Carb Size

[grumpy]

I'm thinking of updating my old Mikuni VM carb to a newer Keihin PWK carb. I know they're easier to tune and are likely smoother across the rpm range. What I'm wondering is, should I stick with the stock 38mm size or go to the 36mm.

The bike uses the std 68x68 bore/stroke but has been bored to 70x68. The reed cage may be one of the largest ever used in a production jap mx bike, it's a huge 8 pedal. With it being so huge, the bike really hits strong mid to top. I can't afford to get it ported right now and am hoping to shift some of it's power to the low end (hence my consideration for the 36mm).

Would going to the 36mm carb actually improve the low end?

[dogger315]

Would going to the 36mm carb actually improve the low end?

Switching to a smaller carburetor may help throttle response thus
improving the low end of the power band slightly, but it may also
effect (negatively) the mid and up for your application.

For "power shifters", consider a torque pipe or a VForce 3 reed valve.
To smooth the hit, try a flywheel weight and/or steel clutch plates.

You're right, the PWK is a nice move up over the VM and will provide
good carburetion once dialed in.

dogger

[grumpy]

Thanks.

I guess I should mention that I ride at 7000'+ and that I have the only aftermarket pipe available without getting a custom built one, flywheel weights are not available and I'm not gonna fabricate one plus there are no reed valves available for this application just reeds and I've got TDR in there now.

So there's very little in the way of bolt on mods to shift the power. I know getting it ported is the true way to go, but I can do the carb swap pretty cheap and figured with a little more vacuum force of a smaller carb that I stood a chance of shifting some of that power. I don't mind losing some on top. If the power shift is gonna be negligible then I'll do a 38mm as they're easier to find.

 I'm not a racer and don't plan on being one, just out having a good time is all.

[2T Institute]

Port work won't help low end power increasing the compression ratio is what you want at high altitiude

[125mx.com]

You could take 0.5+mm off the cylinder base and the check squish and work head to adjust, this gives better low-mid delivery, also advance your ign timing a touch

[SachsGS]

From the square bore/stroke and huge reed cage it sounds like you've got an early YZ250? To smooth the top end hit you might be able to retard the power valve timing slightly. With a 70mm bore I think you are at your boring limit,when it comes time to replace that cylinder you could choose another year that has smoother power characteristics.

[grumpy]

Yea, I'm on the last bore and it's an '83. It's barely at min. spec so it's got a while. I was considering re-sleeving it and having it ported, but now porting doesn't sound like the right option. I think I can change cylinders up to '87 with 0 issues. '84 & up have a smaller reed cage, so I'm guessing there's a little more low end with them. Dunno power characteristics of each year tho.

I'm thinking a 38mm Mikuni TM with a Thunder products VIP (engine side venturi splitter) might be the way to go for the carb. Should give it the throttle snap of a 33mm without losing top end of the 38mm. I experimented with a home made venturi splitter (engine side) and it was more responsive on the bottom without even making the necessary jetting changes. I think with the flatside, I won't have to go as radical on the pilot and I can get a faster air flow. I'm already at a 35 on the pilot and would need to be a bit leaner with the venturi splitter if I stay with the round slide.

So I guess it's time to research and see if I can find the power characteristics of the 84-87's.

Lol, I'm already in the process of switching to the cartridge fork & front disk of the '88, putting an '87 swinger/shock for the better linkage system. I've got plans on switching the internal rotor for the '85-'86 flywheel ignition system, so I can power lights going street legal to bypass some local bs laws that restrict riding. Plus I'm using a '92 number plate & front fender. I'm literally converting this into a bastardized bike to suite my needs.

[SachsGS]

The beauty of an older bike is that the sky is the limit when it comes to modifying it to suit your needs.I've built a few small block Ford and Chevy hot rods and really enjoy doing the same with dirt bikes. The vintage guys are very creative and I understand the appeal of that segment of the sport, you can do a lot of things to your bike without the huge expense of ,say, a modern four stroke.

[grumpy]

Yea, the cool part is that I can sell my good parts on ebay for what I buy the newer model parts for. When it's all said & done very little money outta pocket. I was gonna re spring my forks & shock anyway, why not do that to a better fork and shock. Plus no downtime waiting to rebuild, I just rebuild & put them on the bike once they're done after that I sell the old parts.