Unraveling the 961

I'm a numpty on engines, especially injected ones but some thoughts.
From the manual the inlet valve opens 36 BTDC and closes 60 ABDC.
If you translate that onto a 720 degree 4 stroke rotation, 0 degrees is TDC, and add 270 degrees to piston 2 to get it to the same reference then
- inlet valve 1 opens at 684 degrees and closes at 240 degrees
- inlet valve 2 opens at 234 degrees and closes at 510 degrees.
- both valves closed 510 - 684.
I don't know if my 1 and 2 are the same as Norton's references? If so then it is beyond me to imagine how 2 is robbing 1?
If air is being supplied via the IAC and the air line was too small you could imagine that 2 could be fighting a partial vacuum on the air supply?

Fast Eddie said:

Perhaps I’ve missed the introductory lesson gents, but, can someone please explain how the fuel robbing works?

The injectors inject fuel (atomised?) into each port. So each port has a charge of a given air fuel ratio.

I can understand a cylinder robbing charge, but this charge would contain air and fuel in the above given ratio... so such robbing would not dramatically effect air fuel ratio.

How does one cylinder rob only the fuel from the other?

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I keep questioning myself as to whether or not I've made some kind of mistake with my setup. Have I got an air leak in the exhaust or intake. It all looks ok. And when I give the engine some beans things do tend to even out - so that would seem to imply the IAC pipe between the throttle bodies. I suppose I could put it to bed if I ripped everything off and blocked the balance pipe as Ken has already done. But Ken is getting his own dual wide band setup shortly so I'll leave it with him to validate it! At this stage I've got the engine idling nicely and although I've made an interesting observation I'm not intending at this point to do anything about it!

Another piece of the puzzle filled in today. So far I've confirmed cylinder 1 runs lean at idle with my wide band o2 sensors. I've tested the injectors and confirmed they're both flowing perfectly. Today's test involved using a synchometer to measure the air flow into each throttle body, if out of balance then this could explain why one cylinder is running lean and rule out the role of the IAC valve assembly.

Multiple tests were run, some with the IAC electrically connected, some with it unplugged. Some at idle with the butterflies closed (even with cardboard physically blocking the throttle bodies), some at 2,500RPM with the throttle butterflies partially open to check their balance (with the IAC valve physically blocked).

The result of all this was yet again confirmation that the balance pipe between the throttle bodies is the source of my problems. I say my problems because I can't vouch for the balance in your own throttle bodies. But the tool is relatively inexpensive if you have the desire to check your own. The fix as I've posted previously is to either remove the IAC valve and block the balance pipe or if you wish to retain the IAC valve then you'll need to introduce a method of controlling the airflow in the balance pipe. If your ECU has a fit when disconnecting the IAC valve and your idle is driving you mad the latter solution should give you an engine that behaves like any other.

Is this the problem that if resolved fixes the hot motor high Idle that some experience ? The IAC is not closing enough I suppose , letting too much air in . But why when hot , most owners say that they have good idle when starting off when cold ? Just wondering , and also why do some work well like FE and Blightybrit ?

TonyA said:

Is this the problem that if resolved fixes the hot motor high Idle that some experience ?

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There’s a number of factors that could trigger the high idle when hot - but I doubt this is one of them unless it’s been a problem from the get go. There’s no user adjustment to balance the throttle bodies in any case. High idle when hot = to much air. So it could be the IAC valve, bad engine or intake temp sensor, throttle position screw or an air leak somewhere.

The ECU has a target idle speed programmed into it. The IAC valve motor controls the amount of air entering the engine to achieve the programmed idle speed. To much air passing through the throttle bodies will prevent the IAC valve from being able to achieve the target idle speed.

The ECU is getting some duff info from a sensor that means it does not realise the engine should be idling or the software has a glitch and has missed some data and ignored it. The fix is to turn the engine off and restart so more likely a software glitch.

Then maybe (hopefully) the heat insulator I've installed will correct this. Less heat to the throttle bodies and maybe less expansion ? Just guessing without testing.