Installing a 390 jug on a 250?

When I started porting cylinders I did one change at a time. With each change that was added each engine ran better. My husky '81 250cr had all the porting changes in one engine. This 250 was an animal. If you had the experience and stones to push it. Now I'm thinking of porting the cylinder I'm going to use. Now what if I add a 40mm carb, boyseen reeds and an after market pipe. Advance the timing a tad and off we fly. I think a 400cc would do nicely too?

mine has plenty of go! did u check out the harrow vid? front tyre has helium issues

The 1980 390 cylinder has the matching cylinder stud pattern that would allow it to be installed on an 82 or newer engine. The cleanest would be on a 430 lower end and that would give you a 400 aircooled as mentioned by Kartwheel.

I would have tried it but the 390 cylinder I had at the time was 1978. The one I have now is 1979 so that still leaves me out on that one anyway

I think a 390 cylinder on a 430 crank with a 250wr tranny using the bigger cr front 14t sprocket so we don't over rev it with lots of porting would be the hot ticket. In the past all of my 390cr' s had larger front sprockets. Since these big bores had a larger envelope on the power band the larger front sprocket never hurt the lower end of the power band.

My guess is a 400 cylinder on a Mag lower - the best of all worlds. Do tell us more!

The 250 transmissions are the same as the big bores, only the primary gearing (crank gear and clutch basket) is different. I use the 250 or LC400 primary gearing in all my 430s, they I start much easier. If you use the 250 primary gearing in the 400/430 you have to use the '84-up clutch plates that allow an extra plate in the stack.

The cylinder showed up today and my gut feeling was right she's a 390 cylinder. I'll break out the inside and outside Mic to see what size it is first. I can hone it to clean or bore it. I have everything here. I knew when I seen the head it wasn't a 250.

No - We make new crank cheeks at 72mm stroke....

Andy

Ahh...OK, I misunderstood.

I need to look for a donor bike now. But with winter coming soon I'll have plenty of time to look.
The 390 cc cylinder with the compression release sounds good so far it just depends on what crank.

The 430 has the smaller fly wheels and the 360/390 has the heavier larger fly wheel. Do you think the heavier fly wheel makes the 390 such a ripper? The rear wheel is always spinning.

The 390 has the heavier flywheel because the crank is a "T" style crank and is much lighter than the 430 crank. The 430 is considerably faster than a 390, the type of power is the same, the 430 just has more everywhere.

I understand what your saying the 390 has the "T" shaped counter balancers while the 430 has the full circle crankshaft. So the weight is still there it's just on the inside rather than the ignition flywheel. The 390cr is one of my favorite rides. The 390 is a wild ride while the 430 is like a tractor just point it were you want to go and wick it. My 86 400 wxc was also very fast. Maybe the 400cc had the best of both worlds. The wild ride plus it was very controllable and tractable too. I liked them all but I mainly rode the 390.

Now as we go bigger with the bore x stroke we lose the rpm's off the top but we gain in bottom end torque. Plus the power band is much wider. I have both but can't. So choosing the 400cc setup she's in the middle were I have a mix of both worlds. Someday s we want to ride in the slow lane and other days we hit the fast lane.

Right, the total weight of the rotating mass is what counts.

And balance is important as well to quell the vibrations big bores are capable of

Yup the 430 crank requires the xc or cr ignition while the 390 requires the larger Motorola flywheel.
I can't mix and match the flywheel due to upsetting the balance other wise things will vibrate apart.

Quick question. I have an 82 250 wr . If I understand correctly, I can do a 430 crank and cylinder on the 250 cases?

There is a cast in web of sorts to reduce the crankcase volume that would have to be cut out. The cases for the 400 (applicable era),430, and 500 are the same, two stroke standard shift models. I think 83 and up the intermediate shaft for the starter gearset has two little bolts to hold in in better.

this is a 4 year old post and Bill lost me on the Motorola cell phone /walkie talkie ignition flywheel crap

I removed that wall that separates the 250cc crank from the larger 400/430/500 crankcase. I made a 430 from a 250 case.

Not a 430 cylinder.. looks like a 400 or could be a 450 cylinder.. but with an aftermarket reed..