What drives the speedo?

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What drives the speedo?

MarkPBG
Is there a plastic gear like on cars that gets worn out. My speedo isn't calibrated, and when riding it, I notice the MPH occasionals jumps around. I'll maintain a constant speed, but the speedo which was reading a steady 40MPH, suddenly jumps to 50MPH, then back down to 45, so basically, I'm not really sure how fast I'm going.

What should I look for as far as adjustments, or parts that wear out that may cause this (like a plastic drive gear?)

Thanks for any help. :)
Mark Davis
Palm Beach Gardens, FL
amateur photographer, hot rodder, motorcyclist, adventurer
"Four wheels move the body. Two wheels move the soul."
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Re: What drives the speedo?

shinyribs
Administrator
Look at the wheel side of the speedo drove and you'll see a grease seal. That plastic gear you mentioned is right there. I have a couple drives laying around. Never rode with one or took one apart to check it out, but you're welcome to one if you want to test it out. One of them came off a 16k mile bike so it probably isn't too wore out. I'm supposed to be sending you something anyway, if you wanna give it a shot ill toss it in the mail .
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Re: What drives the speedo?

cdaiscool
They get really messed up with the grease, and causes the bouncing. I remember reading a thread on here about it...

http://honda-cb750-s.456789.n3.nabble.com/Fixing-servicing-a-DOHC-speedometer-td3891671.html
Turbos, Hondas, 4-bangers, what could go wrong?

Fuelly

Shiny: [...] Considering the weather you've had to put up with I'd say you get an Iron Butt award and a Frozen Nipple trophy to go along with it. First time I've ever posted the word nipple... it ends here.
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Re: What drives the speedo?

MarkPBG
In reply to this post by MarkPBG
Thanks guys. From the link, I can eliminate the cable, that's new. The gauge is fairly poor condition so it could be the gauge or the gear. I have a small chrome speedo I bought for the cafe project, I'll temporarily mount that and go for a ride, if it still bounces, I KNOW it's the gear. :)
Mark Davis
Palm Beach Gardens, FL
amateur photographer, hot rodder, motorcyclist, adventurer
"Four wheels move the body. Two wheels move the soul."
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Re: What drives the speedo?

seestheday
When I took apart my speedo while I was writing that post I noticed that the speedo wasn't seating properly.  It could be as simple as pulling the speedo cable off, loosening the front axle bolt, making sure that the speedo gear seats properly and tightening it back up.

If you do need to repack it, it is still an easy job.  I was worried it wasn't going to work well because the gears were visibly worn, but it's been rock solid since I did that cleaning.
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals

My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.

My cb750 video site
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Re: What drives the speedo?

Thorby
I would check the connection at the wheel first.  Mine should have had a rubber ring connecting the wheel to the speedometer gear but the rubber had broken off or disintegrated, breaking the connection.  I did a cheapo fix by packing paper where the rubber would go and it lasted several months.  Yesterday I tried using silicon but it's not registering speed so I need to take it apart again and see what went wrong (I should remember to take pictures for you guys).
Proud owner of stock '76 CB750.
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Re: What drives the speedo?

seestheday
Thorby, have you seen this: http://honda-cb750-s.456789.n3.nabble.com/Fixing-servicing-a-DOHC-speedometer-td3891671.html

Did your seal disintegrate?  Everything else is metal...
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals

My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.

My cb750 video site