neversaydie Posted October 9, 2012 Share Posted October 9, 2012 I'd appreciate a little advice with this if anyone would be so kind. The motor has cut out on me 3 times now, slight stuttering like on 2 cylinders, then cuts out, this is after a mile or so. Wont restart with the petcock in the normal position, but starts up fine if I turn it to "prime". Runs fine if I leave it on "prime". Plenty of gas in the tank. Have taken the tank off and removed the petcock on the assumption that in run position the carbs are not getting sufficient supply to maintain run. The petcock doesn't look like the one in the parts manual, but I don't think that is the problem. Slight film on the filter, that's now cleaned off. When I removed the outlet pipe from the body to check/clean the hole, a very small disc dropped out of the body, I'assume that it sat at the bottom of the brass outlet pipe and that it is a small filter. I am thinking that I could run without the small disc filter in as the petcock filter should be good enough, this should allow better fuel flow through to the carbs Is the vacuum created by the carbs as I cant find any reference to a vac pump in the manual. Any comments greatly appreciated. Link to comment Share on other sites More sharing options...
dt502001 Posted October 9, 2012 Share Posted October 9, 2012 The vacuum is from the eng via the carbs, Check the diaphram for pin holes and the round rubber with 4 holes and the hose from the carbs to valve. The small screen won't make it flow less installed the flow worked well for years thats not the problem. either the diaphram has a micro hole or the vac line is pluged /cracked not sealing tight,or the rubber with 4 holes has shifted. Link to comment Share on other sites More sharing options...
neversaydie Posted October 15, 2012 Author Share Posted October 15, 2012 Still no joy Cant quite get my head around how the fuelling should work The petcock has 2 outlets, 1 for normal running via the vac, and 1 tagged as "prime, from which the fuel flows unrestricted when the petcock is turned to that position. Th outlet spigot form "prime" is the same size as the T inlet to the carbs. Th outlet spigot from the normal run position is smaller and the same size as the spigot on the rubber between the carbs and the cylinders. Since Ive had the bike, the spigot from the prime position has been connected to the carb inlet. The bike appears to run fine with the petcock in the prime position, but splutters to a halt if I turn it to run position. Is this connected up correctly? Link to comment Share on other sites More sharing options...
dt502001 Posted October 15, 2012 Share Posted October 15, 2012 No/yes the small sipgot on the back petcock is for vacum not fuel,,all the fuel comes out of the same spigot weather it's in run prime or reserve. So your selector valve must not be opening in run or reserve this would be because the vaccum valve diaphram (the flat rubber with a spring) is not moving,,, get a syringe or just suck on the line and hook it up to the vac line and draw vac with the syringe if no fuel or little flows from the petcock in run or res than the diaphram has a leak or the hose is nfg. Run the bike in prim and pull the vac line off and see if you have vac commin to the valve the hose might not be sealing tight to the rubber intake manifold ,if you have enough hose cut off 1cm of the hose on both ends and reconect it if its not all cracked up,if so replace the vac line.If still no joy then replace the diaphram in the petcock In the prim pos it dosen't require vac the fuel is alowed to flow freely.. the vac is so you don't have to turn off the gas everytime you park the bike.So if the diaphram is not opening effectively you are shutting off the gas every time you switch to run pos. Hope you get it now. Link to comment Share on other sites More sharing options...
neversaydie Posted October 15, 2012 Author Share Posted October 15, 2012 No/yes the small sipgot on the back petcock is for vacum not fuel,,all the fuel comes out of the same spigot weather it's in run prime or reserve. So your selector valve must not be opening in run or reserve this would be because the vaccum valve diaphram (the flat rubber with a spring) is not moving,,, get a syringe or just suck on the line and hook it up to the vac line and draw vac with the syringe if no fuel or little flows from the petcock in run or res than the diaphram has a leak or the hose is nfg. Run the bike in prim and pull the vac line off and see if you have vac commin to the valve the hose might not be sealing tight to the rubber intake manifold ,if you have enough hose cut off 1cm of the hose on both ends and reconect it if its not all cracked up,if so replace the vac line.If still no joy then replace the diaphram in the petcock In the prim pos it dosen't require vac the fuel is alowed to flow freely.. the vac is so you don't have to turn off the gas everytime you park the bike.So if the diaphram is not opening effectively you are shutting off the gas every time you switch to run pos. Hope you get it now. Yes, that's much clearer now. So the vac is generated by the motor, sucks through the pipe that connects the rubber (between the carb and the cylinders) to the cock. The cock itself only lets fuel through the one outlet, but depending on what position the switch is in, could be vac pulled or free flowing. I had the carbs off at the weekend and couldn't fault them save for a bit of shite on a couple of the filters. The motor pulls like a train on prime, so I'm assuming that the carbs are ok. Im going to look again at the hoses, the one I took off wasn't too tight onto the carb inlet, and the one I put on is a tight fit under the tank and the bends might be restricting flow. Aftre a trawl on the net today, a direct replacement cock may be difficult to locate in the uk, but apparently it can be replaced by a non vac operated cock that is just either on or off. Ill take the cock off again and double check the guts in the petcock I don't think there's much wrong with this now Thanks for your help so far Ray Link to comment Share on other sites More sharing options...
dt502001 Posted October 15, 2012 Share Posted October 15, 2012 Don't take the cock off just put vac to the vac line on the cock and see if fuel will flow out in on or reserve. If not the diaphram needs to be replaced they are avaible from yamaha. As it stands you now have a on or off petcock so why change it you just dont have reserve. Link to comment Share on other sites More sharing options...
neversaydie Posted October 15, 2012 Author Share Posted October 15, 2012 Don't take the cock off just put vac to the vac line on the cock and see if fuel will flow out in on or reserve. If not the diaphram needs to be replaced they are avaible from yamaha. As it stands you now have a on or off petcock so why change it you just dont have reserve. Yes, and the carb floats still control the level of fuel in the bowls, as if vac pulled? Link to comment Share on other sites More sharing options...
dt502001 Posted October 15, 2012 Share Posted October 15, 2012 Yes and no the floats can ony hold back so much weight best to turn it to off when parked. Link to comment Share on other sites More sharing options...
neversaydie Posted October 17, 2012 Author Share Posted October 17, 2012 Blanked off the vac port on the rubber sleeve, went out for a trial with cock on prime, ran super Even with vac on the vac line, no fuel flows. Looking at replacing the guts of the cock/replacement cock Thanks very much for your help. Ray Link to comment Share on other sites More sharing options...
dt502001 Posted October 18, 2012 Share Posted October 18, 2012 Noproblem Ray glad you have it sorted out now Link to comment Share on other sites More sharing options...
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