38mm SBN questions (NOT OEM's)

[fastgtfairlane]

MTRHEAD

What was your final verdict on oem vs 8113 sbn38’s?

Trying to decide on which version for a 550 reed motor. I would rather have the quick acceleration than the ultimate top end.

Thanks,

I bought a pair of gold 38s several years ago from ebay. they were 75 bucks shipped per carb, no chokes, installed primer fittings, 90 degree fuel fittings and extended throttle shafts for using a dual carb setup, and t handle mixture screws. I used them on a ported 550 reed engine. I ran dual feed, dual pumps and just ran a tee to the pulse fitting to both carbs. they come with a 2.0n/s 65 pilots and 107.5 main jets. I cant remember what spring for the n/s they came with though. I used stock metal 440 flame arrestors with the screens removed. I think I was running 70 pilots, 120 mains. 2.0 n/s and 65g springs.

 

[MTRHEAD]

Update: was able to remove, by drilling out, the oem plates on a set of rough old oem 38mm carbs to get the shafts for nice oem style dual setup. Key is a drill slightly smaller in dia than of the threads and they come out easily. Then I drilled out these 15 degree plates since they didn’t have dual shafts. Now these can be setup like oem with the new 15 degree plates. Had a set of screws from my Novi 50’s. Got the oem speed plates off the other carbs too. Just a longer bolt and nut and it’s good. I really want to set these carbs in reality down on the manifold without that plate. But it will require custom plate and then a new throttle adjustment bracket. Easier to replicate oem first to test back to back.

 

[Quinc]

Can you post some pics of the before and after?

 

[Roseand]

I just bought a superjet that has these carbs on it. It's in parts so I can't attest to how it runs yet
Previous owner said it ran good but the pop off is 18 psi with a 2.0 needle. Seems awfully low? Unless this version runs that much different.
135 main/75 pilot with a 61x/62t b pipe setup.

@MTRHEAD I've never seen 38s until yesterday. I'm used to singles and speedplate setups. Looks like the stock 38 setup just uses a riser(individual speedplate?) kinda deal?
Looks like the previous owner just cut the shaft and ran the set screw style mikuni coupler.
Why go through all the effort to swap to oem linkage; is it that much better at reducing wear?
Without a speedplate/carb alignment bracket is it too hard to get decent alignment with an aftermarket coupler?

 

[JetManiac]

I also bought a pair of like new Mikuni non-OEM 38mm Carbs for $50 each. They are both set-up as Slave Carbs, meaning both had the Throttle Shaft shortened for Shaft Couplers.

So because none of these Carbs I bought came with a Throttle Spring, Throttle Stop or Throttle Cam, I need to order one new Mikuni Throttle Shaft Kit which includes the Threaded Shaft, Bushings and Throttle Plate. WSM sells the Standard MIkuni 38mm Throttle Shaft Rebuild Kit. What Throttle Plate Nominal Degree comes with this Kit? Where do I get a 15 degree Throttle Plate to match the other Carb Throttle Plate?

Does the degree "rating" indicate how close to zero degrees- CLOSED- they are? Bevel Angle?

WSM shaft kits are not Genuine Mikuni. We have the genuine 38 kits and Throttle Spring, Throttle Stop and Throttle Cam.

 

[HyEsToN]

i wanna buy parts off you

 

[MTRHEAD]

Update. I put these 38’s finally on my sj. They had drilled out return restrictor so I had to run a jet. Ended up at 85. Smooth idle
I only had 70/130 jets on my and I will say they were rich compared to Oem 70/130. I think 65 or 67.5 and 125-127.5 will be perfect
I pulled 50-60 more rpm and the hit was different to the point the overall carb experience was better. Maybe the 4 holes and the 12 degree plates? Little chugging when transitioning under normal steady riding (rich if not running hard). So far the time and funds (the $125+ plus time) has been worth the rpm. I’ll rejet leaner this week I hope.

 

[Quinc]

Thank you for keeping us updated!

 

[MTRHEAD]

Oh another note. Much more sensitive to needle turns than Yamaha Superjet oem 38’s. The Yamaha superjet oem 38’s are hard to screw up as 70/130 jetting works on about 95% of every combination imaginable.

 

[Storbeck]

Interesting stuff. What is the reason for running these over say a set of aftermarket 44's? (by aftermarket I mean straight from mikuni, but not the oem44's from a yamaha or some such). Just an experiment? Or this is an application where the smaller carbs are preferred for whatever reason (fuel economy, or ease of tuning or power delivery or whatever).

 

[MTRHEAD]

Interesting stuff. What is the reason for running these over say a set of aftermarket 44's? (by aftermarket I mean straight from mikuni, but not the oem44's from a yamaha or some such). Just an experiment? Or this is an application where the smaller carbs are preferred for whatever reason (fuel economy, or ease of tuning or power delivery or whatever).

#1 reason fuel economy where we ride
#2 experiment and curiosity
#3 I had 44’s on the ski and it hit harder and pulled over 100+ rpm More. Fuel economy was significantly worse when riding “aggressive”
#4. When I was racing I always rode with 48’s on my limiteds and 50’s on my supers. Now that we just ride for fun. We go farther

 

[Storbeck]

I don't know if I'm the only one that's enough of a carburetor nerd to find this interesting, but I was comparing some old Mikuni 38's today. This is a carb that is actually from an early 70's snowmobile and two BN38's from old Kawasaki watercraft.

The two watercraft carbs are slightly different, one has the big blocky thing as an add on item with a screw, the other has the boss for the screw but it isn't drilled and tapped. So whatever the reason is for adding that block thing it pre-dates OEM Yamaha SBN38's.

The even older snowmobile version of the carb has the subventuri offset towards the side of the carb bore, and it uses a ball style check valve rather than the little plastic/fiber flappy thing, and it doesn't have a fixed jet at, ALL of the fuel is metered by the adjuster needle.

They all seem to use 15 degree plates and it's hard to tell but I think they have the same pattern of three progression holes, I don't have an aftermarket sbn38 but I'm guessing the throttle blade/progression holes are basically same as the old school carb.

 

[TL91]

@MTRHEAD with your "racing" SBN 38 set up on your SJ, were you running dual pulse lines or are you able to use the OEM 38 fuel pump covers and only run 1 and then tie them together via the OE pump cover? Curious if you can run this set up with an OE tank (only one "fuel out" line) and single pulse fitting or how you achieved it.

 

[MTRHEAD]

On that setup I ran dual because each carb had a pump. Yes you could easily convert to one using oem backs

 

[TL91]

Ok, thanks for the insight. I figured that'd be the easiest way to do it. You run dual pulse and dual fuel feed? Dual fuel feed meaning individual fuel lines for each carb coming from the tank.

 

[MTRHEAD]

Single feed and dual pulse. Only ever ran dual feed on 50’s on my 8mm dry piped race boats.
I like to use the petcock so I keep it to single feed with normal builds.

 

[TL91]

Cool. And yeah I hear ya.

So I'm assuming you T'd your fuel in line after the petcock but before the carbs then? Sorry to keep prying on so many questions, but I've been thinking about doing the same thing and I've put a fair amount of thought into how I would go about it (single pulse or dual pulse, single fuel in line or dual, OE tank fitting or aftermarket for more fuel out options, etc.). Similar to you, I have no necessity for doing this but just more so for fun experimentation.

 

[MTRHEAD]

Yes a T

 

[Storbeck]

I'm curious if you drilled the internal restrictors. Kinda looks like there might be a jet in the t on the return lines.

 

[MTRHEAD]

I'm curious if you drilled the internal restrictors. Kinda looks like there might be a jet in the t on the return lines.

Yes I did. I didn’t have to but there was a time I was using them on my blaster and that has a remote pump with a Novi vapor separator so it’s mandatory with that setup.