Starter Mechanism Grrr Noise

I recently converted my existing starter to the 57807BN four pole along with new leads and battery and it works very well. However occasionally when pressing the start button I get the occasional grrrrr noise which seems to be coming from the primary drive. Any thoughts on what it could be? I am thinking it could be the sprag clutch on its way out?

More likely it's the backfire overload device in the starter motor drive train that is releasing under load. It should be set to release at 50FTlbs torque. Some owners set it slightly higher to combat this clattering noise. It won't harm anything if you leave it alone. I bet it only does it when the engine is clap cold (like first thing in the morning) Your converted prestolite, does it still use the plain bush bearings? If so, make sure the rotor cannot contact the drive end plate, as the bush snout wears into the rotor allowing the windings to contact and short out. When checking the backfire device release torque you have to hold the gear wheel in a vice (using soft jaws) and turn the splined shaft with the torque wrench.

It seems to occur hot or cold. If engine is cold I generally give the engine a little prod first before hitting the button. When I had the starter apart for modifying everything appeared to be of a nice fit, I saw nothing that alarmed me. I would almost say the starter had not much use in the past. Plain bush all standard apart from the now 4 pole. Late 850 1977.

Lovel said:

I get the occasional grrrrr noise which seems to be coming from the primary drive. Any thoughts on what it could be?

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As dobba99 says, it's probably the anti-backfire device slipping as it is supposed to occasionally if the engine kicks back and is not a fault as such. If it does that regularly then it might be worth checking the torque setting, however, I suggest you avoid overadjusting the slip torque as a kickback could flip or destroy the sprag clutch if the anti-backfire device doesn't slip.

Lovel said:

It seems to occur hot or cold. If engine is cold I generally give the engine a little prod first before hitting the button. When I had the starter apart for modifying everything appeared to be of a nice fit, I saw nothing that alarmed me. I would almost say the starter had not much use in the past. Plain bush all standard apart from the now 4 pole. Late 850 1977.

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Next time you have the starter apart, measure the run out of the commutator. The last few NOS prestolites i have converted have shown a commutator run out of anywhere between 7 thou to 14 thou. They were like new inside and had not done any work at all.
The prestolite starters were not made very well and have three built in faults. 1. The commutator run out causing the brushes to oscillate in the holder and wear out. 2. The drive end plate wearing into the rotor causing the windings to short out. 3. Motor not sealed from the elements and the petrol wash from the Amals.

Well in the end I opted for fitting a new sprag clutch which certainly seems to have done the trick.