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Author Topic: 1977 Yamaha XS750 2D Carb Repair  (Read 2342 times)

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #15 on: Sep 06, 2016, 03:30:11 »
Very nice write up!  Seems like you could use the standard VM pilot jets as is. 

Thanks mobius, I was a bit unsure but I now I think its fine.  They worked which is the important thing!

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #16 on: Sep 06, 2016, 03:34:01 »
Nice thread.  Mikuni made a bunch of different BS series CV carbs and they used different pilot jets.  The usual VM22/210 won't work well in a BS because it meters fuel at a different point in the jet.  BS30/96 with holes but different is used in BS40 on a GT750 Suzuki.

And then there is the N151.067 without bleed holes which is apparently used on many 4 strokes including Suzuki GS1000 and Kawasaki KZ1000.  They are available at most jetting sources.

BTW, the choke (enricher) jet is also in the bottom of the float bowl and gets plugged with debris over time. Check out http://pinkpossum.com/GT750/carb/BS40carbs.htm for details.  That shows a BS40 but the enricher jet is in that same place for many (all?) Mikuni carbs

Thanks for the info teaser.  I looked at the N151.067 jets but they looked a little different - especially around the nozzle/tapered area.  Perhaps that is not important?  If not, then maybe they would have been a more accurate replacement.

Why do you think Yamaha used so many different carb set ups on this one model?  I read that later set ups had a 2-position choke as well...

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #17 on: Sep 06, 2016, 03:39:24 »
I got new mixture screws from cmsnl.com, pricey but the right stuff. Braigasen.com has a nice jet reaming kit you can order, its nice for opening up pilots and mains, has already saved me some kronors 😀
I removed 2 broken tips using nothing but pliers and cut off nails btw, but the holes had to be fixed with jb weld and reamed after, your method seems a bit easier on the carbs! 👍

Well at least you got them out!  Common problem huh.  I can't imagine trying to get them out from that side, must have been damn difficult?!

I have ordered stuff from Braigasen - awesome to have someone in Sweden with the right stuff!  The only issue I have with modifying jets is thinking about the poor bastard that works on the bike after me!  Imagine trying to tune a set of carbs if you have changed the set up and you realise you are running rich with 130 mains, so you go down to a 127.5 and run waaay lean because little did you know, those 135s had been reamed up to a 150 or something!  I'm sure it saves a lot of money if you're not planning on selling the bike though!

Offline datadavid

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #18 on: Sep 06, 2016, 04:05:22 »
Well at least you got them out!  Common problem huh.  I can't imagine trying to get them out from that side, must have been damn difficult?!

I have ordered stuff from Braigasen - awesome to have someone in Sweden with the right stuff!  The only issue I have with modifying jets is thinking about the poor bastard that works on the bike after me!  Imagine trying to tune a set of carbs if you have changed the set up and you realise you are running rich with 130 mains, so you go down to a 127.5 and run waaay lean because little did you know, those 135s had been reamed up to a 150 or something!  I'm sure it saves a lot of money if you're not planning on selling the bike though!
Yes you have to inscribe the new value on the jets..
Yes my lady's xs11 had a few carb issues.. 😀
Edit: i pushed the ends back out the way they came from of course, centered a small piece of nail on the tip and used a small polygrip, holding on the lip of the carb, pop they went! Took the damaged orifices to braigasen for the final mending and reaming.
Its sad that the people least apt for bike mechanics usually are the ones who prefer to do it all themselves, destroying almost everything they touch in the process. I had that punishing learning curve in my early teens so nowadays at least more things get fixed than broken! 😁
« Last Edit: Sep 06, 2016, 08:06:52 by datadavid »

Offline datadavid

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #19 on: Sep 06, 2016, 08:12:43 »
Nice thread.  Mikuni made a bunch of different BS series CV carbs and they used different pilot jets.  The usual VM22/210 won't work well in a BS because it meters fuel at a different point in the jet.  BS30/96 with holes but different is used in BS40 on a GT750 Suzuki.

And then there is the N151.067 without bleed holes which is apparently used on many 4 strokes including Suzuki GS1000 and Kawasaki KZ1000.  They are available at most jetting sources.

BTW, the choke (enricher) jet is also in the bottom of the float bowl and gets plugged with debris over time. Check out http://pinkpossum.com/GT750/carb/BS40carbs.htm for details.  That shows a BS40 but the enricher jet is in that same place for many (all?) Mikuni carbs
The important word here is OEM, all stock carbs on all bikes are OEM carbs and specially jetted for the particular engine they were designed for, so mikuni bs carbs differ in all their OEM variations AND different generations.
Quite a jungle.. cant just fit a carb for a different engine and expect it to work the same.
Parts availability and price makes it unreasonable to work with old oem carbs, best bet is usually to buy new stock carbs. Every part needed is on the shelf to a lower price.

Offline datadavid

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #20 on: Sep 06, 2016, 08:26:29 »
Thanks for the info teaser.  I looked at the N151.067 jets but they looked a little different - especially around the nozzle/tapered area.  Perhaps that is not important?  If not, then maybe they would have been a more accurate replacement.

Why do you think Yamaha used so many different carb set ups on this one model?  I read that later set ups had a 2-position choke as well...
Yamaha used different racks because the first generation were never any good.. the second gen mikunis are considered best and the last hitachi's they used was mainly because of pollution restrictions and they ran very lean.

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #21 on: Sep 09, 2016, 04:55:54 »
Yes you have to inscribe the new value on the jets..

Its sad that the people least apt for bike mechanics usually are the ones who prefer to do it all themselves, destroying almost everything they touch in the process. I had that punishing learning curve in my early teens so nowadays at least more things get fixed than broken! 😁

Ahhh, well that makes sense then.  As long as theres a way to identify what is what.

I totally know what you mean.  There are too many people that lack the 'feeling' for working on bikes and break and strip stuff all the time - fucking things up for the next unsuspecting owner.  I shouldn't speak too badly of them though, I have made a few blunders myself!  And still do.  But I do try to fix them properly afterwards  :D

Offline Psycrow

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #22 on: Sep 09, 2016, 14:42:34 »
Great thread. I got my gen 2 Mikuni carbs clean and running with some coaxing. Haven't rejetted for the PODs yet so I imagine ill be digging into them again soon.

Psy

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Offline SprayKat

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #23 on: Apr 02, 2017, 05:00:35 »
Hey Jadus. Interesting post :) I myself have a XS750-2D with Mikuni MK1 (BS34 CV). Had it half caffeinated a couple of years but now its getting the big remake including removing the airbox ofc. So I see you have 3-1 exhaust. I have the same. Im thinking KM-airpods and then ofc it needs resetting. Do you think its a easy as just adopting your jet-sizes?

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #24 on: Apr 02, 2017, 09:43:30 »
Yeah should be!  Did I share the final settings somewhere?  I cannot for the life of me remember what they were but if you hassle me a couple times I am sure I could dig up the piece of paper I wrote them down on from somewhere in my paper mess pile.  No reason those settings wouldn't work for you.

Sweet you're doing a rebuild-ish!

Offline Ryan Stecken

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #25 on: Apr 03, 2017, 06:53:22 »
Yeah should be!  Did I share the final settings somewhere?  I cannot for the life of me remember what they were but if you hassle me a couple times I am sure I could dig up the piece of paper I wrote them down on from somewhere in my paper mess pile.  No reason those settings wouldn't work for you.

Sweet you're doing a rebuild-ish!

I dare to hassle you too  ;) would be interesting what you ended up with too!

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #26 on: May 03, 2017, 16:32:36 »
I dare to hassle you too  ;) would be interesting what you ended up with too!

Alrite, after a lot of digging and no luck, I eventually stumbled across this piece of paper by accident.  I knew I wouldn't have been foolish enough not to write down the settings I ended up with.  I was however foolish enough to misplace the piece of paper!  Anyway, check out attached.  Be aware that those pilot screws I used were not standard, so they will have a different setting to the stock ones.

Offline SprayKat

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #27 on: Jun 29, 2017, 05:06:48 »
That is amazing! Thx. My bike will not be ready until next year, but I'll definitely try removing the stock air box and getting pods :)


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