1984 CR250 tuning info

Right I have a couple of 84 CR250 LC - both have very little bottom end and come in on the midrange pretty aggressively. They ride like an oversized 125 in my opinion. Not always easy to keep on pipe or feed the power in as they come on the pipe... The bikes are not exactly the same spec but both display these traits so im starting to think its porting related.

So can anyone throw any light on what the tuning below would or does actually do to this model? Maybe this will fix some of the issues?

Also i recently read a 30 year old test on the bike and they recomended removing a tooth from the front sprocket and adding 3 to the rear. Pretty major gearing changes. I currently run a 12 front and a 53 rear so am considering trying an 11 front? Any thoughts? The MXA write up said the bike was majorly overgeared.

thanks

11 will eat chain rollers and the swing arm pad will be gone fast as hell
I like 12 56. 56 tough to find but they are out there

On my '84 250wr I'm going with 13-55 on the gearing. It has a cr cylinder. I ran 11's on my other bikes and it ate the 11 sprockets and chains due the tighter wrap on the 11 teeth. The teeth got hooked very quickly but we rode at least five times a week back then.
You could raise the exhaust port. I like to arc it. A small amount can make a difference in port timing. I do example 1/16" up in the center and arc it to 1/32 at both sides. I polish the port to the pipe bracket. I open up the bottom a little were the pipe bracket is in the lower side of the port. The knife edge on the bottom of the vertical runners is good too. Just feather the knife edge into the port. Don't just make the edge a small vee. At the same time you need to kick the timing advanced one line too. We're advancing the port timing and increasing the flow so advancing the timing works with it. I'd go slow with little changes in porting at a time. It's going to wake up in performance.

I also make a 1/8" to 1/4" spacer to install in front of the reed block. This pulls the reed block away from the cylinder and gives more room for the gas flow.

On the cylinder base gasket check the block to see how close the gasket fits. Now lay the gasket on the cylinder. Scribe the excess material to be removed to match the gasket to the cylinder for even unrestricted air flow from the bottom end through the transfer ports.

Where the knife edge is on the cylinder area a like to remove some material and polish the cylinder side of the transfer vertical ports. Looking at the bottom of the cylinder.

Porting is about speeding up the unrestricted flow that's fast in and fast out the pipe does the rest. It's free power. Porting does make it a tad pipey but the throttle response is greatly increased. It will make the throttle response like it knows ahead of time your going to wick it the reel once is that quick. Just hang on and enjoy the ride.

Any porting to the transfer ports in the cylinder you must deburr that sharp edge where the piston rides inside the cylinder. I use a small piece of sand paper to put a chamfer on that sharp edge.

The close ratio (CR) ported bikes are motocross bikes your most of the time on it. There isn't the bottom end like a (WR) wide ratio woods bike. Your either on the cr bike or off there's no in between with the 250's. The bigger bores have a wider power range from the bottom to the top.

Porting the 250cr you may like it or not she becomes a more wild ride. To me I love burping the throttle and gears ripping her through the tight woods it's fun knowing we built it.

Thanks Bigbill! - one of the CRs is freshly rebuilt, ported (cleaned up heavily and exhaust port sorted and manifold),gaskets are matched and even the cases have had slight work . It runs very strong and lively on the pipe. Will try a bigger rear sprocket to try and keep it 'singing ' in tight corners. And yes primary use in MX


Still interested to know if anyone has used the technical bulletin for this model though? As I have a spare barrel hanging around.

I've tested my porting a little on every engine I freshened. The two bikes I applied everything on was a '85 Honda 125cr and my '81 husqvarna cr 250. The Honda ran awesome beating 250crs. The 81 husqvarna ran awesome but as we know it's the pilot too. Once the engine is ported its not for the beginner rider.

I had a Suzuki T500 we were going to build to race at Daytona. I had all the info on how to port it. One guy who raced one down under was nice enough to share his winning info. One guy followed his advice and started winning races. One trick was porting and the other was adding two million 40mm carbs.

I see your print calls for raising the transfer ports. Again this is advancing the port timing.

That is how all the '82-up 250s run, even the WRs, they are mid-top engines. The last thing I would do is raise the exhaust port on those bikes, if I did anything to the exhaust port I would widen it 1-1.5mm and work on the other ports, which if you look at your specs, that is exactly what Husky recommended.

But if you raise the transfer ports you need to raise the exhaust port too.

I remember to have seen in a service bulletin some porting on 250 cr.
It used a wr sleeve and 2 little exhaust ports were added
beside the main exhaust ( 1 to the left and 1 to the right).

Your best shot at changing your power delivery is a differant pipe.I came across a nos dyna port pipe that has a much fatter mid-section.I'm hoping to find a 84cr someday and try out this pipe.

You had better send me that pipe for testing!! Put some pics up out of interest

I know HVA have plans to produce some pipes for the 84 CR250, but am not sure if it will just be a replica of an original! Andy?

Have not seen any other pipes available for this model

No you dont, especially if the transfers are already too far advanced in relation to the exhaust. For example, if you look at the official Husky porting specs for the '79 250 you make no change to the timing of the exhaust port, but you widen it and you modify the other ports. In the official porting for the 390, you mill .5mm of the base of the cylinder, then you raise the exhaust 1-1.5mm. What you are doing there is lowering the timing of all the other ports and bringing the exhaust back to just barely over stock, a .5-1mm change. Another example is the KX100, the easy way to set that bike up is to put the 2mm stroker crank in it, but dont raise the cylinder with a spacer, you machine the head to accept the piston at TDC and use a Pro Circuit power valve that gets exhaust timing back to original. This does the same thing as the 390 Husky porting, you are lowering port timing for all the other ports and use the valve to keep the exhaust stock.

New pipes for the '84 250cr air or liquid cooled?

The last pipe I welded up was from other pipes. I sold the bike.

The bikes are LC's

There were some air cooled 250WRs early in the year I think, but later in the year all models even the WRs (except the 500s) were water cooled in '84.

yep there were aircooled 240 wr's in 1984...plastic tanked but 83 frame geometry (I think) effectively run out models till the single shock LC's arrived but issues with the frame on the single shock saw them wack a few L/C motors into a twin shocker till the single was r&d'd. mid 85. That's how I understand the transition....I am not often right and I could be wrong again!

I think thats correct. There were left over '83s 250WRs with '84 plastic and air cooled engines, and there were LC 250WRs with '84 frames and plastics, which were sold into the '85 model year until the single shock was ready. There was only one 250CR in '84 and it was water cooled.

Mmmm, the mod with 2 more exhaust holes was for the 250 cr 83.

All there mods work together. If not your like installing headers on a car without a cam and four barrel. Everything works together to improve the performance. I'm thinking of doing it all to my 250 air cooled. I'm thinking of going through the bottom end with new crank bearings and crank seals at the same time I port the cylinder to the drawing above.
Now I'm not sure if the bike is a wr or xc yet I guess it depends on if it's a sem or Motoplat with the larger flywheel.

Would installing a cr motoplat which were removing the flywheel make the wr 250 with the cr cylinder too pipey? It would be like a light switch it's on or off like a 125 but the power of a 250?

I have both an '82 250CR with the internal rotor Motoplat, and an '82 250XC with the external rotor Motoplat, both are stock and the power is not all that different, both bikes are mid-top bikes.

If you want more low/mid try lengthening the header on the expansion chamber 20-50mm. I got that from a 2 T roadrace fanatic