Throttle plate fitting

Linkert M88 Carburetor

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old man emu
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Throttle plate fitting

#1

Post by old man emu » Sun May 13, 2018 3:35 am

My friend is wondering why the throttle plate in his M88 is not closing off the carburetor. When the throttle shaft is placed in the "closed" position, you can see daylight all around it.

My thought is that the screw holes are offset from the centreline of the plate, and my friend has simply fitted the plate the wrong way. Is that the likely reason?

I have a picture to show the situation, but for some reason it won't upload here (does not meet pixel size limits)



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Re: Throttle plate fitting

#2

Post by RUBONE » Sun May 13, 2018 4:14 am

Throttle plates have a bevel on them to seat against the body. It need to be in the right orientation to seal. The number stamped in should be on the side facing the manifold opposite the idle slot. With the screws loose it should be held tight to the body to seal, the shaft pushed up from below for no play, and the screws tightened.

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Re: Throttle plate fitting

#3

Post by liberator » Sun May 13, 2018 8:36 am

In a picture (this is a simular carb);

http://www.linkertcarbs.com/carb934.html

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Re: Throttle plate fitting

#4

Post by old man emu » Sun May 13, 2018 12:06 pm

Thanks for the written explanation and thanks for the picture to illustrate the point.

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Re: Throttle plate fitting

#5

Post by Mark44 » Sun May 13, 2018 5:13 pm

If you can see daylight all around the throttle plate, and the problem isn't as Robbie described (throttle plate backwards or upside down, or not set in the throttle shaft slot just right), another possibility is that the venturi is too large. The venturi for an M-88 Linkert is 15/16". The venturi for an M-54 Linkert is 1". Both carbs have a bore of 1-1/4", so if a 1" venturi were used with a 15/16" throttle plate, there would be a gap all around the throttle plate.

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Re: Throttle plate fitting

#6

Post by RUBONE » Sun May 13, 2018 7:42 pm

If you can see daylight all around the throttle plate, and the problem isn't as Robbie described (throttle plate backwards or upside down, or not set in the throttle shaft slot just right), another possibility is that the venturi is too large. The venturi for an M-88 Linkert is 15/16". The venturi for an M-54 Linkert is 1". Both carbs have a bore of 1-1/4", so if a 1" venturi were used with a 15/16" throttle plate, there would be a gap all around the throttle plate.
Actually no, there would not be. The venturi is independent of the throttle plate which seals to the carb body, regardless of the venturi size fitted to the body. All 1-1/4" body venturis are the same OD despite being different IDs. (15/16",1", 1-116", or 1-1/8").

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Re: Throttle plate fitting

#7

Post by Mark44 » Sun May 13, 2018 11:48 pm

My mistake...
i was thinking that the throttle plate sat at the narrow part of the venturi, which of course isn't true, and in spite of having taken several Linkerts completely apart.

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Re: Throttle plate fitting

#8

Post by kitabel » Mon May 14, 2018 5:06 am

Seeing light all around the OD of the disc should not happen, the top & bottom of the disc should completely close off the bore to within a few thousandths.
The throttle disc never closes. The number stamped is the bevel angle of the disc's edge. Only a 12° is available for all 1" Model M, only 13° for 1-1/4", but there are 4 for the 1-1/2": 8°, 9°, 12°, and "12X" indicating 12° with a small shallow relief ground in the disc adjacent to the idle and transition holes. Smaller angles are richer.
These angles affect low speed response and discs should always be replaced with the correct number.

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