brianZortiz Posted July 3, 2011 Share Posted July 3, 2011 My early 260z has been converted with 72 round tops amongst other light mods. My problem is that I can only get to 4,500 rpm before I hit a wall. It will not go any higher. I've done : Adjust float bowls to 14mm Cleaned gas tank and changed fuel filter. Bought new ngk bpr6ey11 Adjusted timing to 12btdc Carbs are synced Checked for vacuum leaks with a carb cleaner and water in spray bootle. (no leaks) Well the 260z has an electronic distributor and the vacuum advance is working. The wires are stock aswell as the coil Found a small kink in fuel line and that helped a little in improvement. Ok so the early 260z has a 5/16 fuel return line and a 5/16 feed. As I remember with my 240z (dual webber downdraft) it had 1/4 fuel return line. I am running out of options. My question is should I switch to 1/4 fuel return lines? Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 4, 2011 Share Posted July 4, 2011 No, that defies common sense. (And the 240 had 3/16" return line, not 1/4"!) The fuel pressure is gained through a restriction in the return line at the front of the engine, that hole allows the pressure to go up as delivery from the mechanical pump rises faster than (hopefully) the float bowls suck it down through the needles and seats, giving more fuel at higher rpms. Did you: 1) Check the inlet screens on the carburettor to see if they are free and unclogged? 2) Made sure your starter system is working properly---if one of the jets on either carb is lower than the other, it will 'hit a wall' and not rev any higher just like an ignition-controlled rev limiter. Quote Link to comment Share on other sites More sharing options...
jasper Posted July 4, 2011 Share Posted July 4, 2011 (edited) IF you have a catalytic converter, make sure that it is not a restriction, including any exhaust issues. Also important, monitor fuel pressure when the problem occurs. Use a known good guage. If you were not aware, there is a fuel filter in the bottom of the stock electric fuel pump,....possible restriction........ Edited July 4, 2011 by jasper Quote Link to comment Share on other sites More sharing options...
brianZortiz Posted July 4, 2011 Author Share Posted July 4, 2011 Tony D, I have not checked the starter system and both my carbs are set 3-1/4 turns down and my jets are the same height. I have never seen a inlet screen, I will try to find it and clean it. @Jasper I do have a small exhaust leak in the exhaust manifold and downpipe. Right in between the donut gasket. I will check my fuel pressure as for now I have a brand new mechanical fuel pump and am not using the electric pump. I did clean the electric fuel pump and changed the filter and will install and hook it up tomorrow. Quote Link to comment Share on other sites More sharing options...
brianZortiz Posted July 4, 2011 Author Share Posted July 4, 2011 I cleaned the bonjo fitting screen and all the surrounding areas. The front carb was dirty, I drove it and it felt a whole lot better. It pulls to 5,800rpm now, but at the top there isn’t much power. My 73 with DGV Weber pulled hard all the way to 7,000rpm and the power was there, and SU's are better than the Downdraft so I know my car has some more power to be freed up. Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 5, 2011 Share Posted July 5, 2011 Now it the time to check your fuel pressure under full load at the top of third gear...that is when it will start to fall short... Quote Link to comment Share on other sites More sharing options...
brianZortiz Posted July 5, 2011 Author Share Posted July 5, 2011 ok sure will, should I hook up electric or keep running mechanical when I check fuel pressure? Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 5, 2011 Share Posted July 5, 2011 Technically the mechanical is supposed to supply the engine. You need to see if you are making 7psi at 5500rpm+. If you're not, chances are the float bowls are running dry and causing problems. An electric may help with delivery, but in reality that's just band-aiding a bad situation. If you had plugged inlet screens you would be falling flat at higher rpms under load, but fuel pressure would be O.K., maybe even a little high. If the suction screen on the electric pump was restricting it, you will have low fuel pressure (now that the inlet screens are cleaned)... so maybe you will have to bypass the electric pump out back (put in a different fuel filter) and make another pass to see what happens. If you have 5-7psi at 5500 you should have enough fuel. I think 1.5-3.5 at idle speed is the low end. With the stock system as the RPMS rise, so will the fuel pressure. This can be a real pita. I had a plugged premuffler restricting my engine on the top end, but how the hell do you find that one quickly? (You don't! } Good Luck! Quote Link to comment Share on other sites More sharing options...
brianZortiz Posted July 5, 2011 Author Share Posted July 5, 2011 Thanks, will give a test and try tomorrow. Quote Link to comment Share on other sites More sharing options...
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