This is on a 1980 F150, 302, 2150 carburetor. Near as I can tell, all pretty much factory.

It dies at idle on cold engine. Once it warms up, it is fine. Also, it backfires, to the point it put a hole in a muffler. I am waiting to fix this condition before replacing muffler. And the backfire happens when it is warm, not sure about cold.

I took the plugs out to do compression check (150 on all cylinders, cold), but the plugs were fouled. This after the backfire, replacing plugs, wires, distributor cap and fuel filter.

Here are some pics of choke, 1st engine off, cold:




Engine off, cold




Engine off, one tap of acceletor pedal.




Engine on, warmed up, idling fine.




Any tips on where to start?

I have replaced a whole bunch (maybe all) vacuum lines.

 

I'm new to this truck, but it sounds like it may be carburetor related. When was the last time you rebuilt the carburetor?

 

I have had the truck for about 10 years, never rebuilt in that time.

 

The rebuild kits are cheap and was easy enough to do on my 4.9. Not to mention educational as well. Might be worth doing IMO if it's ten years old.

 

Except that is has no tag. I'm not sure how to know what kit to get.

 

Also be sure to check the timing. The timing chain may have jumped a tooth.

 

A quick search shows there's 3 different rebuild kits for the 1980 302 on Parts Geek. For around $100 you can buy all three kits. I would buy the three kits, take apart your carburetor, and see what kit fits from the parts you have taken out from your carburetor. Just return the unused kits.

 

Great idea, thanks!

 

I would say as a start to rebuild the carb just so you know it is clean inside, don't forget the fuel filter.


When taking it apart take pictures just incase you cant under stand the build sheet the kit comes with.
Also note the choke black housing. It may have a mark on it that lines up to marks on the metal choke housing. This way you know where to start at when putting it back together.
As a guess I think you could go a little richer on the choke as it looks like it may not be closing like it should and pulling open too soon.


Check the timing chain for stretch. Get the motor to TDC, Pull the dist cap and with a breaker bar on the crank bolt turn the crank just a little till the dist rotor just turns. Note the timing mark, now turn the crank the other way till the dist rotor just turns and note the timing mark. The different between the first timing mark and the 2nd is how much the chain is stretched, let us know what you get.


Once running, motor up to temp, carb adjusted, check the timing. You should have a base timing @ idle, mach. timing @ 2500 (no vacuum) and total timing - vacuum hooked up and @2500. Note the timing and get back to us but as a guess base about 10*-12* BTDC
Dave ----

 

I got the 3 rebuild kits (Hygrade) but I notice they do not come with the float. Is that something I should change as a matter of course?

 

no kits I have ever used came with floats.
only reason why to change it would be because it is bad Ie no longer floats.
Dave - - - -

 

Turns out I have a 351, not 302, so slight delay while I returned the unopened 302/2150 carb rebuild kits and got the 351/2150. Got it rebuilt and mounted back on truck. It runs worse. It will start, but runs very rough maybe can keep it running at high rpm, but dies easily.


I think it may be vacuum after all, at least that needs to be right.

2 issues I am aware of after being under hood long enough to get carb off and back on:

1) I found an old hose I had not replaced that came off front of engine tree that was just loose. I capped that until I figure out where it goes.

2) I found another hose coming off block with screw in end of it. I doubt that is how it came from factory.

The diagram under hood:




and the hose plugged with bolt coming off block is running under fuel filter:




Can anyone tell me based on routing where this should be going?

Re #1 above. there are 3 trees at front of engine with vacuum lines. One tree has 3 connections, first appears to be plugged originally, the second runs eventually to the air cleaner, but stops at a green valve connected to 3 other hoses. and the third is the older hose that was hanging loose.

 

How dirty was the inside of the carb?
How did you go about cleaning the carb?
After you rebuilt the carb how many turns out from lightly seated are the 2 idle mixing screws?
Was the choke on and was the fast idle cam working and the fast idle speed screw hitting the cam?


To check for vacuum leaks if you can keep it running start to cover the carb opening, "choke it off" and see if it idles better of RPM picks up.
If the motor will idle some what but ruff try carb cleaner around the base of the carb and where any vacuum hoses hook up to see if it changes how the motor runs.


FYI I had 1 carb (Holley 600)that would not idle and covering the carb would fix it. I rebuilt the carb and it still had an internal vacuum leak so I replaced it with a used Ebay one and the motor run great now.
Dave ----


edit: have you checked into the EGR valve not closing all the way? That will make it run ruff or not idle.

 

Not that bad. The carb spacer had quite a bit of carbon build up. Took some work with a screwdriver to get it knocked down. The carb was soaked in Berryman for about 10 hours. Hosed off, used spray can carb cleaner to blow through all orifices, blew out with compressor.

right screw was at 2.5 turns, left was 2.25 turns. I looked at the cam but not really sure it was right. I know I didn't take any of that apart. I did remove the choke housing before soaking.



It won't idle long enough to do that test.



How do I test that?

 

This not getting any better. Right now, symptons:

Starts, will not idle at low rpm at all, press accelerator to bring up RPM (no tach) and stays there for a few seconds, then drops RPM to almost stalled and almost always goes back up in rpm and repeats. Surges like this.

What I have done:

Rebuild carb, replaced EGR valve, plugs, distributor cap, spark plug wires, fuel filter, replaced leaking fuel pump (about 6 months ago), . Checked and rechecked vacuum lines, stared at the chart under hood until I was dizzy.

This started after I rebuilt carb. So I removed carb (and made dry float adjustment that allowed it to start). Saw no other issues.