Mac exhaust + Stock airbox: Jet Check

[Greggy]

Hi Guys, I have an 82, XS400 DOHC I am thinking about installing a Mac Exhaust with my stock airbox.

1. I need some advice on mainjet size for both left and right if I go this route.
2. Also is 127.5 left and 117.5 right, the correct main jet sizes for Stock?

Thanks!

 

[NewHavenMike]

What ever size the jets are, they should be matched. Ive never heard of mismatching jets . Even if there were cases of different size jets, a 117.5 and 127.5 are worlds apart. I could see them being a 130 and a 132.5 which is only a one step difference..

You will need to buy a few sets of jets and tune based on your bike. There is no magic setting based on modifications and too many variables. You should get your carbs to factory settings and work from there. My 360 has a stock intake setup with aftermarket exhausts. I ended up running smaller jets before the bike ran well. Going up in jet size isn't always the answer.

 

[xschris]

The dohc bikes with stock air boxes did come with two different size main jets. Your bike is correct. I would install the mac with what you have now and do some plug chops to see what needs to be changed. Just throwing large mains in it most often is the wrong fix.

 

[Greggy]

Thanks guys, I will try it with what I have now.

- xschris: could you please define ‘plug chops’ For me ?

 

[NewHavenMike]

Ahh lol I didnt see the DOHC part... It still doesnt make sense to me though. A full 5 steps difference?

 

[Greggy]

Apparently both sides are 127.5 Jets. I called my Yamaha dealer and they confirmed for this year. This doesn't mean it will work proficiently with new pipes, there may be some trial and error. From what I have read some guys are using different size jets when running aftermarket pipes and pods as well? I'm just going to try it with my stock jets and airbox and see how that goes. I can tweak after the fact.

 

[xschris]

From the parts manual the 82J had 125 L and 115 R main jets and the 82 RJ had 127.5 L and 117.5 R mains. This was because the way the air box was setup with one side being a bit more restricted than the other. Others in the past have stated the same finding on their bikes.

 

[BBS360]

Careful with anything the dealer says.
Yamaha made both sohc and dohc bikes in '82 and the dealer won't know any better.
May be safer to tell them you have an '83.

 

[NewHavenMike]

Oh OK i understand it now..

Also, plug chopping is when you are trying to tune your bike. You look at the plugs to see if you are running too rich or too lean. You should also be able to get an idea on the timing as well just by looking at the plug.. Id get a couple packs of plugs. But the actual chopping part is where you cut the threaded portion of the plug off so you can view the ceramic insulator. If you have a good light and a magnifying glass, you can probably avoid wasting a plug. I had a thread on here a while back showing a chopped plug..