Discussion ZXR250A Vs ZXR250C

Hi All,

In the last few months I have become immersed in trying to determine what (all) the differences are between the ZXR250 A model (1989-90) and the C model (1991+). Why? It's a long and often boring story. Reader's Digest version: Building a drag bike that started life as an A model and due to a dud engine now has a C motor in it. Net result...runs like ****!

All along I've been told that the engines in all of the zxr250 models are the same and the only "real" difference is the CDI units. A different CDI unit was apparently introduced around 1993 to restrict the engines from their original 45hp to 39hp. There are 3 CDI units across the ZXR250 models that I can personally (visually) confirm...21119-1263 in the A model and 21119-1322 in the C model and another one that I forget the code for. The A model CDI has a 6-point and 4-point plug: the 6-point plug only has 4 active points. The 4-point plug also has 4 active points. The CDI from a C model (21119-1322) has 5 active points in the 6-point plug (and 4 in the 4-point plug). The wiring diagram that's floating around the web is for the C model...5 & 4 active points. I'm not a sparkie, so I have no idea what does what in these things (yet). I'm trying to contact a CDI testing centre to ask them for actual technical details for each unit.

The main point of this post is that today I was told the A model engine is different to the C model engine. The C model engine is basically the Balius engine. I need someone to confirm or deny this. From what I've read/discovered, the A model engine has a bore and stroke of 48 x 34.5 and the C model has a bore and stroke of 49 x 33.1

Many engine parts are the same from the A to the C model, but apparently the barrels and pistons are not.

I will keep this thread updated as more information comes to hand.

Sorry if someone has already covered this in another forum post. I looked but couldn't find anything.

Cheers

Frank

 

Hi All,

Update to my previous post. The zxr250a engine IS different to the zxr250c engine.

I'm trying to confirm if the carbs are configured the same...I've heard/read main jets 122,125,125,122 in one model and 125,128,128,125 in the other. The zxr250r is different again (bigger carbs - 32mm).

Cheers

Frank

 

Chris,

If you're asking "what's different about the engines?", then the main difference is bore and stroke. Thanks to Doug for confirming there is also a difference in the carb main jetting is from the A (125;128;128;125) to the C (122;125;125;122). On top of this is the different CDI's. I will post some source info in response to Doug's request.

 

Hey Doug,

I have uploaded information from the Japanese sales brochures. I don't read Japanese, but I can tell what the bore and stroke is for the A and the C from these documents. Also, thanks for confirming the main jet sizes are bigger on the A. Interesting side note is that the C model has 45HP even with different bore/stroke, main jets and CDI. The restriction (to 39HP) might have come from a further CDI change. Does anyone on this site know how to configure a CDI? The A model has a slight bit more torque and also makes it's 45HP at 1,000rpm less than the C model. Given the main jets are different it would be good to know if any other jets are different from the A to the C carbs.

Regarding my drag bike running like crap... it's always had a problem revving. It revs cleanly, but it won't rev past 16,000rpm. My mechanic says it needs more fuel (we're running 125 main jets on all four cylinders). He might be right, but we don't have a baseline (he never ran the engine in "stock" configuration) to measure changes against. The mechanic has also stripped the wiring out of the bike and run basic cabling to support running the engine. He's managed to use the CDI from an A bike with a C engine (picture is on this site with one of my other posts). The carbs are of an A engine as well. The drag bike runs on battery power only, so there's no alternator. The engine has very good compression. We run a straight thru exhaust and a standard airbox with no filter.

I have 1989 A model street bike that I'm going to take to the strip and see what she runs. It weighs 30kg more than my drag bike, so it will be interesting to see what the outcome is. I will post as soon as I have details.

Cheers

 

Murdo,

Haven't done that. Might be part of the issue. Are the pickups different from the A to the C?

Personally, I don't think there's too much wrong with my engine (or carbs), I think the problem is electrics based in one way or another. We'll figure it out sooner or later.

 

Doug,

The CDI from an A a different number of points to one from a C. My mechanic is a whiz (apparently) at electronics, so I assume he knew what he was doing. I may have to get a wiring loom, CDI etc from a C model and retrofit it onto the bike to see if that fixes the issues. Please let me know when you have checked the timing. Between your mechanic and what Murdo and Chris think, we might be on the right path.

Your comment about running "on the battery" is well taken. I have asked my mechanic to put the alternator back in the bike so we can eliminate that as a possible source of the not revving issue. It's difficult to know as the standard loom is long gone and I'll be relying on him to connect it correctly. I had read somewhere that the CDI draws from both the battery and alternator, but I've never had that confirmed or denied. We run the bike with the battery only for weight saving. Waste of time if the bike doesn't run properly!

 

Chris,

I've seen this issue on heaps of bikes. the RC17 CBX750F has the same engine (same bore, stroke, carbs, exhaust etc) as a later RC45 engine, but they use different pickups and CDI. End result is different firing order and 25HP less! BTW, the cams are also different in the RC17.

Let us know what you find. I feel we're on the right track to resolving these issues.

Thanks

Frank

 

Chris,

I don't have A model harness. I have A model CDI, Coils & Carbs. The original harness was ripped out and replaced with very basic wiring to allow the bike to start and run. There's no ignition switch...to start the bike I flick the kill switch to RUN and hit the starter button. That all works fine, so the basics are right (I assume!).

 

Hey Murdo,

Still have the drag bike? What was your PB?

We run one of those trick racing Lithium batteries. It weighs 400grams and has over 200 CCA. It does eventually go flat, so I stick it on a trickle charger between runs/rounds. It's funny when you pick up the battery, you think "the bastards...they sent me just the shell!". It's that light.

Having said that, I still think putting the alternator back in makes sense. If anything should go, I would think it would be the starter motor.

Cheers