Hitchcocks Forum

As I ponder the exhaust & silencer change on my '07 350 Classic, but mindful some articles (one back in Real Classic) suggesting that some shorter silencers can be too loud and offensive, but with my desire to remove the PAV gubbins as that seems a common thing to do - thoughts turned to carbs and air filters. I may change the air filter too and 'reclaim' the 'toolboxes' for their intended purpose. For air filters I am pondering the 50's look with the filter in a "container" like our hosts 50's conversion vs the K&N / SB type. Whiche thenled me to thinkking about the carb on the 350. After reading the most recent 'Real Classic' magazines there has been a series of articles on 'fettling' a Bullet (although in this case a 500 and not a 350 in-case that makes any difference) and a change from the Mikcarb to an Amal (firstly a concentric I think) and then a monobloc. It seems there were problems with the concentric and smooth running was a problem it appears and the writer then went for a monobloc that needed some 'help' to fit with/around cables, and due to it's size and the available space and I think the manifold. The monobloc seemed to work best [and I wonder if our hosts have soleved the manifold side/problems maybe with a special manifold (for the 350 too?)] but wondered for those that have done this the benefits of going the Amal route. Can anyone advise? M

The 389 fits straight on although I recommend some form of heat sink. I made one out of industrial formica or tufnel (whatever the phenolic resign and cloth stuff is called) I believe our hosts have one that you can buy off the shelf. The 389 I used was 1 1/8" so I ported the head to make which I think will definitely improve gas flow. Breathes deep now!

Simon an interesting point you make, but I have never understood why anybody recommends the fitting of a tufnol or similar insulated spacer between the carb flange and cylinder head. The running temperature of a bike engine is below 120C. Indeed for comparison look at a car temperature gauge and most top out at 120C as that is the boiling point of water under pressure! I have a collection of singles and twins of all makes and have had maybe 40 or so bikes in my life time and none ever had the insulated spacer. There may have been bikes with them but I never owned one. I have often wondered if any that had them as OEM were simply being fitted with a cheap alternative to extending the length of the inlet tract without needing to make a new head casting or new manifiold. So with that in mind I would be interested to have your views on why you recommend the tufnol spacer as an insulator. It is not truly a heat sink as a HS is designed to dissipate heat rather than insulate it. A zener diode would have been fitted with a heat sink for example........sorry that was the pedant in me coming to the fore. Wasn't meant to be rude or patronising. Apologies if you thought otherwise.

No offense taken Mike. I have had quite a few machines and some have had isolators (better word?) and some have not. Obviously the boiling point of petrol is well below that of water and having a carb at the full running temperature of the engine makes me a little nervous in terms of vapor. I know a chap with an A65 BSA which doesn't have any and it pongs when stopped after a run. During the fast run of course the carb is likely to be refrigerating slightly so isolating it from the head is probably not a bad idea from that point of view either. finally I like them from a purely aesthetic point of view. My Ducati came with rubber ones which crush and fail so I put tuffnel ones on it and they look very workman like.

Funny world! Just met the chap with the A65 which he still owns and has installed isolating spacers between the head and the concentrics because apparently it was impossible to start when hot.

Hi guys---Our bikes, cars, trucks, trains & planes are all HEAT ENGINES---in fact , so are we. Aluminium is a wonderful conductor of Ee-lec-trickery and also HEAT. And the carb is usually just behind the HOT cylinder head with the HOT "cooling" air from the HOT engine fins passing over it. Petrol as we know is a volatile liquid from the upper end of a "Cat-cracker"-----(LES H, a précis on that for the lads, please ? )and readily vaporises by the addition of HEAT. So anything that can minimise the unwanted addition of HEAT to the fuel has to be a good. I seem to remember back in the day the aftermarket people sold finned induction manifolds?? So yes, the tufnol spacer IS fulfilling a real function.SIMON---that A65 is experiencing a fee-nom-e-num called VAPOUR-LOCK where the petrol in the system is turning to gas before it can get into the needle jet etc. MAD MIKE--don't forget that the car cooling system is, in general, a low pressure one ie typically at 15psi. The boiling point of fluids under pressure will increase proportionally as the pressure in that system increases. Incidentally---I think the concentric type carb was initially developed by the racing lads because, with the monoblock's off-set float bowl and leaning the bike over you would get fuel starvation one way and a rich mixture the other--- plus a more compact design overall. Apart from that "50's" look, I can't understand why folk want to take a retro step in carburation with the monoblock. But then I started to learn ESPERANTO years ago---so what do I know ?? Have a nice day !!

Simon, Alan amongst other things I have a 1967 BSA A65 Thunderbolt whic is all original, ahving never been restored. It does not have a tufnol spacer between the manifold and carb flange. It starts first kick every time and always has done. If your friends A65 wont start, when hot, without the spacer then there is frnkly something wrong with it. The concept of all that "hot-air" from the cylinder head causing evaporation, vapour lock etc etc is frankly an urban myth. I have yet to feel a carb body anywhere near warm enough to cause any such problem. After market suppliers of course are just what the name says. They have an axe to grind to sell their products, particularly the truly non-functuional ones. I have Ariels, BSA's and Nortons and none has or needs an insulating spacer. They have variously pre-Momonobloc, Monobloc and Concentric carbs. All start first time and none suffer fuelling problems. The Monoblocs work perfectly. Remember that the Monobloc was not designed for racing. It was for road bikes. Carbs like the TT and GP with remote float chambers and no idle jets were for racing. In deed it was, is possible to buy chopped off Monoblocs even today to fit with a float chamber to use for racing. Great debate by the way chaps. Thanks for that.

Arh well MM each to there own. I made my own tuffnel spacers out of an old fuse board so no huge profits were derived:). I've had little experience of concentrics at 1975 my Ducati has just missed them for dellortos but I'm told that they are an exceptionally good design let down somewhat by poor manufacture in that they are prone to distorting because the castings are too light. The 389 I have on the bullet also suffers from this in that a previous owner (of the carb) had over tightened it and pulled the throat out of round. I got it working tolerably well with a squish in the wood vise but my next lathe adventure will involve resleaving it in bronze ( I have a chunk of marine propeller shaft that will do nicely). I thought about finding stainless brake cylinder sleeve for the job but it's such a pain to work with and the bronze will see me out. My friend has done it to a 89 Amal which has a bronze body to great effect but I don't see that the mazac body will cause an issue. The big challenge will be making a jig to hold the body in the lathe. I'll post pics if it works out and never mention it again if it doesn't.

---------how's this for a HEAT ENGINE ?? ------http://en.wikipedia.org/wiki/Saturn_V -------.and I bet there's some Tufnol in there somewhere ??