robbie
New member
I did a speed test yesterday with my son riding the mini bike and me following in the pickup truck. I was impressed enough that I thought I should post my results here, for whatever it's worth. I know most folks who are inclined to buy a new engine at Harbor Freight will probably get the 6.5 for only a small amount of extra money. I can see myself doing that in the future, but there are 2 good reasons why you may want a 2.5 instead.
The first reason is that maybe you are putting an engine in a small bike frame that doesn't have enough room for a larger engine. Remove the tank and muffler, and the 2.5 has a very low profile. It's less than 10 inches from the mounting plate to the top of the spark plug. This was a pretty important factor on our bike. The 2.5 wouldn't fit with the tank and muffler, so I'm pretty sure a bigger engine wouldn't fit at all.
The other reason you may want a 2.5 is for a kid's bike. Conventional wisdom says that for a kid you get a big engine and gear it down to go slow at first, then change sprockets for higher speed after experience is gained. The disadvantage to a big engine with slower gearing is that it also increases the torque at the wheel, resulting in death by wheelie for an inexperienced little kid. A 2.5 geared correctly will give good performance, but no chance of unwanted wheelies or sudden loss of traction due to peeling out unexpectedly.
So, here's the review with the bad first and then the good part. I've discovered four unfavorable points about the 2.5:
1. The fuel tank is junk. The bolts unscrew themselves, the tank vibrates, the tabs break loose and cause cracks, then the gasoline runs out. Use a separate tank.
2. The air cleaner is junk. Metal nuts mounted in plastic will pull through easily. The design inhibits flow and I don't think it cleans the air as well as it should.
3. The stock exhaust is too restrictive, and none too quiet. I built a 2 foot 7/8" ID pipe with an 8HP Briggs muffler on the end. It's not much louder, and it increases performance noticeably.
4. The carburetor gets fouled sometimes. I think it got fouled by my dirty used fuel lines, and again by my dirty junkyard gas tank. Sometimes it just gets a bad attitude and I have to take the bowl off and clean it. Between two engines I've had to do a bowl-cleaning maybe 3 or 4 times since early summer.
Now here's the good part. Once you set it up correctly, this engine really goes. I clocked my kid at 20mph on a gravel driveway. That's a 1/3 mile driveway with a slight hill. I clocked him uphill and downhill, then took an average. Also, we went about 1/4 mile on the flat gravel road outside our driveway. The result was consistently 20 mph, which is almost too fast for 11 inch tires on gravel. It feels more like 40. But it's just right for teaching kids to handle a bike.
With a gear ratio of 9.18:1 and 11 inch tires, the engine had to be going around 5500 rpm to go 20 mph. A tachometer would be better, just to make sure the numbers are accurate. I don't have a tach, but Santa Claus might bring us a radar gun this year.
I think that's pretty impressive for an engine with no upgrades other than a better air cleaner and exhaust, and the governor removed. I don't have any regrets at all after buying 2 of these. You can't beat the price.
The first reason is that maybe you are putting an engine in a small bike frame that doesn't have enough room for a larger engine. Remove the tank and muffler, and the 2.5 has a very low profile. It's less than 10 inches from the mounting plate to the top of the spark plug. This was a pretty important factor on our bike. The 2.5 wouldn't fit with the tank and muffler, so I'm pretty sure a bigger engine wouldn't fit at all.
The other reason you may want a 2.5 is for a kid's bike. Conventional wisdom says that for a kid you get a big engine and gear it down to go slow at first, then change sprockets for higher speed after experience is gained. The disadvantage to a big engine with slower gearing is that it also increases the torque at the wheel, resulting in death by wheelie for an inexperienced little kid. A 2.5 geared correctly will give good performance, but no chance of unwanted wheelies or sudden loss of traction due to peeling out unexpectedly.
So, here's the review with the bad first and then the good part. I've discovered four unfavorable points about the 2.5:
1. The fuel tank is junk. The bolts unscrew themselves, the tank vibrates, the tabs break loose and cause cracks, then the gasoline runs out. Use a separate tank.
2. The air cleaner is junk. Metal nuts mounted in plastic will pull through easily. The design inhibits flow and I don't think it cleans the air as well as it should.
3. The stock exhaust is too restrictive, and none too quiet. I built a 2 foot 7/8" ID pipe with an 8HP Briggs muffler on the end. It's not much louder, and it increases performance noticeably.
4. The carburetor gets fouled sometimes. I think it got fouled by my dirty used fuel lines, and again by my dirty junkyard gas tank. Sometimes it just gets a bad attitude and I have to take the bowl off and clean it. Between two engines I've had to do a bowl-cleaning maybe 3 or 4 times since early summer.
Now here's the good part. Once you set it up correctly, this engine really goes. I clocked my kid at 20mph on a gravel driveway. That's a 1/3 mile driveway with a slight hill. I clocked him uphill and downhill, then took an average. Also, we went about 1/4 mile on the flat gravel road outside our driveway. The result was consistently 20 mph, which is almost too fast for 11 inch tires on gravel. It feels more like 40. But it's just right for teaching kids to handle a bike.
With a gear ratio of 9.18:1 and 11 inch tires, the engine had to be going around 5500 rpm to go 20 mph. A tachometer would be better, just to make sure the numbers are accurate. I don't have a tach, but Santa Claus might bring us a radar gun this year.
I think that's pretty impressive for an engine with no upgrades other than a better air cleaner and exhaust, and the governor removed. I don't have any regrets at all after buying 2 of these. You can't beat the price.
