Hiya,
Earlier this year I picked up a 1991 ZX-750 J Model that was in need of some care.
The tank was FULL of rust and obviously the carburetors needed to be serviced. I de-rusted the tank, replaced all the fuel lines, and then had a pinhole in the tank welded at a radiator shop. I fought this bike, tooth and nail, over constant fuel delivery issues and odd-running. I also cleaned the carburetors up as best as possible and replaced the float bowl gaskets. I left the float seat o-rings and a/f orings alone. I replaced ONE bad OEM spark plug boot and all of the plug wires with NGK wires and installed new NGK plugs in every cylinder.
I have a lot of bikes so it kind of got put on the backburner. I got the bike to run ok enough to test ride it, but as soon as I gave it a good bit of throttle I would get backfiring out of the carburetors and lose power, but the bike would continue to run on 4 cylinders (I know as it was running on 3 at one point and would barely run.) The idle was unpredictable and would constantly raise the longer you rode. You could start at 1200 and once the bike got warm it would get up to 2000+. Backing the idle screw off fixed this . . . until you took off and stopped at the next red light or stop sign . . . then it was back up to 2,000. Rinse and repeat forever.
I recently pulled it into my work garage and started working on it today. The bike is running stock/oem jet sizes, stock needles, etc. I replaced an "intake pipe" today, that was broken, and dremeled the gasket out of all of my rubber intake boots and replaced them with O-Rings. I adjusted the float heights to 13mm and installed some factory pro needles that came with the bike. I set the needle clip to the 4th position (1 step richer due to my carb popping) and set all A/F screws to 2.25 turns. I verified the fuel pump is working. I also performed a compression check and got readings of around "120" psi in all cylinder dry and much higher with some oil. I don't think 120 is a cause for concern - im sure the gauge is inaccurate.
I gave the bike a test spin and it had the symptoms it had in the past plus symptoms of running rich - hesitation. The engine has 14k miles on it. While riding I hear a constant metallic sound from the top end - not the valves. Is it possible this is normal noise from the cam chain or just some vibration that I'm perceiving to come from the engine? I also hear "popping" but I can't determine if its the carburetors or not. I brought it home, pulled the plugs and they were sooted up so I wiped them off and reinstalled them. I ran the bike and nuked the fuck out of the reed valve/hoses and the intake boots and the idle did not move a hair.
I performed a carb sync (again) and got them as close as possible and adjusted the a/f screws to 1.75 turns and moved the needle clip to the center position. I ran the bike only in the garage and pulled one plug which looked like this:
I didn't run the bike terribly long . . .15-20 minutes. Long enough for the radiator fan to come in and blipped it some while syncing the carbs. Is this now set too lean or should I give it a proper rip and then determine?
The bike seems to rev well as it always has. The RPMs drop at a decent rate of speed, but if I "blip" the throttle they will hang around 2k for a moment. Is this because I'm too lean on my a/f screw and don't have enough overlap as a result? Is there somewhere else I should be looking?
Earlier this year I picked up a 1991 ZX-750 J Model that was in need of some care.
The tank was FULL of rust and obviously the carburetors needed to be serviced. I de-rusted the tank, replaced all the fuel lines, and then had a pinhole in the tank welded at a radiator shop. I fought this bike, tooth and nail, over constant fuel delivery issues and odd-running. I also cleaned the carburetors up as best as possible and replaced the float bowl gaskets. I left the float seat o-rings and a/f orings alone. I replaced ONE bad OEM spark plug boot and all of the plug wires with NGK wires and installed new NGK plugs in every cylinder.
I have a lot of bikes so it kind of got put on the backburner. I got the bike to run ok enough to test ride it, but as soon as I gave it a good bit of throttle I would get backfiring out of the carburetors and lose power, but the bike would continue to run on 4 cylinders (I know as it was running on 3 at one point and would barely run.) The idle was unpredictable and would constantly raise the longer you rode. You could start at 1200 and once the bike got warm it would get up to 2000+. Backing the idle screw off fixed this . . . until you took off and stopped at the next red light or stop sign . . . then it was back up to 2,000. Rinse and repeat forever.
I recently pulled it into my work garage and started working on it today. The bike is running stock/oem jet sizes, stock needles, etc. I replaced an "intake pipe" today, that was broken, and dremeled the gasket out of all of my rubber intake boots and replaced them with O-Rings. I adjusted the float heights to 13mm and installed some factory pro needles that came with the bike. I set the needle clip to the 4th position (1 step richer due to my carb popping) and set all A/F screws to 2.25 turns. I verified the fuel pump is working. I also performed a compression check and got readings of around "120" psi in all cylinder dry and much higher with some oil. I don't think 120 is a cause for concern - im sure the gauge is inaccurate.
I gave the bike a test spin and it had the symptoms it had in the past plus symptoms of running rich - hesitation. The engine has 14k miles on it. While riding I hear a constant metallic sound from the top end - not the valves. Is it possible this is normal noise from the cam chain or just some vibration that I'm perceiving to come from the engine? I also hear "popping" but I can't determine if its the carburetors or not. I brought it home, pulled the plugs and they were sooted up so I wiped them off and reinstalled them. I ran the bike and nuked the fuck out of the reed valve/hoses and the intake boots and the idle did not move a hair.
I performed a carb sync (again) and got them as close as possible and adjusted the a/f screws to 1.75 turns and moved the needle clip to the center position. I ran the bike only in the garage and pulled one plug which looked like this:
I didn't run the bike terribly long . . .15-20 minutes. Long enough for the radiator fan to come in and blipped it some while syncing the carbs. Is this now set too lean or should I give it a proper rip and then determine?
The bike seems to rev well as it always has. The RPMs drop at a decent rate of speed, but if I "blip" the throttle they will hang around 2k for a moment. Is this because I'm too lean on my a/f screw and don't have enough overlap as a result? Is there somewhere else I should be looking?