GoGuy Posted May 22, 2011 Report Share Posted May 22, 2011 Hi all, I have just converted my CR 6 to carbs and used the return fuel pipe into the tank (the thin one)as a tank vent to exit under the rear wing. On looking at the PI works manual, I can see no reference to a tank vent for the PI system. Is the fuel cap designed to permit an air-bleed into the tank, such that a separate vent was not needed? Logically, the return fuel must be less in volume so there must be a reason why the tank didn't come under vacuum. I am getting unwanted petrol fumes from the vent I have installed,when working under the car and would be happier without them! Has anybody got the answer? Regards GoGuy Quote Link to post Share on other sites
poolboy Posted May 22, 2011 Report Share Posted May 22, 2011 You might want to put a carbon cannister on the end of your vent hose. I found a rather small one on ebay for a motorcycle application and strapped it to the frame behind the rear fender. Attached the vent hose to it. Originally there was a Carbon Canister with some activated charcoal inside, up by the radiator, that the carbs and the fuel tank vented to. Quote Link to post Share on other sites
Peter Cobbold Posted May 22, 2011 Report Share Posted May 22, 2011 You might want to put a carbon cannister on the end of your vent hose. I found a rather small one on ebay for a motorcycle application and strapped it to the frame behind the rear fender. Attached the vent hose to it. Originally there was a Carbon Canister with some activated charcoal inside, up by the radiator, that the carbs and the fuel tank vented to. AAAgh take care - you might fill the boot with petrol - seen it happen. The top of the PI tank has two pipes- for fuel return from NU and from PRV. One of these I dont know which extends deeper into tank. Should you conect a hose to it and run that hose below fuel level it can fill and then siphon out- into your canister.... Note that fuel will slop in a hard turn and be effectively higher level than when standing- that is when it could make a siphon and fuel could escape into boot while driving. Yes seen it happen like that. Lucky no fire, but close shave. When I removed PI and fitted a carb I joined those two pipes together with a short hose. Quote Link to post Share on other sites
poolboy Posted May 22, 2011 Report Share Posted May 22, 2011 As long as you have the gas tank vented one way or the other. Not so much for venting the fumes but for breaking the vacuum that a dropping fuel level will create if not. I don't know how the PI tank differs in that respect, but a vacuum breaker is needed somehow somewhere. Quote Link to post Share on other sites
Colin White Posted May 22, 2011 Report Share Posted May 22, 2011 My original 1974 CR PI has a vented fuel filler cap - nothing else needed is there? Quote Link to post Share on other sites
poolboy Posted May 22, 2011 Report Share Posted May 22, 2011 My original 1974 CR PI has a vented fuel filler cap - nothing else needed is there? I guess not. We have a different set up. The vent line in my car got blocked when I overflowed the tank during a fillup. Gasoline settled into the belly of the vent line which runs from a separator canister mounted on the gas tank to a carbon canister mounted next to the radiator. The line runs along the frame until it turns upward to the Carbon Canister. Not realizing the consequences of overfilling the tank, I headed home. I reckon I had gone less than 5 miles when the engine started to sputter and nearly die. Then it dawned on me what was causing my engine to starve. Reaching back I flipped open the gas cap, breaking the vacuum, reviving the engine. When I got home, I disconnected the vent line from the Carbon canister, blew the gas back into the tank, which I could hear, followed by the his of pure air entering the tank. The point is if the tank is not vented, I don't believe you'd be able to drive more than 5 miles before the vacuum built up enough to hinder fuel flow to the pump and the engine begins to starve. Quote Link to post Share on other sites
Peter Cobbold Posted May 23, 2011 Report Share Posted May 23, 2011 My original 1974 CR PI has a vented fuel filler cap - nothing else needed is there? So has mine, If not easy to drill small hole. Max air flow needed is around 10ml per second so couple of mmm bore ample. Quote Link to post Share on other sites
rpurchon Posted May 23, 2011 Report Share Posted May 23, 2011 Hi all, I have just converted my CR 6 to carbs and used the return fuel pipe into the tank (the thin one)as a tank vent to exit under the rear wing. On looking at the PI works manual, I can see no reference to a tank vent for the PI system. Is the fuel cap designed to permit an air-bleed into the tank, such that a separate vent was not needed? Logically, the return fuel must be less in volume so there must be a reason why the tank didn't come under vacuum. I am getting unwanted petrol fumes from the vent I have installed,when working under the car and would be happier without them! Has anybody got the answer? Regards GoGuy i had problems with fuel smells on my usa carb car.i blocked the vent pipes. then drilled a 2 or 3mm hole angled 45deg through the neck of the petrol cap.very easy its alloy. just make sure it is outside of the rubber seal where the cap sits. problem solved. richard Quote Link to post Share on other sites
john.r.davies Posted May 24, 2011 Report Share Posted May 24, 2011 (edited) Can't believe that a hole outside the seal will allow air into the tank! But care drilling a hole in the cap. I did so for Silverback after repositioning the filler tube to accomodate the rear air intake. Fuel slop then produced a powerful jet of fuel out of the hole on long right handers. One way valves are available to insert into a tank vent tube. They allow air in, but not fuel out. See: http://www.merlinmotorsport.co.uk/FUEL-SYSTEMS-Tank-Breather-Valve/c26_215/index.html John Edited May 24, 2011 by john.r.davies Quote Link to post Share on other sites
GoGuy Posted May 24, 2011 Author Report Share Posted May 24, 2011 Hi all, Thanks for all the replies and comments re. the tank venting. I decided to use a combination of replies and therefore joined together the two pipes entering the tank at the top and work on the basis that either the cap is vented, or its seal after some 36 years has enabled it to develop its own vent! The good news is that I was able to have a good blast to the garage for its MOT - it passed!- and the above mods had no detrimental effect on the performance at all. Problem solved! Many thanks GoGuy Quote Link to post Share on other sites
rpurchon Posted May 25, 2011 Report Share Posted May 25, 2011 Can't believe that a hole outside the seal will allow air into the tank! a pictures worth ? i have put a small allen key in the hole, to show it better. richard Quote Link to post Share on other sites
john.r.davies Posted May 25, 2011 Report Share Posted May 25, 2011 richard, Ah! So! Picture worth thouasnd words, rittle glasshopper. John Quote Link to post Share on other sites
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