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Discussion Starter #1
I recently purchased a 1971 Honda SL125 with a few issues.

The first issue was that it would not run worth a darn for more than 45 seconds. I determined that the spark plug cable was not attached to the spark plug well enough. After looking at the spark plug, I decided to install a new one. After that, I have been able to start it easily enough. I resolved the first issue on my own.:smile:

Now for the issue that I am unable to resolve: While parked, the motor will idle now that I have adjusted the carburetor. I can also rev up the RPMs at or above redline just fine. However, when I took it for a test drive I could not get it to go faster than 30-32 mph. When I increased the speed, the motor would sputter and start to cut out above 6000 or so RPMs in gears 3 through 5. I tried adjusting the high-end jet screw to no avail, however, while parked, I could see and hear the difference in adjustments of this jet, but no matter how I set it, 32 mph was top speed without causing the motor to sputter and cut out. Any suggestions would be greatly appreciated. Thank you in advance! :confused:

There are other issues, but I feel I can resolve them on my own, such as replacing the light bulb in the headlight and changing the motor oil. I will even add turn signals after the motor will run properly.:wink2:
 

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Sounds like a fuel issue. Possibly float not adjusted properly and starving for fuel or a dirty fuel filter.
 

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Discussion Starter #3
I will check the screen in the fuel valve which is the only fuel filter it has. Also, the previous owner had cleaned the bowl, so I probably should check the floats to be sure they do not have a hole in them or something. Thank you, as I was not going in those directions with my thoughts. Thanks for putting me on a better track.
 

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Because of the age of the bike, it could be possible that the timing advancer [if so equipped] could be gummed up. They operate on centrifugal force and if not working the effect would be worse at higher RPM.

Another age related issue is voltage drop through the ignition switch. I had one bad enough that it lost more than one volt and it created symptoms pretty much like you have. Old Hondas are like that as they tend to use handlebar switches as junction boxes. When you get to the early Gold Wings, they take the load off all that and actually use relays. I keep threatening to make up lighting and ignition relays for the little twin cyl Hondas I have,but never seem to get it done.

What you really have to do is troubleshoot your way through everything fuel related and then everything electrical related. Pick one area and stick with it because if you switch back and forth it is too much like chasing your tail.
 

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Discussion Starter #5
Today, I checked the "fuel filter - screen" at the fuel shut off valve and it was clean. I then checked the air filter and I felt it needed replacing as it was so very dirty it may have been clogged. I replaced the air filter. Then I removed the upper end of the carburetor to check the needle valve. Everything looked good there. I then removed the bowl and found a bit of rust looking material. It may have been residue from when the former owner cleaning the bowl as he claimed he had done. I will need to repair the flat rear tire before I test it under load again, so that is next on my "to do list". Every little step is a step closer to being able to ride it to town and back! If anyone else has any suggestions on things to check and or replace, please let me know. Thank you.
 

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Discussion Starter #6
Hello Slumlord.

Thank you for the advice. After I test the bike again, now that I have checked all of the fuel line/carburetor issues, if it continues to act the way it does currently, I will have the local shop give it a tune-up, as this is something I do not have the tools or experience for. There are many things that I can do, and many that I can not do.

Mike
 

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What Slum said about voltage drop can be critical.
It is amazing what can be lost going thru what appears to be a clean fitting. My 41 Ford tractor has many more years than your bike.
Clean bright and tight, is what we use. By clean and bright, I mean shiny surfaces. Get the file out. Replace switches if you can. Replace the wyres if you can. Old stuff robs volts.

UK
 

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I've got a 1971 Honda CB100 with an SL125 engine about the same vintage as yours. Did you replace and gap the spark plug? Badly worn or fouled plug can cause RPM stumbling. And BTW, if you need to replace your carb, they are still available from the Far East on E-Bay. Folks in the Far East still keep these bikes running. Sounds like you are on the right track, I'm sure you'll get it sorted out soon.

About 11 years ago, I rode the bike to work 12 miles away, wide open much of the time at 55 mph. When I got to work, a few remarked that they learned to ride on such a bike. When they asked me how the ride was, I told them it was like the scooter ride in the movie, "Dumb and Dumber". :tongue:

Still got the ABS small motorcycle hard saddle bags I used when it was my regular rider in college 40 years ago. One of these days I'll paint them to match the bike and have my little bagger. :smile_big::grin:

Congrats on the acquisition, and would like to hear how it works out for you. :wink2::thumbsup:
 

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All good suggestions here. In my experiences, most problems like this with old bikes are carb problems. A good cleaning in my ultrasonic cleaner usually does the trick. Also fuel gumming from the petcock to the float valve can keep fuel from getting in the carb fast enough. It will run fine until you start running out of gas. Check your fuel flow into the carburetor by taking the fuel line loose at the carb and open the petcock. You can also remove the float bowl and check that fuel is coming in sufficiently through the the needle valve.
 

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Discussion Starter #11
I decided to go ahead and purchase one of the carburetors on eBay. It is being shipped today out of China, but at least it is on the way. It less expensive and easier to just replace the entire thing.
 

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Okay guys, yesterday I took the rear wheel to the shop to have them install the a new band to cover the spoke connection as well as a new inner tube as I sliced a one inch hole in the last tube. But today I looked to check the fuel flow as had been recommended above. Since I was not able to easily access the fuel line at the carb, I removed the bowl drain screw to see how much fuel flowed out. I then removed the fuel line at the fuel tank valve and checked the fuel flow rate and I also removed the bottom of the valve to allow the fuel to flow out there as well. In all three locations, the fuel flow rate seemed to be about the same. At least I could not see the difference. I am waiting on a new carb to come over on the slow boat from China that I bought on eBay. It should arrive by Christmas. At least until then I will be able to ride the bike at about 35 mph until then. If the new carb does not work, I will take it in for a tune up since I do not have the appropriate tools to perform a decent tune up.
 

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Hello Mike Maass,

I can tell you this I had one ! yup an SL-125 a 1971 Honda and that bike would do around 65. I bought it from a couple of brothers
in north Attleboro, say around 76.

I had a great time with that bike an on/off road bike w/ knobby's . All i can say is get the service manual for it, I did, it was a Clymers
. could be the original ignition coil, might be good to get a new one. Bike will run good at low speed and crap out as you go faster.
 

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Discussion Starter #15
I just bought an ignition coil on eBay. It will be coming from Texas. I decided to spend the $4 over the price of them to come from China on the slow boat. LOL!!! This way I might be riding it sooner than expected, I hope! One of the previous owners also put on a different set of sprockets and chain so they would have more power for off road riding in the sand and woods of the area around the Oregon Coast where I bought it. If this is all it needs, then I will be super happy, because once I get it riding condition, I will take it apart and repaint the parts that need painting (all of it), as well as replace the light bulbs in the gauges and the headlight. I will also be adding turn signals so I can ride in the dark. I am loving this project bike!
 

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Discussion Starter #16
I should never have checked the needle valve on the carburetor! Now I have fuel gushing from an outlet near the choke. Well, a fast drip actually. I checked the section on carburetors again and it says nothing about this. Any ideas what I did wrong while I put the separate sections back together? I removed and replaced the top of the carburetor and needle valve back on a couple of times, and therefore I am hoping I have that back on correctly. But, this is most likely where my error is. Thoughts and suggestions please. At least until the new carb arrives...


Thanks.
 

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Fuel leaving the carb, is usually a float and needle valve problem. Something is likely hanging up, as in getting stuck. You only choice is to take it apart and figure it out.
You are not alone. Many of us have done it many times.

UK
 

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Discussion Starter #18
Thanks Uncle Krusty. I guess tomorrow when it is light again, I will check whether I have the floats stuck or something similar. If that is not the issue, I will just wait for the new carburetor which is on the slow boat from China. I have enough other projects to keep me busy for a while yet and a camping trip with my wife to the Oregon Coast in December in our pop-up tent trailer which I just rebuilt the roof for. So, this is a test run to see how I did on that project. So many things on my plate, and so many things to look forward to, like riding my little project bike. But, knowing me, I will continue attempting to get this bike running properly before the carburetor arrives!
 

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Have fun. It has been a while since I went to the Oregon Coast. Going to be a bit chilly this time of year. We are sometimes on the boat during the winter months, but we have a diesel heater when not at the dock.

UK
 

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Just wondering if it could be a bad float? pin hole leak and float position being low causing carb to get more gas than it needs.
had this happen on 1 lunger gas engines.

Our Unkle Krusty is of course right, old stuff does rob volts like switches ! And the more current you pull through a switch,
the more volts you will lose.
 
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