Hope this post is not too far off base (I realize that the thread is aimed at achieving better speed through engine performance enhancinhg techniques) so these are my thoughts:

Stock my dax 70 runs pretty well....no real complaints as far as smoothness or power (given the cost of the engine)....Of course we all want better tho!

I did several speed runs (stock engine) and pulled the plug to note it's color....the color was the optimal brownish tan color (for info on color reading you can look online for various photos).....so for my particular engine the spark plug did not appear to be an issue....I have experimented with the smaller Champion CJ7 and CJ8 plugs (they are way shorter than stock and would help in tight installs (you have to change the coil wire thou since the tips on the Champions don't screw off or at least mine didn't want to and the stock wire would not slip over the tip of the Champion).....There is a plug out there that claims to increase spark power but I can't remember what it was off the top of my head....I think it was the E3 by Champion but I could be wrong....

Although I haven't finished working on my custom air intake I have seen the potential for vast improvements here (my intake would utilize the idea of ram tuning the intake charge and since it would be angled to allow the carb to easily clear the seat tube we would be able to run a better air filter (most likely an open element oiled cotton gauze unit such as a K&N Air filter....Of course with some input from Paul (AKA Cruiser) the other opportunity would be for a nice air filter housing to be made in liu of the factory restrictive and rather plain plastic unit....Better filtration makes for longer engine life and the cover may be needed since many use their powered bikes as daily transport through the rain etc...

I have thought about the boost bottle theory and I believe they work BUT I have not implemented the bottle on my intake yet since I want to optimize the runner lengths and diameters before introducing another factor (i.e. the boost bottle itself) into the equation
....FWIW by drastically increasing the intake runner length (in one case to as long as nearly 24 inches), I was able to vastly improve low end torque.... unfortunate by product though was no mid or top end....thus I have to tinker more for a more balanced solution....BUT the increased runner volume did show that the boost bottle (which effectively can be viewed as a volume increaser) should work.

For some (assuming enough clearance to put on a free flowing K&N type filter) one can simply enlarge the carb jet by reaming CAREFULLY....and enjoy enhanced performance with minimal $$.

I am also currently tinkering on a tuned pipe for our bikes (modeled for the Dax 70 in this case since that is what I currently run) and will soon post some pics here for you to see the prototype....have to get some tubing manufacturers to see what they would charge me to have the custom made header pipes bent).....I am very tempted to order Dax's new performance muffler just to see how much better it works VS stock BUT I do realize that it is still NOT the more optimal tuned pipe / Expansion chamber set up for max power / performance....But then again if it does yield some performance it is only $20!...So bang for the buck it may be good!

OF course that brings me to my last point and the reason for my opening paragraph... basically I realize that intent of this this thread is to enhance bicycle performance via engine mods economically, BUT I feel that the best overall speed improvement for the $$ can be had by regearing the drivetrain.

As a rule of thumb for each tooth off the 44T sprocket you remove on a 26" wheel you will pick up 1 MPH in speed....Not only that but you will reduce engine RPM's for any given cruise speed thus lessening engine wear and tear and more than likely improve MPG's.....like running an overdrive in a car VS screaming along without the overdrive on....well you get the idea....

As a FYI - I have run my Dax 70 on my Trek 7000 prototyping bike up the steepest hills here in Charlottesville (36T sprocket) with no problems....without the tuned pipe the
36 T will go up the hills but will bog a bit.....With the Tuned pipe 36 T combo, I can achieve 34.4 MPH at approximately 6300 RPM ( I use a digital speedo and Sen Dec electronic tach with max RPM and speed storing features) and run up any hills faster than than my other stock bike with the stock 44T.....NOTE: while the engine may be able to spin faster I personally think mine doesn't feel like she'll last very long at the higher speeds so my emphasis is on gearing higher (more speed for a given engine RPM) and developing parts and methods to allow the engine to develop more power in the Less than 6000 RPM area.

In summary, I feel that gearing higher (fewer teeth on the rear sprocket) will net the most benefit for the $$ spent....I say this not just because I manufacture custom sprockets but because I don't know of any $50 engine mod that can net you an increase in speed of 7 or more additional MPH in speed without unduly sacrificing reliability / longevity of the engine...Of course I am always open to ideas though and that is the reason we are all here.....Have fun tinkering!

Andrew