Hi Folks. I’d appreciate anyone’s thoughts as to the best way to add some additional fuel filtration to my 1979 V50. As now, the fuel system is as it left the factory with cross piece in place and all original hoses… ie c5.5mm Carb and c7.5mm Tap.
Background to the question is as follows: I’ve had the bike for just over a year and have tinkered with the Carbs a few times trying to overcome a very rich mixture. As now, I’ve all new jets, needles and air tubes, plus choke plungers/springs and following a few sessions with an eBay u/s Jewellery cleaner, the mixture is spot on… …however air screws, whilst initially effective after a mile or so’s riding have minimal ( RHS) and nil (LHS) effect on Tickover, and the bike tends to continually backfire on a fully closed throttle for several seconds…(until any fuel remaining in the inlet tract is pulled through and burnt in the exhaust… I think); I’ve standard front pipes and BSA type mufflers- bike came with them, sound good, so I’m thinking fuel or lack of it is the root cause of the backfire.
Petrol tank looks clean and the (new) gauze filters show no collected dirt, however I’m inclined to think the air screw passages are still getting blocked, hence suspicion that the mixture is too lean to burn on a closed throttle, hence backfiring.
Re getting filters in, it kind of looks like they would have to go sort of behind the Carbs and I guess the other question then would be, do all the pipes/filters have to be inclined slightly ‘down hill’? And I guess filters plus cross piece would be nigh on impossible to fit in.
If the petrol pipes are original then I would start there as the ethanol content of todays fuel doesnt like anything rubberyand they were never designed for todays unleaded. Also have you ensured that theairscrews have the relevant o ring and washer fitted, and that there is not a previous o ring still wedged in there (ask me how I know). Finally have you made sure the exhaust does not have an air leak as that is the usual cause of backfiring on the over run.
Popping in the exhaust is often caused by an air leak where the exhaust pipes connect into the head. It might be worthwhile taking the pipes off and replacing the gaskets. They are available on ebay in multiple different sizes, you should be able to find what you need if you have some calipers to measure the size you need.
Thanks Both. I’d totally forgotten about Exhaust leaks (not done any bike tinkering for decades until last spring) so thank you. And! when I bought the bike, said BSA type exhausts pushed the centre stand down reducing ground clearance, so I modified the rear brackets and yes loosened the Head Studs and moved the Pipes up say 25mm at the rear… Oops. So new gaskets it is… which has the added benefit of changing one thing at a time, then definitely some new hoses. Re Air screws, one has a green washer and the other a black one… so, I wonder and will go find out… is there another one buried in the depths.
Just my 2 penn’orth – assuming yours has the carburettor banjos with the round flat white nylon filters / ‘gasket’ – I always found these worked adequately. Of course they need cleaning from time to time, or even replacing. I suspect yes you may have blockages, my personal experience has been that modern petrol can produce a ‘fall-out’ of hard deposits which can constrict jets and galleries, especially if carburettors are allowed to dry out, I say ‘modern petrol’ because it never happened with the ‘old stuff’, and note this predates ethanol, I think I started noticing it not long after unleaded became the norm. It got so that I drained the float bowls if I knew the bike was going to be left for a few days. Note the hard deposits are extremely difficult to shift, carb cleaner would hardly look at it.
Thanks Mike. Yes, re fuel deposits. I started my voyage of discovery into the Carbs with new needles… one had ‘woodworm, pits and holes… presumably due to standing water in the float bowl and the other one had what looked like hard ‘plating’ peeling off… which I’m sure was said hard deposit. I’m going to start with new exhaust seals and a new set of hoses… £20 with cross over from Gutsibits and gets you the correct diameters.
I bet the sealing between head and downpipe is the right path, but if you won’t succeed, there is a way to fit filters in line and avoid air blockage - you need to take care and sort it the way there is only one “U” on way, so air can escape either way up. In case you leave two "U"s - the air will tend to stay and cause trouble. What I have done, is an original cross piece after the taps, but then, a fuel line AFTER the cross going to the right goes through a fuel filter hanging next to the R/H carb, but leaves the filter and goes down and across to the left hand carb - so what you can see is , say, a R/H carb with L/H filter next to it and vice versa. Works well, no trouble. And yes, modern fuel is such a pig, if I know the bike would stay unused for , say, a month, I will fire it up and let it run with shut off fuel until engine dies off starvation - then I am sure a fuel level no longer reaches the brass idle jet(most prone to get blocked), and this way I have never had a problem to start the bike after even a year long break.
Thanks. I’d not thought of doing it that way, but I can see how it would buy some more room to get the pipe runs in and yes makes sense avoiding a double ‘airlock’.
When I bought the bike, there was all sorts of corrosion on the jets… I think they had stood for a while in something horrid and corrosive, like modern fuel. Running horribly rich and misfiring on day 1…Started changing bits piecemeal, ‘correct plug’, new plug cap, needles/air tubes… then eventually got one side running right with new main, pilot and choke plunger all in one go… and then did the whole lot on the other side; not 100% sure what did it in the end, but still got the off idle popping and feels maybe a bit fuel starved. New exhaust seals and cross over on order from Gutsibits.
And as a quick update. New Cross fuel pipes ex Gutsibits now fitted. I cropped one of the hoses slightly to get the smoothest curves in the fuel lines. Also removed some black granular material (?? what looked like rubber) from the RH, easy to get at), carb fuel filter. So now happy with the integrity of the hoses.
New exhaust pipe seals fitted; they were passing half white/half sooty and one was also kind of gunged in as well with some unknown substance…
But still backfiring, so still need that carb strip.
