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Author Topic: 1977 Yamaha XS750 2D Carb Repair  (Read 3173 times)

Offline Ryan Stecken

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #10 on: Sep 05, 2016, 07:55:48 »
Thank you Jadus for this awesome writeup on these carbs!

IŽm currently struggling with the same issues on my XS 750 (same carbs).
Starts really bad, Cylinder 3 gets hot really fast,even though all mixture screws are 2 1/2 turns out.Cleaned the carbs, but now I will check if the mixture screws tips are broken off too.I need to gas the bike the hold the motor running (event though I choke the bike, so I guess the mixture is way off).

There is  a firm in Germany where I get all my jets and mikuni parts from:
http://www.mikuni-topham.de/ , they might have all the parts!

By the way :-) are these avon road riders?

Offline jpmobius

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #11 on: Sep 05, 2016, 10:58:21 »
Very nice write up!  Seems like you could use the standard VM pilot jets as is.  The perforations for introducing air shouldn't make any difference since there evidently isn't an air supply for the solid ones.  The VM jets are actually pilot jets and emulsion tubes in one part.  The jet itself is just the drilling in the threaded end.  The tubular section with the holes fits into an air well and seals with the conical tip.  Air supply to the well is controlled by the air screw on the VM's, but presumably there is simply a straight bore in your bowls to house the jet.  Potentially there might be a hiccup with there being a tiny fuel reservoir around the perforated tube that the straight tubes don't have, but I bet they work just the same - worth a try as they are very common.
« Last Edit: Sep 05, 2016, 11:08:31 by jpmobius »
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

1973 RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline teazer

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #12 on: Sep 05, 2016, 12:01:58 »
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

« Last Edit: Sep 05, 2016, 17:07:55 by teazer »

Offline datadavid

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #13 on: Sep 05, 2016, 13:23:32 »
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! 👍

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #14 on: Sep 06, 2016, 03:28:10 »
IŽm currently struggling with the same issues on my XS 750 (same carbs).
Starts really bad, Cylinder 3 gets hot really fast,even though all mixture screws are 2 1/2 turns out.Cleaned the carbs, but now I will check if the mixture screws tips are broken off too.I need to gas the bike the hold the motor running (event though I choke the bike, so I guess the mixture is way off).

There is  a firm in Germany where I get all my jets and mikuni parts from:
http://www.mikuni-topham.de/ , they might have all the parts!

By the way :-) are these avon road riders?

Yeah, check those screws man.  I spent a good half a day trying to tune these carbs before realising the tips were broken - rendering it a complete waste of time!

I like that company in Germany - have even bought a carb from them before.  Good service too.  I should have checked with them but couldn't see it listed.

Yep!  Avon road riders  8)  They suit the bike nicely surprisingly!

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.