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'74 R90/6: left cylinder after or back firing above 3k rpm

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Richard Whatley
(@wobbly)
Member Moderator

Just a guess.... The rocker arms may have not allowed the inlet valve to close all the way, or as quickly as needed, thus allowing the valve to stay slightly open 1 out of every 20 (?) combustion strokes.

It will be very interesting to see if the issue goes away with these repairs. 

[color=blue]Don't hide 'em, Ride 'em !!
#15150[/color]

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Posted : 12/28/2020 08:24
Darrell Watt
(@10278)
Active Member Expired Membership

My local machine shop has said the exhaust valves and guides are beyond spec so I'm sending them off to a BMW specialist for further work along with the rocker arms with the broken bearings. My local guys say the cylinders and pistons are fine but suggest a new set of rings as the end gaps measure .021. My problem is I want to sell the bike as soon as I get it running right but I'm not able to ride it myself to break in the rings, so I'm inclined to leave the original rings in it. Any thoughts? Maybe I should post this as a separate question.

Thanks

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Topic starter Posted : 01/18/2021 14:38
Richard Whatley
(@wobbly)
Member Moderator

@10278 Re-using the original piston rings is permissible. But if you're going to go that way, then it's possible the valve work can also be by-passed. I would simply advise the seller that a nice LONG ride should be taken to seat things back in.

 

Obviously the rocker arm bearings and push rod tube seals are a must-do in order for the engine to operate correctly.

[color=blue]Don't hide 'em, Ride 'em !!
#15150[/color]

ReplyQuote
Posted : 01/19/2021 07:58
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