1275 cc idle speed

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alawrence10360
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1275 cc idle speed

Postby alawrence10360 » Sun Mar 17, 2019 10:35 pm

I have a 1275cc with a single 1 1/2 HIF
Idle is rough and I need run it at 1300 plus
It has a low tech electronic ignition which uses points and an lcb exhaust and k & n filter
What should it idle at ?

panky
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Re: 1275 cc idle speed

Postby panky » Sun Mar 17, 2019 11:16 pm

My 1275 Mini idles quite happily at 800 rpm. Has it always bee like that or has it just developed? Could be a number of things first one is to check the oil in the dashpot. If that's OK it could be that the mixture isn't set correctly, my method is to wind the mixture screw out (weaken) until the speed starts to drop off then wind it back in half a turn then reduce the tick over down to where it should be - you can fine tune it later by checking plug colour after a run.
What needle is in the carb? the HIF 38 was usually fitted to smaller bore engines so if it's set up for a 998 mini then it could be running lean throughout the needle range. Also make sure there aren't any air leaks on the inlet manifold and vac line to the dizzy.
The other thing that can affect the tick over is the timing
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alawrence10360
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Re: 1275 cc idle speed

Postby alawrence10360 » Sun Mar 17, 2019 11:34 pm

What should the timing be ?

philthehill
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Re: 1275 cc idle speed

Postby philthehill » Mon Mar 18, 2019 8:28 am

A good question - as the engine is not standard the wksp setting is irrelevant.

I would set the timing to 5 degrees static before TDC and advance the distributer till the engine starts to pink/knock under load when road testing, then back it off until you are happy with the timing.

Before road testing you can adjust the distributer until the engine runs sweetest and then road test for final setting.


IslipMinor
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Re: 1275 cc idle speed

Postby IslipMinor » Mon Mar 18, 2019 10:13 am

Lots of 'depends' here!

If you know the answers to these questions, then the static timing can be a bit more prescriptive, otherwise the process that Phil described will get you to the best combination setting for your engine.
  • Does the distributor have the standard mechanical advance? If not, what is the maximum? A standard Minor distributor has too much mechanical advance for a modified engine.
  • Do you know what cam is fitted? What is it timed at?
The key is that the maximum advance must NOT cause detonation under any circumstances. Without knowing more about the mechanical advance, the maximum advance setting may not be the best for idle, and you could well need a higher idle speed to compensate for it.

Bizarrely fitting an alternator can need a slightly higher idle speed, as it charges more at lower engine speeds, which means at night the engine load at idle is higher, so 100/200rpm more can be required to compensate for it.
Last edited by IslipMinor on Mon Mar 18, 2019 3:02 pm, edited 1 time in total.
Richard



alawrence10360
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Re: 1275 cc idle speed

Postby alawrence10360 » Mon Mar 18, 2019 12:57 pm

the engine is standard, Ivor Searle have confirmed that
It does have electronic ignition with pointd and an LCB exhaust manifold
It does have vacum advance but im not surebabout mechanical

philthehill
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Re: 1275 cc idle speed

Postby philthehill » Mon Mar 18, 2019 1:43 pm

The actual engine may be standard but the additional items fitted are not so they will have an effect upon it running and performance.

The mechanical advance is contained within the distributer body so the mechanical advance cannot be determined without a strobe timing light.

What is the number of the distributer?

The number can be found on the side of the distributer body. Whilst the distributer may look the same there are several advance curves/patterns.


alawrence10360
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Re: 1275 cc idle speed

Postby alawrence10360 » Mon Mar 18, 2019 8:15 pm

I cant see the numbers I was expecting. I thought it was 25D4.
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philthehill
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Re: 1275 cc idle speed

Postby philthehill » Mon Mar 18, 2019 9:00 pm

You have the right distributor 41270 25D4 for the 1275cc Midget engine fitted to your Minor.

https://www.moss-europe.co.uk/distribut ... soc=142188

Static timing:- 7 degrees before top dead centre

Ignition timing (dynamic with vacuum disconnected):- 13 degrees before top dead centre @ 1000rpm.


alawrence10360
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Re: 1275 cc idle speed

Postby alawrence10360 » Mon Mar 18, 2019 9:17 pm

Cheers Phil

paul 300358
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Re: 1275 cc idle speed

Postby paul 300358 » Mon Mar 18, 2019 10:15 pm

Another common cause of erratic idle is the mixture screw on the side of the HIF38 being overtightened, it can cause the bi metal strip to bend. Have you recently adjusted the mixture? If so, unscrew the mixture screw until the idle increases and reset off the color of the plugs. My co2 is set to just under 3.


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