'81 MNS Carb jets

[CatatonicBug]

I just accidentally learned something today that has me a bit worried. When I first obtained my bike, the carbs were a total mess. I replaced all the jets and stuff using all the parts from this kit. Now, this was my first time EVER rebuilding a carb, so I just read the headline for the kit that said this kit would fit my carbs, and got 4 of them. What I just learned is that this kit contains a 110 main jet, and according to the manual (and a tech tip here), the LH takes a 110 on only carbs #1&4. It is supposed to have a 120 on carbs #2&3. Is this something I should take the time to replace? Is it a big deal? Why would the two sets of carbs be different? I was going to set about synching the carbs for the first time, and figured it might be a waste if I had to change out the jets. Your thoughts??

[cywelchjr]

I just accidentally learned something today that has me a bit worried. When I first obtained my bike, the carbs were a total mess. I replaced all the jets and stuff using all the parts from this kit. Now, this was my first time EVER rebuilding a carb, so I just read the headline for the kit that said this kit would fit my carbs, and got 4 of them. What I just learned is that this kit contains a 110 main jet, and according to the manual (and a tech tip here), the LH takes a 110 on only carbs #1&4. It is supposed to have a 120 on carbs #2&3. Is this something I should take the time to replace? Is it a big deal? Why would the two sets of carbs be different? I was going to set about synching the carbs for the first time, and figured it might be a waste if I had to change out the jets. Your thoughts??

IIRC from my reading on here is has to do with running the center cylinders richer so they run a little cooler. I believe that the concensus was largely that it doesn't matter, but you can take that as you wish.

[renegade_xs11g]

cy is correct....the purpose of the inner cylinders, #2&#3, jets being bigger was to keep them cooler....
most everyone here on Channel 11 just runs the jets the same all the way across....
good luck, John

[skids]

Yup. It was factory 110's all the way across on two of my carb sets. 81H and I think 80SG

cy is correct....the purpose of the inner cylinders, #2, jets being bigger was to keep them cooler....
most everyone here on Channel 11 just runs the jets the same all the way across....
good luck, John

[DGXSER]

Hey Catatonic,

My 81 special has 110s straight across, came that way from the PO at least. Went back the same. Only issue I am still dialing in is no 1 running rich. Never have been good with carbs, this tunign by ear must require better hearing than I have.

Like the rest have stated, everything I have read here and other places is that it was to keep the center two cylinders cooler since they get less air flow (being in the center of the other two and all ). Not read to many (if any) that are still running bigger jets in the center.

[petejw]

Hey DGXSER,
stick a plastic tube on the float bowl and check the fuel level that way, you might find that the cylinder running rich might have a higher fuel level that you might not pick up with a visual check.

Hey CatatonicBug,
the boys are right, run 110 all across.
or you can run 115 if the bike has intake or exhaust mods.

[CatatonicBug]

Thanks guys. You saved me about $10 and a huge headache. I'm still all stock with the original black pipes (not that they are very pretty anymore though) and original airbox, so I'll just leave everything as-is, and borrow my neighbor's sync setup in the next few days. Hopefully that will solve my issues with hard starts in the cold.

[Ken Talbot]

Hopefully that will solve my issues with hard starts in the cold.

Aha, a clue! When was the last time you checked out the spooge holes in the bottom of the float bowls ?

[CatatonicBug]

Aha, a clue! When was the last time you checked out the spooge holes in the bottom of the float bowls ?

Great... I had no idea that was even there... Now I'm gonna have to take them all off just to make sure. Thanks for pointing that out though! I'll let you know what I find, hopefully in the next few days.

[Steptoe]

spoodge hole

righto, I,ll ask the dumb question.what does the hole do in the carbs?---------------like I always say a dumb question can save a lot of money

[boyat68]

Steptoe:

Taking a look at page 4-2 in the Yamaha manual will show you that the spooge hole supplies fuel to the starter jet makeup. When you open the starter jet lever it allows fuel to be sucked up from the bowl around the starter jet valve and down through the small hole in the upper third of the throttle bore where it is atomized for initial start. If that hole is blocked, then all kinds of starting problems happen. Also TC has posted a colorized picture of the carb that will show this in detail.

[CatatonicBug]

Wow! I worked on the bowl jets this weekend, and after finally getting a tiny wire through the jets on #3 & #4, the bike actually starts! I pull the choke, and VROOM! It starts right up. The only weird thing now is that after a minute or so of warming up with the choke on, the revs increase steadilly, and won't go down until I push the choke in again. Is that supposed to happen?

[XSPastor]

Normal Op

Yes. That is normal as the engine warms up, the revs will rise until you release the "choke." (It is actually an enricher since it adds more fuel instead of reducing the air supply.)

[CatatonicBug]

Good. Thanks. I just wanted to make sure I hadn't inadvertently discovered a new problem.

[DGXSER]

Hey Bug, great you got the MNS running strong! Typically, there is two stages to the "choke" also, it will pull full out let it idle up to about 2K or so, then push in about half way, should "click" into a notch there, let it sit about 2 minutes on a cool day and should be close to ready to go, push the lever the rest of the way in. Mine runs a bit lean so it still stumbles on take off up to about 3K until it gets really warmed up. Then no problem pulling in 5th gear from 1.5K up.