1977 FJ40 sticking throttle

[cbmontgo]

When driving, my throttle sticks and holds ~2,000 RPM. I have to pull up on the accelerator pedal to bring the RPMs down. If I don't pull up on the pedal, it will hold this RPM for about 5 seconds, and then abruptly drops back to idle.

It is starting to drive me crazy, as I have oiled the linkage down with a dry lube, replaced the torque rod grommet, cleaned and reassembled the accelerator pedal and spring, and swapped the return springs on the linkage.

It feels like something is "sticking" and then suddenly lets go after a few seconds.

I rebuilt this carb earlier this year and the 40 runs beautifully other than this issue, which has progressively gotten worse in the last month.

I am starting to wonder whether this diaphragm is the cause:

This hose (below) goes directly to the diaphragm in question. If I disconnect the hose, the idle jumps up to the exact RPM where it sticks when driving. When I reconnect this hose, it goes back to a perfect idle.

Of course, I can't seem to replicate this with the hood open, and the idle won't stick unless the vehicle is driving.

Is it possible that at a certain vacuum setting, the diaphragm is getting stuck?

Other thoughts?

 

[Poser]

Disconnect the vacuum line at the diaphragm and take it for a ride...

 

[wngrog]

Remove OEM carb. Replace with City Racer Carb.

There. The Nolen Grogan method of carburetor tuning.

 

[cbmontgo]

Disconnect the vacuum line at the diaphragm and take it for a ride...

Well, it sticks at a really high idle without vacuum on that diaphragm tho? Like the diaphragm sticks when I let off the throttle for a few seconds?

 

[spotcruiser]

Are you sure it's the throttle? Have you looked at the bowl level when it was racing? I have seen a float stick in the down/open position (perhaps due to contact with a gasket; found no holes in the float) such that the bowl overflowed and poured fuel into the intake. Just something to consider.

 

[wngrog]

My 74 was doing this and I added a second return spring to the carburetor to pull the throttle back.

If you can pull the pedal itself and it fixed it that tells me it is mechanical.

I took a spring and went around the brake master.

 

[cbmontgo]

Are you sure it's the throttle? Have you looked at the bowl level when it was racing? I have seen a float stick in the down/open position (perhaps due to contact with a gasket; found no holes in the float) such that the bowl overflowed and poured fuel into the intake. Just something to consider.

I'll check that out. Thanks.

My 74 was doing this and I added a second return spring to the carburetor to pull the throttle back.

If you can pull the pedal itself and it fixed it that tells me it is mechanical.

I took a spring and went around the brake master.

Pulling the pedal up lowers the RPMs, but doesn't bring it quite all the way back to idle. Do you have a pic of that by chance?

The part I don't get is that it will stick for about 5 seconds, and then something "releases" and it drops to a normal idle. What is releasing?

 

[Poser]

How about you post up a picture or two of your throttle linkage at the carburetor?

 

[wngrog]

I have to go by and see it tomorrow I'll show you but basically I took a second spring that was stronger than the stock return spring and connected it to the brake master to pull it straight back to stop position

Cured my same symptoms.

I could put my toe under the pedal and it would return to idle. Just needed help.

 

[cbmontgo]

How about you post up a picture or two of your throttle linkage at the carburetor?

Here's a video that gives you an idea of the operation:

 

[cbmontgo]

I have to go by and see it tomorrow I'll show you but basically I took a second spring that was stronger than the stock return spring and connected it to the brake master to pull it straight back to stop position

Cured my same symptoms.

I could put my toe under the pedal and it would return to idle. Just needed help.

I swapped the old spring with a Lowe's version to test it out, but didn't think about adding a second spring. That is a great idea.

 

[ceylonfj40nut]

Is the choke working like it should? It seems to be behaving just like when you pull the choke.

 

[cbmontgo]

Is the choke working like it should? It seems to be behaving just like when you pull the choke.

Funny you mention that. The choke is a little finicky on this truck, as it needs choke for every cold start, almost like it is starting too lean without it.

 

[cbmontgo]

Below is a thread that describes almost exactly what my issue seems to be. I'm going to take a look at the throttle positioner when I get back under the hood.

Need some help with a bizzare 2F carb problem

 

[wngrog]

I still feel that if you can hook your foot under it and fix it you have something external sticking. I forgot to take a pic of what I did on the 74 when I was at 49Tire. Sorry.

 

[cbmontgo]

So I've spent a lot of time messing with this today. After disassembling the carb, driving it with the hood open, looking crazy to my neighbors while driving up and down the street, and trying many different odd combinations of things,I decided to remove the emissions computer. It just seemed to me that vacuum is controlling this issue.

Problem solved. Shifts and drives great again.

Thoughts on why this fixed it?

Does it hurt anything to drive it with no emissions control on a smogged rig?

 

[wngrog]

I can't imagine any of that s*** is still really working.

 

[Pin_Head]

It is normal operation of the throttle positioner, except you have the speed adjusted too high. It is supposed to be 1000 RPM. It is controlled by the emissions computer as you found. Vacuum pulls the TP off its high idle position when the speed drops below 10 MPH. The purpose of the TP is to prevent backfiring through the muffler when engine braking due to unburned fuel hitting the hot exhaust with air injected.

 

[ceylonfj40nut]

Does your tach work when you disabled the computer?

 

[cbmontgo]

It is normal operation of the throttle positioner, except you have the speed adjusted too high. It is supposed to be 1000 RPM. It is controlled by the emissions computer as you found. Vacuum pulls the TP off its high idle position when the speed drops below 10 MPH. The purpose of the TP is to prevent backfiring through the muffler when engine braking due to unburned fuel hitting the hot exhaust with air injected.

So if I adjust the TP speed on the carburetor, do I even need the emissions computer?