Up until last year, my 1985 F150 Lariat ran perfectly. It started right up with even the slightest bump of the key, and the engine was pulling a solid 21 inches of vacuum. The engine ran like new, but the truck looked anything but.

Now, after spending over 5 grand in paint and body, the truck looks like new - but now the engine isn't right (of course). The engine wasn't touched at all by the body shop. But the cab was removed for painting - along with the HVAC housing and the brake booster. Both the HVAC system and the brakes work fine.

The truck sat for almost 6 months during the restoration process, but the engine started back up fine with no leaks or funny noises. It didn't want to idle though, so I pulled the carburetor and rebuilt it and installed new plugs. The engine now runs nice and smooth at idle, with a steady 500 RPM idle speed. The fuel mixture screws are only turned out about 1 turn each (highest vacuum reading), and the choke is working great. No hesitations, no flat spots, nothing wrong at all when cruising. However, it smells EXTREMELY rich at idle and it feels like it is down on power. I checked the plugs, expecting them to be fouled, but they didn't look bad at all.

I checked the vacuum with a vacuum gauge, and I am now only getting a steady 17 inches of vacuum (it was 21 inches before). I checked the timing, and it right where it has always been (20 degrees mechanical timing + 12 degrees initial timing = 32 total timing). HOWEVER, if I turn the distributor to advance the timing, the engine vacuum really begins to pick up. By advancing the timing, I can get the vacuum back up (and the idle speed) to almost 20 inches, but I can't drive it like that or it will start to ping almost immediately. So I put the timing back to where it was. But the engine smells VERY rich at idle where it's at, and it sometimes takes excessive cranking to re-start the engine when the engine is hot. It NEVER did this before. This vehicle used to start right up and almost "jump" to life.

Can someone help me to understand what has happened?

 

Just WAGing, interesting problem.

A vacuum leak of course comes to mind right away, though it wouldn't run rich if that were the case.

Rebuilding the carb makes sense, can see why that was your go to.

The assumption is, you haven't moved to the top of a mountain in the five months that went by since that time, so we'll remove altitude compensation from the list as well.

An engine that has sat for a long time can be down on compression, or not even start. Not sure though, about five months.

Bad fuel? That might do it. Still WAG.

To get really out there, let's think about what work was performed and why that might be a factor here - removal of the cab and such - painting - i.e. have you checked to ensure that all of the ground straps to the frame and cab, block, are present and installed, no paint interfering with our electrons - clean bright, and shiny tight? Fresh paint is notorious for this.

Starving the ignition of all the juice it needs could, in theory, cause a weak spark and rough running, and that in turn, would contribute to what looks like an over-rich condition. Have you looked at the spark?

I'm pretty particular about grounding and ignition and find an engine will warm up noticeably faster with a good hot ignition system. Solid grounding at minimum is the only way to get there. It's conceivable at least that a defective ignition, plus maybe stale fuel, a spacer or gasket leaking, would result in less overall vacuum pulled.

(If this theory turns out, this might finally clinch me the much-coveted "Gary Lewis' Remote Troubleshooting Way Out In The Weeds" Award!) /jk

As a practical matter, you could, or should, be able to bump your total timing (minus vacuum advance) up to 34 or 36 or possibly even higher, unless compression ratio is high. Since you've limited the mechanical advance to 20 there's plenty of room to experiment with the initial at least.

 

I would go back and check for a vacuum leaks just to make sure all is good.


You said the idle mix screws are turned out 1 turn out. Is that where you got the highest vac reading when adjusting them? I use my ear but a tach works just as good to get the highest rpm then turn them in 1/8 to 1/4 turn lean.


After you make that adjustment and if it still runs rich at idle there is something wrong with the carb.
Dave ----

 

Rick - I don't think you said what your initial timing is. Nor where you have the vacuum advance connected. And, do you disconnect the vacuum advance before setting the initial timing?

I'm thinking your initial timing is slow. And, in the back of my mind I think I remember that your vacuum advance is connected to manifold vacuum. If so, there's a vicious circle in all of that, and that could account for the pinging.

 

I got the impression that he's accounted for all of this and/or nothing has changed, the exception being the idle vacuum.

 

My vacuum advance is connected to ported vacuum.

My initial timing is set to 12 degrees BTDC.

Before my truck was painted and the engine was running perfectly, I set the ignition timing for highest vacuum (21 inches), and that ended up at 14 degrees BTDC. However, it pinged under WOT slightly, and so I settled on 12 degrees BTDC and that eliminated all pinging. It ran great like this up until I had it painted. When I got the truck back from being painted, it now smells extremely rich, and the vacuum reading is only 17 inches. Now, the engine suddenly wants much more timing at idle, but when I set it to the [new] highest vacuum reading, the engine pings almost as soon as I give it gas.

I checked all of the grounds, and they are present and tight. The engine runs smooth at 500RPM, it just smells EXTREMELY rich and vacuum is down.

I am running a set of port and polished Ford E7 heads, and 32 degrees total timing is about all they can stand before detonation.

Yes, that's correct.

 

Ok, with ported vacuum and 12 degrees BTDC it appears you have that nailed.

When you rebuilt the carb, which is a 4100 IIRC, did you replace the power valve? It sounds like the power valve is leaking, and I say that for several reasons:

• Idle Screws: Most carbs need the screws open about 2 to 2 1/2 turns for best idle, but yours only wants 1 turn. That suggests it is getting gas somewhere else.
• Smell: You've said it smells very rich, and a leaking power valve would do that.
• Timing: A rich condition likes more timing as the explosion isn't as crisp. And your engine likes more timing.

 

What if a vacuum leak was bad enough to cause a lean misfire? Couldn't that create a confusingly rich exhaust smell?

Considering the history of the cab removal, etc., how about isolating as much of the vacuum lines as possible? For example, as long as the truck isn't being driven, you could easily disconnect and cap the lines to the brake booster, heater doors, etc. I think there's a vacuum "tree" fitting on the back of the intake manifold that feeds all these systems. You'd still need to leave the distributor's vacuum advance connected, but I bet you could isolate almost all of the other user circuits.

Just thinking out loud, and wouldn't cost anything to try except a few minutes of your time.

 

That is a great idea. Thanks!

 

Float levels?

 

I set the float level to factory specs when I rebuilt the carburetor, but that was a "dry" setting. I will check the "wet" setting and make sure it is within specs this weekend.

Thanks for bringing that up. That *could* certainly explain the rich condition.

 

Is there anything that can fail inside the stock [mechanical] fuel pump that can result in it pumping too much fuel?

 

Theoretically yes, but that's not a very common failure mode. Have you measured the fuel pressure at the carb inlet? I'd certainly suggest that before changing the pump on a hunch.

Did you ever get a chance to temporarily block off the vacuum lines for troubleshooting? I'd still be leaning in that direction, considering what had been disturbed since the truck had last been running well.

 

Supposedly these days a lot of the new fuel pumps - mechanical type at least - the pressure is often way out of spec on the high side. It pays to check this, as well as ensure the fuel height itself is not too high in the bowl. Holleys always made this easy to check with the side screw, other types of carburetors need a different routine.

 

***UPDATE***

I finally found the problem - blown economizer valve. Thanks, Gary!

I re-tuned the carburetor, and the idle mixture screws ended up being 1-1/2 turns out on the left side, and two turns on the right side. Engine vacumm is back at 21 in. The engine starts up so quickly now I can't get off the key fast enough!