For the past few weeks I have been battling with the carbs on the XS750 that was built last year. They just haven't been running right and it was time for an inspection. I thought I would share my experience because I think it might be valuable to others who may encounter the same issues.
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 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.
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.
Anyway, to my horror, two of the tips of the pilot adjustment screws were broken off in the the carb bodies (see below). 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. 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!
There seemed to be plenty of info online where people have gotten broken adjustment screw tips out of carbs where said 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, 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.
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…). 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.
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!
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. No where 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 have 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.
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?
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!