Rough running low down and high idle when warm - Page 5

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.

Darren69 · 24-03-2020, 04:15 AM

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.

350TSS · 24-03-2020, 05:36 AM

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.

FrankenDesmo · 24-03-2020, 07:42 PM

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.

Darren69 · 24-03-2020, 08:03 PM

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.

tridentperu · 16-09-2020, 04:23 PM

V interesting post, but did you ever get this finally sorted out? I am having similar ...frustrating...issues with my M600. It is definitely carbs and I am getting the same sort of symptoms. But, mine is definitely running too rich, both plugs are black. I get the same sort of hesitation picking up from low speed 3000 rpm ish, which would seem to me to be the transition point from pilot to needle/slide.

As you did, I have stripped the carbs, new float kits to stop the flooding on one side, seal kits etc etc etc. All seems in decent shape. I have dynojet needles but std mikuni jets.

Needles in 4th from top slot. Air screws 2 1/2 turns out as per Dynojet instructions.

I'm not doing anything until I get it through an mot and can try it on the open road. It did get to the Mot station and back (in disgrace) yesterday and ran OK apart from the hesitation/ fluffing as mentioned. A few things to sort before re test, so will get them done.

FrankenDesmo · 16-09-2020, 04:47 PM

I recently moved the needle clips to 4th from 5th which was a marked improvement (still a slight hesitation but not so much that it concerns me now). My setup sounds the same as yours - dynojet jets, pilot 2.5 turns out. I did replace the needle jets though as they oval out over time (factory Pro nickel-plated ones are the best option though), so you might want to give that a try if you didn't when you had the carbs stripped. Didn't replace the needles themselves, which I believe are the dynojet items still.

FrankenDesmo · 16-09-2020, 04:48 PM

Although with yours being an M600 I'm not sure if the clip and pilots settings would be equivalent, and presumably you've got a different set of jets to me.

tridentperu · 17-09-2020, 10:47 AM

That's the next thing, I'm going to drop the needles a notch and see what happens. No, I didn't replace the needle jets themselves, or the needles, just the floats and all the o rings etc.

I wonder if someone has mixed parts up and left the std needle jets in but just fitted dynojet needles, which from the pics I have studied are much thinner than Mik ones so would deliver more fuel...

Further fiddling required and more unwanted practice taking air box off...although I can do this in about 10 mins now!

J

FrankenDesmo · 17-09-2020, 01:44 PM

I'd recommend replacing those needle jets as they are a really common cause of rich running due to the mixture not atomising correctly, especially at lower engine speeds. That's a good point about the dynojet needle - mine already had mikuni needle jets with the dynojet needles, so I didn't think too much of it.

Tbh I think the next step for me will be a jet kit from Factory Pro as well as sorting out the valve shims (w/ mbp retainers), as they tend to be related to rough running.

24-03-2020, 04:15 AM	#62
Darren69 Transmaniacon MOC Join Date: Sep 2013 Location: Sutton In Ashfield Bike: Multiple Monsters Posts: 6,091	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. __________________ Roast Beef Monster! Termignoni and Bucci - Italian for pipe and slippers! S4 Fogarty, S4R 07T, 748, Series 1 Mirage

24-03-2020, 08:03 PM	#65
Darren69 Transmaniacon MOC Join Date: Sep 2013 Location: Sutton In Ashfield Bike: Multiple Monsters Posts: 6,091	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. __________________ Roast Beef Monster! Termignoni and Bucci - Italian for pipe and slippers! S4 Fogarty, S4R 07T, 748, Series 1 Mirage

24-03-2020, 12:44 AM	#61
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.

24-03-2020, 05:36 AM	#63
350TSS Too much time on my hands member Join Date: Jan 2013 Location: Shipbourne Bike: M900 Posts: 1,422	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.

24-03-2020, 07:42 PM	#64
FrankenDesmo Member Join Date: Sep 2019 Location: Glasgow Bike: M900 Posts: 108	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.

16-09-2020, 04:23 PM	#66
tridentperu Registered User Join Date: May 2019 Location: Gloucester Bike: M600 Posts: 36	V interesting post, but did you ever get this finally sorted out? I am having similar ...frustrating...issues with my M600. It is definitely carbs and I am getting the same sort of symptoms. But, mine is definitely running too rich, both plugs are black. I get the same sort of hesitation picking up from low speed 3000 rpm ish, which would seem to me to be the transition point from pilot to needle/slide. As you did, I have stripped the carbs, new float kits to stop the flooding on one side, seal kits etc etc etc. All seems in decent shape. I have dynojet needles but std mikuni jets. Needles in 4th from top slot. Air screws 2 1/2 turns out as per Dynojet instructions. I'm not doing anything until I get it through an mot and can try it on the open road. It did get to the Mot station and back (in disgrace) yesterday and ran OK apart from the hesitation/ fluffing as mentioned. A few things to sort before re test, so will get them done.

16-09-2020, 04:47 PM	#67
FrankenDesmo Member Join Date: Sep 2019 Location: Glasgow Bike: M900 Posts: 108	I recently moved the needle clips to 4th from 5th which was a marked improvement (still a slight hesitation but not so much that it concerns me now). My setup sounds the same as yours - dynojet jets, pilot 2.5 turns out. I did replace the needle jets though as they oval out over time (factory Pro nickel-plated ones are the best option though), so you might want to give that a try if you didn't when you had the carbs stripped. Didn't replace the needles themselves, which I believe are the dynojet items still.

16-09-2020, 04:48 PM	#68
FrankenDesmo Member Join Date: Sep 2019 Location: Glasgow Bike: M900 Posts: 108	Although with yours being an M600 I'm not sure if the clip and pilots settings would be equivalent, and presumably you've got a different set of jets to me.

17-09-2020, 10:47 AM	#69
tridentperu Registered User Join Date: May 2019 Location: Gloucester Bike: M600 Posts: 36	That's the next thing, I'm going to drop the needles a notch and see what happens. No, I didn't replace the needle jets themselves, or the needles, just the floats and all the o rings etc. I wonder if someone has mixed parts up and left the std needle jets in but just fitted dynojet needles, which from the pics I have studied are much thinner than Mik ones so would deliver more fuel... Further fiddling required and more unwanted practice taking air box off...although I can do this in about 10 mins now! J

17-09-2020, 01:44 PM	#70
FrankenDesmo Member Join Date: Sep 2019 Location: Glasgow Bike: M900 Posts: 108	I'd recommend replacing those needle jets as they are a really common cause of rich running due to the mixture not atomising correctly, especially at lower engine speeds. That's a good point about the dynojet needle - mine already had mikuni needle jets with the dynojet needles, so I didn't think too much of it. Tbh I think the next step for me will be a jet kit from Factory Pro as well as sorting out the valve shims (w/ mbp retainers), as they tend to be related to rough running.