Weber Carb Jetting/Tuning, Need some advice here (1 Viewer)

[Rock40]

I set the carb back up to factory "baseline" settings and surprise! It actually wanted to run. After some tuning, it is VERY close to being right. The speed screw is in only 1/2 a turn too far and the mixture screw is at 1 1/4 turns out. That is with the original idle air jets of 70 primary dn 50 secondary installed. I believe the 32/36 actually needs the idle jets to be jetted leaner and it should fall into range.

Don't know if anyone has struggled this much with a 32/36 install and trying to get it set to factory settings but the PCV valve being hooked up made a huge difference in the tune. With some thought, it would have to since all that extra air being drawn in is going to play a big role in the idle mixture.

 

[65swb45]

Your hands are way too clean for a mechanic!

 

[Rock40]

LOL! You should have seen them during the "cleaning" phase of this project! not so nice

 

[RAYJON]

Cool, amazing sometimes its the little stuff.
now you can fine tune it, remember the idle and primary are wher it will run 90%of the time.. sometimes running the secondary idle jet a little rich helps in transision if you have a flat spot in the middle on full throttle.


once you get it set for the motor they are great carbs.. most people just wont spend the time to fine tune them... always wanted to do a triple setup on a nasty sb chevy..

 

[Napoleon047]

I have never been a fan of PCV valves, especially in a carburetor application. With EFI, the computer can compensate for any failure of the pcv valve, but the carburetor relies on all the airflow going through it and not bypassing. I have a Weber 38/38 on a 20R and the valve cover vents to a catch can with a filter on top.

 

[Rock40]

Cool, amazing sometimes its the little stuff.
now you can fine tune it, remember the idle and primary are wher it will run 90%of the time.. sometimes running the secondary idle jet a little rich helps in transision if you have a flat spot in the middle on full throttle.


once you get it set for the motor they are great carbs.. most people just wont spend the time to fine tune them... always wanted to do a triple setup on a nasty sb chevy..

I agree, I have several leaner idle jets on order at this point from LCE Performance. I have hope it will get the idle into the correct range and I can go from there.

 

[Rock40]

I have never been a fan of PCV valves, especially in a carburetor application. With EFI, the computer can compensate for any failure of the pcv valve, but the carburetor relies on all the airflow going through it and not bypassing. I have a Weber 38/38 on a 20R and the valve cover vents to a catch can with a filter on top.

Completely agree. It's goofy that the carb mixture is relying on the airflow pulled from inside the engine to run correctly.

 

[Rock40]

A little more tuning, and it is SO CLOSE to being correct now. With wanting to add more airflow in mind, I opened the carb up and swapped out the primary and secondary air corrector jets. The carb came with 170/160 air corrector jets and I bumped it up to 190/180. Compared to the idle jet, the air correctors only make relatively small changes in the overall fuel mixture.

Put the carb back together and the truck instantly fired up! After warming it, idle speed set at 650rpm, the speed screw is at 1 1/8 turns in which is well within Weber's adjustment spec. The mixture screw however is only 1 1/4 turns out so it's still a tad bit rich. It's very, very close and I think 1 possibly 2 idle jets smaller is going to solve it.

Odd that the Weber was so rich out of the box, I sincerely hope that this thread assists somebody in the future who is trying to tune one of these. There's lots of 4 cylinder Toyota information out there but very little when it comes to 6 cylinder FJ40 engines.

 

[Rock40]

Here’s a pic of the #6 spark plug after about 10 minutes of running, mostly at idle. Certainly not fouled now

 

[greenbeasty]

Glad to see you got it figured out.

 

[middlecalf]

Wow good job on that, what a pain for sure. So Mark raised a point a few posts past, don’t the rings need to be seated on a new build and that is principally done at moderate rpms (not idling)? I ask only because I may be in this position at some point and I have no engine build experience let alone have only witnessed engine “break-in” on a dynomometer (Ramco piston ring company many years ago).

 

[Rock40]

Yes, they do. After getting it started/idling initially and warmed up, I ran it at 1,000rpm for 5 minutes, 1500rpm for 5 minutes, 2000 rpm for 5 minutes and then back down, 1500, 1000. It SHOULD be broken in/rings seated. I'll do it again since the mixture was initially off. You're correct tho that the engine needs varying speeds during break in. Now it's time to change the initial oil and filter however before any more run time.

 

[middlecalf]

Big thumbs up!

 

[Napoleon047]

Carbs cannot adjust to changes in air temp or altitude, so they are generally jetted on the rich side from the factory, especially if you are at a higher altitude. Also, ethanol gas is not going anywhere, and even small amounts of ethanol like E10 requires richer jetting to reach a stoich mixture, so there are usually fewer problems with jetting on the rich side.

All of the fuel in a Weber carb goes through the main jets, even the fuel used in the idle circuit, so changes to the main jet will effect the number of turns on the idle mixture screw.

The Weber DG-series carbs were designed for engines up to ~2L in displacement, so its not surprising that its difficult to find tuning info on that large of an engine.

 

[Rock40]

Carbs cannot adjust to changes in air temp or altitude, so they are generally jetted on the rich side from the factory, especially if you are at a higher altitude. Also, ethanol gas is not going anywhere, and even small amounts of ethanol like E10 requires richer jetting to reach a stoich mixture, so there are usually fewer problems with jetting on the rich side.

All of the fuel in a Weber carb goes through the main jets, even the fuel used in the idle circuit, so changes to the main jet will effect the number of turns on the idle mixture screw.

The Weber DG-series carbs were designed for engines up to ~2L in displacement, so its not surprising that its difficult to find tuning info on that large of an engine.

Well, maybe. I'm more or less at sea level so we can toss the altitude bit out. I also specifically sourced non-ethanol fuel as I didn't want that crap sitting and plugging anything up.

If the fuel is going through the main jets at idle that's fine however the demand at idle is so low (and the main jets so large) that changing a main jet up/down is very unlikely to make any change in the mixture at idle in my opinion. Per the Weber book I have, the mains really don't do much until 2,500 rpm and up.

I don't know the max size engine you could potentially run a 32/36 on but given how rich this one seemed to be I have to think it'll go much larger than the 3.9 "F135" engine. I also suspect what was stated before, that the majority of folks don't want to screw around with a carb this much to actually tune it correctly. Seems like lots of folks run the 38/38 but I think it would have been WAY too much for this application given what I know now.