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Old 02-25-2012, 06:09 PM   #1
R100RT Mark OP
Gnarly Adventurer
Joined: Feb 2011
Location: Georgia, USA
Oddometer: 310
R100RT Constant Throttle Surge – Cured

Just sharing a problem identified and cured as this might help someone out in the future.

Two weeks ago my 1994 R100RT started to exhibit a gentle hesitation/surging on constant throttle at Interstate speeds, typically starting between 65 and 70mph. Felt like fuel starvation. More annoying than limiting performance, but something that clearly needed attention.
Timing, spark, valve clearances, floats, float needles and diaphragms were known to be good, and jet passageways clear, as I had checked all over the prior two months. Furthermore, there was no real sign of inhibited fuel flow, and a “brake cleaner spray” check for air leaks on/around the carburetor at idle, showed no indications of a problem.

Decided one or other of the idle mixture screw, idle jet, or main/needle jet holder O-rings could be the culprit and I knew they had not been touched in the last two years/5,000 miles. Replaced all last night after ensuring seat/passageways clean. Inspecting the old O-rings showed one of the main/needle jet holder O-rings was a little "gunky" and one of the idle mixture screw O-rings was a little squashed, as if it had been trapped sometime before. I was a little more generous with the silicone grease than I have been in the past to aid installation.

Result - Instant cure. Ran 300 miles today without a hiccup.

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