The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: roadrocket.chris on 05.08. 2011 18:28

Title: twin carb heads
Post by: roadrocket.chris on 05.08. 2011 18:28
Some pictures of my twin carb roadrocket head

(http://<table style="width:auto;"><tr><td><a href="https://picasaweb.google.com/lh/photo/8R2OPIrk484bYje8SuIPZu4-jQ6k7aUcjMec4A8txxo?feat=embedwebsite"><img src="https://lh4.googleusercontent.com/-kw4pvOwZFhM/TjrTYh47ZAI/AAAAAAAABKM/rB7vbFpyCZ0/s144/revclock%252520and%252520carbs%252520010.jpg" height="108" width="144" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="https://picasaweb.google.com/yellowlabrador2007/RevclockAndCarbs?authuser=0&authkey=Gv1sRgCJO2muKA4o77iwE&feat=embedwebsite">revclock and carbs</a></td></tr></table>)
(https://lh5.googleusercontent.com/-NgUntE9fGvk/TjrTWBNC00I/AAAAAAAABKE/MUPcq-UJ-s0/s144/revclock%252520and%252520carbs%252520008.jpg)
Title: Re: twin carb heads
Post by: roadrocket.chris on 05.08. 2011 18:44
(https://lh4.googleusercontent.com/-kw4pvOwZFhM/TjrTYh47ZAI/AAAAAAAABKM/rB7vbFpyCZ0/s144/revclock%252520and%252520carbs%252520010.jpg)
Title: Re: twin carb heads
Post by: muskrat on 05.08. 2011 22:19
G'day Chris,
                looks good but I can't get the pics up bigger, like their not linked.
Have you put in tubes to vacuum balance the carbs?
Cheers
Title: Re: twin carb heads
Post by: a10gf on 05.08. 2011 22:26
found them:

https://lh4.googleusercontent.com/-kw4pvOwZFhM/TjrTYh47ZAI/AAAAAAAABKM/rB7vbFpyCZ0/

https://lh5.googleusercontent.com/-NgUntE9fGvk/TjrTWBNC00I/AAAAAAAABKE/MUPcq-UJ-s0/

Chris, maybe fix the thumbnail links, or upload them as forum attachments.
Title: Re: twin carb heads
Post by: muskrat on 05.08. 2011 23:02
Thanks e.
I think duel carbs do look better splayed out like that, rather than my parallel set up.
Cheers
Title: Re: twin carb heads
Post by: Rocket Racer on 06.08. 2011 10:09
Chris,
 what size are those monoblocks?
I take it your happy with them...
nothing like twin carbs on an A7 or an A10 to start a discussion.
Was the splayed design deliberate or just worked out that way. With the parallel ports they certainly sit in tight.

I have no idea if mine add any performance but I love the appearance
Title: Re: twin carb heads
Post by: roadrocket.chris on 06.08. 2011 17:27
they were splayed that way really for convenience they are 1 3/32 inch carbs  i had a rocket in the sixtys in a similar state of tune but with a single carb it had power lower down than the twin carb it seems to have moved higher up the rev range with this set up.
Title: Re: twin carb heads
Post by: muskrat on 06.08. 2011 22:17
That would be right Chris. It's all to do with intake velocity and cylinder filling. Now that it has two holes the rpm has to be higher to get the same. Coupled with a 357 cam it should take off at about 5000rpm.
 Fitting smaller carbs will give you a little back.
Cheers
Title: Re: twin carb heads
Post by: jjbsa on 16.08. 2011 18:58
Hi Chris, Nice photos indeed.  I'm also one of these people who has been chasing the twin carb head thing.  Attached are some pics of a 650 head that I did in the late '60s.  The bike was built first with a pair of AMAL GPs - the type that were originally fitted to BSA Spitfires about 1967 (you could buy them new for about £12 a pair then as BSA had loads that they were dumping).  The bike would not run below about 50 in top with those, so a pair of 32 mm Mk1 Concentrics went on - much smoother.  The bike had one of the 67-358 cams in and was very hungry for revs and exciting to ride.  Alas I sold it in the early 70s when I was in one of those "must move on" moods.  The guy rode it to and from Vienna in Feb 1974 with his girlfriend on the back(!)
Title: Re: twin carb heads
Post by: Rocket Racer on 16.08. 2011 22:59
Interesting to see a converted single carb head, how did you seal the manifold given there appears to be no meat in the joins at 4 and 7 oclock...
Your carbs sit nicely in the frame space, almost room for a still air box...

As you mention everyone loves the look of GP or TT carbs, but they are not the easiest carbs to live with in a twin carb set up, probably why the a65 spitfires had them for such a short time. Ok if you dont mind fettling but for a bike under factory warranty, you can imagine the customer complaints.

My own twin carb (TT) set up is on the twin port '55 head, so much easier to manage and for a race bike twin carbs are constantly being checked along with everything else and the TT carbs are great with Methanol.

Now a 358 would be nice to have, but probably useless on a sidecar. Needs to go on an A7 race bike to give those gold stars a run for their money.
Title: Re: twin carb heads
Post by: jjbsa on 17.08. 2011 09:28
Hi Rocket Racer,  yes, the metal there was very thin on one side and had a small bit missing on the other.  I fudged the bad side by making up a very twisted looking washer out of soft aluminium sheet, which went under the long nut holding down the rocker box corner.  The washer, which was quite soft, was peened very gently to close the gap and then a dab of Araldite was put on it.  It did work, but not how I'd like to do it nowadays.
Title: Re: twin carb heads
Post by: Jim on 22.09. 2011 17:32
For those with deep pockets there seems to be a genuine twin carb head plus manifolds on e-bay at present.

http://www.ebay.co.uk/itm/330616136466?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_5784wt_1193

http://www.ebay.co.uk/itm/330616137221?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_5657wt_1193

Title: Re: twin carb heads
Post by: muskrat on 22.09. 2011 21:11
yep, been watching that one. What makes it XXX special is the casting # upside down. Never seen original factory manifolds before.
Cheers