21 replies to this topic

[GT Jimmy]

Has anyone ever fitted bike carbs to a mini? Chatting to a mechanic friend who's just built a kit car with a 2ltr sierra engine says it's transformed his engine. He's convinced it could be done on a mini. Bearing in mind I don't have a clue I think I should stick with the SU.

 

What do you think?

[nicklouse]

yes people do fit them to cross flow heads on Minis.

 

not worth it on a road going 5 port.

[lawrence]

Don't do it. stick with the SU. Its much more hassle than its worth and will not unlock any more power than a properly set SU will give. 

[GT Jimmy]

Thanks for the replies. SU it is then

[nicklouse]

as you will notice all the guys on Facebook saying do it are running cross flow engines and dont own Minis.

[hhhh]

An interesting observation here is that the Amal Concentric, which is ubiquitous on classic British bikes, has one needle that suits almost all applications. There is facility for idle adjustment with a screw (but a fixed idle jet,) transition by the amount of leading edge slide cutaway, wide open by the removable main jet, but the entire midrange is handled by very limited adjustment with three different clip settings for the needle height in the slide.  Besides this seemingly primitive arrangement, it somehow manages to do a fairly respectable job providing performance and economy. I've never heard of anybody filing the needle to adjust mixture. This is in huge contrast to the hundreds of needles available for the SU.

[carbon]

The Concentric main jet is available with a range of jet sizes to control flow of fuel.

 

The Amal needle does not directly control the fuel flow in same way as the SU, but as you say it is an elegant and very effective design.

[hhhh]

Yes I mentioned that wide open is adjustable by the removable main jet. Not sure what you mean by saying that the Amal needle doesn't directly control fuel flow the same way as the SU; that's exactly what it does except for the slide being the throttle control rather than a butterfly. In fact the fuel control is more direct by the rider because the needle position in the needle jet is always controlled by the throttle control at the handlebar where the SU's needle position responds to pressure differential.

Edited by hhhh, 03 May 2017 - 06:13 PM.

[mk1leg]

yes I'm running Suzuki carbs on my 7port head

Edited by mk1leg, 03 May 2017 - 08:38 PM.

[carbon]

Yes I mentioned that wide open is adjustable by the removable main jet. Not sure what you mean by saying that the Amal needle doesn't directly control fuel flow the same way as the SU; that's exactly what it does except for the slide being the throttle control rather than a butterfly. In fact the fuel control is more direct by the rider because the needle position in the needle jet is always controlled by the throttle control at the handlebar where the SU's needle position responds to pressure differential.

I'll need to dig out an Amal and check, thought there was an air bleed between main jet and needle jet (bit like emulsion tube).

 

But it's quite a while since I have worked on these and could well be wrong.

[hhhh]

Yes there is a pair of cross drilled holes in the needle jet that do perform an air bleed (leaning) function over the first third of throttle opening, but it's quite subtle. Vincent owners sometimes block them to prevent an issue on overrun with little effect on the running.

[Carlos W]

If you're wanting to gain power on a standard engine the first place to start is modifying the head to improve flow.

 

There's a lot to think about when it comes to fitting bike carbs, manifold, carb angle, throttle and choke cables, etc etc

[GT Jimmy]

I'll be sticking with the SU, the head will be reworked when the engine gets rebuilt, thanks for the replies

[Gr4h4m]

I ran one on my supercharged setup some years ago. The design was an upedated su cV style. I would recommend them they are generallymuch less worn out than the su stuff, I changed it to EFI or it would still be on the car

[tiger99]

I can't see why anyone would want bike carbs now, as EFI has become a lot cheaper, and is easily mappable to get the correct mixture all over the rev range. No simple fixed jet carburettor can do that, for fundamental reasons. The best get it about right at a few points over the range. An SU needle can in theory be made to give a correct mixture over the full range, as it does not need to rely on fixed points set by individual jet sizes.

 

I must admit that a crossflow head, 7 or 8 ports, with a bank of 4 intake trumpets at the front, looks magnificent, but they may as well be throttle bodies rather than carburettors. The uninitiated will not know the difference.

 

Actually a crossflow with the inlet ports at the back, to discourage intake icing, and exhaust at the front, would be better, but they don't seem to make the 7 and 8 port heads that way round. Many of the "foreign" engine conversions do have things that way round, as you can see in the appropriate part of this forum, because that is the way that things are done on many modern cars.