KymcoForum.com

The fact that final drive is engaged by roller weights, belt and clutch due to increased revs.

This has nothing to do with the clutch. I don't know if you have EFI or carb.

If it's EFI, idle speed has to be checked and/or adjusted, along with check of all sensors contributing to idle speed management.

If it's carb, the same should be done. Carbed engines also have idle speed enricher ("auto-choke") that might have gotten stuck and did not lower revs when the engine was warmed up.

Since my engine blew, with the most likely culprit having something to do with the cooling system, I've come to view air cooling as an asset. One member here said that the small coolant passages trap air easily, even after bleeding, which is my best guess as to what happened. And it went from supposedly normal operating temperature to critically overheated very quickly.

Are oil cooled bikes more similar to air or water in terms of tight clearances and narrow operating temperatures? Somewhere in between?

With regards to water cooling increasing power -- not always? My water cooled People 250 produced I think 19hp. Air cooled cruisers (Yamaha, Suzuki) will produce more. Kymco's water cooled cruiser and Hyosung's oil cooled cruisers produce more than the air cooled, but not 40% more. For some reason, the sport bikes will be even more than that -- oil or water.

Small 250 cc cruisers you mention usually have 2 cylinders, and not one, thus benefiting from synergy effect and therefore might be a bit more powerful than 1-cyl 250 cc engine.

Also, one should take into account whether the engine is 2 valves per cylinder or 4 valves per cylinder. Your typical 2-valve 300 cc engine in w/c scoots has 23-24 hp while the same engine with 4-valves has around 30 hp, only because of improved cylinder flow.

I guess, this thread is about scenario 2 valve per cylinder single cylinder that is air cooled or water cooled, and not other technical scenarios that might be possible in engineering sense of the word.

Finally, it seems that most people commenting do not fully understand the following cooling options:

- air cooled with fan (most "air cooled" Kymcos)
- air-oil cooled (with or without fan but with separate direct air oil cooler - VW Beetle and old 911s, Kawasaki sports touring bikes of 70s-90s)
- water cooled (without fan, usually small displacement w/c scoots)
- water cooled with fan (usually 300 cc scoots and bikes and higher)

They are all very different with different repercussions on the final engine power output.

Finally, a comment about two things from your post:

- "quick" passage from normal operating temperature to overheated: unfortunately, this is expected. Coolant temperature is usually 90-100 C. Boiling temperature of the new coolant is around 106 C. If something goes wrong, you will notice it in a matter of minutes in a car, or even sooner in a bike that has maybe 1,5l of coolant in the system.
- Clearances are much tighter in w/c engines because they are expected to work in a very narrow temperature range. A/C engines have looser clearances, the same with oil-air cooled engines. W/C engines have therefore higher compression ratio and proportionately higher power output. A/C engines have lower compressions and lower power output. Comparably and relatively speaking, w/c may be constructed to rev higher than a/c engines.

Respectfully
Agreed - H2O =
  + HP - ✔
  + MPH - ✔
 
Longer engine life?? ..287,000 miles on my a/c VW (then traded for the new Rabbit)
My new a/c Piaggio meets the '18 Euro regs.

H2O = "meh"...for a % of us a/c riders/drivers.

Stig

Well, this is why still everybody can choose a/c or w/c. In a small scoot, if you weigh 110 kgs like I do, 125 cc a/c 9hp does not even compare to 125 cc w/c 16 hp. It makes a world of difference. OTOH, if I lived in a third-world country (in full sense of that world), I would prefer technically less complicated scooter, and this is why in rural Africa and Asia, most scoots and even small bikes are a/c.

By a/c VW I guess you mean Beetle because almost all newer VW cars are w/c?

It is largely irrelevant because they are not only air cooled (like Kymco's scoots) but also oil cooled (different construction *and* they have a separate oil cooling radiator). Also, they were usually 1200-1500 cc 4-cyl air cooled engines developing measly 45-55 hp (<40 hp/l). Kymco's a/c 125 cc 1-cyl engine develops 9 hp (>70 hp/l), so it is already under much more stress. Comparison to a/c older cars is not exactly applicable.

Well, they should be, but they decided to put them somewhere where they cannot be accessed when you need to check them.
Kymco scooters seem to be made in a way that owner cannot do anything by himself. And new ones increasingly so.

Tromper - you are right, they seem to be in the rear section.
Fuel tank is also in the rear. I went outside just now to check, everything is enclosed within plastic panels.
It seems that in order to access fuse box, you need to tear apart... everything. Very useful when you are on the road travelling somewhere.

30 years ago whoever engineered this would have been fired.

OTOH it is virtually impossible to produce for example iPhones and large scale consumer electronics in the West. In China, you can recruit 100.000 or 150.000 assembly line workers in a matter of weeks, shove them in the factory and shiny iPhones/laptops start rolling out in a matter of days. In the West, you have everything working against you as an entrepreneur (I am not entering into discussion about worker's rights now...), from state regulation, unions, higher wages, employee contribution/taxation, to employment process.

Chinese do not have more public holidays than most EU countries (I don't know about USA though). There are 25 public holidays in China, but 5-6 are mandatorily compensated by working Saturdays. When you subtract public holidays that fall on Sundays, it's quite similar.

In EU countries, the lowest number of vacation days (usually for new/young employees) is 20. In Germany, almost all companies grant 30 vacation days (Saturdays, Sundays and holidays excluded). For example, I have 30 paid vacation days annually, and additional 7 I can take as paid days off for specific reasons (appearance at court, wedding, death of family member or education).

In China, for employees working for up to 10 years, only 5 paid vacation days annually are granted. In USA - it's zero, and up to the employer.

So, in most cases, it is much more difficult to organize industrial production, than in China... also evidenced by mass scale transfer of industry from USA/EU to China from the beginning of 80s. We simply do not mass produce anything any more and rely on high technology.

The problem will start when internal accumulation of capital in China and India grows sufficiently along with human capital and knowledge, because it is *us* who will become mass producers, and they will start selling knowledge. The process will reverse itself. I think that "our empire" is in decline, but one should analyse larger timelines (decades/centuries) and not years in order to realize this.

Most people would be suprised that BMW (otherwise in car world usually assumed to be "high quality vehicle") is pure crap in motorcycle world.

According to a number of customer surveys, BMW is usually firmly at the bottom of the list, together with all Italian manufacturers you can think of.

Honda, Suzuki, Yamaha and Kawasaki are the most reliable motorcycle brands.

https://eatsleepride.com/c/203865/most_unreliable_motorcycle_companies

I remember reading one such survey in 2016. They measured "catastrophic failure rate" in a few categories (mechanical, electrical, etc.) over 3 or 4 year period. The best performing brands had failure rate of 13-18 %. Those at the bottom and BMW among them, close to 40 %.

BMWs have also been plagued by fork failure of various models for the past decade and a half.

https://www.change.org/p/bmw-motorcycles-bmw-motorrad-should-investigate-and-report-publicly-on-fork-stanchion-separation-issue

This is something that does not even happen to 400 US$ Chinese scooters.

How is this even possible, I do not understand.

https://www.google.hr/search?q=bmw+motorcycle+fork+failure&client=opera&hs=Sh6&dcr=0&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiWicL78a7XAhWJ16QKHeMkA14Q_AUICigB&biw=1600&bih=835

So, I would say that BMW "premium customers" are being pretty much sold snake oil.

I am careful not to claim something like this for overall brands, but judging by facts, BMW motorcycles seem to be outstanding - crap, overall.