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#712998 - 10/28/17 3:47 pm Tachometer accuracy  
Joined: Aug 2016
Posts: 5
Ehab Sahawneh Offline
BritBike Forum member
Ehab Sahawneh  Offline
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Joined: Aug 2016
Posts: 5
United States
I’m doing my first tuneup on my newly acquired 68 Spitfire Mark IV, while checking the timing with a digital strobe light that has RPM readout, noticed my Tachometer was off by about 500 rpm. At idle the tach was reading 1500 while the digital light was reading 2000. My tach is a 3:1 RSM 3003/06 that was supposedly rebuilt by the previous owner. Any thoughts on the cause and how I can verify / troubleshoot?


1968 BSA Spitfire mkiv special
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#713105 - 10/29/17 6:57 pm Re: Tachometer accuracy [Re: Ehab Sahawneh]  
Joined: Oct 2001
Posts: 237
sloppyoil Offline
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sloppyoil  Offline
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Posts: 237
Monclova
Anymore the original stuff is a crapshoot. I have two bikes, one bsa and one triumph that have the original gauges. They work perfect, Ill not touch them except for greasing the cables every few years. They are not daily riders anymore. Those dam things were nominal when they were new, always off a bit and bouncy. Having them rebuilt is another thing. Just finished a 66 triumph in June for a customer. I had him send his own gauges out as I did not want the hassle. Well one works ok, the tach but the speedo is slow and is bouncy past 35mph. Everything is new with no binding. I, just purchased and installed within the last 3 weeks a set of the New Smiths electronic gray face clocks for a BSA I just finished for myself. They look exactly like the originals except for the mileage digital numbers. They work flawlessly. Speed is right on the money as are the rpms. No bouncing, fluttering or whatever, they function just like all the new speedos and tachs on a new Harley, perfect. Look up Tom Kulen at Smiths gauges and buy a set, you'll be happy you did.

#713106 - 10/29/17 7:08 pm Re: Tachometer accuracy [Re: Ehab Sahawneh]  
Joined: Aug 2001
Posts: 3,408
gavin eisler Online content
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gavin eisler  Online Content
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argyll. scotland, uk
There is an internal adjustment , simply put the magnetic element is moved closer for higher readings or further away for slower readings.
If the needle is bouncing its usually a drive cable fault, not the head itself, these instruments are smooth and steady when working correctly.


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
#713108 - 10/29/17 7:14 pm Re: Tachometer accuracy [Re: Ehab Sahawneh]  
Joined: Oct 2001
Posts: 237
sloppyoil Offline
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sloppyoil  Offline
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Monclova
When was that??? LOL I mean when they work correctly.

#713132 - 10/29/17 11:08 pm Re: Tachometer accuracy [Re: Ehab Sahawneh]  
Joined: Aug 2001
Posts: 3,408
gavin eisler Online content
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gavin eisler  Online Content
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argyll. scotland, uk
My 71 tacho works well, its a black face, i had a grey face since 1982 , it also worked well, I swapped the grey for the black to match the repop speedo ( it also works fine).
not much goes wrong with the tacho cable, its helped by the poor crankcase breathing, the speedo gets a harder time, if the needle is jerking its the cable winding up .


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
#713140 - 10/30/17 12:47 am Re: Tachometer accuracy [Re: gavin eisler]  
Joined: Oct 2008
Posts: 1,654
LarryLebel Online content
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LarryLebel  Online Content
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Canada
I had a speedometer on my B44 that pulsated. I bought a used replacement that worked fine and still is a few years later, so it wasn't the cable.

#713164 - 10/30/17 3:24 am Re: Tachometer accuracy [Re: Ehab Sahawneh]  
Joined: Jun 2002
Posts: 9,307
Stuart Online content
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Stuart  Online Content
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Posts: 9,307
Scotland
Hi,

Originally Posted by Ehab Sahawneh
68 Spitfire Mark IV, while checking the timing with a digital strobe light that has RPM readout, noticed my Tachometer was off by about 500 rpm. At idle the tach was reading 1500 while the digital light was reading 2000. My tach is a 3:1 RSM 3003/06 that was supposedly rebuilt by the previous owner. Any thoughts on the cause and how I can verify / troubleshoot?

With respect, if a Brit twin engine is really "idling" at 2000 rpm, the engine's screaming away and the bike's dancing 'round the garage or down the drive. Even 1500 rpm idle is too high, trying to get it into gear causes noisy crashing and grinding. frown Idle should be 1000 rpm or a little under.

So, if the engine isn't actually screaming away, it's the "digital strobe light that has RPM readout" that's "out"? Ime, out of the box, these are normally set up for a four-cylinder engine with a distributor and single coil; not only is your bike a twin, it also has a separate coil and HT lead to each cylinder; if your strobe's working out rpm from a coil or HT lead pickup, likely it should be set to 'single cylinder'.

Hth.

Regards,

#713194 - 10/30/17 12:57 pm Re: Tachometer accuracy [Re: Ehab Sahawneh]  
Joined: May 2004
Posts: 3,548
BSA_WM20 Online content
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BSA_WM20  Online Content
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Joined: May 2004
Posts: 3,548
Sydney Australia
Have you set your digital Tach to single cylinder ?
From an ignition pont of view A65 ignitions are single cylinder .
next problem is a have you fitted an electronic ignition with a wasted spark ?
If so the readings on your digital tacho will be 2 times what the actual revs are due to the wasted spark.

We have all been caught out with this stuff before.


Bike Beesa
Trevor
#713216 - 10/30/17 4:04 pm Re: Tachometer accuracy [Re: Ehab Sahawneh]  
Joined: Aug 2016
Posts: 5
Ehab Sahawneh Offline
BritBike Forum member
Ehab Sahawneh  Offline
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Joined: Aug 2016
Posts: 5
United States
Thank you all, now it’s clear to me, I did not set the digital tach to single cylinder, it was reading 2000, double, so it was idling at 1000, which makes sense since it sounds like a normal idle. So my tach is reading 500 higher. I will try Gavin’s suggested adjustment to lower it. Thank you all, very helpful!


1968 BSA Spitfire mkiv special
#713998 - 11/06/17 1:56 am Re: Tachometer accuracy [Re: Ehab Sahawneh]  
Joined: Aug 2016
Posts: 5
Ehab Sahawneh Offline
BritBike Forum member
Ehab Sahawneh  Offline
BritBike Forum member

Joined: Aug 2016
Posts: 5
United States
Reporting back. Took the tach off and apart, oh my, it was really tough to get the bezel off without damaging it! But with patience it came off. Used the adjustment screw that increases the gap between the magnet and the needle plate. Was able to get pretty accurate readings between 2-5K rpm but still about 100 high at idle. Thank you all for your help


1968 BSA Spitfire mkiv special

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