop screw or dashpot (if it has one). The choke opens due to heat from the engine and/or an electric heating element in it, it's a coiled bimetallic spring that closes it.

Let me ask this - in order to start it when cold (well, cold is relative in HI!) do you have to pump the gas a couple times, then feather the pedal to keep it running? If so, the most likely problem is the choke is not working at all - it's stuck open. Hold the throttle part way open and see if you can move the choke plate with your fingers. If it moves but won't close by itself, the spring may be broken, or it's way out of adjustment. You may just want to get a manual choke kit.

 

GLC thank you very much for your imput. I am busy at work, but this is a top priority to protect my $850 investment!! I can't believe it runs so well.

-Carb cleaner and gas treatment made engine run even smoother, but didn't solve warm up issue. Still very rough at startup/warmup, 4 minutes later runs like a top at all speeds (a top that is 31 years old anyway!).

-Will try the choke procedure today or tomorrow on startup. The previous owner has given me an old F150 manual - and I see the spring mechanism you are talking about. And some are electric.

 

If you have the manual, also look at the fast idle cam and how it operates.

An electric choke should still have a spring - the electricity is used to warm it up so it opens. It has a small heating element in with the spring.

What does happen frequently is the electric heating element burns out, the choke doesn't open, so someone adjusts the choke so it's open with a warm engine. This makes it so it doesn't close when cold.

What you have to figure out is whether it's running too lean or too rich while it's warming up.

 

So, I did the starting and checking the choke on two cold starts.

The butterfly plate is just open all the time from before starting and thru out warming up. When I go and hold it mostly closed manually the engine warms up better. Putting the gas down 1/2 way, then letting up before starting doesn't move the butterfly. And the engine wouldn't start in idle (without gas pedal down at least some).

I am noticing if I am a little lighter on the revs at warmup it is actually warming up better. But still a dog for first 4 minutes.

I will look at the fast idle cam, and also study this automatic choking mechanism. Thanks! I took it 60 miles r/t today and she ran great again.

 

Sorry for the delay - I was out of town.

Your choke spring is broken or misadjusted. If you pump the gas a couple times, then get out, open the throttle, push the choke closed, then release the throttle, it should start (don't touch the gas) and come up to a fast idle. If it does, the fast idle cam mechanism is working.

 

Thanks again, GLC. I put in manual choke and it worked great!