Timing. Degree or vacuum? Or both

OSS

SILVER Star
Joined
Jun 30, 2017
Messages
4,911
Location
Oblivion
On the next model of the F engine, the carbureted 3F that was available in other countries, there is no mention anywhere in the Toyota 3F engine manual of using the Lean Drop method to tune the carburetor at idle.
The carburetor on the 3F engine has an idle mixture screw like our 2F, but Toyota no longer felt that using the lean drop method to tune it was adequate (because it’s not very precise).

The way they recommended that the 3F idle mixture be set is to use an exhaust analyzer to actually measure the air fuel mixture blowing out the tail pipe. .. And these 3F engines didn’t have smog equipment either.

That being said - there isn’t an exact way to set the idle mixture perfectly on a 2F engine by using the lean drop method. The best you can do is follow the instructions given in the Toyota manual and cross your fingers. - or get an exhaust analyzer.

The idle mixture screw on the USA carburetor was capped with a security plug to prevent tampering for the life of the carb. It’s not like the idle mixture ever needs to be fiddled with once it’s set correctly.
 
Joined
Mar 21, 2014
Messages
416
On the next model of the F engine, the carbureted 3F that was available in other countries, there is no mention anywhere in the Toyota 3F engine manual of using the Lean Drop method to tune the carburetor at idle.
The carburetor on the 3F engine has an idle mixture screw like our 2F, but Toyota no longer felt that using the lean drop method to tune it was adequate (because it’s not very precise).

The way they recommended that the 3F idle mixture be set is to use an exhaust analyzer to actually measure the air fuel mixture blowing out the tail pipe. .. And these 3F engines didn’t have smog equipment either.

That being said - there isn’t an exact way to set the idle mixture perfectly on a 2F engine by using the lean drop method. The best you can do is follow the instructions given in the Toyota manual and cross your fingers. - or get an exhaust analyzer.

The idle mixture screw on the USA carburetor was capped with a security plug to prevent tampering for the life of the carb. It’s not like the idle mixture ever needs to be fiddled with once it’s set correctly.
When you are turning in a mixture screw are you adding more air or fuel? or is it the opposite? I do feel that my exhaust smells rich. the exhaust analyzer would tell me if I am running to rich or lean. Then I would set idle to that? that if my timing was on and cat is working correctly. I don't have a surfer smell. To many setting and systems that will affect others.
 

OSS

SILVER Star
Joined
Jun 30, 2017
Messages
4,911
Location
Oblivion
The idle mixture screw is a needle valve controlling the flow of gasoline. If less fuel is allowed to flow, the resulting A/F mixture will have less fuel in it, so it would be leaner.
If more fuel is let in, the A/F mixture becomes richer.

I’ll see if I can dig up the 3F manual…
 
Joined
Mar 21, 2014
Messages
416
The idle mixture screw is a needle valve controlling the flow of gasoline. If less fuel is allowed to flow, the resulting A/F mixture will have less fuel in it, so it would be leaner.
If more fuel is let in, the A/F mixture becomes richer.

I’ll see if I can dig up the 3F manual…
So all the way in would be the least amount of fuel? Then all the way out would be the richest it can go. These old trucks are going to have a more then pungent exhaust smell right? But what is acceptable?
 

OSS

SILVER Star
Joined
Jun 30, 2017
Messages
4,911
Location
Oblivion
I can’t find the carbed 3F manual for some reason — but I found an unusual 1991 2F manual that has lean drop instructions too.

Attached is the page describing lean drop tuning and below is a link to the whole manual:
43D44F96-1379-499A-8AC3-65390CD283E5.jpeg



I don’t think you can tune the carb by exhaust smell. I know that if the catalytic converter is removed, the exhaust stinks really bad even when the engine is perfectly tuned.

Honestly I don’t recall which direction to turn the idle mixture screw to lean the mixture. I’d follow the instructions in the manual.
I “think” turning it clockwise reduces fuel flow…
F6641E41-A708-4546-9322-DC4A7F0DC019.jpeg


762630D7-4482-4D91-8622-620FB36879EB.jpeg
 
Joined
Mar 21, 2014
Messages
416
I can’t find the carbed 3F manual for some reason — but I found an unusual 1991 2F manual that has lean drop instructions too.

Attached is the page describing lean drop tuning and below is a link to the whole manual:
View attachment 2960048


I don’t think you can tune the carb by exhaust smell. I know that if the catalytic converter is removed, the exhaust stinks really bad even when the engine is perfectly tuned.

Honestly I don’t recall which direction to turn the idle mixture screw to lean the mixture. I’d follow the instructions in the manual.
I “think” turning it clockwise reduces fuel flow…
View attachment 2960049

View attachment 2960050
When setting the mixture screw I think there is some different thoughts. Air cleaner on or off?
 

OSS

SILVER Star
Joined
Jun 30, 2017
Messages
4,911
Location
Oblivion
The manual says with air cleaner installed for non USA.
(With a clean filter of course).
But the newest 2F manual doesn’t specify for USA— so it probably doesn’t matter.
 
Last edited:

2mbb

SILVER Star
Joined
Sep 1, 2003
Messages
5,693
Location
WC, CA
Screwing the idle mixture all the way in (clockwise) will seat the screw and stop the flow of gasoline through the idle circuit. Turing the screw clockwise leans out the mixture.

Also remember the idle mixture screw is only for idle condition. As you drive and put a load on the engine there are other circuits that activate making the richness or lean-ness at idle less important.
 
Last edited:
Joined
Mar 21, 2014
Messages
416
The manual says with air cleaner installed for non USA.
(With a clean filter of course).
But the newest 2F manual doesn’t specify for USA— so it probably doesn’t matter.
You don’t think the air cleaner of will replicate a big vacuum leak?
 
Joined
Mar 21, 2014
Messages
416
Screwing the idle mixture all the way in (clockwise) will seat the screw and stop the flow of gasoline through the idle circuit. Turing the screw clockwise leans out the mixture.

Also remember the idle mixture screw is only for idle condition. As you drive and out a load on the engine there are other circuits that activate making the richness or lean-ness at idle less important.
After timing it to the BB. I here a slight I mean ever so slight ping/knock on a long up hill grade at like 40-45 mph when accelerating
 

OSS

SILVER Star
Joined
Jun 30, 2017
Messages
4,911
Location
Oblivion
Is the EGR system functioning correctly?
Octane 87 or 89 gas? (I used 89).
On the 2F, (at least mine anyway) I was so convinced with my newly rebuilt head that the engine was pinging while driving hard up a long hill - that I took it to a speed shop with a dyno to verify.

After a lot of testing on the dyno, the verdict was: no pinging.

What I was hearing while driving on the road was the exhaust sound (maybe slight exhaust leak) reflecting off the center crash barrier K rail on the road just a few feet away which kinda sounded like borderline pinging.
But it wasn’t pinging, it was just the noisy engine.

You can verify if what you’re hearing is pinging by retarding the timing some more. If you retard the timing and the sound finally goes away - then it is slight pinging.

Personally, I wouldn’t permanently set the timing below 7° btdc. If it sounds like it’s pinging a tad at 7°, I’d rather use better gasoline than retard the timing below 7°.

The air cleaner housing with filter installed (or not) is just a shell around the throat of the carburetor to hold the filter element so air entering the carburetor has to flow through the filter first.

Air cleaner off or on doesn’t affect vacuum.
 
Last edited:
Joined
Mar 21, 2014
Messages
416
Is the EGR system functioning correctly?
Octane 87 or 89 gas? (I used 89).
On the 2F, (at least mine anyway) I was so convinced with my newly rebuilt head that the engine was pinging while driving hard up a long hill - that I took it to a speed shop with a dyno to verify.

After a lot of testing on the dyno, the verdict was: no pinging.

What I was hearing while driving on the road was the exhaust sound (maybe slight exhaust leak) reflecting off the center crash barrier K rail on the road just a few feet away which kinda sounded like borderline pinging.
But it wasn’t pinging, it was just the noisy engine.

You can verify if what you’re hearing is pinging by retarding the timing some more. If you retard the timing and the sound finally goes away - then it is slight pinging.

Personally, I wouldn’t permanently set the timing below 7° btdc. If it sounds like it’s pinging a tad at 7°, I’d rather use better gasoline than retard the timing below 7°.

The air cleaner housing with filter installed (or not) is just a shell around the throat of the carburetor to hold the filter element so air entering the carburetor has to flow through the filter first.

Air cleaner off or on doesn’t affect vacuum.
Always great info. Thank you.
 
Joined
Mar 21, 2014
Messages
416
Is the EGR system functioning correctly?
Octane 87 or 89 gas? (I used 89).
On the 2F, (at least mine anyway) I was so convinced with my newly rebuilt head that the engine was pinging while driving hard up a long hill - that I took it to a speed shop with a dyno to verify.

After a lot of testing on the dyno, the verdict was: no pinging.

What I was hearing while driving on the road was the exhaust sound (maybe slight exhaust leak) reflecting off the center crash barrier K rail on the road just a few feet away which kinda sounded like borderline pinging.
But it wasn’t pinging, it was just the noisy engine.

You can verify if what you’re hearing is pinging by retarding the timing some more. If you retard the timing and the sound finally goes away - then it is slight pinging.

Personally, I wouldn’t permanently set the timing below 7° btdc. If it sounds like it’s pinging a tad at 7°, I’d rather use better gasoline than retard the timing below 7°.

The air cleaner housing with filter installed (or not) is just a shell around the throat of the carburetor to hold the filter element so air entering the carburetor has to flow through the filter first.

Air cleaner off or on doesn’t affect vacuum.
EGR is hooked up. I think working properly. I don’t hear much if any difference in the “ping” regardless of where the timing is. Do you think it’s just that we have old cars and it needs higher octane fuel? In a perfect world it should run great on 87. If I switch out my cat would that help the heavy exhaust smell?
 

Users who are viewing this thread

Top Bottom