XS750 Mikuni mk2 carb questions.

Hey guys, longtime lurker/dreamer, first time poster/owner.

I'm in the process of tearing down and cleaning the carbs on my Yam xs750.

I've been following this guide. http://www.yamahatriples.com/index.php/how_tos/carb-rebuild-mikuni-mark-2/

And this video.
https://youtu.be/ow8H-OCp9Hc

First question. Is it necessary to remove the butterfly valves before soaking/cleaning the carbs? The valves are seated in there pretty strong and I don't want to strip those delicate brass screws. The valves seem to twist freely.

Second question. The PO seems to have stripped out one of the pilot jets. Realistically I could get it out but I will definitely need to replace it if I do. Is it an option to just leave the jet? Or is this going to cause issues with tuning/jetting later on?

Sorry for the novel. Thanks for any help.

Yes, absoloutly remove and replace that pilot jet with a clean/new one. I believe it's a 42.5

I will give you a good used pilot jet if you promise to use it. (You don't have to promise to remove the butterflies)

There are two camps on the butterflies. One group will tell you that you really should remove the shafts to replace the end oil seals. The rest will tell you No worries mate. She'll be right.

I would suggest that it is better to replace the seals and to be very careful with those screws. Butterfly screws are "staked" ie spread out to stop them from coming loose. When they are removed, they can damage the threads in the shaft, but usually they come out with a bit of work. They need to be re-staked or loctited when they are rebuilt though.

The pilot jet has to come out though and can be removed with a left hand drill bit and EZ-out if it's too badly damaged.

Ichiban Moto said:

Yes, absoloutly remove and replace that pilot jet with a clean/new one. I believe it's a 42.5

I will give you a good used pilot jet if you promise to use it. (You don't have to promise to remove the butterflies)

Click to expand...

Man that would be incredible! I'm out of town for another week but I'll be attempting to remove that pilot jet as soon as I get back.

Do you just have a bunch of jets laying around that you give away? I really appreciate it.

teazer said:

There are two camps on the butterflies. One group will tell you that you really should remove the shafts to replace the end oil seals. The rest will tell you No worries mate. She'll be right.

I would suggest that it is better to replace the seals and to be very careful with those screws. Butterfly screws are "staked" ie spread out to stop them from coming loose. When they are removed, they can damage the threads in the shaft, but usually they come out with a bit of work. They need to be re-staked or loctited when they are rebuilt though.

The pilot jet has to come out though and can be removed with a left hand drill bit and EZ-out if it's too badly damaged.

Click to expand...

Okay I'll definitely be removing the pilot jet then thanks.

I'll give the butterfly valve screws another shot but if they won't budge then I won't risk it. The whole butterfly assembly seems pretty solid anyway.

Anyone have a guide on jetting and float settings for me? It's a '78 xs750 non-special with Mikuni mk2s.

JamesRustler said:

Man that would be incredible! I'm out of town for another week but I'll be attempting to remove that pilot jet as soon as I get back.

Do you just have a bunch of jets laying around that you give away? I really appreciate it.

Click to expand...

'70's bikes don't like the modern low octane ethanol blend in my area. The stock pilot jet is the first thing that ends up in the spares bin.. so I have a few million of them. probably. I hand carve new jets from solid billet brass blocks anyway.

Mikuni used three different pilot jets on the BS series of carbs. I don't know which are used in yours but they have to be the right ones or it will never run right.

good point, this is you stock pilot jet part: 256-14142-42-A0

That's odd. Partzilla lists that as fitting the 78/79 650 twin which uses a normal VM22/210 and 81 on 650 which uses a BS30/96

I wonder which it is. Here's an image to help.



Looking on line, it looks like maybe they should be BS30/96... perhaps....

just look at the partzilla cross reference for the Yamaha part number:

http://www.partzilla.com/parts/detail/yamaha/YP-256-14142-42-A0.html

partzilla lists 256-14142-42-A0 for several model/years including his '79 XS750

I checked that listing and it is not consistent with other lists on the net, which is why I raised the question of which is actually fitted. Someone must know which type it is so they can order directly from a jet place like Jets R US. Or I guess you just order from Yamaha and take it from there.

that's why I go with the Yammie #'s i always baggie jets by bike make/model anyway

I buy them by Mikuni part number in "bulk" from a Mikuni dealer - that must be why we end up solving the same problem for different perspectives.

That's a good point though Ichiban San, in some caes the part is not available with a Mikuni part numbers. Case in point #284 series needle jets for RD400E. They are a bleed type jet with a tall spray bar like a primary style jet to make it richer at WOT and high revs. I don't think that they were ver listed by Mikuni.

Needles for late style TMX carbs are the same, it's easier to buy one with a Honda or Polaris number than with a Mikuni part number and no cross reference. Using those carbs, means developing your own database of needle profiles from the limited data on line.