70cc eBay engine build


New Member
Local time
2:17 PM
Feb 3, 2008
The engine arrived yesterday, coincidentally while I was out checking out a bike from Craigslist. The engine is a 70cc from eBay seller boygofast.

There's certainly no shortage of bicycles in Davis, CA! I picked up a clunker that someone had used only for last year's Burning Man for $10.

The most tedious part of the build so far has been the rear sprocket. It took about an hour for me to fit those darned bolts and tighten them. Maybe it would have been faster if I had not been half watching van Damme in Lionheart on AMC. 80's action movies are the best!

I can't figure out how to mount the engine, though. The tube is just a tad too thick. I don't have access to a drill, so I can't use their alternate mounting solution or fabricate my own. I'm going to make a trip to Ace Hardware in the morning to see if I can find anything that'll help.

The engine quality is better than I had expected. After seeing some of the pictures floating around showing all the flash hanging around the ports, I was afraid I would have to do quite a bit of manual sanding. However, the ports on mine are really clean; the edge of the port is perfectly clean! New model, maybe?

Also, I received a funky bolt with the package; I have no idea where it goes. It's really dirty, and it's threaded inside, outside, and all around. I'm attaching a picture of it. Anyone care to hazard a guess as to it's function?

Pics of the build to follow when I get the engine mounted!


  • P2070182.JPG
    152.4 KB · Views: 323
it's finally done!

Okay, it's finally installed! I had to do a number of mods to get it to fit:

  • File down the mounts to accommodate slightly oversized frame tube
  • Hack off the fingers coming off the air filter
  • Bend the muffler out of the way of the pedal cranks

Other than those few problems, the process went without a hitch. The frame was just large enough to fit the engine.

Before I installed the gas tank, I picked up a handful of gravel from the railroad tracks, put them in the tank, and shook it around with a bit of gasoline to loosen up the rust. It's nifty trick I picked up from a small engine restoration book.:cool:

The maiden voyage was quite a ride! Incredibly, it burst to life on the third try. However, by the time I got home, I had lost two screws, the front reflector, and the clutch lever. I had used loctite on all the screws that came with the motor, but I hadn't thought to loctite the bike's screws. Oh well, I guess I'll make a trip to the hardware store in the morning to pick up some replacements.
The clutch lever, however, failed spectacularly. It snapped right down the middle, through the lock button. Luckily, I was just taking off when it broke, after having stopped at the side of the road to look for the aforementioned screws. I was about to use one of the brake levers, but then I saw Wayde's post on how he had used the shifters instead, so I tried it and they work even better than the stock clutch lever!:D

I'm not sure about the engine idle speed, though. It seems awfully high/loud when I take the disengage the clutch at stops. I read a post saying that the the engine should idle at 1400 rpm, but I have no idea what that should sound like. Furthermore, the idle adjust screw doesn't seem to affect it much. Does anyone have any tips on getting it to idle properly?

Also, I accidentally mixed the fuel at about a 14:1 ratio; should I go back to the gas station and thin it out a bit?

If anyone has any tips, comments, or criticism, please throw them my way!


  • broken clutch lever.JPG
    broken clutch lever.JPG
    177.4 KB · Views: 291
  • teh bike1.JPG
    teh bike1.JPG
    115.7 KB · Views: 315
Two stroke noise...

Howdy... Okay, two strokes are notoriously noisy to begin with, but the rattle you're undoubtedly hearing is more that likely "way too much clearance" in the clutch assembly. The original that came with my first two engines had so much clearance, it eventually glazed the clutch material and finally quit engaging. I ordered new ones which came with much better tolerances and work much better. The machining and tolerance quality of these engines is to say the least, much less than I would consider acceptable, especially since 2 stroke engines have inherent high compression levels, vibration issues and are prone to pre-ignition destruction. The carburation unit is prone to terrible siphoning that can drain your tank "sucking fuel past the shutoff valve" and filling the crank case with a load of destructive fuel that will kill the engine and could actually cause a catastrophic failure (can you say shrapnel?) if you manage to start it. Another really bad feature of these engines is the low quality of the ignition. In a word, shock mount the ignition unit or you will be replacing it on a regular basis. The magneto is also only lined up by the mounting screws which can and will allow the center magnets to touch the armature at high RPM. This *will* short your ignition and prevent the motor from running. This component also gets way too hot, and this can cause the windings to open/short. Every time this happened to me, I was a 3 or 4 mile peddle back to my shop, (what a work out). In short, if you're only going to play with this thing, you'll have lots of fun re-building, experimenting, and tooling around but don't expect to ride this thing on a regular basis for distances over 5 miles from where your tools are unless you put some serious engine cleanup effort into the internals. I have finally given up on this engine because of the considerable effort to get reliability and durability out of it. I've switched to a 4 stroke unit that so far is miles ahead in quality and practicality. It probably won't ever go 50+ mph, but it will do 30 mph all day long without the major problems of the 2 stroke. This of course is just my informed opinion.... I could be wrong...
Idle speed: I shoot for as a slow as possible idle speed that is reliably held by the engine.
Wow, if I didn't know better, I'd say you're playing "scare the n00b":D.
But seriously, thanks for the tips. I think you may be right about the clutch clearance. There seemed to be a bit of squealing when the clutch is engaged; I think it may be because I hadn't put enough slack in the cable. I haven't tried it since replacing the clutch lever, but I suspect the squealing may be gone now.

I have no interest at all of going over 25 mph, so I'm not too concerned about overheating or serious breakdowns. I built my MB mainly to cruise around town, pick up groceries, etc. However, if it should break down, I will definitely look into a four-stroke. For now, I will keep an eye out for the problems you mentioned. Thanks a bunch!

Good idea! I shall try that the next opportunity I get.
I've found with both my Happy Times motors 50 and 70 cc, that the idle screw didn't do much until the motor had some miles on it (under 100), then one day that little screw had an effect.
I've found with both my Happy Times motors 50 and 70 cc, that the idle screw didn't do much until the motor had some miles on it (under 100), then one day that little screw had an effect.

Yeah, I just noticed that this weekend, after going through the first tank of gas. I actually had to up the idle speed because it was cutting out. Before, it revved super high at idle, no matter where the screw was set. Phew, I'm glad to know that that's normal behavior.
New modifications:

I've also replaced the stock plug with a NGK B7HV. I was concerned that this lower temp plug might not work as well, but it seems to be even more effective! It used to take me ~3 laps around the block to get the motor started, but now it starts even before I reach the end of the block! The engine sounds deeper now, too. My next step is to find a replacement spark plug wire.

The air filter seemed a bit too porous, so I cut a bit of high density foam that I had lying around and fit it into the filter. Doesn't seem to have affected the engine, and extra filtration is always good.

The brass fuel needle valve that came with the kit is t3h cr4p. It started leaking out of the part where the lever enters the valve body! I replaced it with a proper needle valve from Ace Hardware and changed out the Chinese vinyl gas lines for American-made ones, which are reassuringly stiffer. With liberal use of pipe thread tape and pipe clamps, my fuel system is now leakproof!​


To do:
  • Replace spark plug wire.
  • Adjust the clutch. It squeals quite a bit whenever it is engaged. From what I've gathered from this forum, tightening the nut on the clutch plate will alleviate the sound somewhat. I guess while it's open, I should also take the opportunity to grease the gears.
  • Replace the stock tensioner with a regular bike tensioner sprocket. The sides on the stock tensioner are wearing down alarmingly fast.