85 720 st 2wd

[Flailguy]

Hello all I'm John and I'm somewhat new here. I'm having a few problems I can't seem to figure out... when I get to work in the morning I have to drive really slow for about a minute. As soon as I try to accelerate I loose all power. I then have to push in the clutch and rev up to take off, after I rev up it runs fine.

[ggzilla]

Sounds like the choke needs adjusting (if it only occurs when cold).

 

Tomorrow morn, pump the throttle pedal once, then open the hood and remove the air cleaner lid. If the choke is not 100% closed, then its time to adjust it.

 

Partly closed

 

Fully Closed - not even a tiny gap

[Flailguy]

It is fully open and warmed up.

[ggzilla]

Are you having a problem when the engine is fully warmed up?

 

If you only have the problem in the morning, check it in the morning. Choke should be fully closed when engine is cold.

[Flailguy]

I drive 20 minutes before I get there. The motor is fully warmed up and the coke is open. Something ele I'm confused about is adjusting the idle mixture. I screw it in until it runs poorly then out but no matter how far I go out with it runs the same??

[ggzilla]

Tell us more.

 

Are you saying that when you first start off it runs well, and while you are driving for 20 minutes it runs well? But after you get to work it has the symptoms?

 

How does it run on the way home from work?

[datzenmike]

OK here's how the choke should be working... A cold motor has difficulty evaporating the fuel and air and will seem to run lean and perform poorly. The choke forces a rich condition.

 

On a cold motor that has sat all night. Get in and step on the gas pedal once or twice. This releases the choke plate and it should snap shut from it's previously open position when it was warmed up. Start the truck. Idle should rise to between 1,500-2,200 RPMs. Part of the choke system is a fast idle circuit to aid in running and to warm the engine quickly. The electric choke heater warms up the choke and shuts it off by rotating the choke plate into the full open position in about 8 minutes.

[Flailguy]

Runs fine when cold and on my way to work but after a minute or two of going +-5 mph I attempt to speed up again and loose all power. I understand the choke and how it works. The choke is working properly It is closed in the morning when I start it and opens up in about 5 mins.

[Flailguy]

Overall runs great at cruising speed. Low power at takeoff.

[datzenmike]

Ahhh got it now.

 

If a problem transitioning from idle speed to more throttle it's likely the accelerator pump not working right. The accelerator pump fills in the gap between ide and main jet running by squirting is raw fuel.

 

Take the top of the air filter off and look down into the carb (engine off) and pump the gas several times. You should see a strong squirt of fuel into the primary barrel. A dribble or weak or no fuel is a problem.

[Flailguy]

I checked the low speed jet it wasn't clogged timing is right adjusted the vale lash and float to dot in sight glass

[Flailguy]

I does squirt

[Flailguy]

Also I can't adjust the idle mixture screw.

[Flailguy]

I'll be back on tomorrow time to hit the sack. Thanks guys.

[ggzilla]

Something ele I'm confused about is adjusting the idle mixture. I screw it in until it runs poorly then out but no matter how far I go out with it runs the same??

That is how you adjust the idle screw: turn in until it slows down then back it a bit.

 

If you are expecting it to fix the other problem (trying to speed up issue) - it won't. It only adjusts the idle.

[datzenmike]

Carefully remove the idle mixture screw and spring. Count the turns so it goes back the same way. Get a can of carb cleaner with the red straw. Stick the straw into the mixture screw hole and flush it out. Put the screw back in.

[Flailguy]

...so I just got to work an of course it ran fine this morning... smh... This truck has a mind of its own all problems seem to be intermittent.

[Flailguy]

I want to just get a weber. But I'm in Cali so it won't pass smog.

[datzenmike]

Two things...

 

Just realized you have an '85. The idle mixture screw is set at the factory and sealed with an aluminum plug. If the smog police see that it's been tampered with you'll fail the visual.

 

Do you have an O₂ sensor in the exhaust manifold?

[Flailguy]

Yes I do have an o2 sensor. The plug for the mixture was out when I got the truck, so I know it was previously tampered with.

[Flailguy]

I pulled the screw out and flushed it with chemtool. And it starts to run poorly at 4.5 turns out so I left it at 4.75

[Flailguy]

The sensor light is cut and if I touch the ends together it comes on..

[Flailguy]

For some reason I am starting to think its an ignition problem. What do you guys think??

[Flailguy]

Any ideas I can try? It's running terrible again. I checked the o2 sensor at operating temp with engine idling and got .9volts does this mean it's good or bad?

[ggzilla]

I checked the o2 sensor at operating temp with engine idling and got .9volts does this mean it's good or bad?

It indicate nothing. O2 sensor puts out a very weak signal and the load of a normal voltmeter will interfere with it. To get an accurate reading requires a 20Mohm resistance voltmeter which few are.