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

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2 months 1 week ago - 2 months 1 week ago #6248 by Wobbly

10278 wrote: I checked the exhaust system yesterday and couldn't find any holes.

These are not typically rust holes or some such, but rather leaky pipe-to-pipe joints, like where the cross-over pipe joins the header, or header meets muffler..

10278 wrote: I will check the needle length. I think a micrometer should work.

A micrometer would be over-kill. As I remember the typical distance between notches is close to 1.5mm (~0.060"). With those large increments, even a mark on a pencil will work. The 2 exposed lengths of the needles are either close, or WAY off.

Don't hide 'em, Ride 'em !!
#15150
Last edit: 2 months 1 week ago by Wobbly.

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1 month 2 weeks ago #6420 by 10278
Finally got my hands on a compression tester. Readings are 125 and within a couple of pounds on both sides with the engine warm. The readings are a bit low but I do have low compression cylinder spacers on the bike which were installed in the 80s.
I checked the jet needles and their lengths are a match.
I figured I could try sealing the exhaust joints with heavy grease to see if that makes any difference.

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1 month 2 weeks ago #6421 by 10278
almost forgot, I noticed a fine black residue in the intake tube and the throat of the left carb, which is the side where the problem appears to be.

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1 month 1 week ago #6424 by Wobbly
• 125psi is not great, but it's respectable. Carbs were off while doing this ?

• What about the valves ? Are they correctly adjusted with .004 and .006" clearance cold ?

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#15150

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1 month 1 week ago #6426 by 10278
Yep, carbs were off. valves were set to 004 and 008.
I'll try the exhaust sealing today

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