Hi all,

Sorry for posting that isn't about an NSR250, but does anyone know about the yamaha YZ250? I've got a crazy plan to do an engine conversion using YZ250 barrels, but I don't know about their use in a road bike.

So does anyone know how much power a tuned YZ250 makes, how wide the powerband is (1990's powervalve model). How long between changing rings etc. What RPMs they work at etc.

My crazy engine plan is to fit 3 YZ250 cylinders onto an NS400 engine, which will involve A LOT of expense - stroked crank (which will end up less than 68mm, but still workable), machined cases to fit larger stroke, adaptor plates to allow the barrels to fit, also some machine work on the cylinders, and modding the rear barrel to gain exhaust clearance. The cost is pretty high, so before commiting to it I'm trying to gather as much info as possible from anyone who knows about them.

Yes I am nuts before anyone asks! But seriously any advice welcome....

Cheers, Mat.

Can I ask why YZ barrels Mat? Do CR ones not have the same bolt pattern?
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Andy.
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There's a half finished NS400 c/w CR125 barrels on Ebay right now, see it here
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Nice link Fontyyy!
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Andy.
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ermm - because I'm nuts?

Actually I thought about the CR barrel conversion, I have 2 CR125 barrels at home, The conversion is not as straightforward as it sounds (even though the stud spacing is the same) and the YZ250 conversion give a lot more CCs. If some kind of efficiency can be achieved, it could make big power, for a bit.... or go bang... or vibrate itself to death.

There are a few people doing CR125 conversions on the NS400, but I haven't seen one running yet or seen a dyno curve. If someone builds one and it's the dogs, I'll reconsider.

I rang Dan about the one he's selling on ebay, he said it's cost him about £2500 just on the engine on his 500cc CR/NS400. For a similar outlay I want to get a 650/700cc YZ engine built if possible.

Cheers, Mat.

To get it to all fit together, I reckon your custom ignition, pipes, carbs, heads and effective exhaust port height based on engine stroke, cylinder deck/base, con rod length and powervalve timing will all have massive effects on power delivery.

Therefore if not using YZ250 carb, heads, ignition and pipes then any direct comparison is meaningless.

If you are worried about ring life with big ports, could 1990's WR250 cylinder be used? Then locate a suitable road based piston to fit with harder rings, rather than soft ones that wear out quickly.

Just my opinion, Vincent
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NSRless again...

Hi Vincent,

Yes I was planning on YZ250 heads, pistons and rods, carbs might be harder to achieve because of the angle the inlets end up at. I had thought bored out NSR250 carbs. Pipes will need to be designed as near to the right length and as big diameter as possible that will fit on the bike.

Ring life I'm not that worried about in itself - just would be good to know, it all add to the decision making process. The NS/RGV is currently road legal, but it's a track / weekend toy, rather than practical transport. The oil pump will probably have to go as well as the front cylinders will not have enough space to allow the oil pipes to come past. But I've learnt with the NS/RGV that whilst premix is a pain, having to take off a one piece seat unit to put oil in the tank is also a pain!

I figured it would be enough of a challenge to get the thing to work, I figured port timings, custom ignition etc and optimisation can happen later, as long as the thing is theoretically going to work and technically viable. And gets built correctly enough to work. I have adjustable advance CDIs for the NS, modded by BDK, but I've not played with the curves on them.

I have identified all the steps needed and have a method (and resource) to achieve each part, it's just weighing up the risk, the cost, the effort and probable end result.

I think vibration will be a problem and max RPMs will be quite limited. Porting will have to be set up to suit. But talking to the crank people it may be a case of run it and see what vibration you end up with, even at the very tight tolerances they work at.

The powervalve I'm hoping will really help, but making measurements on the frame and engine, I will have to either not have a powervalve on the rear cylinder, or have a very tight initial turn on the pipe.

No doubt I'll be round to yours at some point for some expert advice! The main thing I'm keen to avoid is a load of money spent and a non working / crap engine.

I guess the real reason for asking is - if someone says 'don't use a YZ barrel, there rubbish' or similar. I rode one years ago and it seemed bonkers fast, but then I'm not very good at riding off road bikes...

Cheers, Mat.

I for one am eagerly awaiting the outcome of your decision Mat! I love the sound of something so radical!
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Andy.
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Hey Matt,

I would check the thickness of the crank web were you will have to press in your relocated pin, you will find on 1 web the thickness is half of the other!

Also the transmission is built tougher than the nsr250 box but I doubt it would handle 120+ hp for long. Honda shorthened the engine by having all three selector forks on the one shaft making it shorter than an rgv250 engine for example, however this may also allow a little more slop and lack of precision under extreme (as 3 yz250's would be!) power shifts. They also didnt machine the grooves in the selector drum far enough resulting in slipping gears because the forks are not pushing the gears in far enough thus bending shifter forks and eventually jumping gears. With shimming I believe this can be resolved and I am putting a modified transmission on the road this xmas for testing.

The clutch seams to slip when on slightly modified bikes although from all reports the ebc heavy duty clutch fixs it. According to part numbers its the same as a 900 fireblade so It may just be the springs.

Good luck with whatever you decide, gotta love that v3.
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If all else fails - gas it!

Hi,

Good point about the clutch and gearbox... My NS/RGV already slips, I've got a set of the EBC hd clutch plates and springs, but I haven't got round to fitting them yet.

I've got a whole NS bottom end set aside for this as it is the only NS engine I've ever had that doesn't jump out of second however hard i tried to get it to jump. A lot of the NS400s with dodgy gearboxes are either the early type or high mileage or both. I can't see this bike getting a lot of miles put on it.

I realise that it's a crazy thing to do , and for what is will end up costing I could just go out and buy a reasonably recent fireblade, but where's the fun in that?

Cheers, Mat.

Anything change over Christmas?
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NSRless again...