Lotus Elan

Engine Idle Speed Again

PostPost by: Concrete-crusher » Sun Jul 07, 2019 4:48 pm

Hi I'm sure I have posted about this before , when I take my car for a drive the idle speed creeps up from 850 to 1500 ish

Next time I start up from cold it seems to be back to its 850.

All the previous comments point towards the fuel system , carbs , air leaks etc.

Is it possible the problem could be the centrifugal advance sticking ? which is something I have not heard much about before .

Thanks Steve
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PostPost by: 69S4 » Sun Jul 07, 2019 5:02 pm

You and me both. Car idles to a stall when cold but progressively speeds up to 1200rpm when hot. I've had the centrifugal advance to bits in mine to change the bob weights - new distributor came with 7 degree weights instead of the 12 degree ones needed. I made sure nothing was sticking and it made no difference to the idle speed.

It is kind of annoying - I've just come back from a 50 mile drive and it's not that big an issue but I would like to work out why it's doing it. The carburation is fine, it's not showing any signs of weakness, hesitation or anything and the timing is correct by my strobe light.
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PostPost by: mbell » Sun Jul 07, 2019 5:44 pm

Knowing which carbs you have would be used useful.

The advance should be easy to check with timing lamp when the fault is occurring.

Does the car return straight to an incorrect idle speed and hold steady or does it creep up or down over time?
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PostPost by: nmauduit » Sun Jul 07, 2019 6:31 pm

what would you call "cold"? I set idle (1000 rpm rather than 900, to be easy on the bearings) when the engine is at operating temperature (i.e. radiator hot, confirmed by the temp gauge). When the engine is completely cold it does not even keep idling (weber, choke/enriching device left closed), so has to be maintained by the throttle till it warms up.
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PostPost by: Esprit2 » Sun Jul 07, 2019 6:48 pm

Which carbs? Weber, Dellorto, Zenith-Strombergs?

It's normal for a carb engine to run slower when cold, and to pick up rpm as it warms up. ALWAYS set the idle speed when the engine is hot... up to full operating temperature. If it then idles too slow and/or dies when cold, use the 'choke'/ enrichment device to keep the idle speed up and the engine alive until it gets up to temperature. Also use your right foot. If the 'choke' isn't hooked up or operational, then it's up to you and your right foot to keep the engine going until it warms up. User involvement is required.

If the carbs are Zenith-Strombergs, then the Temperature Compensator will also help with cold running. Not enough to eliminate the need for the 'choke', but it helps. If it is stuck or has been defeated/ disabled, then the engine will have to rely more on the 'choke'/ enrichment device.

A Weber or Dellorto 'choke'/ enrichment device doesn't include a fast idle function, just a richer mixture. The Zenith-Stromberg 'choke' also includes a Fast Idle Screw that must be adjusted separate from the normal Idle Speed Screw.

Personally, I think you're barking up the wrong tree by going after the centrifugal advance.

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PostPost by: JJDraper » Sun Jul 07, 2019 8:17 pm

If Webers, fuel level in bowl too low at start? After a couple of pumps on the accelerator to start the car the level would be lower, rising after a run.

Just a thought.

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PostPost by: richardcox_lotus » Sun Jul 07, 2019 9:22 pm

I used to experience this with my Weber?s. As Tim advised above, once you get the hot idle sorted, it becomes more manageable.

Doesn?t take long for the cold lumpiness to subside (to coin a phrase.....)

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PostPost by: Concrete-crusher » Mon Jul 08, 2019 6:45 am

Thanks for the replies, for completeness the cars are weber 40 dcoe
Type 31s

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PostPost by: richardcox_lotus » Mon Jul 08, 2019 8:50 am

Steve - I replaced my type 31s with 151s and they are so much easier to balance at idle.
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PostPost by: 2cams70 » Mon Jul 08, 2019 9:48 am

It does sound a lot like air leaks. In particular whilst the engine is running run some oil over the carburetor to inlet tract flange joints. If there is a leak the oil will get sucked in and/or you will get some blue smoke at the exhaust. Check the brake booster for leaks too. Disconnect the hose to it temporarily and plug the opening to see if that makes any difference.

I'm assuming of course you've also done the mechanical check to ensure the linkage is not sticking. Rotate the throttle spindles by hand to see if the idle speed drops.
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PostPost by: Concrete-crusher » Mon Jul 08, 2019 5:30 pm

Oil on the flanges sounds like a good idea I will give it a go

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PostPost by: seniorchristo » Mon Jul 08, 2019 9:06 pm

An old British car mechanic I knew used to spray gasoline on the intakes to isolate air leaks. Very effective as the engine sped up immediately. Of course I always worried about the car and the shop going up in flames but he was never the least bit concerned :roll:
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PostPost by: Esprit2 » Mon Jul 08, 2019 10:38 pm

You can do the same thing by spraying aerosol brake cleaner or carb cleaner... anything flammable, a butane lighter without igniting the flame, even hair spray. With all those sprays, the engine's speed will pick-up when you spray a leak area, and they're all a lot more safe than gasolinie/ petrol.

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PostPost by: 2cams70 » Tue Jul 09, 2019 2:06 pm

Check those headlamps for vacuum leaks too - fortunately something I've never had to worry about!
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PostPost by: h20hamelan » Tue Jul 09, 2019 2:31 pm

Is it something that changes after it warms up?
Vacuum hoses expanding, jets?
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