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Old 11-03-2020, 08:45 PM   #1
FrankenDesmo
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Join Date: Sep 2019
Location: Glasgow
Bike: M900
Posts: 108
Rough running low down and high idle when warm

I thought I'd better start a new thread in the appropriate section for this - I had been posting in http://www.ukmonster.co.uk/monster/s...ad.php?t=57899.

Sorry, this is going to be a bit of a long tale with some backstory. Grab a cuppa and settle in.

My M900, from what I believe, has spent most of the last 10 years in storage, coming out for MOTs once or twice in that time. I purchased it from a chap in Cheltenham, who had only purchased it from someone near Edinburgh about a month or so beforehand (and hadn't really ridden it much - he changed the belts and the oil and that's about it). The owner near Edinburgh appears to be the one that converted from an M600 to an M900 (and then put it in the garage?). I got about a week or so riding out of it in early-mid september before the blown RR came to my attention. I vaguely recall it having a wee bit of a hesitation if you gave it a sudden burst of half or so throttle, but that's about it (I chalked it up to it needing a new air filter and perhaps plugs - maybe a carb tune up).

Now, I was a bit foolish and parked it up with a full tank of supermarket E5 for ~5 months, until I got it to my new house, got the new RR, and got it fired up and charging the battery again. Great I thought, time to ride it! Nope, now it coughs and splutters all over the lower rev range up to about 5k, at which point it pulls like an absolute train, and idles at just shy of 3k (only once warmed up - idle is 'normal' when cold). Giving it more than half throttle helps with the hesitations, as it passes through the rough zone quite quickly, but the high idle is always there when warm.

It really is very powerful, so it seems as if half throttle+ is more or less ok on both cylinders, so I don't suspect anything like lack of compression (this is much more powerful than any of my previous monsters - which were a high mileage 'W' head '99 and an S2R800).

I initially thought I had it when I discovered the vacuum line for the scottoiler was loose, and fell out easily when I went to trace it to it's origin. However, plugging up the L-piece and replacing the fuel changed nothing for the idle, and spraying WD40 around the intake runners made no difference. I also put in a new air filter (pipercross) and a pair of new standard NGK DR9EA plugs (the existing Denso plugs were the equivalent of an '8' in an NGK, so I've gone slightly colder), as well as a bit of injector cleaner in the tank.

The plugs, both the existing Densos and the new NGKs, show the same thing - black on the front cylinder, white on the top cylinder. However, feeling the exhausts, the front cylinder is the one cough and hesitating, whereas the top is very smooth with much more force. Or maybe that's not a 'however' and I should expect that if the front is perhaps a bit rich and the top is lean.

I had ordered a carbtune thinking I need to sync the carbs and possible play with the idle mixture and speed adjustments, however I now discover that because I've got carb heaters I can't unbolt and move the oil cooler out of the way to get at the screws . Not that it probably matters at this point as it really seems like an air leak somewhere, and according to the carbtune manual it should be idling normally anyway when I go to sync

I'm at the point where I think it's time to take the carbs off and strip them for a clean & check (and while I've got the airbox off I can probably do the sync etc), but before I do that, does anyone have any suggestions of anything obvious or simpler that I may have missed?

Last edited by FrankenDesmo; 11-03-2020 at 08:49 PM..
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