Forum

[brian h]

Hi Is it essential to use the insulation spacer between the cylinder head and the carb on a 350 Bullet,the reason i ask is i am going to fit a bigger amal carb as opposed to the standard mikarb.

[PeteF]

It's a good idae to insulate the carb from the heat of the engine.


If you're fitting a bigger carb have you increased the bore of the inlet tract? If not, the bigger carb won't do much.

[MadMike]

What Pete says about inlet tract diameter and carb bore diameter is absolutely correct. In a low state of tune going up one size wont do any harm, but neither will it provide any benefit.


Indeed if the carb bore is significantly greater than the inlet tract then it will undoubtedly cause a drop in performance due in part to the step that you would create in the junction between carb and manifold.


Turning now to a tufnol or similar "insulator" between the carb body and the manifold I have to say that i have never understood why the spacer is fitted, except to perhaps lengthen the manifold tract on the cheap. Traditional British motorcycles, in the main, did not have and did not need an insulator. I have a number of singles and twins and not one of them has an insulator, and none of them suffer from fuel evaporation if that is what people believe might happen. Even high performance bikes like Gold Stars did not need one. The most important issue at the junction of carb to manifold is that it is air tight. So two flat mating surfaces with an O-Ring fitted solves this problem.


I may be wrong but after nearly 50 years I will take some convincing. 


  

[grunda 12]

hi mike ,i agree with you regarding the tufnol spacer i think maybe they were fitted so that the carb o-ring had something to seal on it -gasket at the other side ? my c15 had a insulator but if it was original fitment i could,t say although it was no detriment