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Author Topic: 1977 Yamaha XS750 2D Carb Repair  (Read 2303 times)

Offline JadusMotorcycleParts

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1977 Yamaha XS750 2D Carb Repair
« on: Sep 05, 2016, 06:17:21 »
For the past few weeks I have been battling with the carbs on the XS750 I have been working on.  I thought I would share my experience because I think it might be valuable to others who may encounter the same issues.  I will post each step in a different post and include some pictures.

Something to note with all old bikes is you never know what the previous owners have done to them, even if you get a story about it from one of them.  What I found with this bike was that the engine had been apart at some stage and a mild porting job carried out - whether professionally or not I don’t know, but there was obvious die grinder marks when peering down the intake runners.  Or were they like this from the factory?  I wouldn’t think so but not sure.  There was also a few stripped threads here and there - one being the centre cylinders plug hole.  This had a helicoil in there so seemed to be ok for now.  Another issue was the stuffed carbs.  I didn’t find out the full extent of the issue until I started trying to tune it.

The bike got a free-er flowing exhaust and pod filters installed, therefore requiring a carb retune, or at least a check anyway.  When everything was put back on the bike it actually started pretty easy.  So I warmed it up and did a carb sync - nothing seems to mess up any idea of where your jetting is at than if the carbs are out of sync.  But the bike still wouldn't idle right.  I wondered if the fact that the right carb was way out of sync had something to do with it.  It turns out it would, but not in the way I thought.  After fluffing around with the mixture screws a few times and achieving no obvious effect, I realised a closer inspection of the carbs would be needed.

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #1 on: Sep 05, 2016, 06:20:08 »
I should say that I like these carbs.  They make sense and they are actually not too bad to work on - maybe it’s because I am used to working on CV carbs and especially Mikunis.  These carbs are from the 1977 XS750 2D model - turns out it was pretty hard to find info on them online because Yamaha switched to a different set of carbs in later model years.  But the fact that both the main jet and the pilot jet is in the float bowl I thought was pretty nifty.

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #2 on: Sep 05, 2016, 06:22:28 »
Anyway, to my horror, two of the tips of the pilot adjustment screws were broken off in the the carb bodies.  Some ham hand had screwed them in so damn tight they snapped off.  Turns out this is more of a common problem than it should be…  My options at this point were to junk the carbs and try and pick up a second hand set, try repairing these ones, or do like the previous owner and sync the carbs so that it is biased towards the right hand carb where the pilot screw was still functional - therefore allowing the engine to barely idle by relying heavily on that cylinder over compensating for the other two cylinders which were almost receiving no mixture at all until around 1/8th throttle (thus explaining the discovery I made when syncing).  I thought fuck it, I’ll have a go at repairing them and worst case scenario I stuff them a bit more than they already are!

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #3 on: Sep 05, 2016, 06:24:32 »
There seemed to be plenty of info online where people have gotten broken adjustment screw tips out of carbs where the screw sits a top the carb - inline with the flow and perpendicular to the axis of the throat.  These particular carbs have the adjustment screws on the sides of the tops of the carbs - lying flat with the axis of the flow.  I couldn't find any info about this issue online, I am sure there is somewhere and many have probably solved this issue one way or another (please share if so!), but there seemed to be no way to get at the other side of those broken tips to be able to push them out and remove them.

So my solution was as follows:

Punch a centre mark for drilling right in the middle of the boss that contains the mixture screw on the opposite side. 

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #4 on: Sep 05, 2016, 06:25:31 »
Drill a hole with a 3.3mm drill bit so that you can later tap this hole to M4.  Drill just deep enough that you can see the very tip of the broken tip.  It should be pretty clear because it is brass in aluminium so it will show up goldish against the silver background.  Don't worry, by doing this, you are not actually affecting any of the orifices that control fuel metering (I think - I have looked at it for so long trying to be certain…). 

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #5 on: Sep 05, 2016, 06:28:17 »
Now you have a clean shot at punching out that broken tip.  Depending on how hard the muppet who got it stuck there in the first place screwed it in, depends on how hard you will have to hit it out.  These were stuck real good.  I used a hardened steel mechanics pick to start with, then ground down a small carbide drill bit to a point and continued punching till it came loose.  Notice the drill bit had a slightly smaller diameter than the pick and I think this was the ticket.  Eventually got both bastards out.  Look at the state of these screws!  They are even bent in two places, indicating they have been screwed in so damn hard!

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #6 on: Sep 05, 2016, 06:29:54 »
Once the tips were out, it was time to tap the holes with an M4 tap.  I went as deep as the tap allowed, but if you were really well equipped, you would have a M4 tap set with a bottoming tap to get the thread all the way to the bottom of the hole.  Anyway, I figured it would be good enough/deep enough because that hole just needed to be permanently blocked off.

Then I went to the local bolt supply store and picked up some M4 grub/set screws to screw into the tapped holes.  Here it would have been better to use stainless and to use a shorter length, but this is what they had in stock so I took them.  After applying some permanent thread locker to seal everything up, I screwed in the screws, hopefully never needing to be opened again!

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #7 on: Sep 05, 2016, 06:32:09 »
Unfortunately the problems did not end there.  The next challenge was to find replacement adjustment screws.  I searched real hard to find a new set.  Nowhere still stocks the original ones - they are out of production and not on anyone's shelves anymore.  Unless someone else knows where they can be found?  Feel free to let me know or comment for others!

I ended up ordering all 3 types of adjustment screws Mikuni has made for the CV carbs of that era - check out the image.  I wasn't aware that there was a difference in thread pitch in some of them.  It turned out that the screws that looked most like the originals had the wrong thread and the ones that are stepped (usually an air screw rather than a fuel screw?) ended up having the same thread.  They fit but didn't feel like a tight enough fit in the carbs - they felt like they would back out with vibrations.  So I set them up with the washer, spring and o-ring that is provided (as a kit) but assembled them back to front - as I was just after some tension on the screw rather than a seal in the hole.  This seemed to work really nicely - check out the image for the orientation.  Once I screwed them all in place, they seemed to land at roughly the same location when bottomed out - allowing me to back them out the exact amount of rotation for each one and hopefully flowing the same amount of air/fuel mixture.

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #8 on: Sep 05, 2016, 06:33:36 »
The last hurdle was finding the right pilot jets (mains were easy).  The stock pilot jets have the same dimensions as the Mikuni VM22/210 but do not have any holes in the neck.  I needed to go up in pilot size so I needed to order these ones.  I have no idea if they flow any more fuel or not but I went up 2 sizes to begin with and that was too much, ended up going back down a size.  Anyone else have any knowledge on this?

Offline JadusMotorcycleParts

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Re: 1977 Yamaha XS750 2D Carb Repair
« Reply #9 on: Sep 05, 2016, 06:36:01 »
Final comments…

A couple things would have made this repair even better:

1) Me being able to drill a straight hole haha
2) Having a bottoming tap for the M4 thread
3) Having shorter, stainless set screws

Anyway, once the carbs were back on the bike I set about finding the right size jets by using an air/fuel gauge with an oxygen sensor and this went really well, made the whole process pretty easy and reliable.  I ended up getting lamda to be in the 13s for most of the rpm range, while dipping into the 12s and 14s a bit in some places.  Overall pretty stoked!

Turns out the fix on the adjustment screws must have worked because I was able to get an audible response from the engine when adjusting each of the screws.  Now I just hope the set screws and the adjustment screws themselves stay put!

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!

I´m 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.

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 »
I´m 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!