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RPM don't drop - Engine & Ancilliaries/Gearbox - The Lotus Forums Jump to content

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Hi everyone! 

When my engine has warmed up, it has a nice Idle speed (around 900 RPM). 

However, after a while, if I press lightly on the gas pedal while in neutral, I bring the RPM to 2000, but... it stays there even after release the pedal... Never goes down. 

Same symptom when I shift from 1st to 2nd: rpm don't drop before I engage 2nd gear. 

And third similar scenario: if I slow down, gear is engaged, RPM drop along with the declaration, but as I press on the clutch upon full stop, they (sometimes) go back up to around 1800-2000...

Would you know what can cause this? What controls the rpm beyond the choke and the accelerator? 

I've read with great interest the thread about the throttle solenoid but.. my car doesn't seem to be equipped with one. 

Note that the car has an aftermarket Air-conditioning, but it runs on an electric compressor, so I don't think it interferes... 

Any advice welcome! 

Thank you very much, 

Bernard

 

 

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Could be a week idle mixture or even a blockage in one of the carburettors. Do you ever hear a random popping noise from inside the airbox ? Also possibly could be an air leak on the intake side, which only becomes obvious after heat expansion.

Edited by Blue and Tan dog

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1 hour ago, BernardSG said:

However, after a while, if I press lightly on the gas pedal while in neutral, I bring the RPM to 2000, but... it stays there even after release the pedal... Never goes down. 

Have you checked at the carbs if the throttle has closed or is still slightly open by pressing on the throttle lever?

Does it have a throttle jack fitted which could be holdig it open?


Cheers,

John W

http://jonwatkins.co.uk

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Thank you both, it gives me a few points to check. 

There's no throttle jack, and I believe the throttle closes properly, but I'll have to double check. 

Can't hear any "popping noise", so I'll leave the lean mix option aside for now. 

The air leak is a fair possibility! I'll spend some time looking at pipes over the weekend.

Possible related, When I try to use the choke, there is a noticeable hissing sound, not sure if it is normal or if it is also caused by a vacuum leak? It's my first carburated car in over 20 years, and I have to admit I don't remember if this noise is normal or not :)

Thanks a lot! 

 

Bernard 

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Sounds like a sticking throttle cable or carb.

If you flick the pedal with your foot a few times it may settle back down, a new cable or better return spring might solve it.

These cars are renowned for carb problems such as flat spots, rough or uneven idle, poor cold running, difficult to start when hot etc etc...

Don't expect anywhere near the smoothness of a fuel injection car or even a single carb engine...but when it all comes together....what an superb motor!

 

 

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Thanks again for all the comments and hints! 

1 - good for the hissing noise being normal. One less thing to worry about...

2 - the spring is there. When I action the cable by hand it goes back in position well

3- sticking throttle cable : I'll have a look this weekend when I face the problem again., but again, actioning the lever manually seems to work fine. 

I just had a look at the engine bay for potential vacuum leaks. 

I found other problems ;)

- the cam housing Gasket is damaged. Don't know how I missed that earlier. 

- the crank case Breather hose is not connected to the airbox. As a result, there's a gaping hole in the airbox...the "tube" that is normally attached to the airbox is missing in action altogether.. 

 

 

 

Edited by BernardSG

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Hi everyone, 

I had a look at the hole in my airbox (circle in red on the picture below). Is it important to plug it? 

Is duct tape an option or is it a bad idea? I wonder how much suction there is and if it would likely unstick it...? 

Thank you! 

Bernard 

 

824562145_20200613_1130153.thumb.jpg.2acb19a3fc368e32f79bcff9d1b06f56.jpgHi

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Yes, it's important to close that hole, because it will allow unfiltered air (possibly containing dirt etc) to enter the carbs and engine. I wouldn't rely on duct tape alone. Maybe try and find a rubber plug or grommet that fits. If not a perfect fit, you can always add duct tape to seal it. A small piece of alu or such can be fixed over the hole as well if you can't find a grommet.

As the hole is before the carbs, it shouldn't affect running though.

Filip


I have made many mistakes in my life. Buying a multiple Lotus is not one of them.

 

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Thank you both! I'll try to block the hole for the time being. I know it's supposed to go to the tube, but not that easy to reconnect properly. 

Thanks! :)

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When you check for leaks, have a look at the 'O' ring seals between the carbs and the adapter plate and the plate and manifold. If there is a leak here you can get your symptoms. The other think to check is to be certain that the carbs are not flooding, turn on ign and look into the trumpets if there is flooding they will be wet with fuel.

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On 22/06/2020 at 14:51, Drawfiler said:

 The other think to check is to be certain that the carbs are not flooding, turn on ign and look into the trumpets if there is flooding they will be wet with fuel.

Thank you! Will they be wet all the way till the end of the trumpet? Or where the butterfly valve is? 

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I  usually take off the air box and have a look and feel the inside of the trumpet with my finger. If the pump  is on you can usually see the fuel working it's way down the trumpet.

all this is from bittter experience, my engine just wouldn't slow down so I fiddled with the lever stop screw then  it wouldnt tick over. After trying all sorts of things I bought the better needles and had an instant cure.

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