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Old 05-06-2015, 04:36 PM   #1
RocketMan OP
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Joined: Nov 2004
Location: Woodbridge VA
Oddometer: 769
1975 CB125S project and occational hesitation issue

About two years ago I picked up a nice little 1975 CB125S which i never intended to be a full on restoration project, but rather something to ride around town and take on a few longer rides of say 100 or so miles out into the country side. It was in generally good shape and with just a little fiddling and rebuild of the carb, it been running pretty well. over the past years I've started cleaning it up some and its actually starting to look pretty good overall. Compression is down some but with 17,000 miles on it that's to be expected.
Engine before
[
engine after

bike before

current ( lacking the rack I made)


So far i have replaced the stator with used unit, points, diode, blinker, bulbs, all new, headlight bucket, headlight ears, headlight rim, front turn signals, used but in good shape. Also replaced the Givi rack with a home made unit but can still mount the bags.I have new handlebar switches but have not replaced them yet as they are not quite stock as they were made for bars that don't have wiring running inside. New shift lever and seal and new output shaft seal, used valve adjustment caps, new after market rear shocks, not stock but much better and stiffer. Just rebuilt the front forks with new springs and covers, seals were still good so did not replace them.
The new springs were too short so I made some spacers from PCV pipe but they were .09 Inches to small inside dia. so I had to grid them out a bit to fit the fork legs.
old stuff on top of photo, new parts lower


Chrome actually cleaned up pretty good.
before

after


Bike runs well, starts first kick and I've put around 4000 miles on it. On other leaks have shown up anywhere other than a very slight weeping around one of the adjustment caps, got new o-rings to replace them when I get ready to rebuild the top-end this winter.

I get around 70 miles per gallon and I ride this bike pretty hard as seen here


About the only issue I seem to encounter is that sometimes when accelerating esp when really cranking it on, it will hesitate as if the fuel level is too low, but I've checked that and its right on spec. The harder I push it the more pronounced the issue, but as soon as I back off the throttle and roll it on more gently it clears up. and it only does it in the first few gears and not all the time, some days I can ride 30~60 miles without issues other days its pretty consistent for a time then goes away. Using the blinkers seems to make it worse so that why I don't think its a fuel issue. Timing, carb points, advance all checked and set/working as expected, plug in good shape. I can't get it to do it while sitting only when moving and usually only under hard excel. Battery checks out and voltage to battery is what should be seen All bulbs new and all the same wattage.

Any ideas? shorting somewhere? bad capacitor (since its only under excel that it happens)? Weird that I can't reproduce it while sitting, jiggling wires does nothing and key switch is new. The bike has no voltage regulator it just dumps excess voltage to the battery.
other than that the bike is a total hoot to ride and gets lots of looks and folks asking me about it.
RM
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RocketMan screwed with this post 05-06-2015 at 05:30 PM
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Old 05-06-2015, 06:10 PM   #2
Andyinhilo
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That sort of issue is usually due to a bad battery in my experience with old Honda twins and singles. The ignition system is really marginal to begin with and when the battery can't provide its share of the load you get the hesitation and missing, especially when the throttle is open. More air=higher cylinder pressure=harder for spark to jump. When you activate the turn signal or any other electrical load it will get worse.

My first bike was a 1973 CB175. When I let it sit and the battery would get low it would start and idle well, easy acceleration was OK, but open the throttle and it would miss or just die. charge or replace the battery and all was well. YMMV...
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Old 05-07-2015, 04:00 AM   #3
RocketMan OP
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Quote:
Originally Posted by Andyinhilo View Post
That sort of issue is usually due to a bad battery in my experience with old Honda twins and singles. The ignition system is really marginal to begin with and when the battery can't provide its share of the load you get the hesitation and missing, especially when the throttle is open. More air=higher cylinder pressure=harder for spark to jump. When you activate the turn signal or any other electrical load it will get worse.

My first bike was a 1973 CB175. When I let it sit and the battery would get low it would start and idle well, easy acceleration was OK, but open the throttle and it would miss or just die. charge or replace the battery and all was well. YMMV...
Interesting, you are the second person to mention that, someone else from the yahoo cb group said the same thing. While the battery is fairly new, about a year and half old, it has boiled dry a few times in this first 6 months before I thought keep an eye on it (having gotten used the sealed batteries) and the stator went bad this past winter and it was running off just (or mostly) battery power for awhile. While the battery does seem to hold voltage and measures correctly and takes a charge, it could well be not up to snuff as I know its not just about voltage but other factors such as amperage. I think I'll get a new one as they are only 20 some bux and available locally and see what happens.

Thanks.

RM
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Old 05-07-2015, 10:41 AM   #4
Spurlock
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Hi RM,
From your description it does sound like ignition breakdown is a possibility, possibly a weak coil or resistance in the ignition circuit. I'd clean and tighten all wiring between the battery positive and the coil, and double check grounding of the negative battery terminal and the coil to frame. As has been stated these have marginal ignition power, they work fine of everything is perfect but small resistances can cause enough voltage drop to the coil to cause problems. One possible problem is voltage drop across the kill switch. It's easy to bypass the switch (connect the black and black/white inside the headlight shell) and retest. The coil itself might have been damaged by running with the old bad battery which can cause very high system voltage.

By the way nice work so far on fixing it up.

-Bill
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Old 05-07-2015, 06:35 PM   #5
ericm
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In my experience weak ignition will keep the engine from revving out all the time. Not just sometimes.

The symptoms described sound more like a lean condition. Have you checked fuel flow through the petcock? It or the filter or float valve in the carb may be partially plugged. Also the tank vent may be plugged. When that happens fuel will flow acceptably for a while, until the vacuum forming in the tank slows it down. The clue here is that it runs better when you loosen the filler cap and replace it.
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Old 05-07-2015, 07:59 PM   #6
vtwin
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Try checking the fuel screen on the petcock. May have a bunch of debris in it.
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Old 05-08-2015, 03:38 AM   #7
RocketMan OP
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Thanks all but I believe I have found the issue(s).
while I too first thought it might be a fuel related issue I had ruled that out in the past as I did a complete rebuild of the carb with all new parts, flushed and cleaned the tank ( which is in really surprisingly good shape inside given its 44 years old, its a 71 SL125 tank which is why its green) replaced the screen in the petcock and seal, new gas cap with vent good, fuel line is new. Float bowl and idle and air setting spot on, though it took a while to get a good steady idle and air screw setting. timing is spot on and points clean and set. The fact that the blinkers affected the issue is what lead me to believe it was an electrical issue. Blinker is new and stator and diode new as well.

After the first suggestion of a battery issue and knowing the old stator had fried the battery several times I charged it up yesterday morning and also cleaned and reset the plug gap which had somehow gotten wider(?). When for a ride and no issues with blinkers on headlight on under hard excel. So I got a new battery and have been cleaning all contacts throughout. most again are in amazingly good condition for an old bike. Put a little meter to monitor voltage this morning.

So basically it boils down to the fact that a stupidly simple machine usually has stupidly simple issues and usually just requires a stupidly simple solution! Not sure if the issue was the battery or plug, or a combination of both, but will keep an eye on both from now on.

Thanks again.

RM
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Old 05-08-2015, 05:20 AM   #8
lesman
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I would say battery.1.5 years on an acid cell batterythat has been dry few times. The selenium rectifier does a parasitic draw on the battery. There are cheap new versions that eliminate this problem. They look pretty easy to make , also. The life of a 6 volt acid battery is not very long.

Does your bike miss when you hit the brakes, horn or run the turn signals?
That would be the battery.
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