XJ650 factory mixture settings question

Picked up a nice '81 XJ650 a week ago. This is the first XJ I've worked on that still had the anti-tamper covers in place from the factory. I was very surprised to find how dramatically the settings differed from one carb to the next. Carb #1 was set at 2 turns out, #2 was less, #3 less than #2, and #4 was set to only 5/8 of a turn.

Many things will be altered with the rebuild, but I am reusing the original mixture screws. I'm trying to decide if I should start them all at some common default, or begin with where they were originally set.

Thoughts?

 

You'll never have the screws all in the same position when finished tuning.

After all is said and done with the carbs, cleaning, replacing, adjusting float levels, you'll want to start all the screws at 2.5 turns out. Then a running carb sync and a colortune.

 

+1 to that advice. Each cylinder is a like a fingerprint, similar but not exactly the same. The bench synch treats them all like identical twins or in our case quads, which they aren't-the baseline on screws is the same-'a starting point' from which to fine tune just like the running synch.

 

Thanks for your responses, guys. I understand all of that. By the same logic, starting all four screws at the same point makes sense unless you have some other solid information about where each carb should be set.

Considering that I know the "factory-sealed" settings, this gives me a different perspective on how I might approach this. I've never had that info before, so I've always started at some common position for all carbs.

Plus, I've done a fair amount of XJ carb tuning over the last few years, but I've never ended up with a mixture screw in the 5/8 turns out position. Again, since it was set there at the factory, maybe that would be a better starting point for this carb. I'm still going to tune each carb one at a time, so I'll get there either way.

On a side note, 2.5 turns out is not the recommended starting point for '80-81 650s which still used the coarse thread screws. This is from the XJ4Ever catalog:

Thanks!

 

The adjustment you found was done at the factory with an EGA and the cylinders in brand-new condition.

As soon as the motor broke in and the valves got re-shimmed, those settings went out the window; ESPECIALLY if the carbs got rebuilt in the process.

 

Fitz, what's an EGA?

Also, have you ever had an XJ that still had the factory anti-tamper caps in place?

 

Guessing here but I would think EGA is an Exhaust Gas Analyzer or Evolved Gas Analyzer. Probably more accurate than using an O2 sensor to find max efficiency.

 

-Exhaust Gas Analyzer. That's what the little plugs (bolts) on the bottoms of the headpipes are for.

-Nope. All of mine had been "tampered" with. Not unusual, as the dealers were advised by Yamaha that it was OK to "fatten" up the mixtures by about 1/8 turn if the customer had complaints about long warm-up time or "lagginess" at small throttle openings. The same tech bulletin also advised the dealer to warn the customer that doing so might decrease their gas mileage slightly.