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DB34 with amal GP 1 1/2 #706146
08/25/17 7:43 am
08/25/17 7:43 am
Joined: May 2015
Posts: 37
France
V
vincentBSA Offline OP
BritBike Forum member
vincentBSA  Offline OP
BritBike Forum member
V
Joined: May 2015
Posts: 37
France
Hello,
if the setting of AMAL GP 1 1/2 is the same on DB34 and DBD34 ?
Is it a problem to fit a 1 1/2 carb on 1 3/8 intake port ?

I have on my DB34 a GP carb 1 1/2 with this setting

Needle 3GP (set at the middle notch)
Needle jet 109
Main jet 360
Throttle slide cutaway 4

Thanks for your help .

Vincent

Last edited by vincentBSA; 08/25/17 7:45 am.
Re: DB34 with amal GP 1 1/2 [Re: vincentBSA] #706241
08/26/17 2:40 am
08/26/17 2:40 am
Joined: Mar 2014
Posts: 564
Denver CO
J
Jerry Roy Online content
BritBike Forum member
Jerry Roy  Online Content
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J
Joined: Mar 2014
Posts: 564
Denver CO
This from another thread;
Ive just been reading Roland Pikes memoirs, there's a fascinating section when he talks about Goldstar port venturis, after discovering by accident that a big carb with a step at the port gave more HP than a port blended to the carb .The optimum venturi was found to be 80 % of the choke , best angles 22 degrees / 7 degrees .Theres a great section where he talks about tweaking the GS valve clearances on the dyno at various RPMs, as the revs went up HP went up if the clearances were closed down, he put this down to the valve gear flexing.

Re: DB34 with amal GP 1 1/2 [Re: vincentBSA] #706297
08/26/17 5:05 pm
08/26/17 5:05 pm
Joined: Sep 2015
Posts: 290
Kent, England
J
John Alexander Offline
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John Alexander  Offline
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J
Joined: Sep 2015
Posts: 290
Kent, England
Hi Vincent, the 1956 DBD34 head was modified to feature wider carburettor mounting centres permitting the use of an inch and half GP, so i think it may be too small. On the thread of Roland Pike discovering a small step was found to increase power, my Gold Star has the inlet opened to remove the step and performance has not suffered.
Goldie John.

Re: DB34 with amal GP 1 1/2 [Re: John Alexander] #706320
08/26/17 8:15 pm
08/26/17 8:15 pm
Joined: May 2015
Posts: 37
France
V
vincentBSA Offline OP
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vincentBSA  Offline OP
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V
Joined: May 2015
Posts: 37
France
Thank you,
Very interesting the story of venturi !

Re: DB34 with amal GP 1 1/2 [Re: vincentBSA] #706330
08/26/17 10:26 pm
08/26/17 10:26 pm
Joined: Aug 2001
Posts: 1,864
Elko, Nevada USA
dave - NV Online content
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dave - NV  Online Content
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Joined: Aug 2001
Posts: 1,864
Elko, Nevada USA
Gents .. Re the remarks about smaller valve clearances enhancing power output... It's generally believed that more sturdy pushrods reducing flex due to harmonics and reducing valve bounce on closure enhance power. For this reason I and others have fitted 'double ball ended' chrome molly tubular pushrods and ball socket followers in our Goldies from ABSAF. As there is now no place to fit a feeler gauge setting clearances we follow the OEM procedures but set the clearance to just free spin of the pushrod.
It's a bit of a mystery to me what the running clearance of the valves are due to the difference in heat expansion of the alu barrel and the steel tubes. hmmm
I've long wanted to weld on Triumph rocker box access to the BSA rocker box to allow physical measurement of the clearance at the valve stem.


dave - NV
Re: DB34 with amal GP 1 1/2 [Re: vincentBSA] #706387
08/27/17 3:13 pm
08/27/17 3:13 pm
Joined: Aug 2001
Posts: 3,518
argyll. scotland, uk
gavin eisler Online content
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gavin eisler  Online Content
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Joined: Aug 2001
Posts: 3,518
argyll. scotland, uk
Here is the exact text from Mr Pikes memoirs.
Clipped from chapter 26 , here, http://beezagent.blogspot.co.uk/2009/02/roland-pike-autobiography-chapter-26.html

"One of the strangest occurrences to do with carburettors was when one day Reg Wilkes sent an apprentice from the test shop into our, main shop for a 1 3/32 GP carburettor and he misread the size and gave Reg: a 1 3/16th carburettor and they immediately got quite a jump in power. They reported this to me and I went to the shop and the test was repeated and there was no doubt it was quite a gain in power. I suggested to Reg to open the port to match, immediately we lost what we had gained plus a bit more, so we made a thin sleeve and pressed it in and the power was back. We repeated this on other engines always with the same results. A number of private owners of DBD Gold Star noticed the carb being bigger than the port and opened up the port thinking to gain power, but not having a dyno were not aware of the results. Later on we fitted a venturi behind the carb and picked up even more power. The smallest diameter of the venturi could be 80% of area of the carburettor. We tested this on several engines and it always worked and seemed to improve carburation. The venturi had to have the classic included angles of 22 degree in and 7 degree out."


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