Having high idle in park and neutral with my 1987 D150 with a 318 motor. This only happens sometimes. In park or neutral it will idle very high. Sometimes it will go away if I pump the accelerator a few times and it will even out. Sometimes if I drive it a bit then put it in park it will idle ok.

It idles just fine in gear and does not stall. A friend told me it had to do with the throttle return spring (?) on the carb (maybe an idle speed controller?). This is my first vehicle with a carburetor so please be easy on me. Please let me know so I can get this very annoying problem fixed.

 

that is the way carbs are which is why they were tossed for fuel injection. they need to warm up to run properly. the high idle is engine warming up and choke not fully open yet. when it is fully open it will idle normally. there is no cheap fix other than patience.

 

Even once warmed up it will idle very high. I can drive it for 30 miles and it will idle high once I put it in park.

 

Mine was doing the same thing a little while ago, I took to a mechanic. He said there was too much gas going into the carb (screw out way too far) and also the choke was fully open making a richer mixture. My exhaust smoke was black. So he disabled the choke since I'm in AZ and put idle screw in right spot.

 

Borrow an old fashioned timing light and check the timing. Both with the advance attached and plugged off. The emissions sticker should give the timing settings.

 

have you checked to see if choke is fully opening? choke needs to fully open in order to low idle. you may have to hold the choke open and give it some gas for a second to make it engage the low idle circuit. factory choke systems suck and clogged crossover passage could cause it to take a lot longer for choke to hit operating temp and do what it is suppose to.