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Fast running Bantam.
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Alien
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April 18, 2019 - 5:26 pm
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Hi, I have managed to get my bike running but runs very fast, very hot and is very smokey, I have tried to adjust the screws on the carbureter but not to much joy, and ideas ? 

Also which way does the Choke operate as in on and off??

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AdrianS
Harlaston Staffs
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April 18, 2019 - 6:02 pm
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What model Bantam?

Which carburettor?

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Alien
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April 18, 2019 - 6:17 pm
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Its a D7, with an Amal 626 carbureter.

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cocorico
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April 18, 2019 - 9:11 pm
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1956 D3 in the back of the barn with 6 other bikes ahead of it!

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mikef
Chatham Kent
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April 18, 2019 - 10:24 pm
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Hi Allen.
       A Bantam D7 should not have a 626 concentric carburetor. It should have a monoblock 375.
However I think that the concentric could be made to work with some main jet and needle adjustments.
When you say that the engine runs fast, I assume you mean that it will not idle correctly.
I would suspect that the throttle slide is not dropping down fully when the throttle is closed, something must be preventing it from closing fully.
It could be that the needle did not go down into the main jet tube when you fitted the slide and this is holding the throttle slide up. I have seen that happen before and the needle clip breaks. It would make it idle fast and smokey.
The throttle slide should drop down by the pressure of the return spring when the throttle is closed. The only thing that should stop it dropping completely is the idle adjustment screw.
The choke on the 626 is on when the cable is slack allowing the choke slide to drop down, for normal running the cable is tight pulling the slide up.
Just some ideas.
                Mike.

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Alien
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April 19, 2019 - 10:01 am
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Thanks I will look into your suggestions, did not know it had the wrong carb, its the one that came with the bike.

Roger.

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Peterg
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April 19, 2019 - 10:55 am
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Fast running can have two causes. As Mike said, bikes normally run just off tick over and a ridge can wear into the carb at this point so, when the throttle is closed the slide gets to this point and the suction from the engine pulls it into the groove and holds it there. No amount of adjustments will help. It just mucks up the carburation when it finally drops to the throttle stop. Relieving the bottom edge of the slide on the inlet side can help but it isn't a cure.

The other problem, also caused by wear, is too low a slide stop. Normally, if you drop the slide it will choke the engine through air starvation. But, if the carb is worn, air will pass around the slide, sucking air from the pilot feed resulting in a very weak mixture which runs very hot. The will cause the engine to race and 'run on' (where the heat in the head causes ignition rather than the spark plug), so you can't stop the engine by turning the ignition off. The cure is contra intuative in that the slide needs to be higher and adjustment made by the mixture screw.

As a first step I would check your 626 and set it up for a D10.

The choke is off when the lever is pulled back. Apart from very cold weather you should never need to use it. Flooding the carb is all that is necessary.

Don't worry about 'very smoky' until you have a running engine. Think of all the oil that has passed into your exhaust while you were playing with it. It can only burn off once it has reached operating temperature.

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Alien
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April 19, 2019 - 5:02 pm
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Thanks, I will try anything.

Roger.

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Mags 1
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April 19, 2019 - 11:34 pm
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The throttle cables are different for the 375 and 626 carbs, could well be that the original cable is being used and it's too short? That's my take on it.

Four now on the road and at least several in bits.

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Alien
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April 20, 2019 - 5:45 pm
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It looks like you are right as I took the carb apart and removed the throttle and the slide dropped to the bottom.

I will have to chase up for a replacement.

Thanks roger.

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