Rough running low down and high idle when warm

utopia · 24-03-2020, 12:44 AM

Well, just as I was reading the last couple of posts I was beginning to think .. hang on, I wonder if its still cold/damp enough around Glasgow to cause carb icing.
My 750 seems particularly prone to this and I can tell you that just two weeks ago, en route to a norfolk meet-up, I experienced the symptoms on the early morning outward run, though by the afternoon return they had disappeared.
In my experience, the symptoms usually arise when I slow down following a period of steady throttle, for example when slowing for a roundabout at the end of a dual carriageway.
Tickover is at first erratic, sometimes non-existent and I need to blip the throttle to stop the engine dying altogether but after being stationary (or very slow) for just a brief period (maybe 20 seconds) the tickover returns to normal, presumably as the engine heat soaks back into the carbs.
If there has been insufficient time for this heat soaking to fully take place, then pulling away is also erratic and requires a handful of throttle to avoid stalling.
That's about the best I can describe my symptoms.
It may not be the cause of your residual issues but one fairly simple test would be to dose the fuel with Silkolene proFST, which will eliminate carb icing if used in sufficient quantity.
Folk also say that using premium fuel also helps, but I always do that anyway.
Oh and it seems to be cold and damp conditions which are the most likely to cause carb icing .. and somewhat counter-intuitively with the accent on the damp rather than the cold.
You could try a run on a warm, dry afternoon.

Good call by Flip on the carb pods, by the way.
I was hoping he'd nailed it there.
But anyway, you seem to have cured the majority of the problem.

Here's a bit of carb pondering that I was going to post earlier but held back in the hope that Flip's suggestion lead to a solution....

I'm no carb expert by any means but if there is a hesitation at mid throttle but it pulls strongly on full throttle, my best guess would be that the needle stage needs a tweek.
But please don't take this as sound advice as I don't have the experience to back it up .. its just food for thought based mostly on received knowledge and limited work on my single cylinder dommie which has recently been revamped with a pod filter and free-flowing exhaust.
I only mention it in case you might find it convenient at this point in time (when you are familiar with the carbs and associated bits and pieces) to try a small adjustment of the needle clip position.

24-03-2020, 12:44 AM	#28
utopia No turn left unstoned Join Date: Jun 2010 Location: leicester Bike: M750 Posts: 4,559	Well, just as I was reading the last couple of posts I was beginning to think .. hang on, I wonder if its still cold/damp enough around Glasgow to cause carb icing. My 750 seems particularly prone to this and I can tell you that just two weeks ago, en route to a norfolk meet-up, I experienced the symptoms on the early morning outward run, though by the afternoon return they had disappeared. In my experience, the symptoms usually arise when I slow down following a period of steady throttle, for example when slowing for a roundabout at the end of a dual carriageway. Tickover is at first erratic, sometimes non-existent and I need to blip the throttle to stop the engine dying altogether but after being stationary (or very slow) for just a brief period (maybe 20 seconds) the tickover returns to normal, presumably as the engine heat soaks back into the carbs. If there has been insufficient time for this heat soaking to fully take place, then pulling away is also erratic and requires a handful of throttle to avoid stalling. That's about the best I can describe my symptoms. It may not be the cause of your residual issues but one fairly simple test would be to dose the fuel with Silkolene proFST, which will eliminate carb icing if used in sufficient quantity. Folk also say that using premium fuel also helps, but I always do that anyway. Oh and it seems to be cold and damp conditions which are the most likely to cause carb icing .. and somewhat counter-intuitively with the accent on the damp rather than the cold. You could try a run on a warm, dry afternoon. Good call by Flip on the carb pods, by the way. I was hoping he'd nailed it there. But anyway, you seem to have cured the majority of the problem. Here's a bit of carb pondering that I was going to post earlier but held back in the hope that Flip's suggestion lead to a solution.... I'm no carb expert by any means but if there is a hesitation at mid throttle but it pulls strongly on full throttle, my best guess would be that the needle stage needs a tweek. But please don't take this as sound advice as I don't have the experience to back it up .. its just food for thought based mostly on received knowledge and limited work on my single cylinder dommie which has recently been revamped with a pod filter and free-flowing exhaust. I only mention it in case you might find it convenient at this point in time (when you are familiar with the carbs and associated bits and pieces) to try a small adjustment of the needle clip position.