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High R P M At Idle, Help Needed!

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#1 BarraNeon


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Posted 01 November 2019 - 12:24 PM



I have 998 Mini Neon with high RMP at idle (1100rpm with no vac advance, 1300 with vac advance). These RPM values are with the idle screw fully wound back (basically removed). The fast idle cam screw is also wound back, I've actually disconnected the throttle and choke cable to make sure these are not influencing the idle speed. I have 10 deg advance time with no vac and 32 deg mechanical advance at approx 5000rpm, I don't believe the ignition system to be at fault here. I have very little spark scatter thanks to a pointless metro dizzy. I have even compression (about 170psi) in each cylinder and have checked valve clearance which are bang on. I have no starting issues either, so I dont suspect the engine.


Its running a HS4 carb with standard alloy spacer, I've renewed all manifold gaskets as I suspected an air leak but this has not improved anything. I have also renewed the butterfly valve spindle bushes as these had play in them. I've double checked that the butterfly valve is sealing and the chamfers are right way around. The carb behaves as it should during all driving conditions but not at idle. I believe my fuel mix to be correct as it is at the setting it passed its MOT, plus when using the slide lift pin the revs peak and then fall away to a stall showing the mix is about right. I've ensured that the jet is at its correct location i.e. up against the mixture adjusting nut. 


My only thought is that I have an air leak somewhere but I've sprayed brake cleaner/easy start around all the intake and carb with no change to RPM. I've even checked the servo line to check for leaks and have found nothing.


The only way I can reduce the revs at idle is weakening the mix to the point it starts to hunt and then it coughs under acceleration.


This issue has been driving me mad. Any advice welcome.


Many thanks,



#2 sonscar


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Posted 01 November 2019 - 01:43 PM

To idle the motor needs air,too fast almost always means to much air but I guess you know that.Stick your hand over the air intake and see if it still runs or if you can hear it sucking before it stops.Good luck with the hunt.Steve..

#3 unburntfuelinthemorning


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Posted 01 November 2019 - 02:21 PM

Have you tried temporarily removing the breather hose from the carburettor and blocking the hole into the carb to eliminate the breather system as a suspect?


Do you have one of those silly over-run valves on the throttle disc?  It could be stuck open slightly.

Edited by unburntfuelinthemorning, 01 November 2019 - 02:22 PM.

#4 mini13


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Posted 01 November 2019 - 02:54 PM

yeah could be the popet valve iff fitted, apart fromthat theres obviosly an air leak, youve checked the sevo line, but have you disconnected it and plugged it to eliminate a leaky servo?


if you can get one, a pic of the throttle disk fully shut would be good, aso try holding a torch down the carb an looking at the other side of the throttle to check the gap around the butterfly




#5 Dusky


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Posted 01 November 2019 - 04:36 PM

Throttle cable too tight?

#6 BarraNeon


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Posted 01 November 2019 - 05:52 PM

I've disconnected the throttle cable already to take it out of the equation.

The butterfly is a solid plate, I'll take a photo of the valve sealing. I'll also check the servo again and block it as you've suggested. I've tried removing the breather and blocking it, the revs return to their annoying high state so I think the breather system is okay.

It's a good shout to choke the carb and try and hear a sucking sound, I'll try that too and report back. Thanks for the suggestions much appreciated.

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