Yamaha XJ400 4V7 possible needs rejetting.

So.

I have a Yamaha XJ650 -82 that I´ve worked a lot on last summer.
Valve adjustment to spec, carb rebuilt, vacuum sync etc. And it runs great

So why the XJ400?
Well, the XJ650 is in original shape otherwise than the paint.
I´ve not done anything to it and it will probably be that way in my garage.

But I might have a Yamaha XJ400 4V7 -82 coming in.
And this bike have already been rebuilt into a "halfway" café/scrambler-racer. It is not in original shape and form, and will be a project-bike into a scrambler. (Sorry any purists out there in the XJ community)
The current owner have informed that it might need a rejetting since it does not run that great.
The bike have the original air-box removed and some conical (probably cheap) pod filters have been installed.

Before anyone give the tip to reinstall the original air-box and that this is the only way to get this one to work properly, this is not possible. The air-box does not exist. If it would, I would reinstall it.

So even if this will require a lot of trial and error to find what works, anyone with a rough idea what direction that is needed with the jets? I guess up, but what size to try first?
I will check valve compression, valve clearence also but there will most likely needed a rejetting.

 

From what I understand it's not only jets but also the 'inlet tract' that needs attention. If you can get hold of some carb-airbox rubbers that may help
as they act as a kine of trumpet - although I suspect for an uber-rare XJ400 that may not be easy.
Do you know which carbs are fitted?
I assume Mikuni BS** but what size?

 

As @a100man says it's not just the jets. Plenty of discussion here on it, but my take is that you need some kind of extension to the pods to move them away from the carb - this will catch most of the spit-back, or reversion you get from the pulsed airflow. Then you need to compensate from the slides lifting too early (some disagreement on this), but the usual thing to do is cut some coils out of the slide springs (the vmax tuners add a plastic spacer as well).
I would refrain from drilling holes in the bottom of the slides.
Then you might well need bigger idle jets, maybe bigger main jets, and or move the needles up if your carb lets you.
A wideband lambda, and changing one thing at a time would be the ideal way to do it...

 

I have got the same info that using the original rubbers from the air-box to the carbs might help. But that rubber will be a hard find is my guess also.
The 4 carbs are stock on this bike is the original ones. I have read that the XJ400 and the XJ550 is in many cases identical. So perhaps some rubber from the XJ550 in the same time-era might fit?

Yep. It can be a hard trial and error. But in the end a acceptable result. Have however read that it will not bee as perfect as the original air-box.
Either way, it will most certainly give some knowledge in the end. Even if I do not get a prefect result. And that is always good

 

I´ve found info that for this XJ400 the original Mikuni carb is BS28.
This should be the same as the XJ500 and XJ550. But this maybe some one can confirm since this is from what I have read.

 

the Bs28 was used on teh 550 for sure, might be easier to get some aitrbox rubbers for that model. Good luck.

 

To find rubbers for the XJ550 looks a little easier, but still not on every shelf.
And to try to find this type if items living in Northern Sweden is not ideal.

I need the bike first though.
The seller might not accept from what I understand so will see how that goes first.

 

This might help if you haven't found it

https://www.xjbikes.com/forums/threads/good-information-on-pods-vs-airbox-thanks-sybe.113331/

Along with the video there is a link to a discussion thread

 

Thanks Rooster53!
Yes, have found that before, but looked at the video again and also the discussion.

I kind of like the challenge of the pods, but many members of this forum that know far, far more then me on this bikes makes me not so sure anymore.
I´ve got enough knowledge to change the jets and things in the carbs, but I´m not so sure that I will get to a end result that I am happy with.
I will also come from a XJ650 that in my opinion works great after my work on it last year, but that bike is stock.
To move down to a XJ400 with bods that from the start does not work properly, that might be a big step down.

 

To sum up this thread, if someone is interested
There will be no XJ400 for me at this point.

Will keep the XJ650 (that have all stock filter box, and runs great by the way) and proceed dreaming and building up the courage to make a scrambler of it