Anti backfire gear wear

Hi all, my first post on this excellent forum. I'm in the middle of the dreaded rear isolastic replacement on my Mk 3 and I'm in the process of removing the primary chain case and associated bits. On inspection it looks like the anti backfire gear (06-4707) has been slipping a fair bit as there is a groove worn between the holes that house the ball bearings and some rounding of the hole edges. I'm not sure what is an acceptable level of wear before it needs to be replaced but they are not cheap so I'm not in any hurry to do so. Also I have a 4 brush Prestolite starter and it makes a hell of a screeching racket as it turns the motor over and I'm wondering if this might be related due to incorrect torque settings, even though it turns the motor over OK. Any ideas?

The individual parts of the backfire device are available separately, you don't need to buy the whole device. The wear you see maybe from an old issue that has been resolved.

Thanks guys, that makes sense. I read the Old Britts article, he gets into some detail there but doesn't touch on potential mods after a starter upgrade. I'm picking that the screeching is the ball bearings slipping through the gear then. My only concern is the wear - I note the gear in your photo looks immaculate gtiller. Was that a new part? or is that how it came off the bike?

Personally I would stick with the 50 ftlbs, this setting its designed to protect the components in the starter drive train. I have 4 coil,4 brush starter Motobat battery HD cables all works just fine. Yes the Back fire device operates every now and then, but surely that's what its there for.
If yours is screaming, it's built wrong or buggered

That is so helpful, this is starting to make sense now. Whilst I'm rebuilding the AB mechanism another question - how does the peen on the nut work? do you treat it like a tab washer and ditch it after a couple of uses?

Esmerela said:

Personally I would stick with the 50 ftlbs, this setting its designed to protect the components in the starter drive train. I have 4 coil,4 brush starter Motobat battery HD cables all works just fine. Yes the Back fire device operates every now and then, but surely that's what its there for.
If yours is screaming, it's built wrong or buggered

Click to expand...

Yes indeed It is built to slip at a certain point to protect the entire starter train components from violent kickbacks ( I know too well... ouch ) , so let it slip when it asks to slip. That's it's job. 50 ft. lbs. torque to slip point should be just fine no matter how upgraded the starter , cables or battery. Enjoy.

Balmain Bill
I use the upgraded starter 100% as no kickstart but had similar problems as yourself. Tightened the backfire mechanism right up in desperation (old style crescent in vice) and centre punched threads, crude but effective.
No more screeching but also use original spec 20w 50 mineral engine oil in primary case and most important, set ignition to standard 28-29 degrees max, not 30-31degrees as some advise for electronic ignition.
A healthy backfire will usually take a tooth off the small gear just under the starter pinion although it will still function, sort of.

Time the ignition using the crankcase plug slot on the lower right hand side rather than the alternator side timing indicator on initial setup, mine was 3 degrees over advanced compared to the crank. You can then put the correct marks with a fine centre punch on the alternator and indicator.
Never had a backfire or screech since, even with my Boyer Micro Digital and a half flat battery a couple of weeks ago.

Thanks for that @KiwiNeill, hadn't really put much consideration around preventing backfires as a remedy. I've got Tri Spark installed and they talk about 29 degrees for the timing. I'll certainly use the 20w50 as there's a lot of positive feedback on it as a preferred lube. I plan to build the tool as described in the Old Britts technical note to get the torque right and will post how I go.

Balmain Bill
Hopefully that will solve your problem, I,ve set both Trispark and Boyer to 29 degrees, works perfectly.
The Mk3 starter mechanism is why I ended up using the recommended 20w 50 mineral oil in the primary case rather than ATF favoured for other Commandos although other Mk3 owners may have used it successfully.
Changed every year - 200cc, don,t use synthetic or the screeching will return and the sprag hates it, too slippery.

ATF-F is perfect for pre-MK3s

However, it’s too thin for the chain tensioner on a MK3 - hence the recommendation to stick with the specified engine oil in your primary.

Balmain Bill said:

Hi all, my first post on this excellent forum. I'm in the middle of the dreaded rear isolastic replacement on my Mk 3 and I'm in the process of removing the primary chain case and associated bits. On inspection it looks like the anti backfire gear (06-4707) has been slipping a fair bit as there is a groove worn between the holes that house the ball bearings and some rounding of the hole edges. I'm not sure what is an acceptable level of wear before it needs to be replaced but they are not cheap so I'm not in any hurry to do so. Also I have a 4 brush Prestolite starter and it makes a hell of a screeching racket as it turns the motor over and I'm wondering if this might be related due to incorrect torque settings, even though it turns the motor over OK. Any ideas?

Click to expand...

Does your prestolite starter still use plain bush bearings? You said it screeches when turning the engine over. If the AB was operating it makes more of a clatter, screeching sounds like a dry bearing. Where the motor sits under the carbs it can regularly get doused with petrol, especially amals when tickled, this over time washes the lubricant from the bearings and can lead to screeching, bearing wear etc.
I don't advise 'tightening the AB mechanism right up' Its there for a purpose, to protect the starter motor and its drive train.
When setting the AB release torque you need to grip the gear in a vice using soft jaws and turn the spindle with a suitable tool ( a 06.5652 thrush washer welded to a scrap 1/2" drive socket will do the job) and torque wrench set to 50 FTLBS.
This is a bitch to do without the correct tools as the action of the mechanism when it releases turns the adjusting nut so altering the required setting. I use a different method of locking the nut in place which makes adjusting the setting a doddle, i always had bother with the peening method, its a crap way of doing it .
With the AB mechanism locked you stand a good chance of destroying your starter motor or worse, locked engine etc.
Check the alternator timing indicator reads true using a degree disc and tdc indicator down the plug hole, if out it has slotted holes so you can set it true.

gtiller
that,s true but probably too thin for the starter mechanism too for the reasons mentioned above.
I ditched the original oil filled chain tensioner ages ago and fitted a manual Harley Sportster type.
The last 6000 miles have been the longest period of trouble free starter and primary chain operation my bike has seen, no kickstart so 100% electric.
Have to change the drive chain soon so will also try a Chainman Duplex primary chain.

dobba99
a locked anti backfire mechanism is like a solid gear as in the rest of the starter geartrain, it won,t lock the engine any more than they will although it is a useful device if you have a backfire or overload problem, like a broken sprag or primary chain rollers fouling the mechanism.
The hell of a screeching racket Balmain Bill describes is almost certainly either the anti backfire or sprag but a clacking type screech is usually the former. The left side plain oilite bush or the smaller right side one are unlikely to make such a noise and the small one usually wears oval.
Eliminating the reasons behind backfire, notably incorrect timing or carburation is the best way forward and staying close to the standard settings is pretty good in this respect, even in a mildly tuned bike.

The Mk3 timing plug slot is a good way to set up the timing for normal use, mine was spot on using a dial indicator and degree wheel on the last rebuild and I marked the slight offset needed for a max 29 degree advance.
The out of spec, left side primary timing indicator was corrected to match.