Mikuni Rejetting

[ctavel]

I am in the process of rebuilding my carbs (1982 XS400RJ Seca) and have a few jetting questions. According to the part numbers at http://www.yamaha-motor.com/sport/parts/home.aspx My main jets are 165 and my piolt jets are 42.5. My question is, with the stock air box removed, Uni pod filters installed and no changes to the stock exhaust (standard 2-into-2) what would be a good starting point for me to try with new jets. I was thinking about going to a either 167.5 or 170 for the main and am having issues with finding a size for the pilot. Everywhere I look the smallest that I can find is 50, which seems like a hell of a jump.

Also, as a note, it idles well, revs well, but feels like it is running hot (discoloration on pipes) thats why I think I should rejet. Also, I have spaced the needle down one adjustment. Any thoughts would be appreciated. The bike is getting close so I'll make sure to get some pictures posted-- I think its going to be one of the better lookins 400 secas around-- not to toot my own horn or anything though

 

[cbcafe]

If you have dropped the needle, you have leaned the main jet circuit out. You say the bike idles and revs well, but how does it run under load? In my experience, the pods did not mandate a pilot jet change. The idle mixture had to be adjusted slightly richer to compensate for the increased air flow that the pods give.

As far as the main jets go, if you feel they are lean ( the motor will pop or misfire under load) try raising the needles to richen the mixture.

Because you are still running the stock exhaust, you are likely not that far off with the stock jetting.

Post some pictures of your bike. I have an xs400j maxim that has the seca motor and monoshock rear suspension.

 

[ctavel]

Hey, I suppose, after thinking about how I adjusted the needle. I actually raised it-- I did it with the intentions of richening the mixture. It was at the top of the needle and it is now one notch down (therefore lifting the needle)

I need to get my new headlight and taillight hooked up before I can tell as to whether or not it sounds good under load. Thanks for the quick reply. Oh, when I get back around the bike I will make sure to take some pictures and post them up. I'd love to see what you did to your seat because thats the one place that I am a little stumped.

 

[cbcafe]

I actually haven't touched the maxim yet. I have seen a few cool ones on this web site and have been thinking about mods.

There is one little side note on the jetting of my bike, the carbs were stagger jetted. Your seca may be different, but I had to be super careful that I did not mix the jets side for side. Also, your jets are much larger than mine. I believe mine were 120 and 117.5. The pods likely make removing your carbs easier as the maxim with the stock air box is a pain in the ass.

My jetting troubles came from having to replace a rotted stock exhaust. Trying various mufflers netted much grief as i discovered the bike was sensitive to exhaust changes. The carbs were removed many times trying to find the sweet spot. In desparation, a buddy got me a pair of ninja 250 mufflers and the bike ran perfectly with the stock jetting. Hopefully your bike is not as fussy on the intake side.