Fuji Carb jetting

[steffan]

I've been slowly dialing in the new build (rebuilt, 30 over 2F, mild cam, headers and exhaust) and monitoring the AFR, it seems to be running on the lean side: usually in the 16's (16:1 AFR) when cruising (idle is fine in the low 14's). Pulled the plugs and confirmed they were a little white/ frosty.
I pulled the jets and best I could tell was the primary is 123 (1.23mm - closest drill bit I could find, just a little play) with a secondary of 191(1.91mm). No markings on any of the jets. Backup jets same size as the mounted jets.
Picture of the inside of the carb/ machining

 

[steffan]

My question is, how much bigger of a jet should I get in order to richen it up appropriately? Thanks in advance

 

[charliemeyer007]

Wow, that a very clean carb, and nice pics too.

I don't have the answer other than go up in actual drill diameter one size at a time - much easier to enlarge the hole than make it smaller. SAE number drills go from 1 to 80 being the smallest.
1.23mm is 0.0484252" which is close to a #55 drill. #54 is 0.0550 #53 is 0.0595 I have the harbor china # drill set just for this purpose. And a Starrett pin vice for hand reaming out jets.

Maybe there is a formula for calculating the jet size. Or perhaps assuming all factors remain about the same except the area of the hole so 0.0484^2 x 3.1416 /4 = 16 then 14 = X^2 x 3.1416 / 4 says X is 0.0453 which sounds like # 54 to me . That's fuzzy math at work and you know that they say about how assume is spelled.

Google/youtube has lots of Jetting content.

 

[steffan]

Thanks for responding. I reached out to JetsRUs website but Mike didn’t think they had anything compatible although I thought one of the threads said Mikuni jets would work. I’m waiting for confirmation on that.
Agreed, I have same drill bit set - ultimately, just start going up one size at a time. I was just hoping to do it a little more strategic…. Also, I’m lazy and hate to tear apart the carb 2-3 more time to sort it out

 

[charliemeyer007]

Yea dialing in things often is taking things apart several times. Drills often aren't exact - they can make under or oversize holes, which must be why they make decimal reamers for more exact holes. Big fan of reading the spark plugs - they tell the truth for each jug.

 

[brian]

get a pile of mikunis.
The heads are a slightly different size than oe...but it has zero effect on function.

 

[steffan]

I was considering that but then I started reading that the Mikuni jets have a thread pitch of M5x.7 whereas the Fuji is M5x.8
Can anyone confirm the Mikuni size? Thanks

 

[Pighead]

I was considering that but then I started reading that the Mikuni jets have a thread pitch of M5x.7 whereas the Fuji is M5x.8
Can anyone confirm the Mikuni size? Thanks

These guys say M5x.7

Genuine Mikuni large round jets N100.604 - Jets R Us

 

[65swb45]

ROW 2F carbs have a 1.50 primary. 75-78 US market 2Fs ran between 1.24&1.44. 81-87 ran a 1.47

If you have the tiny drill bits, why not just slowly drill yours out? Start with the 1.36 and see how it does.

 

[steffan]

just a little update: just for the community I ordered 2 different sets of jets for Mikuni carburetors. There are a few different sizes but M5 x 0.8 is one of the common ones. A 16 piece set is just under $10 on amazon. I was worried about the quality of machining etc but they look every bit as nice as the original, maybe nicer. This will give me a few different options to go up in size for the primary jet but the secondary jet will have to be drilled to size (plenty of smaller sizes that won't be used will suffice).
The aftermarket ones have threads that are 1mm longer which works just fine. Total length is 9mm vs 6.5mm - again, no issue with that.
I have the other set arriving soon to see how those compare and then will decide which to install

 

[steffan]

I did this just for the MUD community: here is the second set of jets I purchased. These are from eBay for $8 - I purchased them mostly for due diligence and rule out all options.

 

[steffan]

here you can see - quite a bit smaller head. also, if you look at the comparison, the threads and machining are much rougher than even the other aftermarket jet. Now we know - not something you'd want in a real carburetor

 

[steffan]

I put the 40mm in for the primary jet but the largest in the set was 150 which left me hanging for the secondary. I took the smallest jet out of the set, knowing I'd never use it and proceeded to enlarge it. One size at a time I hand drilled the opening - just to make sure I kept it going straight etc. once the hole was enlarged I used power to polish it out. here is the end result at 2.04mm and how they look in the carb just a small protrusion into the bowl.
stay tuned (no pun intended) for results

 

[mattressking]

I've got so many correctly sized 200/220/230 jets I had to start throwing them away. I also have 136/138/150s for primary if needed also.

 

[steffan]

Mattressking-now you tell me
For which carb is that? I thought the Fuji was a little different in the thread pitch from the original?

 

[steffan]

Got it back together and drove around for an hour or so or mixed driving. At idle it’s running in the low to mid 14’s (ideal is 14.7:1)
Cruising around 60mph it’s in the high 14’s to mid 15’s so probably about as good as it gets at current elevation. Full throttle pulls stay pretty consistent in the 14’s.
I was actually surprised how much better it ran. I thought it felt pretty good before (granted, I’ve got no baseline or other references) but now it just felt like it had a deeper/ fuller power range. Can’t imagine trying to sort all this out with the AFR gauge.
Hope this helps someone

 

[EWheeler]

Got it back together and drove around for an hour or so or mixed driving. At idle it’s running in the low to mid 14’s (ideal is 14.7:1)
Cruising around 60mph it’s in the high 14’s to mid 15’s so probably about as good as it gets at current elevation. Full throttle pulls stay pretty consistent in the 14’s.
I was actually surprised how much better it ran. I thought it felt pretty good before (granted, I’ve got no baseline or other references) but now it just felt like it had a deeper/ fuller power range. Can’t imagine trying to sort all this out with the AFR gauge.
Hope this helps someone

Do you have a wide band sensor and digital gauge permanently installed in your exhaust?

 

[steffan]

Yes and no: there’s no getting around the O2 sensor so I had to weld a bung at the end of the collector. That’s easier plugged if I ever decide to pull the sensor etc. I wired the gauge separately for the main harness and kept the wires off to the side so that if desired, it too could be easily removed and returned to stock

 

[Gotta40]

I put a 1.36 in my fj with a 2f and header. it runs real good now, and the plugs look good. it was super lean til I did that.

 

[ryanwk628]

What jets did you end up using, the Amazon ones? Do you have a link? I have a Fuji carb as well and may want to try this