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Topic-icon Automotive Dwell/tach for use on our airheads??

  • Big Bill
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2 months 2 weeks ago - 2 months 1 week ago #5173 by Big Bill
Can an automotive type dwell/tach be used on an airhead. (R90/6). My tach on the bike is not working and I need to check the RPM for a planed tune up. Most of the units I have looked at are for 3,4,5,6,&8cylinder engines but don't mention a 2 cylinder setup. E-bay has a few small electronic tachs which would work for what I need, but would like the dwell function if possible. I wonder what the dwell would be, on an older small block Chevy I would set the dwell at 30 degrees, is there a spec.for our bikes. Thanks for any info.
Last edit: 2 months 1 week ago by Wobbly.

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2 months 2 weeks ago #5174 by Wobbly
No need to buy anything.

Electronic tachs work by counting the pulses of the 12V used to fire the coil. They simply re-calibrate the scale for different number of cylinders. Set your existing meter to "4 Cylinder" and I think you'll be able to divide that in half quickly enough to be able to do what you need.

Hope this helps.

Don't hide 'em, Ride 'em !!
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2 months 2 weeks ago #5175 by Big Bill
Thanks Wobbly, I remember seeing something to that effect on a dwell I once owned. I will need to buy another as mine has been MIA for quite sometime, Thanks.

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2 months 2 weeks ago - 2 months 2 weeks ago #5176 by Wobbly

Big Bill wrote: My tach on the bike is not working and I need to check the RPM...

Allow me to suggest an entirely different approach....

Let's not loose sight of the fact that we are trying to drain the swamp, not buy extra tools we use infrequently. The BMW electronic tach is one of the more robust parts of an outstandingly robust motorcycle. So if your BMW tach isn't working, it's probably a simple wiring fault between the coils and tach. You won't be needing a EE degree to do this job either. This is a single wire running between the tach and coils. On my 1979, the tach wire is clearly visible at the RH coil.

If you repaired the dashboard tach with the same money you're about to spend on a tool you'll use every 5 years, then you could 1) complete your tuning work, AND 2) have the engine speed information as you drive down the road.

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Last edit: 2 months 2 weeks ago by Wobbly.

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2 months 2 weeks ago - 2 months 1 week ago #5178 by Big Bill
My bike is a 1976 R90/6 cable driven tach unit, the speedo still works (about 15 MPH fast) but the tach just swings about the face of the meter so it has been unhooked. True it would be nice to have them both working correctly but the down time and the $$$ holds me back at this time, mostly the 2 or 3 month repair time. I will this coming winter dive into the bike a bit and at that time send them off for repair, or the grand plan is to locate another R90 that is in better shape than mine and have a better base to start off with. Thank You for the info, I have learned a lot from many posts on this site.
Last edit: 2 months 1 week ago by Wobbly.

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2 months 1 week ago #5183 by Wobbly
The vast majority of Airheads on the road have the electronic tach. So when you say "our Airheads" people assume you are talking about the majority.

1. Are you sure you don't have a busted drive cable ?
2. When was the last time you lubricated the inner drive cable ?

A broken or poorly maintained inner cable skips and jumps as it drives the instrument head, often resulting in the needle jumping and fluttering about.

Don't hide 'em, Ride 'em !!
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