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Change my GP carb
#660851
07/16/16 3:00 pm
07/16/16 3:00 pm
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Joined: May 2015
Posts: 37 France
vincentBSA
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OP
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Posts: 37
France
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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
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Re: Change my GP carb
[Re: vincentBSA]
#660896
07/16/16 10:07 pm
07/16/16 10:07 pm
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Joined: Aug 2001
Posts: 5,019 Stone Creek OH USA
Rich B

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Joined: Aug 2001
Posts: 5,019
Stone Creek OH USA
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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.
Life is too short to drink cheap, bad beer.
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Re: Change my GP carb
[Re: Rich B]
#660914
07/17/16 12:10 am
07/17/16 12:10 am
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Joined: Aug 2001
Posts: 86 Yellowknife, NWT, Canada
Dave - North of 60
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Posts: 86
Yellowknife, NWT, Canada
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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=item5b278acd32  4goAAOSwEjFXds3Q&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
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Re: Change my GP carb
[Re: vincentBSA]
#660942
07/17/16 2:11 am
07/17/16 2:11 am
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Joined: Feb 2012
Posts: 1,225 Middle East,
Kerry W
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Joined: Feb 2012
Posts: 1,225
Middle East,
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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.
No generalisation is wholly true, not even this one. Oliver Wendell Holmes
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Re: Change my GP carb
[Re: vincentBSA]
#660951
07/17/16 5:02 am
07/17/16 5:02 am
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Joined: Feb 2012
Posts: 1,225 Middle East,
Kerry W
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Middle East,
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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!)
Last edited by Kerry W; 07/17/16 5:03 am.
No generalisation is wholly true, not even this one. Oliver Wendell Holmes
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Re: Change my GP carb
[Re: Kerry W]
#661005
07/17/16 12:54 pm
07/17/16 12:54 pm
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Joined: Nov 2011
Posts: 4,279 U.S.
Magnetoman

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B ... a 2 stroke version ... The difference, other than the usual adjustable needles/jets, is the 'spray tube' in the bottom of the bore, 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.
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Re: Change my GP carb
[Re: Magnetoman]
#661020
07/17/16 1:47 pm
07/17/16 1:47 pm
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Joined: Aug 2001
Posts: 5,019 Stone Creek OH USA
Rich B

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BritBike Forum member
Joined: Aug 2001
Posts: 5,019
Stone Creek OH USA
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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
Life is too short to drink cheap, bad beer.
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Re: Change my GP carb
[Re: vincentBSA]
#661039
07/17/16 4:08 pm
07/17/16 4:08 pm
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Joined: Aug 2001
Posts: 1,871 Elko, Nevada USA
dave - NV
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BritBike Forum member
Joined: Aug 2001
Posts: 1,871
Elko, Nevada USA
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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.
Last edited by dave - NV; 07/18/16 1:48 pm.
dave - NV
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Re: Change my GP carb
[Re: vincentBSA]
#661042
07/17/16 4:29 pm
07/17/16 4:29 pm
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Joined: Aug 2015
Posts: 1,086 Bolton Lancs UK
Andy Higham
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Joined: Aug 2015
Posts: 1,086
Bolton Lancs UK
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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
1955 BSA B31 500cc "Stargazer" 1962 Greeves 200cc "Blue Meanie" 1962/67 Greeves 350 1967 Greeves 360 Challenger 1984 Suzuki GSX1100 EFE "Sorcerers Apprentice" GM500cc sprint/LSR bike "Deofol" Jawa 500cc "Llareggub" 2003 Aprilia RSV Mille "Lo Stregone"
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Re: Change my GP carb
[Re: vincentBSA]
#661045
07/17/16 4:46 pm
07/17/16 4:46 pm
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Joined: Feb 2011
Posts: 3,421 Orygone
Boomer

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Joined: Feb 2011
Posts: 3,421
Orygone
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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...
Boomer
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Re: Change my GP carb
[Re: vincentBSA]
#661090
07/18/16 2:17 am
07/18/16 2:17 am
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Joined: May 2015
Posts: 37 France
vincentBSA
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OP
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Posts: 37
France
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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.
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Re: Change my GP carb
[Re: vincentBSA]
#661101
07/18/16 6:03 am
07/18/16 6:03 am
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Joined: Sep 2010
Posts: 57 Denmark
Ole K
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BritBike Forum member
Joined: Sep 2010
Posts: 57
Denmark
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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...
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.
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Re: Change my GP carb
[Re: vincentBSA]
#661139
07/18/16 1:42 pm
07/18/16 1:42 pm
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Joined: Aug 2001
Posts: 1,871 Elko, Nevada USA
dave - NV
BritBike Forum member
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BritBike Forum member
Joined: Aug 2001
Posts: 1,871
Elko, Nevada USA
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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...
Last edited by dave - NV; 07/18/16 1:44 pm.
dave - NV
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Re: Change my GP carb
[Re: vincentBSA]
#661192
07/18/16 9:51 pm
07/18/16 9:51 pm
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Joined: Oct 2009
Posts: 765 Santa Barbara, Cal.
Bodger
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Joined: Oct 2009
Posts: 765
Santa Barbara, Cal.
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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.
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Re: Change my GP carb
[Re: vincentBSA]
#661198
07/19/16 2:37 am
07/19/16 2:37 am
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Joined: May 2015
Posts: 37 France
vincentBSA
OP
BritBike Forum member
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OP
BritBike Forum member
Joined: May 2015
Posts: 37
France
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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 ($$$$$)...
Last edited by vincentBSA; 07/19/16 11:05 am.
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Re: Change my GP carb
[Re: vincentBSA]
#661279
07/19/16 4:55 pm
07/19/16 4:55 pm
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Joined: Jun 2005
Posts: 96 montreal
johnnie690
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BritBike Forum member
Joined: Jun 2005
Posts: 96
montreal
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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.
Last edited by johnnie690; 07/19/16 5:34 pm.
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