RZ250 / RZ350

[BRG1200]

Just cleaned up some 250 carbs and got onto the head, made some comparisons between the 250 and 350 heads.
The 350 head has a wide squish band and a very domed comustion chamber.
The 250 has virtually no squish band at all.
Hunh?

I hear tell the 250 is quite a perky motor, and I can see the inlet side is very different. To be honest the 350 seems better in all details.

What's going on with the absence of a noticable squish band on the 250?

[arrow]

Is there the possibility that the 250 head has been modified at some time in it's life?

[BRG1200]

Really doesn't look like it. Have a couple of others, will compare.

[JonW]

Which head codes are these, i'll wager the 350 is a 31k and the 250 is a 51L?

[BRG1200]

Yeah, bigun is a 31k, will go have a look at the 250 later tonight.
What are your comments on 250 heads?

[BRG1200]

29l1

[JonW]

comments... hmm... 51L is the one for the 85 cyls, rest are 29Ls AFAIK.

[BRG1200]

Well there you go, the 51L head has a scaled down version of the 31K squish band. 29L doesn't have a noticable squish band at all. Weird.

[JonW]

hmm... we need pics... the 51L should look more like the 1UA, the 29L should look like the 31k... or my memory is slipping... :/

[BRG1200]

I must get botophucket sorted out.

Quickest descrip is the 51L is virtually identical to 31K head in shape, but smaller, whereas the 29L head is basically a dome with a radiused edge, no squish band to speak of.

Both sets of barrels are on the bench too, no immediately visible diffs, but haven't measured ports yet.

Ignoring porting I can see how the heads might make a difference to the performance, will measure capacity for comparison.

My recollection and observation is that 31K heads turned up on later bikes, using up stock I imagine, so there's a blur factor between models.
The last of the Aircooled Daytona's with earlier ignitions I think would be another example of clearing out stock. Baffling choice otherwise! This subject is like mechanical archaeology.

[JonW]

To be honest you would just the use the best of what youve got condition-wise, youre not really going to notice the 3ish bhp difference the later RZ350s made over the earlier ones in everyday use.

[James W.]

To be fair Jon, the later RZ 350s did put out a bit more than that.. & right across the 'powerband'..
http://www.rd350lc.net/Magazine/MCI0986-5.jpg

[JonW]

Dynos and mags... hmm... officially 31k is 59, 1UA is 63.... I make that 4bhp... "3'ish" was my guestimate... I'd say that i was quite close. If that dyno read at the wheel on both days then id be blown away that the 86 model made what it did. It looks like one was crank and one was at the wheel to me. LCs were 47/48ish at the crank, not the ypvs bikes.

We should remember journalism in the 80s was make do and mend really, some mags made claims like the 85 swingarm was alloy, this one claimed the switchgear was the same between 85 and 86, which it's not. We all know the squish is different and the cyl was reworked but I think the real difference with the 86-on models was the pipes which work better than the 31k style. But, yam cheaped out and while the PVs should have been reprofiled to take into account the 1mm changes in the cyls, yam didnt bother to make new ones to capitalise on that change. Yes the heads are different, yes the wheels are lighter etc etc, but really its an RZ350, most people's bum dyno wont detect the differences. More power is always desirable, but my 2c is that working with better cyls you have when on a budget is the clever option to get a runner etc.

[James W.]

Well Jon, since the article states it was done on the same dyno.. I guess you're duly "...blown away..."

I've owned a few 'stockers' of various model RZ's ( & at the same time) over the years,
& for sure my 'bum dyno' can tell the difference, ( & all with good comp/tune)
its so obvious really, since that torque boost allows taller gearing, without losing pep..
.. so will let you cruise at freeway speeds under the PV opening zone,
& yet run well past the 200km/h mark in top..