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ET: Engine (Twin) Running Weak on Part Throttle


Rob H

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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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VOC Member
VOC Forum Moderator
............... 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

Well Known and Active Website User
VOC Member
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

Well Known and Active Website User
VOC Member
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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VOC Member
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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VOC Member
VOC Forum Moderator
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.
 

BigEd

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VOC Member
VOC Forum Moderator
I should have gone to School !.
I doubt that you missed any lessons on Vincent carb settings as they wouldn't have been in the national curriculum. That is assuming that there even was a national curriculum back then.;)
 

roy the mechanic

Well Known and Active Website User
VOC Member
Try polishing the needles with fine scotchbrite or a scouring pad. They tend to corrode and weaken it out. Also put fresh fuel in.
 

Bill Thomas

Well Known and Active Website User
VOC Member
I was thinking more about "Specific Gravity ", Just wish I could Spell it !.
I had a good idea when I was 16, To Tune my 250 BSA, I rigged up a lever to hold the tickler down, To let more Petrol into the Engine, When I was Flat Out, 65/70 mph.
No it didn't work !!, But good Fun. Cheers Bill.
 

peter holmes

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VOC Member
It might have worked if you had devised a way of getting more air into the engine at the same time, like some sort of fan that worked of the exhaust gas flow maybe!
 

Sakura

Well Known and Active Website User
VOC Member
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.
I don't get this? Surely the float level is influenced by the amount (volume) of fuel in the float chamber, not the sg of the liquid? Isn't the top level constrained by the needle seat?
 

Bill Thomas

Well Known and Active Website User
VOC Member
It might have worked if you had devised a way of getting more air into the engine at the same time, like some sort of fan that worked of the exhaust gas flow maybe!
I gave up and bought a 1000cc Vincent Outfit, When I was 17, With L plates !. Cheers Bill.
 

Chris Launders

Well Known and Active Website User
VOC Member
Sakura, yes it is set by the needle seat, but you have to set the float height to match with this (valve closes when the float is level with the chamber top). You do this to set it, but in use the petrol is lifting the float and how high the float sits in the petrol depends on it's specific gravity. So if it's SG is higher the float sits higher and the valve closes with less petrol in the chamber. I know the floats can be different as I had a new one that was fouling the float chamber before it lifted the needle off it's seat, this is why I had contacted Amal and was told about the float height and alloy float needle scenario.
 

greg brillus

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VOC Member
On both the 276 and 289 carb's the lower brass cap nut that holds the jet block into the main body come in various heights as well. On the 276's I have 3 different types all differing in height......... This will have a large impact on the float level............. More than meets the eye here.
 

mercurycrest

Well Known and Active Website User
VOC Member
(The carbs were refurbed by Martin Bratby who made the slides, the cutaway measures somewhere between 3.1/2 and 4.)
Why not do the obvious and try 3.1/2 slides? With the engine running and riding down the road, the specific gravity of different petrols isn't going to be any kind of issue at all.
 

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