ET: Engine (Twin) Running Weak on Part Throttle

Rob H

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With the motor up to temperature as you start to accelerate it is a little hesitant and hiccups/stutters. This does not happen if I apply a some choke so assume it is running a little weak on part throttle. The carbs were refurbed by Martin Bratby who made the slides, the cutaway measures somewhere between 3.1/2 and 4.

Was running fine last year on standard cams with needle raised one notch. Have 105's fitted now.

Any suggestions what might be the best way forward, will look to try and raise the needle once again and if that does not work do you think it would it be best to go up a size on the needle jet or take a little off the bottom of the slide to reduce the cutaway?
 

Bill Thomas

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I would start by making the pilot a little richer, Screw in a touch.
With the Mk2 Amals, I have found the pilot can alter way up the rev's, More than older Carb's.
Sounds like the slide is too weak, Can Martin supply one size richer ?.
Chris Launders said the other day, He sent a letter to Amal's, And they said the NEW Petrol needs a higher float level with Concentrics ?, We are about to try that with a friends Mk2s.
On Older carb's, Like I think yours are ?, You could twist the carb body a bit, To get the same effect ?.
Good Luck, Bill.
 

BigEd

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............... He sent a letter to Amal's, And they said the NEW Petrol needs a higher float level with Concentrics ?, We are about to try that with a friends Mk2s.
On Older carb's, Like I think yours are ?, You could twist the carb body a bit, To get the same effect ?.
Good Luck, Bill.
That sounds really strange. I have always thought that the correct petrol level in a carb was a fixed level. Maybe Amal know better but I would be interested to know what you find when you try it.
 

Bill Thomas

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I hope Chris can tell us.
I think it said we should adjust the float to the top of the chamber, With the new stay up floats, Concentrics, Mk 1 and 2.
You can now bend the tag.
Nortons have needed this for some time ?.
Cheers Bill.
 

Bill Thomas

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The bit I am talking about " Mk1 concentrics bad starting from warm " Page 38,
Sorry Eddy, I don't know how to move it to this thread.
Cheers Bill. Feb 20th 2019.
 

Chris Launders

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Yes that's what Amals told me, the float top should be level with the top of the float chamber, of course if the specific gravity of the petrol has changed that would affect float height in relation to the petrol level, as would the weight of the float, or a combination of both so the petrol level could be the same but the float height could be different !!
 

BigEd

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Yes that's what Amals told me, the float top should be level with the top of the float chamber, of course if the specific gravity of the petrol has changed that would affect float height in relation to the petrol level, as would the weight of the float, or a combination of both so the petrol level could be the same but the float height could be different !!
Now that makes sense. If the specific gravity of the fuel is higher or lower then the float will sit higher or lower accordingly. Thank you.
 
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