RZ250 what mainjet you running?

jools · Post by **jools** » Wed Mar 14, 2018 12:50 pm

Just wondering what mainjet RZ250 owners are running. I have the 180mj* in and it seems to be a little lean at around 7000-7500 rpm, pulls sweet and hard up until then, but then fades a bit up there. (Motor is nice and fresh), I know a plug chop is required but... any thoughts?
*I think I read that the later 250R used 155 and the earlier model like mine used 180 as standard,
correct me if I'm wrong.

Gavin · Post by **Gavin** » Fri Mar 16, 2018 6:58 am

I have a standard 155 in my ‘86 RZ250. Mine also fades around 5500, then pulls out of it. It does clear a little when it has been running for a while. The carbs have been off and on so many times, I’m just living with it as a characteristic of the bike.

Up around 7000 the ypvs should be moving. Is it moving freely and the cables adjusted correctly?

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JonW · Post by **JonW** » Fri Mar 16, 2018 9:08 am

Ive pulled a bunch of these carbs apart over the years and found a lot of 185 jets in them, maybe worth a try?

jools · Post by **jools** » Fri Mar 16, 2018 9:50 am

Interesting, I wouldn't mind knowing for sure what the original mj was, I read different opinions. Yes everything is adjusted and moving freely. I wonder if a 185 would make enough difference (over a 180) Jon, maybe it's worth a try.
But the difference between a 155 and a 180 is much more. I wonder if the later expansion chamber zorst as opposed to the earlier 'cigar' one might have made enough difference to cause MJ going from 180 or 185 to 155.
(It's an '84 RZ250YPVS - separate fairings)

JonW · Post by **JonW** » Fri Mar 16, 2018 1:42 pm

The MJ changed on the later model as they used IGA PJ carbs, these have an extra jet (the PJ) in the bowl. you add these together to get the size and in the 250s its fixed and ISTR its about a 30. Those 1GA carbs came out in 85 and were used on all the later models (1XH etc). The later bikes do all use the seperate pipe/silencers, but our 85 also uses the 51L cyl, this has a Boysen port and is pretty trick for any main manufacturer as the licence cost them money, they also use a head with the better squish design, ala the 350s of 86-on*. These cyls came out on the Japanese 84 model, but they use single pull carbs, similar to the USA models. While the dome shape lived on, the 51L cyls started and ended in 85 and the 29L design came back for the 1XH as by then Yamaha were saving money on the 250 RZ as we were one of the few markets who took them.

* - Wicked in the USA is now doing cool heads for the 250 with a choice of dome sizes, check out his Fb for more on that : https://www.facebook.com/photo.php?fbid ... 7693430658

jools · Post by **jools** » Fri Mar 16, 2018 5:15 pm

Very good detail Jon thanks. I guess I'd better do plug chops and have maybe a pair of (say) 190s to try... just a bit richer. But first I thought I'd better just do a test running on Prime pos -just in case it's a lazy diaphragm that's not allowing enough fuel flow when pulling at high rpm.
(Though people are saying that plug colour ain't what it used to be when using today's petrol).

JonW · Post by **JonW** » Fri Mar 16, 2018 10:41 pm

One thing... have you cleaned the air jet by removing the brass balls? if not start there first.

jools · Post by **jools** » Sat Mar 17, 2018 4:43 pm

So Jon. would that Air Jet affect it at WOT?

JonW · Post by **JonW** » Sun Mar 18, 2018 8:52 am

The airjet usually is the cause of the stumbling around 5500, but Im no carb expert and it could well be a problem if its dirty at higher revs. it would be good to know its clean and that everything is good in the carbs so you can eliminate them as a source of the problem. It does sound like not enough fuel at WOT, but could equally be either a stuffed stator or blocked pipe(s)... old bikes are funny systems :/

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