Mark III - anti-kickback device and sprag

Looking for help with a mystery.....

I bought my Mark II just over a couple of years ago. The e-start was working fine (not sure if the PO upgraded the starter or not, but it worked perfectly - luck me!)

A few months later, I was replacing the clutch center, tightening the inner primary, etc. When I pulled off the engine sprocket, the Sprag clutch/bearing unit fell apart on my bench.

I bought a new one (the 18 tooth version, vs the original 14 tooth) from OldBritts. Bolted everything back up and had several months and a couple of thousand miles of happy Norton e-start bliss.

Then the starter would occasionally just sit there and grind away for a few seconds before turning the engine over. And when I hit the kill switch there was a whirring/clicking under the primary, as if something was spinning for a couple of extra seconds after shutdown.

The starter performance got progressively worse and about 4 months ago, it wouldn't turn the engine over at all. Recently decided I wanted the e-start luxury back, and stupidly, instead of taking off the primary case to investigate what was really happening when I pushed the starter button, I blamed the starter itself, and ordered one from Dyno Dave.

I bolted his on and ------ same problem. Shit. By now the primary was off and I could see the starter motor spinning the starter gear (clockwise), which in turn was spinning the engine sprocket (counterclockwise). Both were spinning strongly, but the engine was not turning over.

Pulled the engine sprocket off, and once again, the sprag unit came apart in my hands. Or at least, 3 of the bearings fell out and I gave up trying to reassemble it.

I figured there must be a reason for the sprag to turn to crap so quickly. It's inside touches the crank gear, and the outside engages the engine sprocket, so I:
Checked the outside diameter of the crank gear - perfect at 1.625"
Checked the inside diameter of the engine sprocket - oops, it was 2.285, and thus .004" over spec.

Ordered a new engine sprocket and new sprag unit (ouch -- $200!!).

Reassembled and the engine spun right up. However there was still a brief click and whirring noise on shutdown, and I could see that the gears where spinning for a second or so on shutdown.

After a couple more starts and stops - I was back to the original status. The sprag was not engaging the engine sprocket. Just spinning wildly.

Shit - what's going on?

I decided to try earlier advice of Fred Eaton, and per one of his technical articles on adjustment of the anti-backfire device, but of course, that changed nothing.

This morning I pulled the sprag unit out. Before taking it out, I confirmed by hand that the crank gear would spin freely in both directions, indicating that the sprag was not grabbing the drive sprocket. Upon examination on the bench, one of the 18 "teeth" was on the sprag way out of alignment with the others, which led to a diminished external radius, which meant it couldn't possibly engage. I wiggled a small nail inside the unit, jiggled it a bit and got the offending "tooth" to go back in place. Reassembled the crank gear, sprag, and the engine sprocket, and I was unable to spin it counterclockwise, by hand - as it should be.

Two questions:

1) anybody have a similar experience? - my first 18-tooth sprag unit failed after a few months, and this replacement didn't even survive a few minutes. Hmmmm..... ??

2) I both tightened and loosened the nut on the anti-backfire device and no longer know where it is supposed to be adjusted. I understand the concept - it should be tight enough not to slip, but should allow the unit to slip at 50 ft lbs of torque. By the way, I carefully read Fred's article on this unit's assembly and adjustment process: http://www.oldbritts.com/ob_start.html , but since I don't have his handy little home-made tool, I am looking for the suggestions of others on how to set the right torque.


Hope to figure this out soon, my right leg is getting tired.

Thanks.

New sprags are pricey. I actually bought a new one (before the last Andover price hike) when I started rebuilding my MKIII and when I went to replace it, I noticed I already had a newer 18 bearing version already so I left it alone. Previous owner must have changed it at some point. At least I have a spare now but I hope I don't encounter anything your dealing with. I never messed with the anti-kickback either. I took it out and simply popped it back in the way it was.

Hi pkeithkelly,

I had a similar experience rebuilding my mk3 last winter. The sprag came apart in my hand. I ordered a new one (Andover) from Fred and used it a couple dozen times, also with a Dave Comeau starter, when it crapped out (spinning its little heart out), and then, like you, discovered the sprocket ID was out of tolerance; same reaction (ouch, $200). I've used the starter heavily since then bringing the bike to life, say 40-50 cranks, and its still holding together. I sent my anti-backfire assembly to Fred for testing and adjustment (it was a bit low at 43 ft-lbs) and I recommend the same process for you. Fred turns the assembly around in 1-2 days.

I'm getting a bit nervous now hearing your story, since it is so close to mine and without a happy ending so far. I don't believe that the anti-backfire device could cause this problem; its such a dead-simple device. Still, Its worth the shipping and time to have Fred set it up. Fred is also a good resource for Mk3 advice, as is Stan at Rocky Point Cycle.

Are you certain that the crank gear OD is not a bit undersized? You may want to ship it to Fred as well to have him measure it.

One last thing. When I finally got the entire sprag assembly together and began exercising the starter, I was often greeted by an awful grinding noise, say every other start. My clutch plates were not releasing (too old and wet). I replaced the plates and the starter has been very smooth since (I always start with the clutch pulled in). I also use a Pazon Surefire that operates with an advance curve that produces zero degrees advance at zero rpm, a reassuring anti-kickback feature.

You'll get it fixed with only a little luck needed. Keep us Mk3 guys informed of your progress.

How much is it for them to check it?

Rick -


Thanks for the info on your situation. Sounds familiar.

I am certain that the OD of the crank gear is fine. I checked, double-checked, and triple-checked it with a high quality caliper borrowed from my neighbor. It is exactly where it is supposed to be, at 1.625"

And the ID of the brand new drive sprocket ID is exactly on spec, at 2.281"

Instead of sending my anti-backfire device to Fred to adjust, I will figure out a way to set it to slip at 50 ft lbs of torque. I see from Fred's technical article that he welded an old 12 mm socket to an old thrust washer. But I don't have an old thrust washer to sacrifice to the cause. I will figure out some way to get a torque wrench on the shaft and the gear fixed in a vice. Fred's article also said that some Mark III owners do the adjustment while the device is installed - first by slacking the nut quite a bit, so it slips, then gradually tightening it until it will just turn over the engine. I can see a challenge in this in that the nut can keep slipping out of position as you apply the starter. That way you would lose your previous adjustment point from which to apply a bit of fine tuning. I think I'll do the adjustment on the bench instead.

What I still don't understand though - why did my brand new sprag clutch/gear unit start slipping right away, given that the two gear faces it engages with are perfectly in spec (and one of them brand new)? (Yes - the sprag unit was inserted properly - to give the right direction of rotation to turn the crank!)

Your point on noisy clutch plates is a good one. But I always have the clutch in when I start the bike with the starter. And my clutch is in good shape.

Anybody else with ideas on the mystery of the sprag unit?

Thanks

Keith

pkeithkelly said:

I see from Fred's technical article that he welded an old 12 mm socket to an old thrust washer. But I don't have an old thrust washer to sacrifice to the cause. I will figure out some way to get a torque wrench on the shaft and the gear fixed in a vice.

Click to expand...

I did it using some short lengths of welding rod that I fitted into the shaft splines, and then jammed a socket over them.


Note that Norvil motorcycles will not offer a warranty on the sprag parts unless all three parts of the sprag assembly are replaced together: http://www.norvilmotorcycle.co.uk/064733.htm

Quote:
_________________________________________________

"Please note, if you need to replace any of the following, you will need to replace ALL of the following at the same time:-
Sprag Clutch (064733)
Sprocket (064681)
Gear Assembly (064731)
The reason for this is that they all wear at the same rate and failure to replace them together often leads to premature failure of the new part
For this reason, there is no warranty on any of these three parts unless they are replaced together (normal warranty rules apply for parts that are manufactured incorrectly e.g if a part is faulty upon receipt, it will still be exchanged under warranty)"
________________________________________________

L.A.B. -

thanks for the tip - using welding rods to wedge a slightly oversize socket over the shaft. I was just thinking about doing that about 15 minutes ago.

I looked at Fred's notes. He welded an old thrust washer on an old socket to create a custom tool.

I noted the price of a thrust washer on OldBritts, at $6.20 and am tempted to order one and sacrifice it to the god of "Special-Tools-For-One-Time-Use-That-Will-Sit-In-The-Back-Of-A-Drawer-For-20-years"

Also - thanks for the quote from Norvil. I did order a new Engine Sprocket Gear (06-4681) with the new sprag unit, but since the ID of the Starter Drive Gear (06-4731) was perfectly on spec, I opted not to replace it.

Thanks for the pointers.

keith