STARTING FROM SCRATCH

[Goldie]

Can anyone offer advice on fitting a carb to a machine that is not totally standard.

I have a BSA DB32 Gold Star 350, that has been bored out to 400cc.
The cylinder head is a DB32 scrambles, and i wish to get the carbruation correct.

Is it best that the carb choke size matches the choke size on the head?

I have been told that the Monobloc is far superior to a concentric, is this true?

I do have a couple of Amal GP carbs, which i could play around with, but i have been told that they can be tempermental if not set up correctly.

I have settings for 350 motors but not 400.
                                                                                                                                 I am at present running the bike in, and have already done nearly 400 miles It seems to pull fine up to 4000rpm, but to get it up to 5000rpm, a lot more throttle is required, which to me does not seem right. I thaught it should be a steady progression with the throttle to increase the revs, past 4000rpm.

There is no spitting back through the carb or backfiring, in fact it does not miss a beat. The timing is spot on, which makes me think that it is the carb.

I am at present running a Concentric 930 with 280 main jet and 3 1/2 throttle valve, the needle has 2 rings on top, 3 grooves, and is set in the middle groove. The needle jet is 106 and the pilot jet 25.
The carb choke matches the head perfectly.

Any advice would be appreciated.

[riflegreen]

Hi Goldy,

My 350 runs a 30mm Concentric and has a 230 main jet , other settings similar to yours , I've had a monobloc on it with a 280 jet and ran just as well as the Concentric .

Personally I prefer the Monobloc , I think it is a stronger body and not so prone to distorting like a Concentric .

Also have the GP carbs for both my Gold Stars and find the initial setting up is a pain , so changed to a big Concentric for the 500 and the smaller for the 350 .

Perhaps you could drop down 10 at a time on the main jet and see if it improves .

Chris

[riflegreen]

Missed the carb/mainfold question , I believe GS had a slight step , ( smaller inlet than carb ) , originally but would have thought it would not be noticeable unless you are racing it and wanted the last ounce out of it .

I can vaguely remember reading that BSA removed the step and lost a bit of power of the top end - altered the fuel flow turbulence ?.

Chris

[henry_norton]

I've used monoblocks and concentrics and they both seemed fine to me. The general consensus seems to be that monoblocks are a bit better dealing with the vibration over a period of time than the concentrics, whilst the latter give more consistent carburation because the float chamber is in line with the main components rather than being off to one side. I don't know how anyone worked these things out but it's likely it was worked out over a pretty long time and at very high speeds so probably doesn't affect most of us in the real world. I like monoblocks as they are a bit less fiddly to work on and they look more 'vintage' - whereas I find the concentrics look a bit too modern on old bikes.

Good luck with the setup. HN

[Goldie]

I am running a 34mm Mikuni on the 500 Goldie, and find it faultless, with easy starting and good response through the rev range.

I would be tempted to buy a Mikuni for the 400 Goldie, but it may not cure the problem.

[twolitre]

It seems to me that (all things being equal - but they aren't) that you have increased the air intake by about almost 15% throughout the range and the bike will therefore be 'strangled' at higher revs/speeds unless the carburettor venturi and inlet tracts are increased by a similar percentage in cross sectional area.  Where that leaves you with jets and needle I would not like to guess.  Such things would be settled originally by experimentation on a dynamometer.
  Remember also that you have probably have increased the compression ratio which will need timing amendments.  How do you KNOW the timing is spot on?

Jim.

[Goldie]

Does anyone know if cam repacement on Gold Stars can be carried out without dismantling the top end?

It does not say you can't or can in the manual.

[chris p]

hi goldie as twolitre sugests you have probably increased the compression and the only thing that influences jet sizes needle positions  is compression  i dont think it influences ignition timing  and beleive it or not  the more compression you have the smaller jets you need and a lower needle position  regardless of which carb you use  set it up as if it was bog standard and work from there weakening the jets and needle one step at a time  if you ve got a longish slightly uphill straight near you that would be ideal for plug chops  hope this helps good luck