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Old 04-08-2014, 11:33 AM   #1
190e OP
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Joined: Feb 2013
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idle speed lift test

Engine idling normally and fully warm. Flat top Bings


Test:
I lift the throttle cable slightly on each side in turn.

left cylinder gentle and relatively smooth increase in idle revs.

Right cylinder reluctant increase in revs and relatively rough response.


So I have poor pick up from idle on the right cylinder. I'd welcome your diagnosis. My thoughts are blocked transfer ports (I checked them recently though) or weak idle mixture on the right cylinder.

Anyone remember carbs with lifting pins that could be used to diagnose idle mixture. At idle lift the carb piston by 1/16" with the lifting pin or a screwdriver, which leans the mix a tad.

  • RPM's rise and stay up, that carb is rich.
  • RPM's rise briefly, then drop, mix is about right.
  • RPM's fall, engine gets rougher- mix is lean.

Is that test true for Bing CV's ?

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Old 04-08-2014, 12:50 PM   #2
bpeckm
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I think you are referring to the old SU carbs, which operate on a similar principle.

There is a good book out there on Bings, available from Bing for a quite reasonable price. It will tell you more about tuning, jetting, setting etc than you might need to know!

In general, a stumble off-idle on these is because of something in the idle circuit. The idle circuit has both a fixed jet and an air-mixture screw. Most likely cause would be either a blocked air port, or a blocked jet. If messing around with the idle mixture doesn't do it, then pull the carb and use carb cleaner and compressed air. The idle jet lives up in a vertical tunnel, and has a pesky o-ring around it, check into that as well...






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bpeckm screwed with this post 04-08-2014 at 12:58 PM
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Old 04-08-2014, 01:58 PM   #3
supershaft
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During idle our Bing slides are bouncing all over the place. I don't see how what you are asking about is going to tell you anything.

If your carbs check out and it sounds like they do, the very next step whenever chasing an idle or just off idle issue is to do a leak down test since leaking valves first and foremost effect idle and just off idle. No matter what else is going on, no engine is going to idle well with leaking valves. It could be that you have leaking valves in that cylinder. Very common scenario in my experience. The trouble is is that it takes experience diagnosing a leaking valve unless it is leaking very badly and they can just barely leak and effect idle considerably. I have fixed a number of bikes that wouldn't idle well after they had had the carbs and a bunch of other stuff gone through numerous times since it wasn't the carbs or ignition at all. It was the valves leaking and nothing else that was the cause of the trouble.
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Old 04-08-2014, 02:03 PM   #4
Bill Harris
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Agreed with both posts. An idle transfer port is a prime suspect but the problem may be in the idle jet or the idle air jet (or drilled passage).

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Old 04-08-2014, 03:18 PM   #5
190e OP
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I've checked the transfer ports are clear so we'll see how it runs tomorrow. If it's not that I'll mark the mixture setting then try it richer. This is a very low mileage engine which runs well and the carbs are generally in good order. I'm aware a full carb clean and tune up will probably cure it but I want to try one thing a time and particularly want to know for future reference whether the symptoms were caused by a weak or rich mixture. I want to learn what the test of lifting the idle speed on one side indicates.

Having run right through the winter there is a possibility it's engine breather condensate blocking something. I've already dropped the pilot jet and needle jet to clean out the emulsion tubes once but that was a couple of months back.
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