Mk3 Starter Drive Issue

Hi all,
I'm attempting to fix an electric starter issue in my 1975 Mklll in which the starter simply "spins" when button is pushed.

I am looking for advice and suggestions to address reasons why the starter won't engage.

On rare occasions, it will engage, but it's rare and hot or cold motor does not seem to make a difference.

I have the starter drive assembly apart. One thing I do plan to do is adjust the torque setting on the backfire gear assembly, per Old Britts recommendations (50 ft-lb torque).

Sprag bearing is OK and the tolerances are all within specs.

Thanks!
Stephen

chasesa said:

Sprag bearing is OK and the tolerances are all within specs.

Click to expand...

The crucial part is whether the sprag (and the rest of the starter drive) is turning the crank? You can observe this by operating the starter with the primary cover removed (whilst keeping all body parts out of harms way).
If the large drive gear outboard of the engine sprocket turns when the starter is operating but the crank does not turn then the sprag is not engaging.

Thank you, L.A.B.
I shall give that a try.
If that's the issue, it would obviate the need to disassemble the backfire protection device.
Stephen

chasesa said:

If that's the issue, it would obviate the need to disassemble the backfire protection device.

Click to expand...

I would suggest you check the slip torque, regardless.

L.A.B. said:

chasesa said:

If that's the issue, it would obviate the need to disassemble the backfire protection device.

Click to expand...

I would suggest you check the slip torque, regardless.

Click to expand...

+1

I found mine would not release at all. I disassembled and polished all the parts and was able to salvage it. You could see where it was all galled up from slipping until it locked up tight.

Best price I've found on a new sprag:


http://www.vxb.com/page/bearings/PROD/O ... 7Aod5TsAZw

Twice in my MK3's life the starter would engage the crank, just spin. The first time it was the sprag and a new one cured that problem. I still don't really know why it just slipped though as it looked fine. I do suspect the automotive grade of oil though and now I use a bike specific 20w50 synthetic in the primary. The other time the backfire device itself failed so check that as well. If I remember correctly a diaphragm spring in that bugger had cracked effectively relieving it of all it's tension so It refused to ever lock.

L.A.B. said:

chasesa said:

Sprag bearing is OK and the tolerances are all within specs.

Click to expand...

The crucial part is whether the sprag (and the rest of the starter drive) is turning the crank? You can observe this by operating the starter with the primary cover removed (whilst keeping all body parts out of harms way).
If the large drive gear outboard of the engine sprocket turns when the starter is operating but the crank does not turn then the sprag is not engaging.

Click to expand...

Hi L.A.B.
As suggested I tested the crank for spin with the installed sprag and it did not turn when the starter was engaged.

The sprag that has been on the bike seems to be OK, no cracks in housing, scored parts, etc. Actually, I had installed it 3 years ago.

In addition, the drive sprocket ID seems to be in spec at 2.281" and crank gear OD is 1.625".

So, I installed a new sprag, Stieber DC 4127 (3C). The crank gear would turn clockwise but not counter-clockwise (no ani-backfire ass'y installed). That's correct according to Old Britts. Then installed the anti-backfire ass'y to test starter and voila, the crank turned and no spin, as before.

What is puzzling is why a seemingly good sprag bearing with low, low miles will not work. I wonder if it's oil, except I use Castrol GTX 20-50.

chasesa said:

So, I installed a new sprag, Stieber DC 4127 (3C). The crank gear would turn clockwise but not counter-clockwise (no ani-backfire ass'y installed). That's correct according to Old Britts. Then installed the anti-backfire ass'y to test starter and voila, the crank turned and no spin, as before.

Click to expand...

I take it you meant the crank gear turned but the crank didn't?
I can only think there's too much clearance?

Edit: It seems I misunderstood what chasesa said, from a PM I received from him it appears the starter is now turning the crank and the new sprag is not slipping.