Hey folks, haven't been on in awhile, which is a good thing I suppose- means the old truck hasn't broken lately. It survived the entire winter as a long-commute daily driver, though I've probably spent thousands in gas in the process. Last month I bought a second vehicle, so now I have more time to spend on the truck.

I've had this nagging issue ever since doing a bunch of work to the truck (it runs 10x better than before the work) that's never really gone away, and seems to have gotten worse.

When accelerating normally from a stop (or just romping on the gas at idle) while the truck is warming up, the engine falls pretty much flat on it's face. It acts like it's starved for fuel, or flooded. If I carefully ease into the throttle, I can get it going without any real noticeable symptoms. But if I take off normally, and especially if I get on it, the power will cut out momentarily.

Once it does that, if I hold the gas to the floor, it will act dead for 2-3 seconds and then come alive. So if I'm at 1200 rpm or less, a press to full throttle is enough to kill the motor completely, but if I'm at say, 1500rpm, it will have time to recover and then takes off like a bullet once it catches.

It's really bizarre, because when it's first started (dead cold) it will not exhibit this symptom. It's only after driving the first 1/2 mile that it starts. But once it's warmed up to where the temp gauge is at about 1/4 (10-15 mins of driving), it will run normal.

I posted once about this and was told the choke wasn't adjusted right. But now that temps are routinely in the 70's, it still does it. I'm not much of a carburetor junkie and don't know much about setting chokes, or if the choke even applies at 70 degrees ambient temp.

But I do know that I rebuilt the carb personally last summer, changed timing chain, all belts, hoses, ignition components, the whole nine yards. Basically the only thing I haven't done is rebuild the thing [engine]. Which I don't think is necessary as compression is decent and engine only has 115k miles.

I've noticed my mileage is down too. Before, I could pretty much bet the house that it would return 190 miles per tank (give or take a couple %) regardless of driving style, which averaged out a tad above 10mpg. Now I'm in the neighborhood of 9mpg which is a hard pill to swallow when that's roughly a 10% decrease.

Please offer any input / suggestions / things to try as I don't want to throw parts at the thing, or make needless adjustments if it could be avoided. It runs like dynamite when warmed up...

 

Sounds to me like your choke isn't behaving correctly.

 

Because the engine is good when warm, there's probably no problem with the accelerator pump.

The choke coil (inside the round, black cap) is a bi-metal spring that, when cold, puts tension onto the choke flap to close it for starting.
There is a mark on the cap that lines up with the marks on the choke housing, according to the specs on your rocker cover.

If the choke coil has weakened with age, or the setting is toward the 'lean' side of where it should be, the choke will open earlier than it should.
This would be consistent with the engine running OK at first with the choke still closed, but then hesitating as the choke begins opening (early).

By loosening the locking screws & turning the cap backwards (top towards the cab), more tension will be put on the spring & the choke will take slightly longer to open.
I would check to see if you still have the original specs on the rocker cover first.

The center mark on the housing is a reference, then you have 1-rich, 2-rich, etc putting more tension on the spring; & 1-lean, 2-lean, etc in the other direction (top turned away from the cab) reducing tension.
This adjustment doesn't affect how much the choke is applied (if working properly)......just how long it takes to open again.

It can be necessary to adjust the cap summer/winter, with bigger temperature changes - so it might be worth noting where it is first.

 

quakerj,

I am confused. You mention that you rebuilt the carburetor and are talking about the choke, but the truck description at the bottom of your thread states "EFI". Perhaps you are describing the timing set?

Is your engine a 460 with a carburetor?

Do you have the original air cleaner and the associated preheating hoses attached? If not, the lack of warm air feeding the engine, especially when cool and damp, can cause internal carburetor icing. Carburetor icing can cause symptoms somewhat like what you have described. If your truck is still "original", check that the preheating system is working correctly.

 

X2, very important!

David

 

You might check your mechanical and vacuum advance for correct operation.

I recently did mine and discovered the vacuum advance was sticky and needed a little shot of lube under the reluctor (the star wheel gear looking thingy that triggers the magnetic pickup) to free it up.

I haven't put a lot of miles on the rig since I did that but early indications are a pesky little off idle stumble is gone and mileage improved.

 

So the choke is in the system even when temps are in the 70's? That I don't understand.... I can't fathom how there would be carburetor icing on a warm spring day.

I've heard of people pulling their chokes off their carb and still being able to start and run reasonably well in cool weather. I suppose I will need to run the truck and watch what the choke is doing from under the hood on a cold start. When I say "cold start" I'm not implying cold weather, just that the truck is not warmed up.

Oh and yes, this is a 460 with stock 4180 4-barrel, rebuild last summer. The "EFI" in signature is describing the timing chain set only, as it's not at stock 1986 cam timing any longer.

I think the choke IS adjustable, but it doesn't have the marks that one post described. This was one that was "preset" from the factory and not intended to ever be adjusted.

 

Carb icing can occur in moderate temps. It does not have to be freezing. It is caused by the venture action, the air temp drops as it goes thru the venture.

Which is why most warm air systems keep the incoming air temp at between about 95 degrees and 105 degrees.

It also why intake manifolds are also heated under the carb. Blocking off those passages to "make more power" render other driveability issues.

David

 

Many people live with this behavior from a carbed engine. It wasn't that abnormal in the old days when carbs were on all vehicles. Now fuel injection has spoiled everyone. They expect to jump in and take off no matter what the temp of the outdoors and the engine. It can work that way if everything is exactly right.

You can get rid of this if you really want to. Everything has to be in place and be working. By everything I mean;

Choke. Of course I see you have one and it's sort of working. Does it have a hot air tube going to it and is it still in place? Does it have electric assist also and is it hooked up and working?

Heat riser. Does your truck have a valve or valves in the exhaust manifold, and are they hooked up and working?

Air cleaner. Is it all original, hooked up with the temp sensor and the vacuum lines, and is it working? As soon as you start it up from a cold start, go out, open the hood, and check to make sure the flapper door is closed in the snorkel of the aircleaner, it should be. Most of these regulate the temp to 100 degrees entering the carb. The carb is calibrated for this. You need the tube going down to the exhaust manifold so it can draw warm air up into the carb like someone else mentioned.

If you have all this in place, and everything is adjusted and working correctly, your stumble should go away. But it's not easy and takes a lot of fiddling to get it right. Nobody can start a gasoline engine in a vehicle and take off without a choke. If they baby it and pat the gas it's possible, but it's not easy to do when the engine is cold.