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Idle Speed Better With A Full Tank Of Fuel


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#1 Boycie

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Posted 07 May 2018 - 11:23 AM

Hi all.

I love blatting about in my 1976 Mini, but something I've never quite understood is why the hot idle speed/ quality changes in relation to how full the fuel tank is.

 

With a full tank, the hot idle is smooth and a little too fast for my liking.

 

When the tank gets to around 1/3 full, the idle progressively gets slower and becomes choppy to to the point I give it a little throttle to keep the drops gears from chattering.

 

There are no other running faults.

 

The spec is as follows-

Standard 1098cc with 12G202 head, Minispares st1 kit, standard HS4 carb with red jet and original small fuel tank.

 

Any ideas why the hot idle is so variable?  Nothing else seems to affect it, just the level of fuel in the tank...



#2 nicklouse

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Posted 07 May 2018 - 11:37 AM

two things that I can think of. crap in the tank that gets moved when you fill.

 

or the pump is not working correctly.



#3 Wim Fournier

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Posted 07 May 2018 - 12:02 PM

The fuel line from the tank to the pump is clogged with debris that sticks in the line due to the quality of modern fuel contaminated with ethanol.

You will be very happy after laying a new line.

I was.



#4 phillrulz

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Posted 07 May 2018 - 12:05 PM

more fuel slightly higher fuel head pressure? maybe fuel pump is starting to struggle?



#5 Boycie

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Posted 07 May 2018 - 01:27 PM

Thanks guys.

I'm pretty certain the fuel lines and tank are as clear as they can be as I've had everything apart over the years and it's never made any difference.  I did consider the fuel pump but it runs strongly at high rpm so thought that should be ok.  But... thinking about it, the pump will be operating at a higher speed as the revs go up which would possibly be masking a slight delivery pressure fault?



#6 Mervyn

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Posted 07 May 2018 - 03:12 PM

Does the cap go whoosh when you take it off? Could be tank breather clogged and the pump struggling to suck the fuel

Merv

#7 Boycie

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Posted 07 May 2018 - 07:10 PM

No, nothing like that Merv, the cap tends to 'fit where it touches'!






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