Making assumptions

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Sometimes when I book a bike in for a certain problem I just head off on the usual path based on the assumption that it'll be like all the others and it turns out to be completely the opposite. I had an ST2 in for running badly around 3,500 rpm. No surprise there, sounds like your normal, not set up well ST2. For good measure I started with the fuel filter, then did the TPS baseline and throttle body set up and figured it'd be good to go. I'd made the expected changes along the way which helped reinforce the assumptions.

Then I went for a ride. And it did indeed run badly 3,500 rpm. Fine at 3,000 and coming good at 4,000, but rubbish in between. Which I found a bit odd. First I tried adding some ignition timing to the eprom mapping because the ST2 can be a little light on advance in that area, but that made little difference.

So I taped the old FIM hand held terminal to the fuel tank and went for a ride. Generally if they're running this badly you need to make fuelling changes of at least 20% to get a change you can feel, and it felt to me like it was rich. So I went leaner and it got better. By moving the zone borders around on the FIM Megazone system you can isolate bits of the fuel mapping as desired to find out where the problem begins and ends. The result was good, but very wacky.

The ST2 has quite close rpm and throttle break points in this area of the map. And the 45mm throttle bodies see quite a bit more throttle opening for a given condition that I'm used to seeing as most of my work like this has been 4V based with the 50mm throttles. Cruising around 80km/h was using about 10 degrees of throttle opening, whereas on a 4V it'd be 6 or so. The rpm break points in the area are set at 3,000, 3,600, 3,800, 4,000 and 4,400 rpm. I didn't touch the 3,000 and 4,400 rpm lines, or any either side of them, and only few points on the 4,000 rpm line. But at 3,600 and 3,800 it was up to 51% leaner up to and including the 25 degrees throttle opening line, generally 30% or so.

On the 3D graph of the fuel map there was a trench in the map at 3,600 and 3,800 up to 32 degrees of throttle opening. Meaning that in this particular area the bike was trapping much less air or had become very efficient. It didn't feel any different, and part throttle performance is hard to quantify in seat of the pants terms.

The only factor I could see for this were the very loud D and D mufflers fitted, but even that's just a stab in the dark. It was nice everywhere else, so it's not like it's an overall issue. Just a very specific rpm related issue at low throttle openings, which given these systems are defined by throttle position and rpm makes it an issue. If they used an air flow sensing system (or carbs, which are air flow sensing devices) this sort of thing probably wouldn't happen.

Made no sense to me at all, and was certainly not what I expected when I made the booking. But you get that, and overall it was quite a bit of fun. Which I can say because I worked it out - it could have gone the other way fairly easily.
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