Tuned S4Rs stutters below 3K, rough in steady cruising: not the coil, after all

[Howie]

Is your local high octane fuel ethanol laced?  Back in the old days before E10 using fuel with a higher octane than needed was just throwing away money.  Not so today.  Ethanol does not atomize as well as gasoline. 

[Moronic]

Howie, it's not stated on the pump that the 98 RON fuel here has ethanol in it, and so I think it is ethanol-free. Good thought though.

She seemed to go bad again on the freeway today. Something tells me this isn't over yet after all. 

Okay, if it's NOT the fuel, and there is no error history showing up on the diagnostic tool, what else could it be?

Just to reiterate from earlier posts, bike has 55,000km, new fuel filter was on the service sheet from 7000km ago, when the valves and belts were also done. Went into the shop at around 51,000 running as well as ever on the stock system, came back with an Arrow pipe and remap and this intermittent problem, which for the first 3000km was simply a problem (which I thought was a mapping problem) then came good intermittently.

Today I was riding along the freeway thinking: no this still isn't quite right, the motor feels "tight", like it is too tightly bound to the throttle and there is no overrun nor that feeling of ease a duc has when running well. Felt lean to me. Still felt smooth though, which meant it hadn't gone fully bad. Then my right hand went to sleep, a tell-tale sign, and I realised it probably had (just) gone fully bad. Was heading home and so turned onto a normal metro road soon after.

Water in fuel? Dud spark plug? Gunked up injector? ECU responding badly to the Tuneboy treatment? Dodgy fuel pump?

When she is running well, she sort of squirts off the bottom in the lower gears when you use just a little bit of throttle. When running badly it is as though you have to haul her along with the throttle. (The difference is much more subtle than I've made it sound, though. It is more like the contrast between eagerness and indifference.)

Shop tested the TPS, said it diagnosed fine, but recalibrated it.

[Speeddog]

How big of a deal is it to re-flash the ECU again?

I'm thinking there's something dodgy there.

The ground wire under the aft bolt attaching the ECU is there, yes?

[Howie]

How big of a deal is it to re-flash the ECU again?

I'm thinking there's something dodgy there.

The ground wire under the aft bolt attaching the ECU is there, yes?

Good possibility considering the time line.  Might not be a bad idea to empty your fuel tank and in stall a new filter first. 

[Moronic]

Thanks guys very much for the thoughts, I will have a look at the ECU but I don't think they've needed to move it so it should be the same as it was beforehand.

Re the flash, I think they reflash it every time they adjust the map, and they made a couple of changes the first time I brought it back, thinking just the map was out a bit for steady cruising. So unless there is a real problem with the ECU, I think that is okay.

I am starting to think it must be a fuel filter issue, maybe getting cleared sometimes at least a bit when I fill the tank.

Shop will fit a fuel pressure gauge and also clean out the tank, and we'll see. When they've a slot available. Sigh. This is getting very frustrating. I appreciate the support. Still seems weird it made good power on the dyno, but felt odd when I rode it away from the shop. Yet has come (very) good a couple of times.

[Howie]

Are they running the bike on the dyno simulating throttle opening and load where you are having the problem or mapping at wide open throttle full load?

[Moronic]

Not quite sure how they do it, but from discussion it appears they do run it on the dyno under a range of throttle settings, then test-ride for the very low-end and steady cruise stuff.

Point I was making though is that it made 128hp on the dyno with stock airbox and with the baffles/db killers still in the Arrow mufflers. On a dyno that measured it at 115 stock. And even more to the point, on those occasions when it has come good on the road, it has been very good right through the range.

So it seems implausible that they could have tuned it so well if it was exhibiting this issue all along. And then test ridden it and concluded it was all good.

On the other hand, she felt less than stellar to me on the suburban ride home after i picked her up.

So, either something has happened between the tuning and my arriving to get it.

Or, the implausible is nevertheless actual and the bike ran well enough in its rough state to make that number at the top and produce a good looking curve. (Which I'll admit is wavy at the low end, so perhaps a sign all was not right. Sorry, don't have an electronic copy.)

Or something in between, I suppose.

Which still raises the question: it was running fine stock, so what has happened since? (Remembering that it's not the map itself, because it still runs fine sometimes.)

[Howie]

Do ask them what they find when they clean the tank. 

[Moronic]

Okay, it feels like I have been chasing this problem forever but I suppose it has only been a few months. And I think my shop, Pro Twin in Perth, Western Australia, has got to the bottom of it.

It took me a while to convince them there really was a problem. But once they got onto it, boy did they look into it thoroughly.

The misfiring and stumbling arose at very small, constant throttle openings and revs under 6K. When they put the bike on the dyno to check it in these conditions, the fuelling on the rear cylinder showed up as very spiky and lean.

What was causing that?

Pro Twin checked, adjusted or changed just about every possible problem part. That included ECU, coils, injectors, fuel pump and filter, crank sensor, compression, valve clearances and valve timing, retune with open airbox and no muffler baffles. No joy.

Finally, it was observed that the rear throttle body was sucking in air through the butterfly pivot bearing, which when removed proved quite worn.

A fresher set of throttle bodies was substituted.

Lo and behold, the stumbling and misfiring diminished dramatically. I have put 1200km on the newer throttle bodies, and while you can still feel a stumble when cruising, a lot of the time the bike feels much like it had on those few occasions when, as I noted in previous posts, it felt like it had come good.

Pro Twin has proposed that the problem originates with the Arrow exhaust. They think it is generating a very low pressure zone downstream of the throttle butterfly at small openings, which is drawing air through the bearing and leaning out the mixture. They think the effect is more pronounced on the rear cylinder, which is where most of the misfiring occurs.

Or at least, that's how I understand what they told me.

I think the "coming good" was a matter of the butterfly spindle from time to time settling differently in the worn bearing and closing or partially closing the air leak. At least, that hypothesis seems consistent with the range of performances the bike has exhibited since the exhaust went on.

The bike is now much better, but given the riding conditions over here - lots of boring stuff on the way to the corners - I would love to be done with the stumbling altogether.

So ... does anybody know of an S4R/s Monster with a full Arrow exhaust that doesn't stumble in steady cruising around the 70mph mark?

And secondly, what do people think of the suggestion that the Arrow is short on back pressure and has anyone found a way to compensate for or resolve that?

BTW: the upside of the Arrow is obviously the free flow when you open her up. It is an exciting exhaust when you're on it, and I'd not be at all surprised if it offers a few more ponies up top than its rivals.  

[koko64]

Wow.
So your model's TB's don't have a seal on that part? You would reckon there should be a seal on a moving part that could leak air under vacuum. Brad and others would know. I ask because I have seen TB seals available through one of the forum sponsors CCW, but I think they were butterfly shaft/spindle seals. You said butterfly pivot bearing. You would think there should be a seal in there somewhere. I'll go and have a look and come back.
Anyway someone will chime in, or you could contact Brad, Speeddog, etc. Might be worth hearing what Chris at CCW has to say?

I checked and found a seal "throttle shaft Viton U Cup Seal". I don't know if it relates to your TB part. It is in the Californian Cycleworks MAINTENANCE Section under 'Seals and O-Rings'.
What part does your manual identify?

Interesting to see how servicable the throttle bodies are.

[Moronic]

Thanks for the suggestions. Yes I meant the spindle, which rotates in a bearing of some kind I assume. So a spindle seal would do the trick, all else being equal.

Problem is though, it is not often that seals outlast their bearings. I saw the original rear throttle body and there was A LOT of play at the spindle. Depending on how the seal works, I imagine you'd need to do the bearing as well. A bit disappointing at "only" 55,000km.

Looking at the parts diagram on Ducati Omaha's site, it appears the throttle bodies come as a set and Ducati doesn't supply basic components for them. Don't know whether you can get parts elsewhere. Can I assume they are off-the-shelf bits from Magneti Marelli?

The thing is, the motor wasn't stumbling when it was running the stock exhaust, even with the wear on the throttle body. That is why the diagnosis was so long in coming. So I'm guessing the seal isn't designed to be under much pressure. Happy to be corrected on that.

[jerryz]

The TPS is also critical for correct low speed running

[Moronic]

Thanks jerryz for responding. They have checked and recalibrated the TPS also, and then just in case the diagnostics missed something they substituted another TPS for the original and tried that.

I very much doubt Pro Twin has overlooked a parts failure. They have been over the bike very carefully, putting in big hours - and on their own time, not mine. I've named them here only because I think they deserve public credit for an incredible level of dedication to satisfying the customer. And it is not as though they are short of things to do - they are a very popular workshop and you can wait a month for a booking there.

And to reiterate from my earlier posts, the bike had been running great on the stock exhaust when I brought it in.

However, the implication of all their hard work is that the uneven running does originate with the exhaust.

So, either

(a) there is a big bunch of stumbling, Arrow-equipped Testastretta Monsters out there getting steadily worse as they wear, or

(b) someone, somewhere has found a way to tune out the stumble.

If it's (b), I'm sure the shop will be almost as happy to hear about it as I will be. No-one knows so much that they can't learn something new.

Relatedly, a search on the net has shown up a few threads in which Arrow-ed S4R/s owners complain of a cruising-speed stumble and/or poor low-speed running. In most cases no resolution is reported, while in one case it was reported that the stumble had been fixed by a workshop but the bike's owner had no idea what they'd done.

I should probably get along to the local Ducati club and ask around. A lot of knowledge there. Coincidentally, I was stopped for lunch at a country town last weekend when a group of half a dozen Ducs went through in the 50km/h zone, including a Monster like mine with the same Arrow pipe. it was misfiring audibly just like mine would in those conditions. None of the others was. 

[CDawg]

Late to this thread, but will toss in my 2 cents:

I had similarly rough low end on my S4RS.  Thought it was the nature of the beast, then was about to spring for the ECU re-flash to tame it.  Turned out that my fuel sending unit had a tiny, slow leak. It was small enough that there was never any seepage on the ground, but the wire housing was damp.  Long story short, ECS replace the fuel sending unit and it now runs very well.

[Ak1nza]

I have an 07 S4RS at 10.5K miles with a full Arrow system, stock air box, and no re-flash that I know of.  I run with the DB killers installed.  Running without the DB killers causes a lot of popping and was more noise than I cared for.  The popping is nearly completely removed by leaving the DB killers in. 

I don't have any cruising issues at 70MPH or up to 100MPH that is mentioned.  I do have a low speed issue (more like a slight surging) in 2nd gear just under 4K RPM.  I figure this is a TPS issue and I have to get my TPS re-calibrated since it's easily reproduced and occurs at a consistent RPM. 

Have you tried going back to a stock exhaust to rule out everything BUT the Arrow exhaust?  You did say that the bike was running great with the stock exhaust before, but then the diagnosis goes onto other parts of the bike other than the exhaust.  Maybe you can go back to the beginning and rule things out by reverse elimination.  Not ideal I'm sure but just an idea.