How to sync a 71 XSB1 Carb

[Quoco]

Hey guys, I was wondering if you could give me a helping hand. I spent all afternoon trying to get my 1971 yamaha xs650 to idle right, yet it was a failed process. It would start up evey kick and run yet it has a lot of back firing to go along with it. I try to adjust the mixture screw but it doesn't seem to work. I then proceeded to take out my sparkplug, to fine it wet, so I'm guessing now the fuel got into the motor. I adjust the brass floater to 25mm measurement also, but no luck. Then some one on here told me to sync my carb, yet its a 1971 style and the carb doesn't have a vacuum barbs until the 75 model. So if I were to sync my carb, how would I go about doing it? I made the tool that everyone is using with a yard ruler and ATF coolent along with clear tubing...though I don't know where to stick it to...any respond would be great. Thank you

 

[TwoManyXS1Bs]

Hi, Quoco.
There is a procedure for syncing, but your description of the idling sounds like you need to overhaul the carbs first. If idle mixture screw doesn't change anything, then you've got some clogging in the idle circuit. Best clean/overhaul carbs first, then good tune-up, set timing. Until you do that, you can't sync carbs...

Hope this helps.

 

[tonyc]

Check out the tech section at the top of the page. Also read the carb guide more than once. If you do not have one get a manual.

 

[Quoco]

hey thanks for the reply guys really great,... ill try reading it again and check back here.

 

[XSLeo]

Once you get the carbs cleaned. you need to do the basic tune up procedures to eliminate any mechasnical issues from upsetting your carb work.
Do the steps in this order.
#1 adjust cam chain tension.
#2 adjust valve clearances
#3 adjust the points gap
#4 adjust points timing
Once you have these right then procede to carb tuning.
In your repair manual it will exlain the "dead cylinder method" Follow that procedure will sync your carbs.
To adjust the idle mix screws start with the factory spec of 1 3/4 turns out. Now during the "dead cylinder method" when you have the bike idleing on one cylinder at it's lowest rpm you can adjust the mix screw. Turn it in till the bike stumbles, then turn it out till it stumbles. Set the screw half way between these points. The idle speed should be at it highest. I then set it 1/8 turn out from there.
Leo

 

[GreasyC]

There's a big gotcha on the 71. Look on the head between the carb mounting points and you'll probably see a black hose running from one side to the other. This connects the two intakes together. Playing with the mixture screws or trying to balance the idle speeds side to side will be a confusing mess if this hose is in place because each piston can draw from both carbs at once. You can turn the butterfly closed all the way so the screw comes right out and that cylinder may keep on running off the mixture from the far carb. Remove the hose while you're tuning and seal off the holes. Don't use anything that could be sucked into the engine, there's vacuum here.

Once those are blocked off your adjustments will make more of a difference and you can do a rough balance with the dead cylinder method. Or you could hook up a manometer to those spigots so you can do a proper open throttle balancing of the dual throttle cables but getting the intake crossover hose back in place without upsetting the cable balance is kind of tricky. It tends to change whenever you disturb the carbs.

 

[TwoManyXS1Bs]

Hey, GreasyC
Good to catch that...
I use hemostat, clamp it off...

 

[JasperLP]

That hose should have a spring in it to keep it from collapsing.

 

[iRacer]

Once you get the carbs cleaned. you need to do the basic tune up procedures to eliminate any mechasnical issues from upsetting your carb work.
Do the steps in this order.
#1 adjust cam chain tension.
#2 adjust valve clearances
#3 adjust the points gap
#4 adjust points timing
Once you have these right then procede to carb tuning.
In your repair manual it will exlain the "dead cylinder method" Follow that procedure will sync your carbs.
To adjust the idle mix screws start with the factory spec of 1 3/4 turns out. Now during the "dead cylinder method" when you have the bike idleing on one cylinder at it's lowest rpm you can adjust the mix screw. Turn it in till the bike stumbles, then turn it out till it stumbles. Set the screw half way between these points. The idle speed should be at it highest. I then set it 1/8 turn out from there.
Leo

Thanks!

https://photos.app.goo.gl/UCrWz66fVb1DFvQK9