Hello,
I 'm Looking for changing my GP carb...
Very difficult for me to find the best setting. Without idle it's difficult to ride...
It's à beautiful carb but very hard to ride with.

What do you think about other carb ? I want to keep a look like GP carb (not mikuni for exemple)
Maybe AMAL 389 ?
what carb is the best compromise ?
It's for my DBD34 500 clubman (RRT2 gearbox and BTH TT magnéto)

Thank you

Vincent

The Gold Star book has settings for a 389. You can get bell mouths for them and they look the paat. You would need an adaptor as the spacing for the distance between the mounting studs is different. I put one on my BB34 GS and it bolted right on due to earlier head but starts and runs great. I bought it new from JRC with all the settings I wanted.





Bill B...

I have a 1038 Concentric on my GS. Looks decent, works great. But may be a bit hard to find.

Reality is, snort of a 1000 series Concentric, gonna be tough to find anything that looks the part. Dellorto or AMAL MK II are about your only choices short of Mikuni.

I highly recommend the AMAL 1038 - looks the part although pretty scarce, especially for 4-stroke. I have one on DBD Clubman - starts and idles. Mikuni's are great but to me look wrong.

Concentric 1038's hard to find but one on eBay right now:
http://www.eBay.ca/itm/AMAL-1038-4T-CARBURETOR-BSA-GOLDSTAR-VELOCETTE-THRUXTON-NORTON-MATCHLESS-RACING-/391505431858?hash=item5b278acd324goAAOSwEjFXds3Q&vxp=mtr

It has tickler on the left (better on the right) but otherwise will work. Lots of recommendations on this forum for jetting.

Cheers

Be careful with the choice of 1038 Amals - if the idle screw is on hr left, it is probably a 2 stroke version, and possible Spanish made. The difference, other than the usual adjustable needles/jets, is the 'spray tube' in the bottom of the bore, where the needle goes through - they can be changed reasonably easily though. With the 2 stroke fitting, I read that the bikes just will not run right.

Thank you everybody !
Hope to find a 1038 carb...

I'll be in England in a week or so - will see what I have left- might have a 1036 or 1038...can't recall..might be a two stroke one though. (the 1036 will be just as good, if not better in the sorts of uses most of us put the bike to these days..perhaps 2mph less at the top. A guess, might be better!)

Thank you Kerry,
Yes 1036 or 1038 but maybe 1036 is the best solution.
Vincent

My stash of 1036 & 1038's are all 2 stroke carbs with LH idle screws and tickler. Plenty of room on a GS to access from the left.

Both are easy to convert to 4 stroke. There are plenty of junk 930's around for organ donors. There have been a number of threads on jetting. Definitely use a new needle and needle jet.

I am running scrambles cam set. Started with a 1036. I just didn't like the way the bike ran. It ran ok, but just did not seem to run like it should in the mid-range. Tried a 1038 and never looked back. I am quite impressed with the 1038 on my GS

While this is true, I wonder how relevant it is for a given Gold Star.

The reason I say this is AMAL made four spray tubes for their Concentrics: generic 4-stroke with a flat top; Triumph Trident with a slash starting halfway back on the tube; 850 Norton Commando with a step cut out of the back of the tube; and generic 2-stroke with a slash starting at the front of the tube. The fact that specific 4-stroke engine/exhaust configurations (i.e. Trident and 850 Commando) required different spray tubes to, presumably, fine tune the behavior in some rpm range says that a generic "4-stroke" spray tube may or may not actually be optimum for all, or any, Gold Star cams and exhaust systems.

It's not like the generic 2-stroke spray tube is completely different than any of the 4-stroke tubes. In fact, it is less radical looking than that of the Commando, and not very different than that of the Trident. Absent back-to-back comparisons across the entire rpm range of otherwise-identical carburetors I'd say it isn't possible to know if the performance of the generic 2-stroke spray tube is worse or better for a Gold Star than the generic 4-stroke tube. Further, to know the ideal spray tube shape for a Gold Star engine with particular cams and exhaust system would required doing tests with "home made" spray tubes having other shapes as well.

I tried all 4 variations with my GS with the Concentric. The generic 4 stroke gave the best mid range settings/response followed by the triple spray tube. The others ran like crap. Actually the triple spray tube wasn't all that good, but definitely better than the Commando or 2 stroke tube.

But I would agree, different cams/exhaust/etc. may work better with one of the other spray tubes

I may be lucky but I used a 32mm Stepped spray tube Commando body on my B44 and once I had the main jet dialed in and the needle position sorted it has clean carburetion through the full throttle opening and rev range. I used the corresponding needle that the 850 Commando would use with a stepped spray tube, main jet is 310 without mute and 290 with mute.

The stepped spray tube was the fix for a roll on noise test where the throttle required to be snapped open, without the step the bike would not accelerate, with the step it could. Hardly an everyday fault and in normal riding if your bike bogged down from snapping the throttle open you would just close the throttle until it caught. But it was in the test so it had to be able to accelerate.

For what it's worth, the 1036 I have on my Special Competition came with a 2-stroke tube and it works nicely. The bike has 65-2446 inlet and exhaust (as far as I have been able to determine stock inlet probably would have been 2442) and, as far as I can tell, it has an original (not repro) 'twitter' silencer.

It's worth mentioning that the shape of the spray tube affects the air flow over it and hence the pressure drop that draws fuel into the air stream. Because of this, all jetting can be expected to be affected when a spray tube is swapped so experimentation with spray tubes requires experimentation with jets, slide cutaway and needle position as well.

Vincent ... As I've mentioned here before I've replaced GP carbs with AMAL 38mm MKII 'Smooth Bore racing carbs' on 3 Goldies. We fit a 'conventional' 40mm MKII on the dirt tracker to be legal in AHRMA races but a smoothie in all other races.
BTW, this seems a 'age related' issue in my case.

The smooth bore MKII is a much higher quality instrument than the conventional pot metal model but others have found it satisfactory. One issue is the pot metal throttle slide is notorious for hanging up giving you a quite high idle rpm when closing the throttle. I've wondered if a chromed brass slide is available as is fitted to the smoothy.

The MKII smooth bore has the same issue as a GP without a idle throttle stop. However it's easily modified. But another issue is the need to fit a flange mount to spigot adapter. The Mikuni adapter is available and correct but pricey seems to me. This model carb is available with both left and right idle air adjusters. But of course the RH version is best on a GS, but the LH model is tolerable.

I M N S H O the AMAL mk1 has no place on a quality bike, it is (by design) cheap and nasty. If you can find a Monobloc in good condition use it. The Dellorto is a good option, especially with the accelerator pump, you can snatch a big handful without stumbling or bogging down

You can buy a brand new AMAL Monobloc directly from AMAL for 170 quid or about $220. The one I got even has the "correct" Gold Star number stamped on the flange.


Bill B...

Thank you everybody.
The problem with the Monobloc is on the diameter limited to 1" 3/16 (30 mm).
i have 34mm on my inlet.

Why not just keep the GP.

I have made a stop for the throttle valve by adding a blob of solder to the end of the cable, so that it will hit the roof of the carburettor. Of course you will have to adjust the idle speed by filing a bit off the blob.

You don't write what your problem is with the setting. When I made the setup of mine, the basic problem was that it was running too rich. So I moved the needle clip from the middle to the top notch and that cured it.

I have also made other changes to the jetting, but that is only fine tuning, it ran fine by just moving the needle clip.

The lowers rpm not easy with the GP.
He run great up to 4000 rpm and i leave in south of France and my sinuous road (with my RRT2 long gearbox) is not appropriate for this...

Re the lack of GP idle stop:
The crude method I've used for years on my highly modified GS 'works'.
But prolly not good for everyone. My GP is on a 'tuned' length carb spacer and it sticks out behind the smallish alu BSA comp fuel tank. The throttle cable needs to go over the top of the tank and is secured with 'modern high tech' Walmart suction cups. I adjust the idle speed with the carb top adjuster.

A much better method was used by our departed friend GStarRon (RIP) and I'm sure many others. I want to use this method someday. Ron had a small hole drilled in the bottom edge of his throttle slide that passed enough air for a dependable idle. It would be tricky arriving at the proper size hole. hmmm
Interestingly Ron's Goldies nearly always started on the first kick. hmmm

Neil Keen a tuner/racer from The Era described to me his method for devising a throttle stop on his GP when on the street. He drilled/tapped his carb top for a #6 screw. Then a long screw went down and through a hole in the top of the slide. The idle speed was than adjusted from the top and the screw was locked with a nut

The 'secret' to easier starting a tuned engine is to leave the throttle at idle when kicking over. Difficult when manually operating a GP throttle but we've all done it for years.
Did I ever tell you the story about my 7 month preg bride push starting me on my flooded Goldie at the Boise TT? sheeze...

Interesting thank you,

What do you think about my GP setting (DB 34GS) ?

GP carb 1 1/2
Main jet 340
Needle jet 109
Needle 3GP6
Needle position 2 (from the top)
Throttle slide 4

On the GS book is indicate (for a GP carb 1 1/2)
Main jet 350
Need le jet 109
Needle position 4 (from the top or bottom ?)
Throttle slide 4

From an East Coast BSA Service Bulletin:

all this, most interesting!

I want to cast my vote again for the GP with the idle adjusted by the cable adjusters...then again, my name here says it all, lol.

What about a Gardner carburetor..if you are going for something off grid.

Your Monobloc will probably be a good choice.

'In the day' the Monobloc, GP and del Orto were the ones most seen, then the Concentric started to weasel it's way in..I sure liked my GP tho, never had a complaint with it, sure heard a lot, but never from me. Do I get a badge or something?....:>

I think if I wanted a GStar that idled I would just get an electric bike, lol...I'm kidding, well, maybe not.My neighbor's BMW is so quiet he keeps sneaking up on me with it.

Monobloc it is then.

very interesting thank you !

I think the hole on the slide is more precise than adjusting cable.
But diificult to take the decision to make a hole in the slide ($$$$$)...

Hi VincentBSA ,I drilled the slide on my 1959 DBD many years ago and have had a steady idle since.
I've never seen the BSA Bulletin but the hole doesn't need to be that high. My slides have a 3/32 hole that is 5/32(center of the hole) above the downdraft edge of the slide.Your jetting is pretty well the same as mine ,I'm using the steeply tapered 3GP needle,NOT the fat needle.The hole allows the fuel to make it's way in as the slide closes.It is interesting to watch at night with a flashlight lighting up the velocity stack.Even coming hard off the throttle and gearing down for a stop the engine will settle down to a reliable idle.Mark the center of the slide with it installed ,then remove and center punch and drill the hole ,deburr both sides ,reinstall and your ready to go.It is also very important to have the float level correct.Good luck.
I also thought of the money if it didn't work ,but figured the hole could be silver soldered closed.