Amal GP Carbs

[oldmikew]

Anyone used them on a Commando ?

 

[Rohan]

Peter has ??

http://peterwilliamsmotorcycles.com/wp- ... a-Side.jpg

 

[johnm]

Yes the original JPS used them.

But the production racer machine that came to NZ with Peter Williams and Norman White a few years ago had Concentrics.

And I specifically asked Norman White why they had been changed. Was it because the factory wanted to use the Concentrics because that's what the street bikes had?

He said they did use GPs but had gone from GPs back to Concentrics because they were easier to run and there didn't seem to be any reduction in performance.

A friend of mine had arranged their visit down under and we had dinner with them one night and spent all night talking racing Nortons. (Just 5 of us so we were able to have a good talk). So Im pretty sure what Norman told me was correct. At least as well as anyone can remember 30 years later. :lol:

I could add from a personal point of view that during several years of racing we won a number of races on Concentrics while our rivals were sitting in the pits trying to sort out their GPS. From what I observed they would be about number 48 on my list of priorities for creating a fast Norton.

 

[acotrel]

Surely a range of needles with a variety of tapers gives a more flexible means of tuning than a tube with bleed holes ?

 

[oldmikew]

Yes the original JPS used them.



And I specifically asked Norman White why they had been changed. Was it because the factory wanted to use the Concentrics because that's what the street bikes had?

He said they did use GPs but had gone from GPs back to Concentrics because they were easier to run and there didn't seem to be any reduction in performance.

A friend of mine had arranged their visit down under and we had dinner with them one night and spent all night talking racing Nortons. (Just 5 of us so we were able to have a good talk). So Im pretty sure what Norman told me was correct. At least as well as anyone can remember 30 years later. :lol:

I could add from a personal point of view that during several years of racing we won a number of races on Concentrics while our rivals were sitting in the pits trying to sort out their GPS. From what I observed they would be about number 48 on my list of priorities for creating a fast Norton.

Many thanks for posting this -must have been an interesting night. The advantage that I can see is that G.P s will with remote
float tolerate down draft. Croxford turned up at the Festival of 1000 bikes with a JP Norton a couple of years ago and that
certainly had MK1 Concentrics but Mk2s are perhaps more prone to flooding if mounted at a similar radical angle.

 

[oldmikew]

Surely a range of needles with a variety of tapers gives a more flexible means of tuning than a tube with bleed holes ?

Yes I would have thought so - I suppose its a question of time and patience and as old have plenty of the former and still some of the latter. Whether its really all worth the candle is a different matter but I fancy having a fairly radical amount of downdraft.

 

[acotrel]

There are Mikunis which are designed to be vertical down draft with the float bowl in a different position to normal. If you are somewhere in between horizontal and vertical, you still end up with a compromise. The remote bowl is a better option - perhaps Del Ortos ? I think I'd go mad playing with that tube and the parallel needle in the GPs. Getting the jetting right all the way along the needle is of critical importance, and VM Mikuni needles work very well in Mk2 Amals - could be good in Del Ortos too ? From memory, I think most 32 to 36 mm carbs have 3mm needles, regardless of brand. Mikuni provide the biggest variety of precision ground tapers.

 

[oldmikew]

There are Mikunis which are designed to be vertical down draft with the float bowl in a different position to normal. If you are somewhere in between horizontal and vertical, you still end up with a compromise. The remote bowl is a better option - perhaps Del Ortos ? I think I'd go mad playing with that tube and the parallel needle in the GPs. Getting the jetting right all the way along the needle is of critical importance, and VM Mikuni needles work very well in Mk2 Amals - could be good in Del Ortos too ? From memory, I think most 32 to 36 mm carbs have 3mm needles, regardless of brand. Mikuni provide the biggest variety of precision ground tapers.

Del Ortos are worth a look certainly and didnt realise VM Mikuni needles would work in Mk2s . Live not too far from one of Uk
main distributor so that is well worth knowing. Thanks.

 

[madass140]

when used on twins did they use one or two float bowls?

 

[gripper]

GPs don't have an idling circuit i believe. So it will never tick over.

 

[madass140]

maybe that is why BSA replaced them on the Sspitfire with MK1's

 

[Brooking 850]

How did they compare to smooth bores?
I have a couple of sets I want to use on another race motor Im building withe a FA head.
I am using Premier Concentrics on the current race motor and they seem to work fine, woould love to hear anyones experiences comparing all three types.
Regards Mike

 

[acotrel]

I have never used smooth bores. Theoretically they look good, but sometimes benefit is pretty delusional. It might be like fitting stickier tyres - the effect is largely psychological ? There are plenty of guys who cannot competently jet conventional carbs, so GPs would have them completely stuffed.

 

[acotrel]

I use two 34mm MK2 Amals with the ports tapered back to 30mm over the first 20 mm of the port. Using methanol fuel, I never get any hesitation regardless of how I use the throttle.

 

[Brooking 850]

I was looking for responses to do with members using smoothbores or having been around others who have used them
Regards Mike

 

[Chris]

Hi Brooking

I have 34 & 36mm smoothbores on my bikes. I changed from standard Mk2s I cant say I notice the difference other than the needle retaining method is much more secure. All the differences that make them a better carb you can get for the standard carb. ie the alloy taller top caps, the Chrome brass slides. Setting tickover is on the cable adjusters only I would like a throttle screw. Dave Watson ran standard carbs a few times & still won. Nigel dynoed them & found the smoothbores better. Personally I just find the quality better.
GPs make more power but keeping a pair matched is far harder to manage. Jack Shermans worked on Gps before he did his work on MK1s for the triples I will have to dig out the article but I seem to recall they were not worth the effort.
I am a bit taken by Keihn carbs as we are now allowed to run them. Richard Pecket swears by them, cleaner, better fuel, more power, better quality, set & forget. Maybe one day. However saying that all my Race triples are on Mk1 carbs periodically correct & they run well. When they wear out however?

Chris

Chris

 

[Brooking 850]

Thanks Chris, thats the info I was looking for.
Yes my race bike is on 32 mm Mk1 Amal Premiers and they do a fine job.
Have these other 34 mm smoothbores and have yet to use them.
I have had flow tested 2 x sets on the FA head, one set seems to perform better than the other on the bench, however I would probably not notice the difference when racing.
Thanks for your reply.
Regards Mike

 

[madass140]

I havent investigated this (yet) but becoz of the lack of idle or low speed function making the GP's unsuitable for road use I'm sure with a bit of thought and some fine machining a low speed function could be incorporated, to be considered though is a fuel supply
which is not an issue with incorporated float like everything else.

 

[Fast Eddie]

Chris, on top of the differences that you describe, the main difference in a smoothbore, that cannot be replicated in a mk2, is the internal design of the choke (intake) and slide. It’s this design that makes the choke a ‘smooth bore’ at WOT. This causes less disturbance of the incoming charge and therefore leads to increased velocity, flow and ultimately, bhp.

The difference in bhp is small on the dyno though.

They also seem to work differently on different bikes. I had bikes that whilst having a few more peak bhp, the power and torque delivery below that was better without smoothbores. Perhaps these engines ‘liked’ more turbulence of the intake charge? I’ve no idea to be honest!

Ref the CRs, I’ve run them on a triple and they were great! Neil Beadling set them up for me, I suspect I would have had a nightmare doing so myself! Yes, they are expensive though, especially when you’ve got about 30 bikes to put them on like you!!

Let me know before you change your fleet over to CRs, I’ll buy shares in Keihin...!

 

[kommando]

On 4 valve engines they lose low down torque in comparison to 2 valve engines due to lack of turbulence in the incoming charge, it can be gained by making one of the 2 inlet valves a different size which induces turbulence.