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

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Old 11-03-2020, 07:45 PM   #1
FrankenDesmo
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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 07:49 PM..
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Old 11-03-2020, 09:05 PM   #2
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I use NGK DCPR8E spark plugs in my 900, I think those are the right ones. I also take care (now) to order the suffix type 4339, which are the version with the threaded terminal, so you can unscrew the ally nipple and use them bare threaded.

It looks like you have exhausted all the avenues possible to avoid a carb strip and if the cooler and oil lines are spoiling the fun then it makes it even more justified. ( but could the lines be removed from the carbs and be safe to run with the tap turned off?)

Given what you say about an earlier conversion to the 900 engine makes me wonder if the carbs are right for the engine or maybe they have been played with?
I think it's well worth checking all the jet sizes and so forth... Blueprinting the carbs for want of a better expression.
The problems you are having in the lower rev range and lower throttle settings seem to indicate pilot jet problems. Unfortunately there is no pilot jet as such, it's just a drilling which blocks very easily with corrosion from standing and is somewhat harder to unblock! Ultra-sound cleaning is a good option.

You've been very patient and methodical so far. I think you stand a better chance of success with know clean carbs set to the base settings.

Since you have got the electrical side sorted and new plugs fitted, I would suggest checking the valve clearances while you have the carbs and cooler and other gubbins out of the way. The valves and sparks should always be in perfect order before attempting tuning the carbs.
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Old 11-03-2020, 11:23 PM   #3
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You said on the other thread that you drained the tank.
Did you inspect what came out and if so, was there any water in it ?
If there was, or maybe even if there wasn't, I wonder whether there is any water in the float bowls ...?
To be honest, I wouldn't bet on that being your problem but draining the float bowls might be one more thing that you could try before resorting to stripping the carbs.
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Old 12-03-2020, 01:30 PM   #4
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Quote:
Originally Posted by Mr Gazza View Post
I use NGK DCPR8E spark plugs in my 900, I think those are the right ones. I also take care (now) to order the suffix type 4339, which are the version with the threaded terminal, so you can unscrew the ally nipple and use them bare threaded.
I had not heard of doing that before - is that just to remove any traces of resistance?
They seem to be the same plugs, heat range aside, except yours have a Projected insulator (whatever that means), and take a 16mm socket instead of 18mm.

Quote:
It looks like you have exhausted all the avenues possible to avoid a carb strip and if the cooler and oil lines are spoiling the fun then it makes it even more justified. ( but could the lines be removed from the carbs and be safe to run with the tap turned off?)
True, I do appear to have enough access underneath to undo the inlet/outlet lines for the heater, which would be better than having to remove the whole battery & air box when the carbs need adjustment. Will see how possible that is.

Quote:
Given what you say about an earlier conversion to the 900 engine makes me wonder if the carbs are right for the engine or maybe they have been played with?
I think the carbs are correct, but I have nothing to back this up with other than I would think that it was obtained as a whole unit, or they had an m900 donor of some kind, as the only piece that is still M600 (by the looks) is the frame (which is just the stamping on the tube, as we know the frame is no different). But aye, it could very well have been played with. The assumption is that it is a stock '93/94 900 lump & carbs, but there is no way for me to know until I crack things open.

Quote:
I think it's well worth checking all the jet sizes and so forth... Blueprinting the carbs for want of a better expression.

The problems you are having in the lower rev range and lower throttle settings seem to indicate pilot jet problems. Unfortunately there is no pilot jet as such, it's just a drilling which blocks very easily with corrosion from standing and is somewhat harder to unblock! Ultra-sound cleaning is a good option.

You've been very patient and methodical so far. I think you stand a better chance of success with know clean carbs set to the base settings.
That's what I've been thinking - I need the carbs to be in a known state. An ultrasonic clean might not be a bad idea. I had been hoping to not have it off the road much longer, but it is still early in the year so a good time to put this sort of issue to bed properly. Once I've got them open I'll see if I should order some parts from factory pro (such as the emulsion tubes), if they need replacing. Might as well do it with good stuff - and he has a kit suited towards airbox lid chops, which is what I have.

Quote:
Since you have got the electrical side sorted and new plugs fitted, I would suggest checking the valve clearances while you have the carbs and cooler and other gubbins out of the way. The valves and sparks should always be in perfect order before attempting tuning the carbs.
I had wondered if there could be an ignition issue, but I think I'd have problems all the way through the rev range, rather than part-throttle low revs - as that would seem to be firmly in the area of the pilots as you say.

Quote:
Originally Posted by utopia View Post
You said on the other thread that you drained the tank.
Did you inspect what came out and if so, was there any water in it ?
If there was, or maybe even if there wasn't, I wonder whether there is any water in the float bowls ...?
To be honest, I wouldn't bet on that being your problem but draining the float bowls might be one more thing that you could try before resorting to stripping the carbs.
Could definitely be worth a try - I'm guessing that water will tend to float on top and therefore stay there? Though I did run it dry with the tap off after draining the tank.
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Old 12-03-2020, 02:09 PM   #5
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Oh forgot to say - Valve clearances... Ok, yep, I'm up for the challenge XD, need to learn how to do this at some stage anyway. I'll do some searching around to find out what I need & the process. I know CCW have some videos on how to replace the shims, I think that might have been checking the clearances aswell.
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Old 12-03-2020, 02:11 PM   #6
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I don't have experience of the Mikuni carbs as mine has FCRs but I know that carefully setting the balance at idle and the sync whilst opening makes a world of difference to how well it runs at the bottom end.

Especially given that mine has a tendency to run rough at idle, due to various factors; light flywheel, big carbs, short intakes etc.
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Old 12-03-2020, 02:14 PM   #7
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I would love to put a set of FCRs on, but at ~1000 that's a bit much even for me. Maybe once I've sorted every other part of the bike to my liking
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Old 12-03-2020, 02:58 PM   #8
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I am now considering purchasing a small ultrasonic cleaner, seeing as this wont be the last thing I'll want to clean <eyes up the GT550 carbs on the shelf>

They don't seem to be too expensive. But also I'm probably getting carried away as I'll need to acquire some more tools and probably some carb parts which will all add up.
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Old 12-03-2020, 05:04 PM   #9
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Quote:
Originally Posted by FrankenDesmo View Post
I had not heard of doing that before - is that just to remove any traces of resistance?
They seem to be the same plugs, heat range aside, except yours have a Projected insulator (whatever that means), and take a 16mm socket instead of 18mm..
I use the threaded nipple ones because the nipple doesn't fit into my plug caps, I have to remove them and just use the treaded portion bare. If you need the nipple, the fixed ones are a little cheaper.
Don't ask me what a projected insulator is! Probably like overhead mudguards or underslung overdrive? (Chain.)

Regarding your asymmetric mixtures, I wonder if one of the diaphragms is split and not lifting the needle valve/jet properly?
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Old 12-03-2020, 05:06 PM   #10
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Quote:
Originally Posted by FrankenDesmo View Post
..... I'm guessing that water will tend to float on top and therefore stay there?
No, water is more dense than petrol and will sink to the bottom.
Thus it will settle in the base of the tank and since the outlet to the pump is at a low level, it would be easily drawn into the carbs.
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Old 12-03-2020, 07:34 PM   #11
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Quote:
Originally Posted by Mr Gazza View Post
Don't ask me what a projected insulator is! Probably like overhead mudguards or underslung overdrive? (Chain.)
Alright you got me... what's an overhead mudguard?

Quote:
Regarding your asymmetric mixtures, I wonder if one of the diaphragms is split and not lifting the needle valve/jet properly?
Got the carbs off just now, so I checked that first - nope, they appear to be in fine condition (although now I'm stumped as to how to get them seated and the caps back on without pinching them ).

Quote:
Originally Posted by utopia View Post
No, water is more dense than petrol and will sink to the bottom.
Thus it will settle in the base of the tank and since the outlet to the pump is at a low level, it would be easily drawn into the carbs.
Duh, right, yeah, wrong way around... I drained one of the floats into a glass to see if anything separate out - nothing, but that does make sense given I ran it dry before putting fresh fuel in. Maybe the storage was dry enough in Cumbernauld that it didn't absorb too much (the tank was also over half full the whole time it was stored).

---

Aye so carbs are off (those throttle cables are bloody fiddly - I'm not looking forward to putting them back on!). Before disassembling I thought I'd see if it's possible to get the oil cooler out of the way by removing the heater in/out lines, unfortunately I can barely get at the bolts with a spanner and even then there is barely enough movement to turn them and wanting to press on I gave up on that. I think for future tuning it'll have to be with the airbox off... except that also means battery & coils off, too - how the hell is it even possible!?

I've already managed to chew out one of the infamously soft float bowl screws, not surprised as they're the JIS screws I believe, and I only have a PH1 & PH3 (PH2 might have had a better shot, but the PH1 was fairly snug). Anyway off to B&Q tomorrow to get a small hacksaw and cut a slot into them - then they're getting replaced with allen heads.

So I should be able to get them stripped by Saturday, which is good because I'm away Sunday for the week. If I discover anything obviously stuffed (such as emulsion tubes), I can put the order in for while I'm away. Not quite going to get the bodies off for ultrasonic cleaning though, but hopefully I can find someone local to do it when I'm back.

Took the opportunity to remove the tank and properly drain it, as I'd previously drained it in situ, which didn't get everything. That also opens up access to the valves on the top cylinder, so I'll also be picking up some feeler gauges to check the clearances as suggested.
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Old 12-03-2020, 08:09 PM   #12
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Any had any experience with seal kits such as this: https://www.ebay.co.uk/itm/1-x-Ducat...4383.l4275.c10

Ordinarily I'd just put it all back together, being a bit of a cheapskate, but this time round I'd like to make sure I'm replacing things that should be replaced when one strips the carbs down.
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Old 12-03-2020, 08:10 PM   #13
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You might find these 'how to' videos useful when you come to reassemble your carbs:

http://www.ukmonster.co.uk/monster/s...554#post573554

I came across them by chance the other day...
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Old 12-03-2020, 08:15 PM   #14
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Thumbs up

Nice one Luddite
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Old 12-03-2020, 08:25 PM   #15
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[QUOTE=FrankenDesmo;573551]Alright you got me... what's an overhead mudguard?/QUOTE]

Sorry mate I though everyone knew that? Mudguards usually go "overhead" the wheels, but it was often claimed as a feature by owners of lowly basic models when trying to impress owners of smart models with things like overhead camshafts.

Next you'll be telling me you don't know what a Rolls Canardly is..
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