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Discussion Starter · #1 ·
Hi everyone. Just stumbled onto this forum and so glad I did! LOTS of knoweldgable people on here...

... I've got a 72 CB 450. Overall it's in very solid shape (10,800 miles). It's also a daily commuter for me. Lately, When accellerating in first gear from a stop, it feels like it's missing a tooth or two--the engine will race for a split second almost like it's slipping out and right back into gear. Only happens in first, and if I'm really light on the gas, I can avoid it from happening.

So, is that a whole new Gear box? Something a barely functional mechanical literate can handle? Any ideas on cost to repair? Or does someone know of a decent, reasonably priced mechanic in the Denver area?

thanks so much for any guidance y'all can offer
 

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Do you hear any noise at all? How about if you put it on the center stand and put it in first gear, any sound that would indicate a broken tooth? Clunking?

Could just be that the clutch needs to be adjusted, might be slipping under acceleration.
 

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Sensei
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Sounds like it needs the clutch adjusted to me as well.........But, could be oil additives..... What type oil are you running?....
 

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Sounds to me like worn/rounded engagement dogs on a gear and a damaged or bent shift fork. And either of these items will cause the other to fail. possible shift drum damage as well. If this is the case you'll have to split the lower end apart, probably cheaper to put in a whole nother used tranny/forks/drum rather than buy new parts. Broken gear teeth are a little less common but possible.
 

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Discussion Starter · #5 ·
Thanks for the replies (and PMs) on this one.

No clunking or other weird sounds, so that's good. It sounds like my best (cost-effective) plan is to start with he clutch and work my way down from there...

I'm using a synthetic 4T the guys at Performance Cycle in Denver recommended-- changed it out about 1,000 miles ago.
 

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If it were the clutch it would be worse in the higher gears.
I can't think of any clutch issue that would cause the symptoms you describe.
"-the engine will race for a split second almost like it's slipping out and right back into gear"
I've cratered a few transmissions in my days, this really sounds like a transmission problem.
 

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Not to argue with Tom, but if "easing" into the throttle makes the symptoms go away, it's likely the clutch..... IN MY OPINION, synthetic oils are great, but too "slippery" for these older bike clutches...... Put in some "dinosaur" oil and I'd bet the problem vanishes/diminishes.... (may take two changes to get rid of ALL the synthetics effects....)
 

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( Didn't "steal it"....Just beat you to it....LMAO!)..... :lol: :D
 

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Well OK,
but a slipping clutch is worse in higher gears, not just first gear.
Rounded engagement dogs or broken teeth are a problem in 1 gear only.
Having trashed a few transmissions drag racing, you can ease into the throttle and the gearbox won't skip unless you put some load on it. If you look at what's happening with the shift forks and dogs you can see why this is the case if the dogs are rounded.

Mobile 1 synthetic 4t is specifically made for wet clutches in bikes and should not be a problem, even with a stock clutch. I use it in my zx14 with no issues.

Just sayin......

I promise not to say "I told you so" when Dflo digs in to his motor and finds the problem.

He's gonna need more than clutch work.
 

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Tom....Like you said, bent shift forks or rounded engagement dogs could be the problem...I agree with that, ....but it also could be a worn detent, simply a worn pin on the shift drum, or a number of other things...... just starting with the clutch/oil because it's the cheapest fix, and a bike brought to my shop last week had exactly the same symptoms and an oil change fixed it.....
Also, this only happens from a full stop, when the clutch receives maximum torque against it, and these old bikes barely make enough HP/torque to break a clutch free except in first gear....
A zx doesn't qualify in my mind as a vintage bike (yet), and there have been signifigant materials differences incorporated into its construction, so the syn may work fine in it....
I have been wrong before, and will be again....I never take it personally.... :D ;)
 

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The clutch is on the input of the transmission, slippage is related to engine torque regardless of what gear you are in. Slippage will be more apparent in higher gears. It's easier to load the clutch in high gear than in first gear.
It's not the clutch.........
 

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Sensei
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Half of the clutch is connected through the transmission to the rear tire, which at rest provides the most resistance to movement, hence the most likely slippage point.... It's a physical law called "moment of inertia" or simply inertia.....
I do see your reasoning, but when the wheel and transmission shafts are already spinning in the same direction as the engine is trying to spin them (rolling in higher gears), It requires less "grab" to overcome the smaller resistance due to the difference in spin rates than to go from ZERO to whatever the engine revs are......
I would agree however that higher engine revs (which are generally associated with more torque) would make any slippage more apparent..... But this applies in any gear.....
"A body at rest tends to remain at rest until acted upon by outside forces.... A body in motion tends to remain in motion"....Simple, basic physics.....
I could still be wrong and you correct in this specific case, (trans gear problem)... we'll just have to wait and see....I am however enjoying the exchange of opinions/ideas on the matter...... :D :D :D
 

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Discussion Starter · #14 ·
Wow.

this is a great exchange... learning a lot.

I promise to post back here when I get it sorted out!
 

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Long shot here but could it be the drive chain jumping a tooth, is the front sprocket worn?
I know its only in first gear when pulling away but that is when max torque is on the chain.
I always go for the simple things first.
 

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The facts and only the facts ma'am. You have been only riding the bike to work and the only thing that has changed is the oil, then it's most likely the oil.
I totally agree with Steve and also understand what "gs1327" is implying, but slipping gears will be heard as well as being felt. In these older bikes synthetic oils will cause clutch slippage and are best avoided, only use 10w40 and you will be rewarded with a long lived engine and preferably use an oil suited to high temperatures, for example an ATV or Quad.
 
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