Hi folks fixed one problem and another arises. I found an air leak in the servo hose which I have now fixed. The bus stops and idles alot better than previous now its fixed, the problem now is under braking you get a back fire through the exhaust. Its a type 4 with weber progressive and vacuum advance distributor. Im guessing its running a bit lean, is it just a case of re-timing? Thanks.
If you put your foot on the brake the servo is connected to the manifold so air will enter the manifold so it will go a bit lean for a moment. Although an air leak in the servo itself could make it worse. A lean backfire may also be partly due to the tendency these carbs seem to have which is to go very lean on deceleration. Combine braking with foot off pedal and pop. This will be jetting and idle mixture tweaking needed not so much timing although that should also be checked anyway. As this single centremounted carb and T4 engine combination is one of the least optimum for the bus, you may be better off spending your money on a rolling road tuneup after you get twin carbs ...
Thanks for the reply. Ive checked all the basics, timing, points etc and adjusted the carb to the best of my know how. I've not played with jetting. There is blowing round both heat exchanger/exhaust joints, could this be the problem? Im pretty sure it didnt back fire when I had the servo pipe leak.
The exhaust leak will not help with backfiring. When the mixture does not burn in the cylinder either rich or lean then the hole in the exhaust will alter the mixture in the exhaust pipe so that when it reaches hot gas from a cylinder that did fire then it goes pop too. All the way from a massive bang when there is a lot of fuel, to a popping noise when it is leaner. And some of that fuel helps to clean the oil off the piston rings which will not help wear. When the engine is running just feel how cold the inlet manifold is to the touch. That means you run a risk of ending up with both a lean mixture and a puddle of cold fuel dribbling around as some of the petrol condenses on the manifold. So you need to jet the Type 4 very rich to get it to run well and then occasionally you get an over rich situation when the puddle of petrol flows. Juddering. Hesitation. This is why the Type 4 came as stock with twin carbs or FI. Centre mount carburettors on the Type 1 use lots of heat to make them work.
