Engine size/purchase help, please



OK, I am a road and mountain biker but new to motorized bikes.
Also, I have limit knowledge of small engines (one of the reasons
I want to try this, so I can learn)
My question is about engine size/performance.
I see 50cc and 70/80 cc motors being sold, but some websites say the 50cc will run faster than an 80cc, please explain in lay terms how this can be.
Also, should I invest in a 4 stroke over a 2 stroke, which is better and why?
Do 2 strokes have problems, do they run dirty?

I am 5'8, 140 lbs, riding to school on flat roads in a small town in north Texas.

Bottom line, what is a good entry level engine that won't break my bank.
I don't mind building and installing.
Speed is not a great issue, but we all know going fast is FUN.
Thanks for your help,


The allegation is that the 50cc will rev higher than the 62-72cc which are supposed to be torquier (?). Since you are light and ride flat, the 50cc should be great. I think the 2strokes are simpler and fit the bike better (in the frame mount config). And they are very inexpensive relative to 4strokes. (its not hard to get them on ebay for $150 complete). For a bit more jack, thatsdax or spookytooth (see the sidebar) have a reputation for quality and customer service.

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Available power is a function of both ccs/cui and state of tune...

for example the 1963 sceptre engine of 1592cc with 9.1:1 compression and dual zeniths produced 80hp (when you got the carbs balanced right)

the 1963 Minx engine that was identical except for a different head and a single carb managed a wheezy 62hp at a compression of, I think 7:1

The problem with higher states of tune is that it loads and stresses the engine more.. that said on the flat you should have more than enough power for what you are looking for... my 32cc can pull me and a relatively heavy schwinn to a maintainable 26-27 mph cruise...

good luck with your bike

Jemma xx


Read Read Read and get it set in your mind exactly what u want.By all means ask a few specific questions then and read some more.
Sorry man but with SO many choices u really gotta do your homework....confusing i know but necessary.


HI and welcome,

I personally have not tried a 50 so I cannot say how it runs but I do use a Dax 70 and it pulls great.....I live in a semi hilly area and have even geared my bike higher (more speed fewer engine RPM)....The 70 works great for me...plenty of reserve power!..of course if you live on the flats the 50 may be sufficient....Also, in most all areas I know of the general rule is less than 50 CC and you "street legal" without a license or insurance (DO check the official rules for your area before taking this as gospel though!)....IN any case the 50 would be closer to legal than the 70 or above engines (although you may want to check around the board for a post regarding "effective displacement" that may have an argument for allowing the 70 CC engines to be "legal" depending on how your areas laws read.
hope this helps and doesn't confuse you

PS - The Dax 70 is not marked as a 70....FWIW.


Where in north Texas? I'm in the panhandle if your close enough you could come take a look I've got 1-80cc? and 2-70 cc. You can try them out. I weigh 200 and both pull me fine I'm sure the 50cc would be fine to.

I am in Wichita Falls, quite a drive.

I have been looking at some bikes that look similar to what SpookyTooth is selling but for less $$$.


Although I might have to trim the rear fender.
*** Also, with a rear fender attached, would I still be able to affix a rear rack?

I have found a 50cc engine on e-bay for $180 delivered.
If I could get the bike for $170.

I feel like that would be a good price for a first build.
And I can buy some goodies for my new ride with the money I save.
Lights, Speedo, Goggles, helmet,rear rack, mirror, lock.
Grand total with extra parts: $500.

Be safe,


Well all I can tell you is read read read , you are in the right place, try starting with the stickys in this section Bicycles, Engines, Motors, Parts, & Accessories and then as far as a bike look at your local craigslist for a good used bike.remember that on e-bay buyer beware! look into thatsdax ,he's not only a sponser ,he's also a respected member here.
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Now that you've made i few decisions lets try and narrow the engine down:
If i was starting again i'de definately concider the Grubee Gen-II 50cc(actually 48cc)
It has advantages over our Generation 1's:
Steel lined cylinder wall.
45 degree angle fire head.
203 crankcase bearings.
Needle bearing on piston wrist pins.
Improved carburetor design.
20% more power(so other people say)

I like the idea of the larger engine bearings and the steel cylinder wall and since it's more compact(no intake manifold,angled SP) it would make for an easier/cleaner installation.
I BET it costs more but i also bet it's money wisely spent.....your decision though man. :)

http://www.grubeeinc.com/USA/StarFire GEN.htm
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Active Member
Jan 11, 2008
However, keep in mind that federal guidelines specify no larger than 48cc (and driven no faster than 20 MPH) to qualify as a "motorized Bicycle."

I'm an ex-defensive tackle, weight over 300 pounds, and a Mitsubishi TLE43 (43 cc) pushes me right along.

(plus, the Mitsubishi is clean burning, meeting CARBII emissions)


I ended up with an e-bay special, 70cc center mount, but it has been pre-fired so that makes me feel good.
Still had the smell of gas when I opened the box.
I purchased a Greenline BC-706. (A flat black beach cruiser with larger frame and 7-speeds for easier starts) it is the Qurve at Spookytooth, and after getting the bike and talking to Roland I shouldn't have been so frugal and spent the extra money and bought from Spookytooth and have them prep the frame.
I took a vice grip to the bottom tube so that the engine mount would fit, I also purchased longer bolts to replace the short mount screws that came with the kit.
I am attempting an "early cycle" look.
Engine delivered:160
bike delivered: 200
I will post photos when I get all my parts in, it has been hard sitting here with a bike and an engine. Unfortunately the company I bought the bike from sent 48 spoke wheels, not 36 spoke, they stated they are sending replacements.
So I wait.
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