Honda Aero 125.. Loud bang

[scootn2smoke]

Hello everyone. I bought a Honda Aero 125 a couple weeks ago. It has 4,900 miles and has been kept up very well. With the exception of one small piece busted off, the plastic is perfect. I was excited to get this as I have wanted one for quite a while. So I got my temporary license, plates, new helmet, well you get the idea. All said and done, a week later and I was ready to ride. She ran great in town, I tried to take it outside of town and it got up to 45mph and thats it. If I tried to give it any more throttle it would bogg down. As soon as I let up the power came right back but would top out at 45. THE NEXT DAY. I took a short ride across town and heard a loud bang. It sounded like I ran over a pop bottle or a piece of plastic. I even stopped and was looking back to try and see what I had hit. There was nothing there. As I started to take off again I could hear something rattling around somewhere near the cvt maybe? I was about a half mile from home so I just took it real slow. The motor is still running fine, it still feels like it has power but I didnt want to get up any kind of speed. Could someone point me in the right direction? I have mechanical ability, Im just not that well versed in the engine / tranny department. I downloaded the repair manual and now see that I cant go much further than taking the cover without specialty tools. I dont want to buy them all. If I knew where to start or what to look for I think I would be ok. Parts are another issue but I think I can get parts if needed. Thanks in advance for any help

[motormike]

It is likely that the drive belt is old and worn.
Also likely the roller weights are worn and flat-spotted.
This condition would explain the lack of top end speed...it should do 55-60 mph stock.
Remove the CVT cover, taking special care to match the bolts to the holes from which they are removed.
These 8 bolts are all different lengths...
Inspect belt for width spec.
I am using a Gates Power-Link 799 x 19 x 28
I recommend replacing the rollers with Dr.Pulley SLIDERS
My only size used so far is 19 gram
Other NH125 owners have reported using sliders as small as 13 gram.
I will buy 17's the next time I replace mine.
The difference in mid-range and top-end is impressively better.
Low-end is not affected too badly with 19's.
My top speed is now around 65-67 mph...(however, I have a 24mm carburetor now)
The LOUD POP you heard could be the belt shredding apart and hitting the inside of the cover...maybe.
Only inspection will tell.
ONE THING you absolutely MUST DO ! ! !....drain and refill the gear-box (final reduction)
I destroyed the gears and bearings in mine after 6k miles because I failed to do this easy maintenance.
Parts are NOT that easy to come by...
EDIT : The belt can be replaced by removing the clutch only,
But with the age and mileage, you should really pull the variator.
Impact wrench does the trick taking off the nuts that hold variator and clutch on their shafts.
Should you replace the rollers, re-assemble without grease,...completely dry is the recommended method.
The carburetor may need to be cleaned as the dirty jets will also limit the top-speed.

[patthesoundguy]

every thing mike said and I recommend pulling the driven pulley apart to give it a good clean and re-grease. My aero 80 was very sluggish off the line. Replacing the center spring helped but servicing that pulley really restored my performance to the transmission. I even gained some top end because the halves of the pulley weren't allowing full variation untill I cleaned the crusty grease out and re-greased it. The other day I got to 55mph on a slight hill with stock carb. It was maxing out at 43- 44 mph. With a new belt I may get some more top end as well. With a new belt you may also gain some top end. They wear out and get narrow changing the gear ratio.

[patthesoundguy]

Here is how I keep my trans cover bolts in the right places after getting mixed up I know were the go now especially on the spree cover but this tool sure speeds things up

[patthesoundguy]

If the stock rollers on the aero 125 are made of the same material that my aero 80 rollers were made of, replacing the rollers with new ones especially the Dr pulleys sliders will really improve the variator. My stock rollers were toast with huge flat spots when I got my aero 80 with only 1400 miles them.

[patthesoundguy]

I just remembered that I have a tutorial almost finished that shows how to service the driven pulley assembly. I'm picking up my new belt tomorrow so I'll take the pictures Im missing to complete the tutorial then and hhopefully get it posted tomorrow night.

[breaze]

Here is how I keep my trans cover bolts in the right places after getting mixed up I know were the go now especially on the spree cover but this tool sure speeds things up

Love it - can't say how many times I've placed mine in order only to knock them around later. I keep a similar cardboard cut-out to trace out a new gasket for the side cover - I could use it for both purposes.

+1 on opening that side cover - will never perform right with flat rollers, worn belt, or dirty clutch plates/shoes. This forum has many tips on removing/installing the belt, clutch and variator - can't wait to see Pat's how-to. This is one of those jobs that is easy if you know the tips, almost impossible if you don't. If you don't have an impact wrench, many tools available to remove variator and clutch nuts - strap wrenches, chain wrenches, etc. Remove and install entire assembly (clutch, variator, and belt) together and it goes easily - especially if you pry and wedge the clutch apart so that the belt is loose.

I actually have 12 gram rollers (not sliders) in mine and I love the acceleration - still hits mid to high 50's top end bone stock.

[Johnniespeed]

It is hard to diagnose someones problem from only a popping noise, but it is very likely as Mike stated your belt is shredding. Mike also mentions a very important item, change the transmission oil and check it often to verify it is full.
The variator sliders are probably flat spotted, but that will not keep you from driving around. I use the 15 gram DR Pulley sliders ( 20 15- 15g) I am very happy with that selection.
In stock form without modifications it should do about 57 MPH.
Keep in mind the speedometer reads 5.5 MPH low, that means when the speedo says 50 you are actually going 55.5 MPH.

[Wheelman-111]

Greetings:

A speedometer can't read "5.5 MPH low". The speedometer error is a percentage of the actual groundspeed. Otherwise, when you stop it will indicate you're going backwards at a fast walking pace. A speedometer that reads 10% high will show 11 mph at 10, 55 mph at 50, and 110 mph at the ton.

Anyway, It's very unusual for a street production machine's speedometer to underestimate speed. The DoT checks that along with many other things when it certifies a new model for domestic consumption. The idea is that you want the speedometer to read a little high, if anything. This keeps speeding tickets going only to the deserving. Japanese sportbikes, particularly in the competitive 600 class, are notorious for 8, 10, even 12% optimistic. In the case of the YZFR6, even the Tach was inaccurate. There was a class action lawsuit you can Bing. Among cycles, Harley-Davidson comes the closest to the truth - at least my 2001 Sportster did. No more than 2% high.

Now nobody knows LESS than I about the buck-and-a-quarter Aero, but may I suggest you pull out your GPS and check the assertion that its clock is pessimistic. Unless the tire is oversized or someone modified the cable drive somehow, you may find it inaccurate.

[Johnniespeed]

There IS a Garmin GPS on my Aero 125 and the speedometer is off by 5.5 MPH, both my Aero NH125's are off exactly the same amount.
The speedometer reads low by 5.5 MPH .
Pardon my semantics, perhaps I worded it wrong, and should have said off by 10%, but I did not measure it wrong.
You can do a search on this site about the Aero 125 speedometer reading low, there are many others on this site, who notice the same thing.

[Wheelman-111]

Greetings:

I sure won't dispute your direct findings. Generally speedometers overestimate speed, for the reasons cited. Is the 125 all Stock? If so, How did Honda-san get it so wrong?