MOPED ARMY

Well the vm20 is nice because that main jet swap is so easy. That's a big carb for the stock bike. If you want to go with that carb, why don't you get a pipe first. maybe start with an 80ish jet in the stock carb. Get that tuned in and when you feel ready for the next step - go with the bigger carb. vm18 is nice too but doesn't have the easy main jet swap.

The pipe will get you much more speed initially than a new carb. (edited)

Sounds fricken dangerous too me :)

If you don't increase the amount of air the engine consumes, you will create a lean condition because you increased the diameter of the carb. The carburetor's venturi reduces the air pressure inside the carb, so that atmospheric pressure can push the fuel (from the float bowl) and air (into the carb) into the engine.

But if you don't increase the engines demand for more air, or find a way to increase air pressure, the pressure inside the carb will increase, and less fuel will be pushed into the carb, and that is how the lean spot is created.

It has been 100 years since I worked on a QT :) They have a great stock exhaust pipe, and it works well the stock cylinders porting. A tuned pipe does not, or at least in my opinion, the power gains of a tuned pipe are not worth the money unless the exhaust ports blow down time (BDT) has been increased to 21 to 23 degrees. 18 degrees of BDT is the absolute minimum, but I wouldn't waste my time building or selling a tuned pipe unless the customer knew that changing BDT was essential to the performance of the engine.

I don't know these companies, but if you are under the opinion that you can bolt a pipe on to any stock cylinder and get decent HP, that isn't true.

With a stock cylinder, you are better off with a 1/2 pipe that only improves the engines scavenging abilities.

Bill M.

more mlm circuit pipe goodness

Blowdown would not change as all ports were raised 1.5mm (the same amount) by the 1.5mm spacer although, come to think of it, since the purpose of the spacer was to expose more of the exhaust port (as the piston covers quite a bit of it at BDC), the spacer may actually have increased blowdown. I removed 1.5mm or thereabouts from the top of the cylinder. Just using stock head so squish would be the same assuming I took off the perfect amount from the top of the cylinder - which I probably didn't. If anything, you err on not taking off enough so you don't have any issue with the piston hitting the head - the minor loss of compression is actually helpful in fighting high engine temperatures. so if you lose 10 or 15 psi, no big deal.

You can always shave the head down to regain that lost compression if it's keeping you awake at night or take a little more off the top of the cylinder.

Anyhow, I haven't even tried to widen the exhaust port yet. I found a machinist who charges me $20 a cylinder to take material off the top of the cylinder. Beats the hell out of spending $200 on a bigger kit and matching head and I could probably get it to a top speed similar to that $200 kit. (edited)

Thanks for the advice. I've ordered the mlm exhaust.

material was taken off the cylinder. Extra gasket was accounted for. see my previous post. This is not my first rodeo.

Simple pipe, 14.12 SHA and a little exhaust porting. 40s.

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