HOW TO: Bench-synch your carbs

A bench synch just gets you close (sometimes spot on if all the cylinders and valves have equal wear). You will still need to do a running synch with your preferred vacuum comparison device.

 

the pics were lost when the forum made a recent migration to new software and the op hasn't been around in a couple years.

FU

 

Thank the gods almighty

The pics have returned to us

 

awesome! Great pics too!

 

I was cleaning and adjusting the carbs of my 1982 XJ 750 today and was also fiddling around with paperclips like described in the How-to. After a few minutes I just tossed them away out of misery and I think I've found a much quicker and way more accurate way to calibrate the carbs.

I adjusted carb 3 with the idler screw so that the top of the valve aligned with the (correct me if i'm wrong) canal for the idle gas you see in the top of your carb when you look into it from the engine side. I took a screw driver and adjusted the rest of the carbs in the same way with the synch screws and I was done in under a minute.
Mounted the carbs back on and oh boy what a difference!

 

That is method #3. There is a thread around here somewhere detailing that method. It's the one I use.

 

I just did this very thing. Quick and easy. No fussing with paper clips or drill bits.

 

I missed this comment. Throttle linkage was bent. I bent it back to vertical, or close. The idle speed 1000 after closing the choke. at operating temp, it races up to 2000 RPM. Is there an easy fix?

 

An easy fix? Yes, do all your settings again - carb synch, mixture, idle speed, carb sync, mixture. Until you get it right.
If you can’t get it right (assuming valve clearances correct?), then strip the carbs to find the issue. Then do the above again.
So potentially not so easy a fix, which is why most posts on here concern carb issues. You have to be methodical and somewhat clinical. Any one failure can cause an issue, if you have two or more it becomes impossible to diagnose.
You have to become a carb expert.

 

P.O. had all work done by shops. That had me taking for granted most adjustments were reasonably done correctly. WRONG!!! I had to dis regard the conventional means of bench synching and use the visual method to uncover the holes evenly. Then use the idle knob so they were all were half open. Reinstalling carb bank today and test. Possibly the last good day of the year before we get the 1st snow. I'm not rushing to beat the weather, just don't want to wait till spring to button this up.

 

P.O. had all work done by shops. That had me taking for granted most adjustments were reasonably done correctly. WRONG!!! I had to dis regard the conventional means of bench synching and use the visual method to uncover the holes evenly. Then use the idle knob so they were all were half open. Reinstalling carb bank today and test. Possibly the last good day of the year before we get the 1st snow. I'm not rushing to beat the weather, just don't want to wait till spring to button this up. Thank you.

 

I don't know about conventional, but I've only ever done a bench synch by lining up the hole edges. And my colourtune went in the bin years ago, long before I bought the first Yamaha.

 

Regarding "conventional", I'm only referring to the start of this thread. IE: Paperclips, strips of business cards and such. This Bench synch was nearly spot on! Only had to back off the idle knob 1/4 turn to get 1000 RPM, and it stayed there after riding up to operating temp. Noticeable horsepower and torque increase! Mixture screws are set at 2 and 3/4 turns out from bottom. It's a bit rich. Going to give it more air, by turning out the mixture screws 3 full turns. I have a Colortune but it's not newbie friendly. Doesn't seem very precise I guess. I'll go by smell, sound and feel. I gotta trust my instincts more. This is my 1st attempt at tuning a motorcycle. She's a real beauty. Totally worth the effort. Thanks for replying. This site has some pretty old forums but it has helped a lot!

 

Wouldn't you be better off turning the screws in a bit to make less rich?

 

Yes, in is weaker.....
Only do it when the engine is good and hot. If it speeds up chances are it was rich, keep going, all equal amounts, untill is starts to slow down. Then back out until it speeds back up.
If you have a good ear you can do each one individually, but you can't do running synch by ear, you have to have a gauge/gauges.

 

Thanks. Not knowing how fuel travels through the carb bank, I thought turning the mixture screws IN = Less air. So, please clarify:...IN = rich and OUT = lean. Correct?

 

In = lean, out = rich. As you turn the screw in it plugs the tiny fuel hole more and more, therefore making a more lean mixture.

 

Sorry Huntchuks, that's what I meant to type and that's what I did. 1/4 turn IN equally. It's only 40 degrees so not sure how "good and hot" it got, but the brief ride after adjustments smelled less rich, idle is on point and, compared to this summer, It is scary fast and smooth running. Come spring, I'll take it in to the shop for checking vales and pro carb synched. I had a friend who works at the local Yamaha dealer help me do it at home with his own garage made stuff. Hard to believe it will have even more to give after that! Thanks again.