Peculiar problem with ITB's

[moro]

I'm having a weird problem with my Suzuki ITB's and I've run out of ideas on why it's happening and how to deal with it. First, car details to save you from going through my build thread. 1.8 engine, skimmed head, aftermarket camshafts with more lift and duration (264 degrees, 10 mm), Suzuki GSXR throttle bodies on Danst intake adapter/manifold, aftermarket exhaust manifold.

When pressing the loud pedal, there's a fair bit of resistance in it at the first few millimetres of movement. It's especially pronounced when going on throttle after engine braking (as opposed to when going on throttle from idle when it's not really noticeable). It almost feels like engine is sucking the throttle butterflies shut and I have to overcome this by putting more pressure on the pedal before it finally starts moving. When the pedal goes past this point and opens the throttles a bit this weird resistance disappears and further pedal travel happens normally. To give you an example. I'm approaching a bend, I stay in gear and brake a little. When it's time to accelerate I move on the throttle and try to press ir a little, but the pedal resists. I have to increase pressure until the pedal finally yields, but the resistance abruptly disappears the moment the pedal has moved and of course it means the pedal jerks forward, adding too much throttle opening. Not ideal, especially on a wet road  It also seems to be most prominent in the middle of rev range but less pronounced when over 4.5 thousand revs and as I've said, not really felt when going from idle. This makes me think it might be related to the way the engine breathes, the torque curve lies low below 4000 rpm but at 4k the engine wakes up and goes like stink. It seems that the "throttle sucked in" effect is most noticeable in the rev range where volumetric efficiency is low. Could there be a connection? Has anyone experienced anything similar? Any idea why this might be happening? What could be done about it? I'm really at a loss here and everyone I've spoken with locally just shrugs.

[joeytalent]

What kind of rpm do you idle at? I get similar behaviour, but you can actually hear the point where the butterflies open (and you get a sudden change in pressure).

[josho]

Sounds to me like theyre being allowed to close just a little too much, I had the same thing on mine, adjusted them so that they sat with no resistance on opening (then faffed around resetting the idle) and after that it was spot on.

[joeytalent]

Interesting - I'm going to have a fiddle with mine tonight, and then trying adjusting my throttle bodies

[moro]

It's kind of reassuring to hear someone else's had similar issues. Idle is around 900 rpm on hot engine. Butterflies are indeed almost closed at minimum position and adjusting them for bigger opening makes idle too high. I suppose what's you're saying is to open them more but limit idle revs with ignition retard?

[joeytalent]

Yeah. I ended up with my idle around 1200rpm - it hasn't solved the problem, but it is nowhere near as bad now.

[moro]

Thanks, very interesting to know! I'd prefer to avoid high idle as it's a bit of a pain in a car used everyday. Will have another go with adjustments and see what could be done.

[josho]

Mine still idles pretty low. Around 750-800 warm, but now have no problems with sticky throttle

[atlex]

what is the airsource when the throttle bodies are closed ? is it big enough ?

[moro]

atlex , there is no external idle control, just throttle stopper screw determining the minimum opening and amount of air when idling

[atlex]

another thing to try, if you haven't already done so.. is to clean the throttle bodies with carb cleaner. it'll make the engine stall if you just put it in at idle but the trick in my experience is to open the throttle up so the revs are over 3K, let go so the butterfly snaps shut and then give it a good spray so it sucks passed the parts where it should suck past.

also worth having a go at the lips of the butterfly with an old toothbrush socked in carb cleaner, just make sure it isn't the kind that will shed plastic molding everywhere if it meets sharp metal lips, or melt in contact with carb cleaner.

[atlex]

I should give reasons :-) ^^

point is that intakes tend to get gummy, and where it is an issue is at the face between the lips of the butterflies and the inside of the intake :-) I can imagine if it gets particularly gummy you might end up with a situation where it actually makes the throttle feel sticky like you mentioned moro.

[moro]

Had a few free hours to tinker with this. The problem seems twofold. First, the throttles indeed had mechanical resistance at the start of the opening, though very very slight and certainly not to the extent for it to be the only reason for my issues. Now cleaned and reassembled they move freely and nicely. This made a difference, but only little. I've went for a test drive then and tried to replicate the problem by accelerating, braking then accelerating again and paying attention to how the pedal feels and how engine is reacting and now that the mechanical issue wasn't there it became obvious the problem is in mapping. I've played with acceleration enrichment a bit but haven't had enough time to make a considerable improvement. Hopefully will have a couple free hours tonight and see if I can map it out.

[moro]

I've played with accel enrichment a lot but to no avail, it seems that initial opening of the throttle just overwhelms the engine with air. Then I've read the latest PPC mag where Dave Walker explains the same issue being usual for bike throttle body conversions. The reason is the size of throttle cable pulley, it's just too small and thus too sensitive. I will have to look into either adapting a different cable pulley or making my own, though the original GSXR pulley being pressed, not bolted on the butterfly shaft complicates matters a bit.

[joeytalent]

Oh that's really interesting! The TT600 ones I'm using have a bolt on pulley - I might see if there's something I can replace it with.