Technical Carburettor help please

[nerodino]

Hi all,
Wife complaining (unusual!) today re her car having no power.
Tried it and found that it appeared to be firing on one cylinder!.....removed plugs and both very sooty. cleaned and refitted and engine ran fine.
Thought i would look at carb, found slow running jet partially blocked so cleaned and refitted, adjusted idle speed (Generator light just flickering as per Haynes) BUT when trying to adjust mixture screw, no difference all the way in or out! removed screw, needle has a flat end, is this correct? should it not be a sharp point (it does taper down at the end but has a distinct flat face) as with other carbs i have messed with?
Any help appreciated.
Regards, Graham

 

[Bambino]

needle has a flat end, is this correct? should it not be a sharp point (it does taper down at the end but has a distinct flat face) as with other carbs i have messed with?
Any help appreciated.
Regards, Graham

Graham,

I've attached a photo of a new and an old IMB idle screw. Yes, it does have a sharpish point though it is the taper that does most of the work in regulating the idle mixture. If yours is so flat that the taper is missing most of its length, then the screw is useless.

I'd wonder how tightly the screw had been fitted in the past as these ones do not have a collar and seat that are supposed to prevent damage to the taper. The second set of idle screws in the pictures are from a DCOE carby and show the new style with a collar and the old style, like yours, with just a naked taper.

If you rebuild the carby, you will get a new idle screw as part of the rebuild kit. BTW, unfortunately the old style and new collared screws are not interchangeable. The collared screw has a much finer taper and point to allow tighter control of the idle mixture in the newer 'anti-pollution' carburettors.

Chris

 

[nerodino]

Nice work bambino, Mine is a collared type as depicted in r/h photo
(l/h screw of the two) It appears to have a sharp point which mine has not. Having said that the taper is there, it is only the first 0.25mm or maybe less which is flat.
I guess the best thing to do is buy a carburettor repair kit from Ricambi.
Regards, Graham.

Graham,

I've attached a photo of a new and an old IMB idle screw. Yes, it does have a sharpish point though it is the taper that does most of the work in regulating the idle mixture. If yours is so flat that the taper is missing most of its length, then the screw is useless.

I'd wonder how tightly the screw had been fitted in the past as these ones do not have a collar and seat that are supposed to prevent damage to the taper. The second set of idle screws in the pictures are from a DCOE carby and show the new style with a collar and the old style, like yours, with just a naked taper.

If you rebuild the carby, you will get a new idle screw as part of the rebuild kit. BTW, unfortunately the old style and new collared screws are not interchangeable. The collared screw has a much finer taper and point to allow tighter control of the idle mixture in the newer 'anti-pollution' carburettors.

Chris

 

[Bambino]

Having said that the taper is there, it is only the first 0.25mm or maybe less which is flat.
I guess the best thing to do is buy a carburettor repair kit from Ricambi.
Regards, Graham.

A new kit is the go - especially as the carby will have to be stripped to be cleaned.

I've attached a photo of the idle screw out of an IMB 10 from a later model 650cc 126 engine next to the older style screw out of a 500F IMB 6. The screw end is collared, has a flat tip and only a slight taper.

The idle mixture screw on these later carbies in contained in a well and sealed with an O-ring, presumably to protect the threads from dirt - or maybe, to discourage amateurs like me from tampering with the setting

Good luck with the rebuild,

Chris

 

[nerodino]

Chris, Let me get this right, You say the right hand screw is froma 650 engine? Mine is a 500 but definately has the collared screw on the right in the carb!
Also i note that that screw DOES HAVE a very small flat at the taperof the needle as has mine.
I wonder if mine has the wrong carb, How can i tell?
Regards,Graham

A new kit is the go - especially as the carby will have to be stripped to be cleaned.

I've attached a photo of the idle screw out of an IMB 10 from a later model 650cc 126 engine next to the older style screw out of a 500F IMB 6. The screw end is collared, has a flat tip and only a slight taper.

The idle mixture screw on these later carbies in contained in a well and sealed with an O-ring, presumably to protect the threads from dirt - or maybe, to discourage amateurs like me from tampering with the setting

Good luck with the rebuild,

Chris

 

[Bambino]

Graham,

From the information I have, Fiat fitted the following Weber carbies as standard equipment to the following cars -

Nuova 500: 26 IMB 1
500 Sport: 26 IMB 3
500D: 26 IMB 4
Wagon: 26 OC (basically a side draft IMB)
500F: 26 IMB 6
500L: 26 IMB 10 (also fitted to the 126s)

The major differences were in the jetting and on later models, the presence of anti-pollution 'CO limiters' and fuel return lines. My 1968 500F has an IMB 6 and the 1990 126 (650cc) I bought for parts had an IMB 10.

The type is stamped on the side. I would think that your carby is probably correct for your car, particularly if it is a late model. If you are going to change carbies, you would probably not replace one IMB with another (they are not that great) - more like you'd change it for a Weber 30 DIC or a Dell'Orto FZD.

Hope this helps,

Chris

 

[nerodino]

You are correct Chris, It has a 26 IMB 6 as should have,
Maybe i will look to change it!
Graham


QUOTE=Bambino;2553091]Graham,

From the information I have, Fiat fitted the following Weber carbies as standard equipment to the following cars -

Nuova 500: 26 IMB 1
500 Sport: 26 IMB 3
500D: 26 IMB 4
Wagon: 26 OC (basically a side draft IMB)
500F: 26 IMB 6
500L: 26 IMB 10 (also fitted to the 126s)

The major differences were in the jetting and on later models, the presence of anti-pollution 'CO limiters' and fuel return lines. My 1968 500F has an IMB 6 and the 1990 126 (650cc) I bought for parts had an IMB 10.

The type is stamped on the side. I would think that your carby is probably correct for your car, particularly if it is a late model. If you are going to change carbies, you would probably not replace one IMB with another (they are not that great) - more like you'd change it for a Weber 30 DIC or a Dell'Orto FZD.

Hope this helps,

Chris[/QUOTE]