I put together a Grubee Skyhawk about a month ago on my old Columbia mountain bike. I did all the basic mods, cut piston skirt, intake and exhaust manifolds were opened up etc, along with a 41 chain as the kit came with the worthless bicycle chain. Up until yesterday, I just went on 1-5 mile rides trying to determine whether or not it was dependable and what tools and parts I should carry. Got 38 mph out of it once. On these short rides the only problem I had was four stroking almost all the time except when there was sufficient drag on the engine, inclines, hills. I adjusted and shimmed the needle so many times I could do it blind folded. Never did get it dialed in. It was either too rich, plenty of torque for hills but 4 stroked on the flats, or too lean better on flats with less 4 stroking but no torque for hills. Well, chose a happy medium and decided yesterday to go on a long ride, 42 miles round trip. It really was no fun. The further I went the more i began to hate it. This engine sounds like it gonna blow at any moment. I averaged about 15 mph on this trip due to the 4 stroking and fear that it was going to throw the rod. Downhill is agony trying to keep fuel flow so it doesn't lean out too much and a reasonable speed so it doesn't blow up. This engine makes some very scary noises for a 2 stroke and it is unsettling. The bottom line for me is this, yes the 4 stroking can be dealt with, and the engine is what it is, a cheap Chinese engine put together with kitchen cabinet hardware, and measured with a yard stick. To get my moneys worth I am going to set the bike up on a home made (Chinese like dyno) and see what it takes to blow it up. I will post a video as long as the shrapnel doesn't hit the camera. I am done with this engine. I am looking at the Harbor Freight 2-1/2 HP 4 stroke. Film at 11:00.