Battle with My Riva 125

This year, 2012, I bought a very first scooter in my life, Yamaha Riva 125 with 22000 miles on it. High mileage scared me but it seemed have been well taken care of.
300 miles since I got it, the intake valve got stuck and it was bent.


Got new valves, in and ex, installed after the head was cleaned and inspected. There were cracks on intake valve guide which were visible. I thought it will be OK. 100 miles after new valves installed, the two valves decided to hit each other.


So, this time, I got used cylinder head with valves and other parts still installed. The intake valve shows un-even heat transfer.

The head was disassembled and cleaned before installation.

Now, the scoot is on the road and has two issues. Starter clutch does not engage on first try, on occasions when hot. It catches eventually, and I can be on my way home. I know this clutch should be replaced soon.

Second issue is the engine coughs when at steady speeds between 35 to 45ish miles. A few times, I had the engine quit on me when I was decelerating for stop light from 50+. At about 40ish, I lost engine and had to coast to stop. When this happens, I have to wait like 1 to 2 minutes before it starts again.

Does any one have any idea? Carburetor needs cleaning and adjustment?

Took the scoot for errands. The throttle cable broke prior to the last place for the day so I didn't make it there. Parked the scoot without shutting down and raised idle speed. Good thing I didn't have side covers installed. Doing this didn't give me enough RPM to roll on its own but I was able to limp forward with light kicks.

When there was not much cars, I was able to use my finger to open the throttle a little. Good thing the cam on the carb was on left side. Also, I had riding gloves on!!. The throttle cable guide on the grip was pinching the cable. The cable was flexing every time throttle was twisted instead of rotating on its bud end.


Now, I need to order a new throttle cable, adding to do list for the scoot.

Waiting for new throttle cable and new tube, I replaced gauge cluster.

Some one must have used wrong bulb.

This is what I got.

The scoot lost almost 9500 miles.

Finally the throttle cable and throttle tube had arrived yesterday. Installed them today along with heat wrap on exhaust pipe. Took it for a ride. The wrap was white when installed. Smoked a lot. Now it is tan.

The starter clutch didn't engage when the engine was hot. It took like 10 tries to get engine started.

The battle continues.

My scooter's tires are in need of replacement. Currently the front tire is dry-rotted Bridgestone, Molas-8 and rear is worn Chengshin something. Riding with these on wet roads is like riding on icy roads.

I am thinking of getting Kenda K329 or Kenda K413 due to their price.

What are your thoughts and actual experience on these? Or, any others you recommend?

Thank you.

Wow. I have no tire recs but I will give this a little bump with a BIG hearty 'thanks' for posting such clean, detailed pics of that engine and other things, as well as your explanation of what is going on and what went wrong.

Very interesting for this mechanical newb.

Thank you!! DudeClone. Your post on this thread is very first one made by someone else other than myself after about 650 views later. Although I started doing this for my own sake, I was beginning to think of abandoning it. I am pleased that you found this thread very interesting. It gave me some motivations to continue.

Now, the engine quitting problem is, I think, due to auto-choke not working properly. It has gotten progressively worse. Newly installed spark plug is removed when the engine quit. It is really black and wet. I will perform troubleshooting later.

ANY new tire will be better than what you have! i've had good luck w/ Pirelli and Michelin. They're all fairly inexpensive for the size you need.

Thank you, nedc. Just checked prices and reviews on one of on-line stores. Pirelli SL26s are $6 more than Kenda K413s. I think I'll go with Pirellis since they are supposed be excellent in wet weather. Also, they have your un-biased suggestion.

Checked the function of auto-choke today. Heat element is working as it should as per service manual. The cam is pushing the plungers. Took the carb apart to see if anything is clogged or something. Didn't find anything eye catching there except rusty crud on the bottom of the bowl and wear on needle valve. The scoot had in-line fuel filter installed when I bought it. So, I don't know if the crud was there and then the filter was installed. Or, the filter is not so effective.

Got other kind of filter from a local cycle shop. The shop owner says on those filters, there's something pass through the filters. :huh. Really? Why bother installing it then? Why is he selling them many kinds and sized filters?

The carb had signs of some one being there before me. There's extra washer under the main jet. An O-ring is missing between the main jet and needle jet. A blank plug is removed for pilot jet.

The battle continues.

Good luck with the scoot. I miss my old 125.

Sent from my Galaxy Nexus using Tapatalk 2

Tires: I use Kendas and they work fine. The price is right and the grip and appearance is OK, too.

Starter clutch: This is a problem with bikes generally. You have to go pretty far into the engine to get to the problem. I'm not sure how Yamahas work, because I mostly understand Hondas. There is a service manual at:

http://www.motorscooterguide.net/Manuals/Yamaha_XC125_Service_Manual.pdf

Thank you, Alex.

Thank you, Wentwest, for the link. The scooter came with service manual, but having it on PC come in handy as well.

Put the carburetor together. The engine started fine and idled fine, too. Let it warm up, and revved engine. It seems good. So, I did test ride the scoot. It run fine for 10 minutes and then the old problem came back. The engine hesitates at speeds 30-45 mph. If I keep at that speed. The engine dies. If I accelerate, it runs good.

It seems like the pilot jet is clogged. So, I took it apart again and blow compressed air through it. Visually inspected. It seems fine. I am lost for now. I wish there's a detailed "how to" on adjusting this carburetor on the service manual. Float level and fuel level has been checked and adjusted to spec per the book.

If it does indeed run fine for 10 minutes, the problem is not your pilot jet. It's got to be something that is related to heat. If you electric choke was malfunctioning, it would be apparent immediately after the scoot warmed up, sooner than 10 minutes. Maybe your valve clearances are slightly too tight and after the engine gets good and hot (10 minutes running or more) they are hanging slightly open. My $.02.

Thank you for your input. Adjusting those clearances were really difficult for me because of their sizes, too small!!

I'll check on that sometime this weekend.

There's another Riva 125 up for sale. I think I will look at it on this weekend as well. Use this one as parts, or the one I have for parts,,,,,. It would bites if both becomes "dead".

My 125 Riva also has a lean surge from 35 to 45. It even occurs during full acceleration, but to a lesser extent. It also has some odd high at times idle characteristics combined with extremely low idle at other times. The carb has been well cleaned, set up per the service manual and a fuel filter is installed. The surge begins when the engine is fully warmed. I've yet to figure if its is related to a vacuum leak. The carb adapter may leak at the head when warm.
The carb is very simple and relies on the main jet for the entire useful rev range. E10 fuel may be making a lean designed system too lean. I will get a can of no ethanol fuel later and see if the problem resolves itself. That will be after the warmed engine is tested for vacuum leaks. After that, the float level will be raised to see if a bit more weight will reduce the issue.

You could buy a second one, or you could look for a Honda CH150. They are watercooled, and a bit bigger than the Yamaha 125, but they are more reliable in a lot of ways. Yamaha has always had trouble getting the carburetion right on the Riva. Look for 85, 86 or 87 Honda Elite 150.

Everyone has their favorites on a forum like this one, and mine is Honda scooters from the mid 80's. The 250 is even more powerful, but costs a bit more, too. Maybe only a couple of hundred dollars.

Make sure the vaccuum petcock is working properly, it may be running out of fuel at high rpm . Also check float level for richness problem and irratic idle. I just bought a 150 Elite with 18k on it and what a sweet running scoot, I swear it will out run my 250 Elite up to 60mph.

I got a used un-molested carb off of eBay, came in on Friday. Installed it. Run it. And it did the same thing as the original carb.
I feel now the engine is running too rich when it is surging and eventual shut down. Or oil is finding it's way onto the spark plug and killing it. After the engine is dead, I would wait for 3 to 5 minutes for restart. Then, engine starts OK, as if the plug needed the time to dry off. :huh

Reading "cheap bastard"s post makes me to think this is a defect. The second carb installed did exactly what the "bastard" says including high and low idle speed.

There was a write up on CV carbs saying that on these carbs, pilot jet is supplying fuel up to 3/4 of the way. Rather than main jet.

Vacuum leak has been checked ok,even even when hot. Fuel supply to carb has been verified good while surging is going on. I have a large see-through fuel installed.

The guy who was selling his Riva has not returned my call, so I don't think I am getting that one. Which is OK with me.

I like Honda. I own three Honda cars. There are quite a few Metropolitan available but it's too small/slow for me. Elite 150? I'v never seen it around here. There was Elite 250 up for sale a while ago. I may go for this one if it is still there.

Mean while I'll continue battle with this Riva.

The Riva and I went on a 75 mile round trip to Blackhawk Farms raceway yesterday. with no traffic to contend with, it was easy to concentrate on the surging issue. The engine seems to have a changing load against it. Maybe its not fuel or even engine related at all. As kids we worked on and beat old mini bikes, crude go karts and old under powered snowmobiles. The centrifugal clutches were often worn out or replaced with cheap low quality parts. They sometimes acted in a similar fashion. There was even drivetrain squeal when chains were used. My riva has a new Gates belt, but there is drivetrain squeal. When the that occurs, the surge is greatly reduced. If there is an incline, strong headwind or need for quicker acceleration, the problem is greater. The oldest boy weighs about 80 lbs less than me and has no surge complaint on the bike. Any slight decline or strong tailwind results in no surge.
The clutch may be glazed or the friction material worn out. I will be pricing an upgraded clutch such as the Dr Pulley unit before taking the gearbox apart to look at it. I guess while its apart, a new set of roller weights or the lighter slides may get put to use.