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Hole In The Throttle Plate/disc


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#1 stormtrooper

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Posted 14 August 2015 - 08:12 PM

Hello all.

I hope someone can help me with a answer to a question which I can't seem to get a conclusive answer on on-line anywhere.

I have a problem with not being able to reduce a fast idle on a 1992 Mayfair. Checked timing plugs mixture air leaks and have had the carb off to clean it. What I have noticed is a small hole approx 2.5 mm in the lower half of the disc which doesn't appear on any of the Haynes or other diagrams on line. Should this be there? Could it be causing the fast idle.?

#2 Mini ManannĂ¡n

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Posted 14 August 2015 - 08:20 PM

I wonder if it had that wossisname, the anti-run-on valve (?) at one time?  Solder it up or put a new throttle plate in.



#3 KernowCooper

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Posted 14 August 2015 - 08:25 PM

Solder it up sounds like they had the valves in the plate at one time



#4 Spider

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Posted 14 August 2015 - 08:26 PM

It's there to reduce NOx on over-run.

 

As Mini Manannan and KernowCooper suggested, solder it up.


Edited by Moke Spider, 14 August 2015 - 08:44 PM.


#5 stormtrooper

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Posted 14 August 2015 - 08:34 PM

Cool guys thanks. I'm on it!

#6 dklawson

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Posted 14 August 2015 - 09:26 PM

If the car continues to have a high idle speed after soldering the disk up, go back to the carb and completely re-do its choke adjustments.  Make sure the jet is able to move freely up and down with the choke knob and that it is fully up when the choke is pushed in.

 

Another thing to look at carefully when soldering the butterfly disk is the knife edges on its perimeter.  If a previous owner replaced the disk, it is possible they put it in backwards and the throttle is not closing completely.  Basically look through the carb throat with the piston lifted and make sure you cannot see any light around the perimeter of the disk when the idle speed screw is "backed off" and the throttle should be fully closed.



#7 KernowCooper

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Posted 14 August 2015 - 09:46 PM

You'll have to check/tweak the mixture/idle after soldering up the hole


Edited by KernowCooper, 14 August 2015 - 09:46 PM.


#8 peter-b

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Posted 14 August 2015 - 10:02 PM

You should only have to set idle, as it's on the engine side of jet. I was told by a car by guy it was a base idle port and as the spindle and butterfly wear they let extra air/fuel past. ??
Another thing to check re high idle is the spindle and ensure the butterfly is centralised correctly. Oh and solder up the hole:-)

#9 carbon

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Posted 15 August 2015 - 09:35 AM

SU carb butterflies come in various types, I have seen at least 4 different fitted in the H4 and HS4:

- solid, ie no holes or cutouts

- small 'half round' cutout about 2mm wide at lower edge

- small hole about 1.5mm about 15mm up from lower edge

- valve and spring set-up

 

Chances are the hole in your butterfly spindle is correct and the fast idle problem could be down to worn spindle bushes?

 

Also before making any changes would also suggest checking that the throttle return spring is working properly, and there are no issues with sticky throttle cable.






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