UK Monster Owners Club Forum .: Technical :. Engines, Clutch, Gears » Rough running low down and high idle when warm

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Old 23-03-2020, 11:44 PM   #61
utopia
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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.
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Old 24-03-2020, 03:15 AM   #62
Darren69
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As per Utopia and Flip if its not carb icing or the pods then you could try dropping the needle clip to raise the needle. I'm no carb expert myself but that's what I would try.
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Old 24-03-2020, 04:36 AM   #63
350TSS
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The fact that it occurs at consistent revs/throttle position seems to suggest carburettors and at 4000 to 4500 rpm I think he would be on the transition between the needle and main jet. If it is popping a bit then as well that suggests a weak mixture so raising the needles would be exactly what I would do as well.
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Old 24-03-2020, 06:42 PM   #64
FrankenDesmo
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Interesting idea re: carb icing as that is not something I've ever experienced (at least, I don't think so - I used to ride my original M900 year-round in NZ, and in Wellington the winter days do get down to low negatives on occasion). I think I've still got the heater tap off, so while the 'outlet' will be pumping oil in, it wont be going anywhere, so I'll try a ride with the heaters properly on and see what happens there.

Otherwise I had also come to the conclusion that I either need to adjust the mixture screws or adjust the needle clip (or both).

Funnily enough I had been reading around various places and managed to glean that this is probably a needle to main transition issue. I need to be a bit more attentive to ensure it is only in the 4-4.5k region, and not around 3.5k as well, which would apparently indicate problems in the pilot circuit also.

Obviously adjusting the pilot is a bit easier than moving the needle clip, but hey-ho

Btw the clip is currently sitting in fifth from the top.
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Old 24-03-2020, 07:03 PM   #65
Darren69
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i wouldn't rule out carb iceing. It used to happen to both my SS 750 and 900 and on my commute I would regularly have to stop in the same layby in the morning for a ciggie break while the ice melted and this would happen predictably at around the same place on cool damp mornings after an extended run on fast a-roads then slowing down for a village. If I would stop for 10 mins then it would be ok for the rest of the run into town. Symptoms are like running out of fuel and the bike will not idle without throttle so it may not be the exact problem you are seeing and I would occasionally pull the choke on to keep the fuel flowing and the bike running.
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