Very rough idle then runs fine?

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Very rough idle then runs fine?

I bought a nice clean example of a Bandit 1200N last November, which behaved as it should the first few times I rode it. After a while though, it got very difficult to start and warm up.

It came with an SP Engineering Tri-Oval can which without the baffle created an insane amount of popping and cracking on deceleration and felt a bit fluffy. Knowing Bandits can take a race can or performance air filter but not both without rejetting, I pulled the filter and found a foam Pipercross one in there. After putting a standard filter in its much smoother and only has a little pop and crackle which I enjoy 

Only thing is, when I come to start it, I use the choke but I find I have to hold the throttle partially open until its warm or else it stutters and cuts out, taking some work to get it going again. Once it's warm it runs like a dream. I would rather not have to spend 10 minutes watching the rev counter like a hawk when I want to ride it, so is there anything I should be looking at?

Well the obvious thing to do to help it start is to richen the mixture up at idle - this can be done by opening up the pilot screws by a turn or two. There are four of them one per carb, underneath the inlet tracts. If it's running that lean though then it could well be running lean at other points in the rev range. It'd be worth checking the fuel level in the carbs (no need to take the carbs off) or getting it dyno'd.

I have thought about getting it dyno'd, what would a rough cost be for this? I'm hesitant to richen the mixture because I'm sure when it's warm it's running a little rich anyway. As I say, there is popping and crackling on deceleration already. I tried adjusting the idle screw which works until its warm, then the idle speed creeps up too much.

The pilot screws should only affect the idle, besides far better to run rich than lean believe me. Popping and cracking on overrun are common when using a different exhaust to standard. Make sure there are no induction leaks and the timing is spot on before going further. Try simply putting fuel cleaner in the tank first (such as redex), a concentrated dose, give it a good run and see if any better. Tappy is spot on about fuel level too, have the carbs been set up and balanced as this would be my first choice for all the symptoms you describe.

Thanks for the advice. I do love the pop and crackle on deceleration, even though it was massively excessive with the Pipercross filter in, it sounds much better now with the standard filter. I have got a can of Motul fuel system cleaner and a week off work coming up soon, so was thinking this should be the first port of call. I'll fill the tank, chuck it in and run the whole tank through to see if it helps. If not I'll have a go with the pilot screws. Just seems odd to me that it idles perfectly well when warm. I'm not sure if the carbs have been set up properly (I'm leaning more towards not), so I'll look into getting them balanced. Someone mentioned that there may be a Dynojet kit fitted, but with the engine running much better since putting the standard filter back I don't think so. It seems like the last owner just threw on the can and filter to make a whole lot of noise!   One other thing I noticed that I need to sort, that may be unrelated is the terminal ends on the wires to the carb heaters have had water in them so are wet, slimy and a little green. I'll clean these up too.

If it was popping and banging more with the freer flowing filter in then it's probably running lean, not rich. Both will cause popping and banging. Incidentally - running rich isn't much better than running lean, at least not in the long run. The excess petrol washes the oil off the bores and causes fast cylinder wear. As Oldboy says - adjusting the idle screws will only affect idle, and the first tiny bit of throttle movement. If the revs creep up once warm then it's likely that your throttle stop is just set too high. I have to say tho' - these kinds of adjustment to an engine are done by ear, and if you don't have the experience then taking directions over the internet from people that can't hear it isn't going to teach you much, and probably won't improve the bike much either. A basic dyno run can be anything between 20 and 50 quid. Work to correct problems will then depend on what needs doing. It would probably be worth taking the carbs off and seeing whether they've been re-jetted or adjusted so that you at least know where you're working from.

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