I would recommend you break-in the engine first, and here is why..... Keep in mind, this is just a general reply not only to you, but to anyone else who may be reading this with the same questions.
First off, since some people will be using a brand new engine, and assuming you/they have a reciept for it, the engine, or part of the engine may be under warranty. If the engine, or parts of the engine have any kind of defect, you want to find it before you do any port work. Once you port an engine, you may as well toss out any hopes of getting anything replaced from the manufacturer under warranty.
Second- Break-in can be hard enough for some people with a stock engine, much less a ported engine. An engine that has been ported CORRECTLY will break-in just like any other stocker, but the factory recommended needle settings may not be optimal, and some people may struggle with getting the engine tuned in.
Third- Once you have your new engine "broke in" run at least 5-6 tanks of fuel through it after break in, tuned for good power. Generally, fuel will leave stains on the outside of the sleeve that will help you see where the transfer ports are in the crank case, and how they flow up the side of the sleeve. Also, this staining may allow you to notice areas where the back plate shrouds the transfer ports above it. Basically, its as easy as "coloring between the lines" with your dremel tool.
A word of caution-
When you take your engine apart to port it, only work on one part of the engine at a time. Put all the parts of your engine in a sealed plastic bag to keep any metal shavings out of them, and to keep any other part of your engine from getting contaminated with metal shavings
Clean each part of the engine you are working on with brake cleaner before you start grinding on it. That way all the metal shavings fall out or off of the parts instead of sticking in the oil from your fuel.
Never, ever drill, or grind on a crank case that still has bearings in it. The slightest piece of metal or debris in a main bearing will spell disaster as soon as you pull the throttle open.
Clean everything as many times as it takes.... I use Q tips to swab everything inside and out. Its amazing how much crap you can find in parts that actually look "clean".
An engine that has been ported correctly will idle better, have better fuel economy, run cooler, and make more power.
Will they last as long as a stock engine? YES, or maybee LONGER. The thing is, a modified engine that makes up to 30-50% more power than it was designed to make, and or turns an extra 5-10,000 rpms more than it was inteded, may not last as long if the person operating it doesn't respect it.
Most stock engines dont make enough power to hurt themself under normal conditions, however a fully modded engine CAN make enough power to hurt itself. :juggle:
