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Old 08-29-2014, 12:40 PM   #1
swanker OP
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78 R100 Left Cylinder Bogs down until warm

I've been struggling to understand a problem that has creeped up.
The left cylinder bogs down and back fires when given gas.
The right side runs fine and is enough to overcome the issues with the left in order to ride.
After a few minutes the left side runs fine.

After a prolonged ride, >30 min the left side will bog down again with throttle.

Any insight?
I've checked the carb diaphragm and it's not ripped, and is seated correctly.
I made sure the clamps are tight from the airbox to the carb and the carb to head.

Interestingly, this morning after revving the right side a bunch the left side kicked in much faster.

Here are a couple of videos.
Left side 1:

Left side 2:


Right side Fine:

swanker screwed with this post 08-29-2014 at 01:04 PM
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Old 08-29-2014, 01:04 PM   #2
swanker OP
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fixed vids
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Old 08-29-2014, 01:07 PM   #3
hardwaregrrl
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Hows the rubber boot on the left, carb to head?
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Old 08-30-2014, 07:54 AM   #4
Whiteguyphil
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Did you check the valve clearance on each cylinder's TDC (TDC after the intake valve closes)?
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Old 08-30-2014, 09:18 AM   #5
Stan_R80/7
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I recommend swapping the left spark plug wire with the right and see if the problem persists. Good luck!
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Old Yesterday, 03:58 PM   #6
swanker OP
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the rubber boot on the left, carb to head is 4 months old.
put new plugs in (old ones looked decent, left side was a little oily), swapped plug wires, tested for vacuum leaks by spraying carb cleaner around enricher housing and all carb connections - no change in idle speed.

I have not yet checked valve clearances. The left side does seem clankier than the right so will do that soon (wanted to wait until I get another peanut cover so I take care of it in one go and replace the gasket).

Any other ideas?
I've yet to do a compression test, could poor pressure explain this?
I replaced the rings last summer on this side as I basically had no pressure (like 40psi? and it went back up to the low 100s maybe?).
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Old Yesterday, 05:51 PM   #7
Bill Harris
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Another thread had a "hesitation when warm" symptom, it was a carb problem and eventually turned out to be a bad plug cap, too.

http://www.advrider.com/forums/showthread.php?t=998450

FWIW.

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Old Yesterday, 08:38 PM   #8
supershaft
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I have had spark plug caps act like carb issues a number of times. You can most always see a bad spark plug cap arcing as the engine misses in total darkness. Real handy trick diagnosing V8's! Airheads too.
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Old Today, 06:00 AM   #9
swanker OP
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I've swapped the plug wires and there was no difference.
I'll check that other thread to see what the carb issue was.

Could a cold compression be just enough below where it needs to be and then as it warms up compression rises to where it needs to be for proper combustion?
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