Reed Valve vs. Piston Port Carburetor Jetting

On Sat, 01 May 2004 02:23:03 GMT, Rick Cortese
<ricortes.TakeThisOut@earthlink.net> wrote:

 >Just finished bolting together a 'mostly' 1974 Yamaha DT360. One of the
 >parts I used to replace missing Yamaha carb was something off a 70's
 >Suzuki TM250. Huge carb anyway, looks as big as one on my TM400.
 >
 >The thing runs way to rich at idle and idles way to fast. Idle air screw
 >is just about all the way out to keep the plug from fouling and at least
 >get it to smoothe out a little. Slide is dropped all the way down.
 >
 >It went through ~10 oz of mix in the time it took to put a timing light
 >on it, check for the advance<wasn't advancing at high RPM for some
 >reason>, and do a quick carb adjustment. Bight side is it is so rich, it
 >is pretty much a one kick starter w/o the choke on.
 >
 >Has anyone had experience rejetting going to a reed valve? I'm hopping
 >it isn't some other gotcha like floats since I triple checked it and
 >used new parts for the valve.

I'm guessing it's a round slide Mikuni?

0 to 1/8 throttle; the pilot jet and air screw controls the mixture

1/8 to 1/14 throttle the slide cutaway controls mixture

1/4 to 3/4 throttle the jet needle jet controls mixture

3/4 to wide open the main jet controls the mixture.

All these ranges overlap. and the pilot circuit dumps in fuel across
the whole range. A way too rich pilot circuit can mask a too lean main
jet, be careful. Too rich a mixture in any one range can mask a too
lean condition on either side of it.

With the right pilot jet, your air screw should be between 1 and 3
turns open. Right now you're too rich.

Start at the bottom end and tune for a smooth idle and good throttle
response up the scale. If the old carb and the new one were both
Mikunis and the new one is just bigger, you can use the old jetting as
a guide and just make everything a little richer across the board as a
place to start.

This where everyone's eyes start to roll; I like to use an exhaust gas
temperature gauge to jet an engine. My experience with these is solely
street and road racing engines so I don't know how effective it may be
for a dirt bike. Find a place where you can run the bike wide open in
top gear. The EGT will respond as quickly as a tach and it will tell
you what's going on with your mixture. Once you have good throttle
response off the bottom, run the bike at full throttle in top gear and
observe the max temp. 1100 deg F is usually the magic number, but be
very careful approaching this by doing plug chops and looking for
signs of detonation and the state of the mixture on the plug. Once you
have a good main jet, get the bike up to full throttle in top and very
slightly roll off the gas observing the EGT. If you see the temp spike
upward your jet needle/needle jet combo is too lean. I've seen road
racers seize and crash in just this situation. because they had no way
to observe this problem.

It is a characteristic of these carbs that, at full throttle, when you
have a main jet that's just right at the max rpm it will be a little
too rich at the bottom end of your power band. That is, the lowest RPM
that you can use full throttle.

Having said all that, a dirt bike just doesn't get ridden that way. If
you're a little too rich, it's okay. Those last few precious top end
HP so important too a road racer just aren't as important on a 30 year
old dirt bike. As long as it has good throttle response and the plug
looks healthy, you're okay.

Pressure check your engine when you put it together, air leaks will
make you gray before your time, trust me. When you're finished with
this you'll know way more than you ever wanted to know about carb
jetting. If I got a dollar for every hour I spent f***ing with carbs
I'd be a rich man now.

Bill Smith

Disclaimer; All this applies to two stroke engines, mostly Yamaha road
racers and street bikes of 70s vintage, your mileage may very and be
very carefull approaching limits. A little too rich is safer than too
lean, always approach from that direction.<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Reed Valve vs. Piston Port Carburetor Jetting