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Automotive Dwell/tach for use on our airheads??

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Bill Casey
(@big-bill)
Posts: 14
Active Member
Topic starter
 

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.

 
Posted : 08/07/2019 20:15
Richard W
(@wobbly)
Posts: 2546
Member
 

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.

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

 
Posted : 08/07/2019 23:38
Bill Casey
(@big-bill)
Posts: 14
Active Member
Topic starter
 

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.

 
Posted : 08/08/2019 10:23
Richard W
(@wobbly)
Posts: 2546
Member
 

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.

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

 
Posted : 08/08/2019 14:22
Bill Casey
(@big-bill)
Posts: 14
Active Member
Topic starter
 

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.

 
Posted : 08/08/2019 19:51
Richard W
(@wobbly)
Posts: 2546
Member
 

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.

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

 
Posted : 08/10/2019 13:06
Bill Casey
(@big-bill)
Posts: 14
Active Member
Topic starter
 

This one has been loud, and jumping about since I bought the bike in Jan. of this year. I have lubed the cable, and been inside the tach. housing and lubed what little I could and while it did help a little it still is very jumpy and loud, the complete gauge package really needs to be sent to a repair shop/person. I should mention that the PO glued the needles back on when they fall off at some point. The person I bought it from didn't really take care of the bike, and the deeper I get into to it the more I can see wrong with it. Thanks again for the info.

 
Posted : 08/10/2019 14:57

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