Yongkang Mavis Power Machinery Co., Ltd.



Now that I got your attention...

While cruz'n the I-Net I found this ad on one of the Chinese products distributer web page.

I have been trying to find out the specs on these little motors to see what they are really able to handle. Here is the info from the YMPMC ad...

Product Details

Bicycle Engine Kits (F50/F60/F80)


Product Description
1. Engine Type: Single cylinder, air-cooled, 2-stroke
2. Mixing Ratio of Fuel(90#) and Engine Oil: 16: 1 For new sets, 20: 1 after running 500KM.
3. Displacement: 48cc, 60cc, 80cc
4. Ignition: C. D. I. Ignition

Is this sounding a bit familiar?

5. MAX. Power(kw/rpm): F50: 1. 6/6000, F60: 2. 5/6000, F80: 3. 5/6000

0r 3.5 * 1.341 = 4.6935 HP... Seem like wishful thinking in both power and RPM for these little engines.

6. Volume of Oil consumption: F50: 1. 5L/100KM, F60: 2. 0L/100KM, F80: 2. 5L/100KM
7. Maximum speed: F50: 30KM/H, F60: 35KM/H, F80: 38KM/H

That caught my Eye! F80 Max Speed (with an average 100lb Socialist Chinese rider) of 38KM/H... I had to find out what that was in USA terms.
I checked into my favorite online conversion site and here is what it said:
38 kilometer = 23.612 105 303 6 mile or under 24MPH! Who is it that is trying to get 35+ MPH out of these little guys?

8. Fuel tank: 2. 0L
9. Net weight: F50: 6. 5kg, F60/F80: 7. 0KG

Here is where I found this info:


Humm 6000RPM... Wonder what that is in Bicycle terms...

Working With 6000 rpm at the engine drive shaft though the 20 tooth "bevel" gear to a 90 tooth large bevel gear (clutch assembly) for a 4.5 / 1 reduction to 1333rpm. Then through the 10-tooth drive sprocket. to the (what most of us have) 44 tooth wheel sprocket (a ratio of 4.4/1) gives us 303 rpm at the wheel axle.

A 26" wheel has a circumference of (around) 80 inches, or 6.666 feet. 6.666 times 303rpm is 2020 feet/min or (*60) 121200 feet/hr.... Let see, 5280feet in a mile into 121200 = 23mph. Just what the ad said...

Working backwards from 35mph with a 36 tooth sprocket...

35*5280=184800ft in 1hr/60=3080ft/min. 3080/6.666 (same old bicycle wheel) = 462rpm... now the other sprocket ratio of 3.6/1 gives an output shaft speed of 1663 through the lgbvlgr/smbvlgr (4.5/1) = 7483rpm...Oops! Over torqued and over reved at the same time.

Not to mention "40mph bursts" which breaks down easily to 7485/35=213.8*40=8552RPM!

I wonder if this is why some of the guys are having trouble with the Chinese engines???...
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You're absolutely right!

:cool:I agree with you 100%.

Happy time owners are zinging their engines WAYYY past their limitations!

Just because the engine can rev past their rpm range doesn't mean you should do so.

I disassembled a 70cc engine recently, and was satisfied with its workmanship, internally and externally.

I've seen a lot more casting flash in VW and CHEVROLET engines than i did in happy time engines.

I'm not too familiar with 2-stroke engines, and I don't have the specs to double check my engine's stats.

HOWEVER, the engines are so cheap, I'd just replace them if they conk out.

Then I'd simply limit engine rpm to 6,000rpm. I believe they'd last MUCH longer if not overrevved...ever.

With 36-tooth sprocket and 4.1 gear reduction(82 lgbvlgr/20 smbvlgr, that computes to max speed of 32mpn.

With 44-tooth sprocket, it's 26mph.

With 48-tooth sprocket, top speed of 24mph.



5-7 Heaven

Thanks for the missing specs on the internal gears... I got my information by trying to count the teeth on the gears on the tiny pic's on Spooky Tooth's parts page. The 20 tooth gear was easy, but the 82 tooth was hard to get right.

You mentioned the casting quality, which I wondered about. My engine doesn't look all that bad, but I can't help but thinking of; what the alloy is that was used in the castings and how well the crank and gears were tempered.

Anyway, my experience with two-cycle RC engines tells me to use enough oil in the mix, don't over-rev (under prop) and that if a part can come loose, it will!


Awesome work guys.

People need to remember that we're riding BICYCLES not motorcycles.
If a person wants extreme performance over and above a bicycle, they should be spending their money on a motorcycle...still 2 wheels, but it'll bring the speed up, lol.

As for speed...has anyone ever considered what the poor bicycle (that we put HT's on) is designed for? In all honesty, I don't think that the wheel bearings on a normal bicycle are designed for continous usage at 30+ mph...that's leaving out the other important things like spokes, rims, tires, brakes, general frame stress, etc, etc.

Good engines will take the abuse of running harder than they're designed for, but will your bike?


good points !!

You guys make some really good points in here.. Chris.. Good points !!


:cool:Good work, Sockmonkey and Chris.

Sock, even with the 82t gear in my hand, it took a bit of effort to count those tiny teeth.:eek:



A few details

Hi all, here are a few things I may or may not have mentioned before

the reduction in the China-2stroke is 4.1 to 1.

A really good running engine can reach 7300 rpm, stock probably no more

NanTong has a dyno calibrated in Watts.

Mr. Chen once told Don that 1.2hp was acceptable for a new engine. (after a new square-head turned that on his Dyno)

every engine is supposed to be fired

every so-often they are supposed to dyno one, to meet ISO standards (I forget the ratio of production number, test to test)

in China they use 20wt engine oil, mixed rich, and smoke very badly

No-one in China tunes these engines up, cares about performance, or even about improved parts (this has all been done for/by Americans)

Also, in the Cities, there are little hole-in-the-wall repair shops everywhere, most no larger than a "FotoMat" (if anyone remembers those)

These kits (complete in box) have been illegal to sell to the Chinese public for at least 5 years that I've known of (tho they can buy any part that they need to keep an installation running, only new installs are illegal)

a blown up engine is worth about 2.50 in trade-in value in South China

Hope you find some of this interesting,



It's interesting that you said it is illegal for Chinese to buy complete new engine kits....why is that?

The most RPM's I've pulled on my Dax 70 was 6340 according to the Sendec Tach.....I try not going to 6000 often because it sounds too fast for the engine....



Thanks for all the input!

I was hopping some of you wrench-heads would turn up if I started this thread. I have no problem with a guy trying to peak out his engine, I have done it a few times myself with RC engines...

Man it just hit me, the last new fuel powered RC engine I bought was about 10 years ago. It was a 4-stroke 40 (6.5CC) and cost $159.99!

Anyway, the trade off was longevity for power.

I've dumped three M-cycles in the past 5 years and don't really want to do that on a bicycle! What saved my hide was the leathers / protective gear that I had on... Can't imagine going down in a T-shirt, Bermuda shorts and flip-flops:eek:..