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Since getting my truck this summer, I haven't had any problems with driveability in the mornings as it warms up. But... now that the mornings are cold and foggy I have some terrible problems with stumbles and misfiring, but the symptoms are flat strange:

1. On firing up from cold the engine runs fine, or as fine as it ever does on choke, which isn't bad by Toyota or Mazda carb standards.
2. The engine starts to miss about 5 miles from home, but runs evenly with reduced throttle.
3. The truck idles just fine at stop lights.
4. The missing continues only at highway speeds or with opening throttle when running up through the gears.
5. The misfiring disappears after waiting for a couple of minutes at a light I seem to always have to stop for about 20 miles from home.
6. The truck runs fine the last 10 miles to work.
7. The truck runs fine in the evening on the 30 mile drive back home.
8. The truck runs just fine on foggy mornings with air temperature above 55 degrees.
9. The truck runs just fine on cold, clear mornings with temps as low as 44 degrees.

I've already checked the coil for cracks and incorrect resistance values, and it appears to be just fine. I replaced it earlier this summer, so I would be disappointed if it was failing already.

I've already checked the points for burning and gap, and although they needed a little sanding and regapping, this hasn't changed the cold, foggy morning misfiring. I sprayed some silicon lubricant into the cap and distributor to try to lock out moisture, just in case... no effect.

My only theory, and one that is hard to verify, is that I'm suffering from carburettor icing. This is the only car I've ever owned that would have suffered from this, so that's why I'm not confident in my theory. I remember reading about airplanes suffering carb icing, of course, and also about icing in high velocity motorcycle carbs from years ago.

Assuming I'm wrong, what else should I look into? I need your ideas.

Scott

Maybe a wet or shorted plug wire? Usualy a bad plug wire will show it's self under a load on the engine, and will miss specialy if the carb is lean at all..

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If you have the stock breather, the valve in the goose neck should close upon starting. This allows air, which is warm, to be pulled from the shroud around the exhaust. Check to see if the valve is in fact closing. Mine had a hole in the diaphram that closes the valve wth vacuum upon engine start.

If it wasn't a temperature effect, I'd say you're starving for fuel. Most of the symptoms you describe are classic partially clogged fuel filter or weak fuel pump. (Hard acceleration or highway speeds sucks more gas, which drains the float bowl. After sitting at idle, it fills back up.)

Maybe the colder air is just enough to push you into that 'too lean' area.

For lack of anything better, I'd try changing the fuel filter and checking that the fuel pump flows well.

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You went there in that?!?!

I have a 80 4x4 that ices up at the srart of winter, I just take the cold
air duct to the carb. out and no more problem, its done it very since
I've owned it (10yrs)

I'm going to check most of these. I ruled out fuel delivery problems because the pump flowed just great last summer when I tested it, and I put a new fuel filter on in July. Perhaps I was too rash to rule that out.

Also, some previous owner played "Mr. Badwrench" and removed most of the emissions gear (air pump, etc.) and lots of the seemingly unused hardware like the pipe from the exhaust manifold to the air cleaner's intake, and the vacuum line that opens that valve. I guess I can look into replacing this missing stuff, at least to bring warmed air to the carb in cold, moist weather. Hmmm... Eric, you're about to get another e-mail.

I'll check my plug wires too, but since they work in cold dry weather or in warm wet weather, I'm thinking that is less likely to be my problem. But, ya never know....

Scott

my truck did that when ever it rained! it ended up being the cap, rotor and points. If you never want it to ice up again move to Phoenix

This morning was cold and foggy, and sure enough about 4 miles from home my truck acted up. I pulled to the side and let it idle, then turned it off, and put the key to the RUN position for about 30 seconds and restarted. The truck ran fine all the rest of the way to work.

So, that confirms 1okey2go's suggestion that I'm starving for fuel. Looks like my fuel pump is lazy in the cold, since it pumped just fine on a warm summer day. I'm guessing it needed a moment to refill the float bowl, and could keep up with demand after that.

So, should I trust a junkyard fuel pump, or just go for the $40 NAPA solid-state fuel pump? I'm leaning to the NAPA pump, but I've heard they're noisy.

Scott

I don't know the particular year, engine or carb we're talking about here, but some have a fast idle solenoid. That could cause a problem similar to what you describe if it went south. Also if the fast idle is incorrectly adjusted, as too much onto carburetor linkage cam, it could contribute to the problem. If the main idle circuit, as in idle mixture jets, are too lean, and fast idle is too strong, then when fast idle falls off at warmup there's not enough fuel moving in idle circuit to allow good transitions from idle to cruise to WOT.

Also the fast idle setting and choke setting could keep the truck running bad one minute and good the next if they're out of balance. Some have a secondary vacuum diaphragm that helps open the secondary throttle blades. If the truck has good fuel delivery during warmup, with choke on, but falls on its face when fully warmed it could be due in part to the vacuum diaphragms. What I'm saying is that these various things acting together to cause an imbalance in the carburetor's fuel delivery and mixture control could cause all of these bizarre problems, and then some!

I'd go at it with a can of carb cleaner and blow out the idle mixture screw passages, and the shooter in the venturi bores, and then adjust whatever you can to spec [fast idle screw/solenoid/diaphragm, choke pull-off, mixture screws] along with doing what was suggested for carburetor air cleaner heat.

#11

I have a Napa fuel pump on my truck and, yes, it's noisy. It sounds like a valve tapping behind me.

I got a junkyard one out of a Trooper, just to reduce the noise, but I have yet to put it in. That's what I recommend, if you suspect the fuel pump. They're cheap at pick-n-pull.

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You went there in that?!?!

I've got a purolator fuel pump. Its still got a little bit of tapping noise but its hardly noticable when the truck is running it cost me about $50.00 at Checker Auto Parts

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