Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature currently requires accessing the site using the built-in Safari browser.
We noticed you are blocking ads. DO THE TON only works with community supporters. Most are active members of the site with small businesses. Please consider disabling your ad blocking tool and checking out the businesses that help keep our site up and free.
Bought a 75 cb 125 a couple months back as a project for me and my kiddo. It ran great when I test drove it, did not go off the side road, only brought it up to 5krpm. brought the bike home, and took it for a ride on the main st. and anything above 6500rpm the bike sputters, and stalls. there is an aftermarket chinese carb on it right now. It starts in the freezing cold with the choke off. when you choke it cold it wont start. also I ran a warm compression test, it was only 100PSI, I believe the mannual suggested 175 as a good compression. What next? rebuild help it run better at high rpms? Or does it prob. need a carb also?
Lots of possibilities here, but by your description of not needing choke when cold it sure sounds like you have an over-rich condition either due to too rich carb jetting or some other carb problem. Another possibility is a restricted air filter, is it stock? The stock filter is a foam sleeve and normally works fine, but check that whatever is on there is not modified with some really restrictive material or a super small aftermarket filter.
Does the bike rev OK above 6500 in neutral at 1/3 to 1/2 throttle? If so the slow jet and slide needle are probably in the ballpark. But if it craps out at 1/2 throttle and above when under load then that points to the main jet too big or possibly the air passage to the emulsion tube or the small side holes in the emulsion tube are plugged. I'd be suspicious of the Chinese carb. Try to find a buddy with a CB/CL/SL 100 or 125 and swap to a stock carb as a test.
Along with being too rich there could also be an ignition problem causing weak spark. Spark problems usually show up at higher rpm/heavy loads. Check points gap and timing, also spark plug obviously. And this bike absolutely needs to have a good fully charged battery to run right. Key off you should see at least 6.5 volts across the battery terminals which should rise to over 7.5 with headlight off and revving to 5K.
People often do bad compression tests by failing to test a hot engine, failing to hold the throttle wide open when testing, and MOST COMMON - using the wrong test gauge setup. Especially on small displacement engines you need a gauge with the check valve directly at the spark plug end of the gauge or gauge hose. If you try to use a gauge with a longish hose with the check valve at the gauge end of the hose rather than the engine end, then you are effectively increasing the volume of the combustion chamber by the volume of the hose. So that lowers the compression ratio, just the opposite of milling a head to increase compression ratio. In any case that engine will still run very well with 100 psi compression so don't tear it down trying to fix the bad running problem just yet. But do check for adequate valve clearance. Hope this helps.
It does have a small aftermarket filter. The bike does the same if you rev it in neutral as if you are driving it. 6500 either way farts out. I did check the engine hot for compression, but I also used an automotive type compression tester with a long hose. I also noticed today that the clutch somehow siezed over the past week, any way of breaking it free without ripping it down?
Hmmm. . . if the problem was running too rich I would not expect it to crap out at the same rpm load or no load. That might point more toward ignition breakdown. But first try running it with no air filter at all to see if the problem might be related to a restrictive filter.
These early 100/125 engines are prone to wear in the camshaft journals and head bearings (the cam runs directly in the cylinder head, no replaceable bearings) if they don't get regular oil changes or if run in dirty air with one of the gauze and screen type air filters that are notorious for passing dirt. When that happens the end of the cam where the ignition advance mounts wobbles around causing erratic timing and points gap. Check for wear in the left cam journal by turning the crank to TDC compression stroke, then putting a small flat blade screwdriver between the points plate and advance cam and gently prying upward. There will always be a bit of clearance but if you see the cam bolt move up and down considerably then you might have a worn head and cam. Combined with points gapped too wide and/or a weak points spring that might cause the points to float above a certain rpm. Try stiffening the points spring (blue steel blade), gapping the points down to .012 - .014", reset the timing and see if that makes any difference. As I said at first there are lots of possibilities, just trying to suggest some ways of eliminating possible causes.
By the way if you need a CB125 shop manual there is a pdf version factory manual here:
http://www.dotheton.com/downloads/Honda%20CB100%20CB125%20CL100%20SL100%20CD125%20SL125%20Workshop.pdf
Im going to check the points tomorrow, The carb with the filter off yielded the same issues. Thank you for the manual, my other half is ticked that I ran the printer out of ink....lol
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
