datsun mechanic Posted October 13, 2011 Report Share Posted October 13, 2011 help! 85 720 calif carb/air fuel solenoid type float level is good at idle then goes low when driving. Where does the fuel go or why does level not stay up?? New fuel pump/filter/needle valve and strainer/pressure and volumn tests are correct/blew out lines to tank and carb/blocked return line. Quote Link to comment
datzenmike Posted October 13, 2011 Report Share Posted October 13, 2011 Really glad you included the year and that it's California electronic feedback carb because there are 3 different motors and 4 carbs not including throttle body injection and 2 diesel motors used in the 720. The demand exceeds the supply. This would indicate that the pump is not supplying enough. No sense checking the filters, just replace them. There's one beside the electric fuel pump (#16400 06W00) on the frame just forward of the right rear wheel. The base of the electric pump also twists off and has a filter (#16404 28530) in there as well. There is a brass screen inside the banjo fitting where fuel enters the carb. It can be removed and cleaned of any trash caught in it. Quote Link to comment
datsun mechanic Posted October 13, 2011 Author Report Share Posted October 13, 2011 Things i have done so far......... New fuel pump ane filter/needle valve and strainer......press and volumn test are correct.....blew out lines to tank and carb....put cap on return line for testing purposes. Quote Link to comment
datzenmike Posted October 13, 2011 Report Share Posted October 13, 2011 Well pull the hose to the carb off down by the frame rail and put in a pail. Turn on ignition and it should shoot out gas from the pump. The return line should only dribble as there is a restriction in it so the pump will build up pressure on the carb inlet side. Any chance that these two lines were removed and hoses put on backwards???? This would put the restriction on the pump side and reduce pressure. As a rule the return line is slightly larger diameter than the inlet line. Quote Link to comment
datsun mechanic Posted October 13, 2011 Author Report Share Posted October 13, 2011 i will try that in the pail tomorrow to ck if lines reversed. This truck had the same problem when i bought it 5months ago/towed it here and i'm just now finished rebuilding engine and a few other things.......................... Quote Link to comment
datsun mechanic Posted October 15, 2011 Author Report Share Posted October 15, 2011 today is saturday.......made alot of progress since i lowered/shortend new(to long fuel hose at fuel tank as it curl up about 4 inches about fuel tank). I will put a 100 main jet in return line to see if that keep more fuel available for float bowl. Quote Link to comment
datzenmike Posted October 15, 2011 Report Share Posted October 15, 2011 If a stock set up, the return line will have a restriction in it. Just check the hoses from under the intake going to the two metal lines on the frame beside the starter. The return line is slightly larger than the one from the electric pump. I think the two fuel hoses are slightly different in size also. If they are reversed the pump will have to push fuel through a mm sized hole to get to the carb. The carb may use gas faster than it can be pumped in. Quote Link to comment
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