Sorry about the length of this post- thought it was better to include more detail than less.

Just got a 1971 F-100 with a 390 engine in it. Ran pretty rough at idle and though it sounded better when it warmed up and sounded pretty normal at 1500 RPM, it had no power when you mashed the gas - It was really hard to get 40 MPH out of it on the flat.

I found some problems in the throttle linkage, fixed them, and took it for a road test. It died after 2 miles as I was
coasting into a turn and it would not start again. Towed it back and checked:

Fuel pump output - Plenty of gas at the carb, pump obviously working when engine cranked.
Distributer - Spark jump from connector to cap on all wires when cranking engine, no tracing or pitting on cap or rotor. Wires look new.
Spark Plug #1 - Looks pretty new and clean and fires when plug grounded to frame and engine cranked.

I figured the engine might be flooded, so I left it for two days. Then I cranked it and it started right up with the familiar rough idle. I quickly checked the timing on plug 1 (vacuum tube connected). I got a quick look at it before the engine died again at idle, and refused to re-start.

The timing on #1 looked like firing right at TDC. There is a yellow line painted on the pulley at 6 degrees BTDC and a blue piece of metal which I assume is an index marker mounted to the engine block (See picture). The yellow line shows up at the TOP of the index marker when the strobe flashes, and I would have thought that it should be at the bottom of the marker.



I checked the compression on #1 cylinder and it slowly built up to 75PSI while cranking.

I can't figure out what the carb is; there are no useful markings on it that I can see. Here's a picture:



So.....

1. Is it possible that the carb is managing to flood the engine at idle?
2. Does anyone know what this carb is and where I can find out how to adjust it? What do I adjust?
3. Do I use the top or bottom of the blue index marker to set the timing? Or something else entirely?
4. Anything else I should look at to diagnose this failure at idle?
5. 75 PSI seems really low for compression. Is this likely to be THE major cause of lack of power, or just a contributing factor?

Sorry if this is really basic. I don't have a lot of experience with this stuff, but I want to learn more. Any help at all is much appreciated.

Pete

 

Six degrees before top dead center will serve you well. You are at TDC but that is not the cause of your problem.
Start with the carb.
On the front of the carb you will see two screws with springs. Get a can of Gumout with the red schnozzle, pull those screws all the way out, squirt the schnozzle in the holes. And then squirt down the throat of the carb to wash the throttle plate at the bottom of the carb. Put the screws back in after you are all done, run them in until they seat, back them out exactly two turns.
Squirt up all external linkage.
Go from there. See what happens. You are out five bucks and if nothing good happens, come on back in and talk to us.
It is pretty clear that you have a fuel delivery problem in or around the carb.

Semper Fi

 

part of problem that center (big)port plug looks cracked and can see metal thru it VACUUM LEAK replace the little one too it looks iffy

 

It is an add-on Carter AFB. Looks and sounds like it needs to be re-built. All that mud on it shows me it's been seeping and leaking for ahwile. My guess is PO had the truck setting for years before selling to you. So probably still has Bad gas in tank and carb. You have to have all 8 spark plugs out to get a correct compression test.

 

... and the throttle butterflies fully open.

 

I would do what Mike said first. All those vacum plugs look dry rotted and cracked. I would replace them all and go from there.

 

Yup, change out any rubber lines, especially those dried out plugs on the carb. That should get your idle smoothed out.

75 PSI is lacking, should be 120+ but who knows how worn out the engine is. Should run better then 40 mph. I'm thinking there is a timing advance issue causing the lack of power.

 

Yes, exactly as above. Get the Gumout and vacuum caps at the auto parts store. Do as I said above with the benzene and while you are in there it would not hurt to tighten down the carb mounting bolts and the intake manifold bolts.
From the look of the choke plate your carb has had no attention since the fall if the Roman Empire.
Come on back in if you are still having trouble.
Semper Fi

 

All good info Guys. I got nothing to ad that ain't been said. Yet.
Do what they all said and come back and let us know.

 

Thanks guys! I have plenty to keep me busy now. I cleaned the outside linkage and the screw ports and then ran out of gum-out. Got some more now but the can says I have to have the engine running to clean the inside of the carb. I pulled all the plugs; #1 was sopping wet with gas and all the others had some gas on the threads, but not as bad. I'll check the compression while all the plugs are out and then see if I can get it running long enough to clean the inside of the carb. More soon.

 

Spray the firing end of those plugs with your Gum-Out and let them air dry. Right now at least one of them is fuel fouled.

 

Long day. First I gapped all the plugs to.035 (they were way over), then I cleaned them off good. While they were out, I checked the compression and got (1-8: 96-85-85-30-50-61-65-75) Not good. I installed 4 new vacuum plugs on the carb and then installed the spark plugs and after a bit of fuss I managed to get the engine started again and cranked up the idle to where it would not die immediately. I sprayed about 1/2 can of gum-out on the internals and they sure look cleaner now. Then I reset the timing to 6 degrees BTDC and took it out on the road. It seemed to have a bit more power, but really jerked when I put it in gear because of the too-fast idle.

When I got back, I tried to turn the idle down and it promptly died. I couldn't get it started again until I mashed the gas down to the floor and cranked for 15 seconds until it started to catch and then another 5 or 10 until it coughed, sputtered and finally roared. I turned the idle back up and looked into the carb and could see gas dripping at the bottom of the left hand primary barrel. No drips in the right side, or in the secondary barrels. I got no joy from adjusting the idle mixture screws, in fact, I turned them both all the way in and the engine did not even change revs. I backed them out quite a ways too with no noticeable change to the idle. I left them set at two turns out. At this point I could get it started reliably when I mashed the gas while cranking.

Then I fiddled with the automatic choke a bit, and with the engine too hot to touch, I had to move the index mark all the way off the scale to get the choke plate to open fully. It looks like it's closing as the engine cools off.

Then I fiddled with the idle screw a bit, trying to measure (and lower) RPM. Unfortunately my laser tach kept giving me silly readings. Eventually I managed to lower the idle too much and the engine died. Now, after 10 minutes waiting between tries, it catches quickly and dies instantly. If I give it gas, it won't catch at all.

From all this, it seems to me like the engine floods at low RPM (wet plugs and dripping gas in carb). Besides pulling the carb and doing a complete rebuild does anyone have any other ideas to try?

Pete

 

I still think ya have a major vacuum leak. Prolly intake or carb base gasket's.
Is the power brakes working right? Is the vacuum for it hooked up properly?
Is the brake booster blown?

All things I would do as well as a kit in the carb.

 

Bad news. The compression numbers are lousy but what the hey, she runs. A tired old motor.
Arctic could be right.
Did you tighten down the manifold and carb base as I suggested above? If not, do it. To check what he has suggested, get some starting fluid with the schnozzle and spray a little around all intake manifold and carb gaskets and vacuum lines to the carb. If the engine flares, she is sucking air.
It is looking like you may have to pull the carb and rebuild.

 

About the only thing that will cause an AFB not to idle is dirt in little tube under the primary venturi (#34 in the exploded view) Pull off the top half of the carb, it's easier to reassemble if you remove two flathead screws on the top of the carb and remove the piston/metering rod assemblies before removing the top half. Pull the primary venturis out and look in the end of the little tube, there's probably a piece of dirt in one of the tubes.
Exploded view...> AFB Exploded View