Mix Screw - gone

[Mickster]

79 BS38 carbs. The bike has been running great since the topend rebuild, about 500 miles worth. Has been sitting for about a week.

Went out to start it and it started right up, running on one cylinder very good but barely anything on the other. OK, got out the flashlight and found there was no mix screw in the right carb. The screw, and the spring, must of launched at some point. No luck finding it.

Mikes sells the screw, with the O ring, but not the spring.

Is there a work around, or a place where I can get a couple of the correct (78-79} screws with the O rings and springs?

Thanks for your help.

 

[5twins]

As far as the spring goes, I think scrounging used ones is your only option. I'm not aware of any complete screw assemblies being sold. According to the parts diagram, the spring is unique to the '78-'79 mix screw only and not used on any other models.

 

[jpdevol]

Not that I'm recommending non-Mikuni, especially Keyster, but - in this case -they do have a kit that includes the fuel screw and the spring:
https://www.mikesxs.net/yamaha-xs650-mikuni-carb-rebuild-kit-78-79-bs-38.html

 

[Mickster]

The supposedly OEM mix screw, o-ring and spring I ordered off e-bay finally came in. It is silver, not brass like the other one I have. The one that got lost was also brass. Are the brass ones aftermarket?

There must be something wrong with the right side carby. The old brass screw and the new silver screw will catch the thread and screw in until lightly bottomed.

However, if I back out the right side to 3 turns the screw will pop out from the spring pressure. The left side doesn’t do that.

I have them both set at 1.5 turns out but that does not leave much room for finer tuning. The right side screw is engaged by just over 1 thread.

This was an abandoned bike when I picked it up so don’t know what the history is. Mis-matched carbs? Damaged carb? Any ideas?

Thanks for any insights.

 

[jpdevol]

This was an abandoned bike when I picked it up so don’t know what the history is. Mis-matched carbs? Damaged carb? Any ideas?

Pics may help identify those. Also, you'd be surprised what a phone camera can do in taking a pic down into the fuel screw hole (might be bunged-up or broken remnants down in there). Of course, the "silver" screw is suspect anyway.....

 

[5twins]

The silver colored screw would be original, the brass aftermarket .....

..... and seeing that the factory setting spec is 2.25 turns out, and many need about 2.5 turns for best running, having it pop out around 3 turns is definitely an issue.

 

[Jim]

I wonder who makes a helicoil that size?

 

[5twins]

Honestly, I'm not even sure it could be fixed. Another carb body is the easiest option I think.

 

[Mickster]

5twins, that is the exact same screw I got. It came in a plain, unmarked baggie but it is the same one.

I've got the screws at 1.5 turns out and took it for a ride today. Some spitting out the carbs until fully warmed up, about 6 miles of normal riding. After that the bike ran very nicely, almost. While cruising at 1/8th throttle in 4th or 5th it still occasionally spits out of either carb. Not bad, but it still should not do that.

I don't know if backing out another 1/4 turn will do anything but I'll give it a try. More than that I'm afraid of loosing the screw again. If it helps that's good, if not I guess I'll just have to live with it until a future date.

I'm not an expert so these carbs are probably not in perfect sync, probably not tuned to perfection, so maybe it's as good as it's gonna get. I really like this bike.

 

[jetmechmarty]

Your pipes are huge. I think your engine needs to turn at 10,000 RPM to take advantage of them. I can't be sure, but it may contribute to the problem. I bought those pipes for my project as well.
A little about that here: https://www.xs650.com/threads/xs6502f-xs650sk-mash-up.58192/page-6