1FZ-F Idling Rich

[thebill]

I changed my head gasket earlier this year and ever since, the idle has been unstable and very rich (I live at 7000’ elevation so it tends to be rich to begin with). I had a shop look at the carburetor last week and he said even with the idle mixture screw turned all the way in, it’s still getting fuel when idling. I confirmed this today. I was very careful to put the vacuum lines back exactly as they were before and have checked that there isn’t a vacuum leak anywhere. Might you have any idea of where I may look to begin working on this? It idled and ran really well before the HG.
Thank you!

 

[JuanJ]

Hello,

Your idling speed should be (650 +/- 50) rpm. Idle speed at altitude is a little higher, (700 +/- 50) rpm.

Check the carburetor base for leaks.

Before tuning the carburetor, it is wise to have the sparkplugs cleaned/replaced and the distributor cleaned.

You have to adjust the idle speed adjustment screw. Then you have to adjust fast idle speed, Dash Pot and throttle positioner.

The full procedure is in the engine manual. It takes a lot of patience. A CO meter helps.





Juan

 

[thebill]

Thanks @JuanJ
I just took it for a lap around the neighborhood and I’m pretty sure it’s a vacuum leak - the revs hang around 2200 and I need to blip the throttle in neutral to get it to settle down.

Is it unrealistic to reuse the insulator and the metal gaskets? It would be only the second use. Is a thin coat of RTV advisable on all surfaces of the insulator/gasket between the carb and the manifold? Again, I’ve had zero problems with the carb since it was installed new 3 years ago…

 

[JuanJ]

Thanks @JuanJ
I just took it for a lap around the neighborhood and I’m pretty sure it’s a vacuum leak - the revs hang around 2200 and I need to blip the throttle in neutral to get it to settle down.

Is it unrealistic to reuse the insulator and the metal gaskets? It would be only the second use. Is a thin coat of RTV advisable on all surfaces of the insulator/gasket between the carb and the manifold? Again, I’ve had zero problems with the carb since it was installed new 3 years ago…

Hello,

The carburetor has a slightly complicated (in my opinion) vacuum circuit, hence my comment about patience.

It is not unrealistic to reuse the insulator. Make sure the base of the carburetor is clean (and the manifold as well) before installing it.

The carburetor itself is not an issue. Getting it tuned properly can be. But once it is tuned, it does not miss a beat.





Juan

 

[RHINO]

the float height may need adjustment,,, your symptoms also jive with a high fuel level

 

[mattressking]

Confirm vacuum is around 16inHg at idle (assuming about 6k elevation). Make sure vac gauge needle is rock solid, if it bounces (hunts) then do a valve adjustment. Also go back and double check all vac lines are routed correctly, most miss one here and there.

Then this is where all carbs function the same. If you can turn the mixture screw clockwise to fully seated and it still runs, your primary jet is too large. Vice versa, if it exceeds 4 turns on mixture screw and the idle never gets rough, you can go up a size for the primary jet.

For your truck, Reduce the size of the jet to the next available lower jet. Rule of thumb is 1.5-3 turns from soft seated (fully clockwise) is the sweet spot when tuning to your elevation and engine.

 

[thebill]

Thanks @JuanJ @RHINO @mattressking
Some more info:
We skied on Sunday. The parking area is at around 10k ft, so 3k above town. It ran fine on the way up but on the way down it wouldn’t hold an idle. I had to keep the throttle cracked while coasting or blipping during downshifts etc. This is the second time this has happened, the first prompted me to bring it to the one Toyota shop in town who would look at a carburetor. He did little more than fiddle with the idle and mixture which I’ve been messing with for a few weeks.
As I said, I did the HG not too long ago so everything on the intake came off but it all went back together as it was - no vacuum lines are misrouted etc. I haven’t had the carb open since I put it in back in 2021 so the float hasn’t been touched since then - does this eliminate the float as the culprit? I replaced the jets(?) in the bottom of the bowl with slightly smaller ones - IIRC, .001 smaller per 1k ft above elevation per @FJ40Jim . I wasn’t able to locate any of the other jets (Covid insanity) to further account for elevation, but it’s run really smoothly without any hiccups this whole time. It idled at 600+\- without bogging or wandering noticeably.

I’m going to start with the simplest thing which is another look at all the vacuum lines and a closer look at the base/insulator. Planning on a thin coat of gas-resistant gasket on the insulator surfaces before putting it back together.

 

[JuanJ]

Hello,

You may find this information useful.

It is good to set idle speed to 700 +/- 50 rpm at altitude.

To be continued.





Juan

 

[JuanJ]

Hello,

Regarding fast idle and related stuff.

The tuning requires quite a bit of patience.

As I mentioned before, it helps to clean/replace the sparkplugs and having the distributor contacts cleaned.

Hope this helps.




Juan