Converting 1980 CR250 engine to a CR420 fire breather!

Alright Gents, I know someone out there must have tried this...

I have this very nice 1980 CR250 pretty well sorted. It runs and handles great. The problem is all my other bikes are open class machines and I find the 250 to be a little short in the power department.

I've done a little home work and it seems to me that I should be able to convert this to a 420.

The 1980 250 appears to share the same cases as the 1980 390 and uses the wider cylinder stud spacing that the Post 1980 390/420 and 430's use.

I also notice that the 390 and 420 share the stroke which makes me think I can use the 390 crank.

I happen to have an extra cr390 crank and also came by a pretty nice 420 top end.

My plan would be to use the 250 cases, the 390 crank and the 420 cylinder. I also understand that the primary gearing is a little different for the 250 vs the 390 but I may even have that setup also.

My question is has anybody else tried this. Am I missing something. Were you happy with the result?

Thanks,
Kevin

I believe it would be better just to get whole lower end with a set of gears clutch etc. First I am more of an 81 to 85 bike restorer than
1980s or earlier.
You want the bigger shaft gear set that come with the 1980 CR. Also to consider is Primary ratios of clutch and primary gear must be different
just as the Mid 80 models.

why not just drop an 83 430 motor straight in?

There is no reason not to try if that is what you really want. I have been working on a 1980 250CR for a friend on the Yahoo forum. It made me wish I had a 1980 390 engine instead of the 1979 390 I do have sometimes. I mated the left 250 centercase to the right 390 centercase and they matched up exactly exceper that the stud pattern on th 79 390 case is shorter front to back. I like the fact that you must have a set of Husky triples bored out to 43mm like I do. Cause the same front end is going on one of my personal Huskys.

Hi Gary,

Thanks for your response.

Not sure I follow your comments fully. The 250 is a 1980. I would be reusing the 250 cases and gearset.

I have a spare 390 clutch and primary gear so can use that as well.

I have a 1981 430 primary kick motor on the bench now, that one is a lot different than the 1980 motor. I would stick with 1978 thru 1980 parts.

The reason to do this is because I have most of the parts on hand in my Spares pile so cost would be low. It would give me some extra beef, especially at the low end. The 250 is pretty pipey.

Thanks,
Kevin

Can't seem to find many of these around anymore for a reasonable price. I 'm guessing all the cafe husky guy have them all under their benches :-)

I'm just as guilty but with Yamaha motors!


The 390 would even do the trick, its just that I happen to have the 420 top end. I do like the rest of the bike.

BTW, I have a 430 also but not finished with it yet.

Yes, that is what I was thinking. With so many parts shared it seems fairly straight forward. Makes sense for me as I have the needed parts already.

On the front end, I am running a yz490 43 mm fork, yz triple clamps and yz DLS front wheel/brake. I machined bushings that allowed me to reuse the husky steering stem in the Yamaha clamps and with the original husky timkin bearings. It's a beefy setup and still vintage legal.

The front end/wheel change over made a big different in handling. That 35mm fork was pathetic. Rear wheel is 18 inch.

Its a very solid bike but needs more steam.

I am going to extend the stem in the YZ triples I got for the IT490 forks I bought from a friend about 7 years ago. That way I can use the front end in both potential builds; A Husqvarna framed HL500 and a personalized 83 Husqvarna 430WR.

dont forget the 390/420 runs a larger drive sprocket I believe it's a 14T, the 390/420 has so much bottom end pulling power and torque a smaller drive sprocket makes it over rev and wind out quicker, I tried my first 390cr and changing the front sprockets looking for the sweet ratio only to go back to the stock 14T. She's a totally different ride than a 250. Anyone going to a 390 make sure the output shaft is the larger one with the larger splines. I ran the '78 390cr hard and never had a problem with the smaller shaft and spline but when given a choice I'd go with the larger shaft/spline.

I became a power nut after I rode my first cr390. My '81/250cr stayed parked after I rode the cr390.

Even the prices on partially assembled husky engines are expensive now( not complete) don't see many complete engines.

Kevin

Kevin,
Yes you are correct. This is an easy conversion for the 1980 250/390
You will need the 390 crank, 390 magneto, 390 primary gears, 420 top end and cylinder studs, 390/420 pipe 420 axc pipe or do the Husky 390 pipe mod for the 420. Rejet the carb.

Marty

Thanks for this Marty and to all others for their replies, much appreciated!

OK, yes it looks like I have most everything. Saved a pair of rotted 390 cases so should be able to use the studs out of that. I did not realize the motoplat was different between the cr250 and cr390 so will have to check my stuff, may be short on a 390 motoplat.

Ive got the 430 to finish up and a maico magnum 490 that is nearing completion. I will want to finish those before tackling the 250==>420 conversion. It will probably be late fall or winter before I can do this.

As a side note, the bikes I ride tend to fall into the 1979 to 1983 range and all have similar capabilities. I figure this way I can ride them at similar speeds, etc without a big adjustment.

I like the earlier short travel bikes too but have thought it would be too easy to overcook a section if coming off a long travel bike. I know many do it though.

Here is a 78 CR390 I redid a few years back. Good power but suspension falls short. I want to keep this one stock though (except for green tank, that John Deere green looks good on the farm!).

Again, thanks for everyone's help!
Kevin

I liked the 35mm forks on my 1978 390 Auto as they were 10" travel front & gas Girlings rear. Nice balanced bike and cornered like a cat on carpet. The 35mm forks got wiggly when hard cornering with 12" travel.

The carb is different needle, needle jet wise too it's probably easier to pick up a 430 carb. Message sent, bill

Yes, good idea Bill. I do have some old 38s and a good size jet box though... Collected from jetting 465/490 Yamaha's! Lol!

There's something special about the big bore early crankcase case design. These wind up quicker to me. I wonder if there is a bigger difference between the 390 and 420? There's less stroke and less pork chop weight in the crankshaft. There's a larger ign flywheel weight.

390,,,,,71mm stroke,,,,,,82.92 mm bore. Older case less stroke faster windup?

420,,,,,,71mm stroke,,,,,,85.92mm bore. Older case

430,,,,,74mm stroke,,,,,,,85.92 mm bore newer case, Slower wind up because of more stroke?

To me the 390 when she's running correctly stick it's a sick little ride.

How about installing the thunder products UFO, give us a before and after report.

good lord

Actual lold

Omg, ?

There is a such thing as excess power. Balance between hp and ability to utilize it effectively is the ultimate goal. Not mere 1/4 mile straight line sprints.