1977 Jet Sizing

[Mantis Toboggan]

Hi All,

Newbie here. I'm getting ready to do my first ever carb clean/tune/sync and need some advice on jet sizes and compatibility. I recently acquired a 1977 XS400D (Mikuni BS34) which came with pods but still has the stock exhaust. After spending countless hours over the past week reading the tech articles and postings on this site (which is amazing!), I believe I need to increase the main jet size to 147.5 from the stock 142.5, although I am not 100% sure. Can anyone tell me for certain what jet size is required and if any other jets would need to be increased?
As far as main jet availability, I've looked through mikesXS.com and haven't been able to find much. Are the early 80's xs650 jets compatible? Are the carbs between the early 80's xs400 and xs650 compatible with the 1977 xs400? Is there another trusted site where I can find jets and other parts?

Thanks in advance!

 

[willem]

Hi and welcome! we like pics around here

The combination pods + open exhausts usually requires about 2 sizes up on mains, 1 on the pilot jets. But with only pods, you may be able to still get your mixture right by adjusting the carbs, or going up 1 size on the main jets, or 1 on the main and 1 on the pilots. But there is no way to be sure because it depends on a whole bunch of parameters including your altitude, what kind of pods, how your float level is set, any tiny leaks in the carbs or exhausts, in what state your baffles are, etc etc

only way to really tell is to pull your plugs and read them. Then adjust with the idle mixture screws, figure out your performance in higher revs (just try and feel) and if you need more adjustment change your jet sizes. It's a trial and error process.

 

[Mantis Toboggan]

Thank you for the reply, Willem! I figured it would be a trial and error process, I guess I was just hoping that there would be a straight forward solution based on my setup. I think I'll just get one size up of both main and pilot jets and start from there with the mains first.
Picture to come!

 

[xschris]

I would start with the pilots because that will help the bike to idle and be able to take off at the line. The mains are for 3/4 to full throttle.

 

[Mantis Toboggan]

Other than MikesXS, what is a good source for jets and other parts?

 

[xschris]

Your local Yamaha dealer. ebay.

 

[Mantis Toboggan]

Has anyone purchased jets from PJMotorsports? Is their measurement system reliable?

Will check with my local yamaha dealer tomorrow...hopefully they have in stock.

 

[xschris]

I have used them before to get a set of hard to find 77-79 air jets. No issues

 

[Mantis Toboggan]

Thanks! Took apart the carbs today and found out that one of the idle mixtures screws has its tip broken off in the carb body and most of the jets and screws are pretty stuck. They are soaking in pine-sol now and hopefully after a good cleaning and dowsing with penetrating liquid they will come free. As for the stuck screw/jet tip...is there a safe way to push it out or do I need to drill it?

 

[xschris]

Try here http://www.xs400.com/forum/showthread.php?p=121303#post121303 Post number 1906

 

[scott s]

I don't see jets listed in the XS400 section at MikesXS. Are you guys using the ones from the XS650 section? The ones for the '80-84 Mikuni 34mm CV carbs (XS650)?

I don't see pilots listed, either.

 

[bcware]

The Main Jet #XXX - Large Round Mikuni Type Yamaha XS650 fit, yes. (#115 through #160)
The XS650 #XX Pilot Jet - BS30/96 Type Mikuni BS34 CV carburetors also fit, yes. (#30 through #50)

Read carefully when choosing. Some look the same.

 

[scott s]

So it should be:

Pilot Jet #45 Part #48-0607

Main Jets
145 Part #48-0123
147.5 Part #48-0124
150 Part #48-0125

 

[bcware]

Yes, those are correct.

Just be aware that those are probably just MikesXS part numbers. For the pilot jets you just want to be sure you're not ordering the VM type, because they look very similar.

The main jets look different enough so that you can't really confuse them with anything else for sale on MikesXS anyway.