205 vs 195 Pilot Air Jets (Hitachi)?

Anyone know why the '82 XJ650J is spec'd to have 205 pilot air jets as opposed to the 195 size all the others use? Looking over the engine specs for all the models, I can't find where a "J" is any different. Is it something in the carbs themselves?

The reason I ask, is that I got a new(used) bank of carbs for my bike, which I've been working on. They were sold to me as "82 XJ650" carbs, but I noticed that they had the 195 size pilot air jets. My original carbs have the 205s. Thinking that the 205s are correct, I put those in the "new" carbs and installed them (after disassembling, cleaning, replacing seals, etc.)

After synching, I couldn't get any yellow at all on the colortune plug, except for cylinder #3, and all my mixture screws had to be WAY out to get it to run. So, I suspected clogs in my pilot fuel circuits. I took the carbs back off, opened 'em up and have now figured out the pilot circuit, and made damn sure it's good and clean. They didn't really seem to be clogged - but could have been slightly clogged - hard to tell. I know they're clean now though.

So, what I'm curious about is - should I retry it now with the 205 pilot jets, or put the 195s the carbs came with back in? It seems that the 195s would give me less air, and thus a better chance of being able to see "rich" (yellow) on the colortune. Is the jet size carb specific rather than engine specific?

I just wonder WHY these "J" models have 205s in the first place. Was it just an attempt to lean down the idle mixture to meet emissions standards in 1982?

 

The 195 air jets came with the non-YICS motors; once the YICS system was fitted to the 650 engine (1982 all 650 models ecxept the 1982 XJ650RJ Seca models), then the pilot air jets were increased in size to 205 (making the mixture leaner, increasing fuel economy, less emissions, etc......exactly what the YICS system was designed to provide).

So your "1982 650 carbs" were most probably off of a 1982 XJ650RJ Seca model.......

 

Thanks for the insight Chacal.

Now, this of course leads me to the next question...

If the 205 jets were added solely due to the addition of the YICS system, and with the purpose of making things run leaner, How is it correct to block off the YICS ports and set the mixture screws to "bunsen blue" (or a hair richer) on the colortune? By blocking off the YICS ports, you effectively now have a non-YICS motor, which normally should have 195 pilot air jets. So it stands to reason that with 205 jets, you WOULD need to turn the mixture screws much farther out to get a correct mixture on the colortune. Yes?

Or, are we supposed to set the mixture to some "lean" setting (NOT bunsen blue) for YICS engines - and if so, how?

Perhaps I'm not understanding this correctly...

 

This is a good observation, but thats why we plug chop after we colortune JUST to make sure everything is ok under real working conditions.

 

I'm not sure I understand yur question? 750 engines used #225 jets. 700 engines used #210 jets. What difference does it make? You set it to run slightly rich using the jets that the carb is equipped with. And the non-YICS engines used coarse-thread mixture screws, so a direct comparison of "how many turns out" is meaningless.........and lets not even talk about the change in the TCI box that occurred with the introduction of the YICS engine (going from the black-label 4H7 box to the red-label 5G2 box, same a used on the 750 "always YICS" engines). I can tell you that via experimentation that a YICS engine performs worse when using a non-YICS TCI box, probably due to differing advance curves.

The YICS system operates on the principle that by creating a secondary intake passage that injects part of the fuel mixture at an angle, and at a higher speed, into the cylinder, it creates "swirl" within the cylinder, encouraging cylinder filling and and promoting better (and more omplete) mixture burn.

A good write-up on how the YICS system operates, technically, can be found at:

http://www.xjbikes.com/Forums/viewtopic/t=14757.html

Also, recall that IDLE mixture screws set the fuel mixture primarily at idle and off-idle conditions, which from a fuel-economy perspective, probably contributes only a minimal amount to overall "tested" fuel economy figures; however, since EMISSIONS regulations and measurements are made at idle conditions, it would vastly effect those readings.

Using the Colortune to set the mixture screw settings to a slightly rich condition is not a "factory" recommendation; Yamaha never seemed to recognize or acknowledge the fact of its existence (assuming it even existed back then). They used a a gas analysis device. And even the tuning guide for these engines notes that if setting the mixture screws to the "correct" emissions-standards (CO, HC) levels results in customer dis-satisfaction with performance, then to adjust the mixture screws to a slightly more richer setting, noting that it will slightly decrease fuel economy, but will make the bike perform better during off-idle operation.