Hills vs. 48cc

[LewieBike]

I had a morning test of my 48cc HT engine a couple days back, we have a 400' elevation hill with a good 3/4 mile long climb that I estimate is roughly 4 percent.

First time I've ridden the bike since I rejetted it's PW50 carb. I was able to maintain about 22 mph with the throttle just under WOT. Engine is running strongly, I have the stock gearing on a 26"x1.9" mtb tire/wheel.

Note I weigh just under 158# and the bike is about 47# so this is lighter than your typical HT bike and rider. I was noticing that increasing to WOT didn't really gain any speed, but that it was knuckling under and just pulling me uphill at a clip I didn't expect.

This is the same hill my 30 cc friction drive will buzz up at about 14 mph and my 125 cc pitbike engined Honda does at about 38 mph. So splitting the difference I'm going to assume that this engine is probably not going to gain much from further work.

 

[Greg58]

There are a few things that will help, I have two 48cc bikes, one is my first bike I built in 2011. A good expansion chamber gained me 4 mph up the hill of the road I live on. I don’t know if you’ve checked if the piston skirt clears the intake port, my no name eBay engine needed trimming, that helped as well.

 

[Greg58]

Cleaning up the transfers and matching them to the cases helps too, shaving the head to increase compression helped mine, probably getting a head flat and true is needed on just about all stock heads.

 

[CrazyDan]

You might see 25+ up that hill with porting, expansion chamber and aftermarket cdi.

 

[LewieBike]

I did do a small amount of deflashing and port base gasket matching, it's not like the engine really needed a lot, as the transfers were clean and the base port match was acceptable. The big issue was the intake manifold gasket was intentionally restrictive.

I'm wanting to keep it stock. the stock exhaust is small and light, not really wanting to snake an expansion chamber though the bike's frame. The PW50 carb is about 12 mm throat diameter compared to the 14mm the NT carb runs, so any tuned pipe would probably not help that much. It also has an enrichment circuit instead of a choke flap, it's a nice feature that allows you to play with your WOT mixture from the handlebar control, using an old 5 speed friction shifter. This carb is the best investment on this bike and it was only $17.

The PW carb meters fuel much, much better than the stock NT. Especially after checking the main jet with my numbered drill set, and filling the jet with solder and drilling it out 2 drill bit steps smaller. Big change in the half to full throttle range which was 4 stroking before.

 

[Greg58]

I solder and drill jets also, the black bike with the expansion chamber is a #72 drill bit, the 24" mountain bike needed a #73. On both I'm running a nt.