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Split Weber Idle Problem When Engine Is Hot


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#1 Lenhamracer

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Posted 25 January 2022 - 03:32 PM

Dear forum users 

 

I have a problem with the idle speed when the engine has run for a longer time. Cold start is fine and after heating up with the choke the engine runs nice on idle at around 800rpm. After 1 hour fast driving on a rally (typical 350km/day on very small roads) idle is completely gone and the engine just dies when you stop the car. Next day it is fine again.

It is a Cooper S engine with split weber.

Any ideas??
 

thank you very much

 

Thomas



#2 nicklouse

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Posted 25 January 2022 - 07:06 PM

What cam?



#3 Tones61

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Posted 25 January 2022 - 07:17 PM

Actual split webers or a single 45?
What's the engine spec inc cam?

#4 Gaz66

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Posted 25 January 2022 - 08:15 PM

Are you using thackeray washers under the nuts which hold the carbs onto the manifold.
Too slack the carb will allow air in via the O rings and too tight the carb will vibrate and foam the fuel in the carb.
Also, after a good thrashing the under bonnet heat could be vapourising the fuel in the carb/manifold.
Does this happen just after its been thrashed or does it happen after youve just tootled around?

Edited by Gaz66, 25 January 2022 - 08:16 PM.


#5 Avtovaz

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Posted 25 January 2022 - 08:45 PM

have you a balance pipe between both sides of the maifold? 



#6 Lenhamracer

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Posted 25 January 2022 - 11:48 PM

The car runs a swiftune sw8 cam

yes split weber 45 on longman manifold 

no balance pipe 

 

As far as I remember I run the little springs from weber, what do you mean with thakeray washer? Are these the washers which look like a sort of mini spring? 
And yes, it happens after thrashing around on higher revs. The engine only looses any idle speed, you can still keep it going with the throttle but under hard braking and no throttle it just dies. 
How can you check that the screws are not too tight?

 

Thank you very much 



#7 Lenhamracer

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Posted 25 January 2022 - 11:51 PM

If there is too much heat under the bonnet, may be it would help to wrap the exhaust manifold?



#8 Gaz66

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Posted 26 January 2022 - 12:24 AM

The car runs a swiftune sw8 cam
yes split weber 45 on longman manifold
no balance pipe

As far as I remember I run the little springs from weber, what do you mean with thakeray washer? Are these the washers which look like a sort of mini spring?
And yes, it happens after thrashing around on higher revs. The engine only looses any idle speed, you can still keep it going with the throttle but under hard braking and no throttle it just dies.
How can you check that the screws are not too tight?

Thank you very much


Yes, the thackeray washers are the little springs.
The tightness of these is not an exact science I found. As I said, too slack they leak air in and too tight they foam the fuel. If they are really slack the engine will suck the O rings in too.
Hard to say the correct tightness but the thackeray washers should not be fully compressed and if you grab the air filter end of the carb there should be a tiny bit of movement where the carb rocks on the rubber O rings.
Weber may have some data on the tightness of the thackeray washers if you look on the net.
I would put my money on heat vapourising the fuel.
What length inlet manifolds are you using?
Short ones will have the carbs over the exhaust manifold so will increase the heat on the carbs thus making vapourising more exaggerated.
Might be worth checking the O rings are in good order too.

#9 timmy850

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Posted 26 January 2022 - 02:10 AM

You have a soft mount kit for the carb to manifold? My new ones came with instructions to leave a certain gap between the spring coils to ensure the correct tightness

My car had a similar issue when the battery died. When the idle dropped it wasn’t making enough power to keep it running. That could be something quick to check

#10 weef

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Posted 26 January 2022 - 12:24 PM

I always used to tighten this style of washer leaving a gap of 0.007" between the coils. Never gave any problems with then coming slack but this was when the washers were made of a good quality, perhaps the "chocolate" ones available today will not meet this specification. 



#11 Lenhamracer

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Posted 26 January 2022 - 04:21 PM

I attach a picture to this post. O-rings were new last year. I think they are fine. I will check if the carbs are too tight. Currently I am in the Netherlands for two weeks. I think the alloy manifolds are all the same. These are old Longman ones.
 

Battery is fine, car always starts….


 

I attach a picture to this post. O-rings were new last year. I think they are fine. I will check if the carbs are too tight. Currently I am in the Netherlands for two weeks. I think the alloy manifolds are all the same. These are old Longman ones.

Attached Files



#12 Spider

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Posted 26 January 2022 - 08:11 PM

My experience with Webers is very limited however I'm wondering if this is a heat soak issue ?

When carburetors heat up, that will of course heat the fuel. This may cause a vaporisation problem. It also lowers the fuel viscosity quite a bit, which usually makes the mixture rich, though I would think, it would need to be very rich to make it stall at idle.

 

Also, if I recall right, Webers need a higher fuel pressure than the SU, I seem to recall in the order of 4 - 5 PSI. Low fuel pressure can cause stalling at idle.



#13 nicklouse

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Posted 26 January 2022 - 08:16 PM

Just to clarify 

91syiXi.jpg

4gHKJox.jpg

You have the rings and either the rubbers or springs under the nuts?



#14 Gaz66

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Posted 26 January 2022 - 08:27 PM

When the engine is thrashed and the underbonnet heat rises the limited amount of fuel from the idle jets would quite easily vapourise in a hot alloy manifold thus causing this problem.
Looking at the picture its the manifolds rather than the carbs that will be getting most of the heat (obviously) so I would be putting my money on this after the thackeray checks have been ruled out.

#15 Earwax

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Posted 27 January 2022 - 06:58 AM

Hi

 

All the heat soak/ electrical suggestions are good fixes.  Mine is more a work-around!

 

SW8 - is a fast road cam - so  little lumpy at idle maybe?  I would adjust idle up to 1000 -1100rpm for a test run in rally mode - see if it makes a difference at the hour mark - turn it down for day to day use if it is too annoying.






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