3-hole vs 4-hole 44mm Mikuni SBN

[SharkB8]

I bought a 44mm Mikuni carb for a 650sx with Factory pipe. Unfortunately I didn't know there were different kinds of 44mm Mikunis and I ended up with the less common 3-hole model off a 6m6. I even sent it to JSS for some TLC before I found out. So I'd like to use this carb if I can.

I assume that all the 650sx jetting advice I see online is for the 4-hole model.

So if someone has this jetting in a 4-hole carb......

high 130
low 100
2.3 needle and seat
65 gram spring pop off at 15 psi
high 1.0
low 1.0

... what would be the equivalent jetting in a 3-hole carb.

I will go through the whole jetting process but I'd like to start out in the ballpark if possible.

 

[Crab]

We need Supertune to enlighten us on this, I have duals that are 3 holers also with reverse jetting.

 

[2XLR8]

Change the spring to a 115, change the low to a 120 and take the main out to a 145. Tune the screws as you need to high no more than 1 and a half. If you have the fine thread low screw you will need to be out more than two turns out on the low.

 

[SuperJETT]

I've never seen a 3-hole SBN44. Pics?

 

[rossthaboss11]

the 3 hole carbs are the better ones. i know it doesnt make sense but the 3 hole carbs are for the true 701 motors and the 4 hole carbs are the 650 carbs. this was for the superjets not sure if its different for kawis but thats what i was told. and off my stock 650 motor came a 4 hole carb.

 

[OCD Solutions]

Is that an old BN as compared to a newer SBN? I never understood the difference.

 

[Matt_E]

We're talking SBN's only.

BN's have a round pump body and the jets are not replaceable.

The holes discussed here are inside the carb throat.

 

[SuperJETT]

the 3 hole carbs are the better ones. i know it doesnt make sense but the 3 hole carbs are for the true 701 motors and the 4 hole carbs are the 650 carbs. this was for the superjets not sure if its different for kawis but thats what i was told. and off my stock 650 motor came a 4 hole carb.

You're probably speaking of a 38mm then, 38mm SBN carbs are 3 hole tops, 44mm are 4 hole, unless there is a SBN 44 that I've never seen/heard of which is possible.

 

[SuperJETT]

We're talking SBN's only.

BN's have a round pump body and the jets are not replaceable.

The holes discussed here are inside the carb throat.

Ok, so we're talking about oem vs true Mikuni? That's a different story.

 

[snowxr]

Maybe he's talking about the tiny low speed circuit holes inside the carb bore. They are by the throttle plate. Yam oem carbs marked 61X (701) have 3. 6R7 (650) have 4. 64x (760) have 3. 61L (650) have 4.

 

[Crab]

Just below the throttle blade.

 

[OCD Solutions]

Yam oem carbs marked 61X (701) have 3. 6R7 (650) have 4. 64x (760) have 3. 61L (650) have 4.

That right there needs to be a sticky in the tech section for identifying carbs.

How many holes in an aftermarket Mikuni? Does it differ by throat size?

 

[SharkB8]

After a little more reading and picture taking, I see I was wrong in my original post. The 3-hole carb is the better one, like a few people here have said.

I'll post some pictures for future reference. You look in the bottom of the carb and open the throttle blade a little and the holes are on the jet side (ie not the fuel pump side). There is one fairly large hole (pilot, I believe?) and then either 3 or 4 small holes.

Also for the record we are talking about 44mm Mikuni Super BN carbs. SupetJet 650's came with the 4-hole model. All others that I am aware of are 4 holes. Hopefully someone more knowledgeable than me can fill in any missing information here.

 

[SUPERTUNE]

Maybe he's talking about the tiny low speed circuit holes inside the carb bore. They are by the throttle plate. Yam oem carbs marked 61X (701) have 3. 6R7 (650) have 4. 64x (760) have 3. 61L (650) have 4.

Yes, this is all correct!
6M6 (650) also a 4 hole circuit.

 

[SUPERTUNE]

After a little more reading and picture taking, I see I was wrong in my original post. The 3-hole carb is the better one, like a few people here have said. I don't agree!

I'll post some pictures for future reference. You look in the bottom of the carb and open the throttle blade a little and the holes are on the jet side (ie not the fuel pump side). There is one fairly large hole (pilot, I believe?) CORRECT, THE BIG HOLE IS FOR THE PILOT ADJUSTER and then either 3 or 4 small holes.

Also for the record we are talking about 44mm Mikuni Super BN carbs. SupetJet 650's came with the 4-hole model. All others that I am aware of are 4 holes. NO, SEE THE ABOVE POST! Hopefully someone more knowledgeable than me can fill in any missing information here.

IMO, the 4 hole carbs work better all around (I'm talking about dual setups) and as you go up in engine size the 4 hole carb works even better, Yes, you can get a little more crispier throttle response with the 3 hole carbs setup on a 701-735cc engine, but just not worth the the time and $$ to use them as you need to buy throttle shaft kits and special work to change over the straight fuel fitting over to the 90* degree fittings, then at this point you have no engine cc upgrades left without buying another set of carbs (46 mm's)
Just go with a new set of aftermarket 46's set up right for the engine and are very user friendly to use on your bigger engine as you upgrade later. You can even use the 46's on a pump gas 1000cc engine.
Advice: Spend your $$ wisely.

 

[SharkB8]

Thanks for the info.

Which is the better "single" carb for a 650sx with a factory pipe and no porting yet (but will probably be ported by you (x scream) later on).

After posting this thread and reading the replies, I'm guessing that either will work, with the 3-hole carb probably able to supply more fuel someday if the motor gets some more aggressive mods.

Is that roughly correct?

 

[2XLR8]

in a single carb application I can't see how you could not tune a three hole just as good a 4 hole. It might take a little different pop and low jet combination but it should be fine.

 

[SharkB8]

I am pretty sure I saw a post by Chuck that said the carb that came on the SuperJet 650 could not be tuned to provide enough fuel for some high-output engines. Of course, I can't find that thread now when I need it.

 

[SUPERTUNE]

You can use 44mm 4 hole carbs for duals as their almost the same as the aftermarket 44, with a different thread pitch on the low adjuster, you also need to modify fuel fittings and replace the throttle shaft for and extended one.
On stock 61x single carb cases (like 2LRX8 stated above) I don't really think it makes a big difference, just jet for the application.
On 62T cases with a conversion single manifold, use only the 4 hole type or aftermarket carb.

 

[Talsivar]

what about 1 3 hole and 1 4 hole conected as duals? i know it's not ideal but will it work?