38dgas high hot idle speed

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halfpenny
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38dgas high hot idle speed

Post by halfpenny » Fri Apr 21, 2017 1:00 pm

With best settings of the volume screws (about 3 turns out), hot idle is 1100rpm - this with the throttle stop screw backed right off, so I cannot reduce it. Accelerator cable is not tight. Choke is disengaged with fast idle screw well away from the choke cam. As far as I can tell, the throttle assembly is right against some other back stop which I cannot see, and cannot be pushed back further. Everything looks to be in good order, and car runs well throughout the rev range. Any ideas on the reason for high idle speed please?
Fuel pump is standard mechanical, and engine starts and stops on command, hot or cold.



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Re: 38dgas high hot idle speed

Post by philhoward » Fri Apr 21, 2017 1:02 pm

Are the throttles synchronised properly? I wonder if one is still partially open - possible if someone has been "playing".


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Re: 38dgas high hot idle speed

Post by Nigel Clark » Fri Apr 21, 2017 1:09 pm

philhoward wrote:Are the throttles synchronised properly? I wonder if one is still partially open - possible if someone has been "playing".
+1

Or possibly a bent throttle butterfly plate or twisted spindle if someone ham fisted has been there before. Can you look down each carb barrel with the choke flaps open and see if one of the butterflies isn't closing completely? That would give you a clue.


Nigel



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Re: 38dgas high hot idle speed

Post by halfpenny » Fri Apr 21, 2017 1:48 pm

Thanks gents, looks ok, but all ideas welcome before I remove the carb from the engine. I guess too much fuel somehow, because I assume with throtte stop backed off, the engine should stop?



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Nigel Clark
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Re: 38dgas high hot idle speed

Post by Nigel Clark » Fri Apr 21, 2017 2:25 pm

Too much fuel on it's own will just make the engine run rich, with a slow lumpy idle. To run faster, the engine must be getting too much air as well, hence the suggestions that one of the butterflies may not be closing properly.

As another possibility, there could be an air leak between carb and manifold or manifold and cylinder heads. I think it's time to get the carb off. Check for:

- Butterflies not closing fully.

- Condition the gaskets and insulator block between carb and manifold.

- Any other potential air leaks, e.g. vacuum hoses to auto advance and brake servo.


Nigel



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Re: 38dgas high hot idle speed

Post by MrGoldtop1960 » Fri Apr 21, 2017 4:45 pm

Just to add to the great tips given by Phil and Nigel, if you do take the carb off, put a straight edge across the mating face that goes against the insulator block. I have seen them banana shaped where PO's have overtighten the 4 x holding nuts and bent the carb base....
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Re: 38dgas high hot idle speed

Post by AJL Electronics » Fri Apr 21, 2017 5:31 pm

Go for the brake cleaner test first. Pay particular attention to the throttle spindle area and the base gasket.


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Re: 38dgas high hot idle speed

Post by halfpenny » Fri Apr 21, 2017 6:44 pm

Problem solved. The air filter on the Lockheed servo had become displaced (it snaps on to the body), so that the servo would not maintain a vacuum, but it would generate some braking assistance so that the failure was not readily apparent from the driving seat. The air leak was sufficient to raise idle speed. Easily confirmed by putting a clamp on the servo hose. Thanks for the input.



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Re: 38dgas high hot idle speed

Post by Nigel Clark » Fri Apr 21, 2017 9:28 pm

Glad it's sorted now.

Nigel



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