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Author Topic: Tacometer reading WAY too high... maybe I didn't hit 12k rpm  (Read 694 times)

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Offline BestBoy

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Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« on: September 16, 2019, 07:02:21 am »
So in a previous post I mentioned that Id accidently hit 12k on my K2.

However, Ive been thinking it was showing the revs to be way too high so I checked today.
It says Im doing 9k at 70mph in 5th.

I know thats wrong. My gearing is: 18/48

Whats going on!? Any ideas?
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Offline ekpent

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #1 on: September 16, 2019, 07:07:31 am »
 Probably should be showing more around 5K at that speed. I would say its broke  :D
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Offline Scott S

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #2 on: September 16, 2019, 08:06:31 am »
 Or the tach is from a different model. What bike do you have?
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Offline BestBoy

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #3 on: September 16, 2019, 08:11:17 am »
1972 K2
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Offline RAFster122s

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #4 on: September 16, 2019, 02:32:09 pm »
Would that K2 be a 500, 750, etc?


72 K2 would preclude it from being a 550...
350K was a twin and 12k might be possible.
« Last Edit: September 16, 2019, 02:36:24 pm by RAF122S »
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Offline Scott S

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #5 on: September 16, 2019, 02:53:34 pm »
 I see from his first post (ever....not just on this thread) that it's a 750.
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Offline MD

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #6 on: September 16, 2019, 07:02:04 pm »
I'll jump in since I am having the same issue.  '75 CB550F Supersport.

Replaced the cable hoping it would solve the issue.   :(  It did not.  What I have been able to glean from yesterdays research is there are two possible issues.  One the mount the the cable connects to at the base of the tac is loose and the rotating magnet is touching the spindle that moves the tac needle causing it display faster.  The other issue can be that the oil in the tac has dried out. 

I found a UK vendor that will rebuild the tac.  Does anyone know of a vendor in the States?

Thanks.

-MD
« Last Edit: September 16, 2019, 07:03:45 pm by MD »
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1975 CB550F Super Sport;  Lake Superior Circle 1000, 45-90 Saddle 1000, All in Yooper 1000 and SS 2000 in 48 hrs:  1985 GL1200A, MN in State SS1K

Offline BestBoy

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #7 on: September 17, 2019, 03:35:16 am »
Thanks!

Ok Im gonna open it up. Yes its a K2 750.
Ive compared engine note to others on Ytube (Tach reading vs speed) and Im not slipping clutch.

Id say at 5k I'm reading 2k over and at 7k im reading about 3k over.

OK lets see whats inside this thing...
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Offline carnivorous chicken

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #8 on: September 17, 2019, 10:18:15 am »
When my tacometer is reading too high, I switch to pozole for a day or two...
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Offline pjlogue

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #9 on: September 17, 2019, 11:32:12 am »
It may be due to the counter balance spring getting rusted and weak inside the tac.  When I restored my gauges I noticed a very fine coat of rust on the spring and thought if it continued the speedometer and tac. would be off.  I coated them with a light oil to help slow further rusting.  I kind of doubt they are repairable unless you found a spring with the exact same "force" and size.  I can't think what else would cause it other than the drive magnet getting physically closer to the driven disc. 

-P.
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Offline PeWe

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #10 on: September 17, 2019, 11:38:22 am »
CB500 tacho wiill show too high rpm. The red area later too, I think 9500.
CB750 tacho has red from 8000.
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CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
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http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
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http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
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Offline Gene

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #11 on: September 17, 2019, 11:53:14 am »
Quote from: carnivorous chicken on September 17, 2019, 10:18:15 am
When my tacometer is reading too high, I switch to pozole for a day or two...

this is under-rated comedy.
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*1973 CB750K3 (Bow)

Offline BestBoy

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #12 on: September 17, 2019, 02:27:30 pm »
Yeah, the inside got a good clean. Took off the extra weight of dirt added to spinning magnet etc. No good. Its defs the spring loosing tension and being easier to coil. Looking at replacing it with a single Speedo/Tacho combo. Probably digital.
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Offline Scott S

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Re: Tacometer reading WAY too high... maybe I didn't hit 12k rpm
« Reply #13 on: September 17, 2019, 03:16:02 pm »
 Trim one coil off of each spring and bend the end back over so it will fit on the post.
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'17 Triumph Street Scrambler
'81 Yamaha XS650
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