Jetting advice for 400 with Yoshimura on it.

[Uncle Bob]

The 400 carbs are working nicely upto about 7/8k rpm, then it stutteres and won't rev out, if accelerating, but seems to hold the revs once there.

Fossie has pointed out that the first place to look at, is the air filter and if aftermarket pipe fitted, then jetting.

The bike came with a Yoshimura full pipe on it (probably fitted in Japan) and the jetting is still with the standard #85 main and needle at standard 3rd groove.

As I haven't ever rejetted for a pipe before some advice on initial values would be handy. Is this something I can get near enough myself or will Dyno time be better (no idea of cost though).

So far someone has said for a performance pipe, go up one main jet size (so #87.5 in this case and raise the needle by 1 notch). How does that sound. Any advice welcome. Then run and check with a plug chop (never done this before either so advice welcome).

I guess you just have to buy the jets and if still too small buy the next size up. I was looking at around £12 a try:
http://www.ebay.co.uk/itm/Motorcycle-Sc ... 1303096201

Cheers

Rob

[johnr]

im assuming that the inside of your fuel tank is spotless and your carbs have been stripped and ultrasonically cleaned first?
7 to 8k does sound like where the main jet takes on the bulk of the fueling as opposed to the needle jet position, but if the holes in the emulsion tubes are blocked or theres a blockage in the main jet then it will pop and fart like an old lawnmower!
or, you know my opinion on 400 carb issues.

[Uncle Bob]

I'm sure all is OK - ultrasonically cleaned and checked with new Viton O rings, and after your last comments I then stripped each float assembly out, cleaned the float valve etc and checked float heights. Does run great upto the 7/8k mark. I think Sparki said that with a performance pipe up one size main and raise the needle. I just need somewhere to start with from some people who might have been there before (on other bikes probably).

I do like reading your comments when something 400 related comes along though...!

[johnr]

as an aside, have you checked that your ht leads are tight in your coils? mine ran better when i took the ht leads out of the coils, snipped half an inch off then screwed it back in.

[Uncle Bob]

I'll give them a check over. I was hoping it wasn't HT related or CDI (esp. CDI due to I guess impossibility in getting another one). Once I got the carbs back on it wouldn't start and I thought no sparks (plug out check, though think that might have been bad earth). I pulled connector off CDI box and put back on and it fired up (on 3 cylinders as one plug was still out) straight away, which made me jump somewhat as I wasn't expecting it! As bike was off road for 5 years I thought (hoped) it was just a bad connection.

On hols at moment so will check when I get back.

Cheers

Rob

[pookie]

[quote="cubeconn"]I'll give them a check over. I was hoping it wasn't HT related or CDI (esp. CDI due to I guess impossibility in getting another one).

There are some chinese guys on e-bad that sell replacement cdi units for both the 250 and 400 kats, you give them the serial number on the unit and about £80 and they send you a shiny new cdi, i've got them on my watch list just incase mine goes pop.

[johnr]

iirc fossie had one of these when they started making them.

[Uncle Bob]

Cheers - I had found some pages referencing them but nothing for sale. I might ask for a link at some point...

[Uncle Bob]

Thanks to advice from Fossie who diagnosed the issue and Sparki on what to do mixed together with a little Google and I put 90 mains in up 2 sizes from the 85's and it now revs cleanly through as expected. I got geniune Mikuni mains from http://www.motocarb.com in the end.

[sparki]

good news then!