Advance unit

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bobert1953
Posts: 2
Joined: Fri May 14, 2021 6:06 pm

Advance unit

Post by bobert1953 »

I'm refurbishing my bike (72 R75/5 90k), rebuilt heads, valves, seats, rings, etc. My issue is with the advance unit. The cam tip had been broken off and correctly repaired before purchase. (I've had the bike for the past 40K). When I seat the unit onto the cam, the fit isn't tight. With the unit mounted and the screw snugged up, I can rotate the unit about 5 - 10 degrees. I presume the detent on the cam tip is buggered. I have been able to tighten the unit down and maintain good performance but I'm concerned it's getting worse. Is there a fix beyond a new cam? I've read about others using shim stock into the gap with success. Thoughts? Recommendations?

TIA

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schrader7032
Posts: 7687
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX

Re: Advance unit

Post by schrader7032 »

Welcome to the forum! Not sure if there's a way to fill in the detent in the cam nose in order to tighten things up. But one thing you should be doing regardless is put the advance unit on and turn to hit the stop of the detent such that the spinning camshaft helps to push the unit against the detent shoulder. That way you get consistent settings and hopefully prevent it from moving.

As for an alternative, the Boyer electronic ignition is an option and is often used when the threads are buggered on the end of the camshaft. The Boyer relies on electronic advance curves, so no need for the older unit.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

bobert1953
Posts: 2
Joined: Fri May 14, 2021 6:06 pm

Re: Advance unit

Post by bobert1953 »

If the detent is buggered then how does the Boyer ignition mount and register on the cam?

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schrader7032
Posts: 7687
Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX

Re: Advance unit

Post by schrader7032 »

I can't say, but the Boyer unit is one thing people go when the threaded portion of the cam has been sheared off. I could be wrong, but I don't think it relies on that detent.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

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Flx48
Posts: 54
Joined: Sun Oct 25, 2020 3:11 pm
Location: NW CT

Re: Advance unit

Post by Flx48 »

You might consider a crank (as opposed to to cam) triggered electronic ignition such as Enduralast, EZA, Vape, etc.
Best-
George

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