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Discussion Starter · #1 ·
I have no idea hot to set my carb screws, the air mix and idle screws. Do they have a set setting you should start off at? Any help will be gratefully received. It is running high and backfires when I let throttle off .
 

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The idle screws should be 1 1/2 turns out from closed for an initial setting. I usually put a drill bit (approx .150") in the lower curve of the carb intake and adjust the slides until just touching , This should give you a base to begin.
 

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Hi John,
I would first:
Check for air leaks at the inlet manifold.
Check the ignition timing
Check the valve clearances
Check for leaks where the downpipes attach to the head.

This thread was for monoblocks, but still works true for the basic setup of concentric carbs:



Regards
Peg.
 

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Discussion Starter · #5 ·
The idle screws should be 1 1/2 turns out from closed for an initial setting. I usually put a drill bit (approx .150") in the lower curve of the carb intake and adjust the slides until just touching , This should give you a base to begin.
[/QUOT
The idle screws should be 1 1/2 turns out from closed for an initial setting. I usually put a drill bit (approx .150") in the lower curve of the carb intake and adjust the slides until just touching , This should give you a base to begin.
Cheers Jake, but which is the idle screw?
 

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Discussion Starter · #8 ·
Referring to Peg's linked carb set-up guide the idle speed screw is the one angled upwards into the carb body. The idle mixture screw is the other one.

Chris
View attachment 754095
Ok, so I set the idle screw 1 and a half turns out from when it is fully in. But what about the idle mix screw? Does that have a starting position too?
 

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Ok, so I set the idle screw 1 and a half turns out from when it is fully in. But what about the idle mix screw? Does that have a starting position too?
Nope, it's the idle mixture screw that needs to be about 1 1/2 turns out from fully in, Jake was suggesting using a 0.015 drill under the slide cutaway as a staring point for setting the idle speed screw. Obviously you need to do both carbs, then adjust the cable adjusters so that the slides lift at the same time with a little free play so that the slides both hit the idle speed screws with the throttle closed. I use a couple of the same size pencils under the slides to check that the slides lift together, the ends of the pencils will dip as they are released - all easier to do than describe! Whereabouts are you in the UK?
 

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Discussion Starter · #10 ·
OMG I’m confused now. So where are the cutaway slides that I put drill under?
 

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Discussion Starter · #11 ·
Can anyone verify which is the idle speed screw and which is the idle mix screw. In my manual it says throttle stop screw and pilot jet screw.???🙈 Just need to know what to do with each screw🤷🏻‍♂️
 

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Can anyone verify which is the idle speed screw and which is the idle mix screw. In my manual it says throttle stop screw and pilot jet screw.??? Just need to know what to do with each screw
The idle speed screw (throttle stop) is angled upwards (the end of the screw holds the slide slightly open to allow the engine to idle)

The mixture screw (pilot jet screw or idle jet screw) is horizontal.

If you remove the air filter, look into the carb and twist the throttle, you’ll see the slide go up and down. It’s the lowest point that it goes which is set by the idle speed screw. You use the small drill bit to set the openings of the left and right carbs the same. There must be slack in the throttle cables when you do this. You also need to check that the throttle opens both slides at exactly the same time when you turn the twist grip.
This will give you your starting point.
 

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Can anyone verify which is the idle speed screw and which is the idle mix screw. In my manual it says throttle stop screw = idle speed screw and pilot jet screw = idle mix screw.???🙈 Just need to know what to do with each screw🤷🏻‍♂️
 

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Hi John,
I would first:
Check for air leaks at the inlet manifold.
Check the ignition timing
Check the valve clearances
Check for leaks where the downpipes attach to the head.

This thread was for monoblocks, but still works true for the basic setup of concentric carbs:



Regards
Peg.
Hi, please elaborate on checking for leaks at the intake manifold. TIA...😎
 

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To check for leaks at the intake manifolds, with the bike running spray some starting fluid or similar around the intake manifolds and listen for a change in RPM. If there is a change, that means you have a leak.

Rob
 

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To check for leaks at the intake manifolds, with the bike running spray some starting fluid or similar around the intake manifolds and listen for a change in RPM. If there is a change, that means you have a leak.

Rob
Thank you much. Stay safe.
 

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Hi SMG
The OP said in his original text that the engine is ‘running high’, I took this to be fast idle.
If you have a leak sucking in air between the head and throttle slide, the extra air will cause the engine to run fast at idle. The extra air will also weaken the mixture as it comes in after the carburettor, destroying any chance of you managing to set the carburettor up correctly. If you have an air leak it is highlighted when the throttle is closed because the partial vacuum is maximised when the throttle is shut and blocking intake through the carb body.
I am not saying there is an air leak, but it is easier to spend 5 minutes checking first, rather than finding one after several days of unsuccessful tuning.
The pre carb tune, ignition timing and valve clearance checks are useful also.
The exhaust/head security I put in because the OP said he had "backfires when I let throttle off".

On overun sometimes unburnt fuel is drawn into the exhaust, without oxygen it just passes through the exhaust, if you have an air leak at the cylinder/head drawing in air to the exhaust, oxygen can allow the fuel to ignite in the exhaust, causing backfiring. This phenomenon is more prevalent when electronic ignition is fitted because a wasted spark is fired on the exhaust stroke.
I noticed in the later photo that stay up floats have been fitted, the float height will need to be checked. Other people that have fitted them say they need a slightly different setting from standard, there are plenty of previous posts on the subject.
Regards
Peg.

754335
 

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Hi Rusty
Oohps, my mistake, I must pay more attention.
I love your Daytona btw, it’s beautifully put together.
Peg
 
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