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1992 90 HP Johnson won't maintain full throttle.

M

maxx90

New member
Hi, I'm new to this forum and hoping some of the experts here can help out. I have a 1992 90 HP Johnson, model no. J90TLEND. When I first launch from a cold start, it will get up to full throttle for about 10 seconds, then it drops back to what feels like about 1/2 throttle. If I slow back down and accelerate again it will go back to full throttle but only for about 4-5 seconds and then back to 1/2 speed. It will continue to do the same thing each time I slow down and accelerate. The VRO pump was already disconnected when I got the motor. Work I've done: Rebuilt the carbs, removed the old VRO pump and replaced it with a new standard fuel pump, replaced all fuel lines (including primer bulb) from the tank to the carbs, replaced the fuel filter, flushed and cleaned the fuel tank, replaced all coil packs, plugs and wires. Checked compression (120, 120, 120, 117). When it slows down it feels like one of the cylinders quits firing or maybe a fuel blockage, but I've replaced everything related to fuel and fire. I've also checked that I have good spark on all cylinders with a spark tester. I thought maybe it was overheating but I have good water flow from the pisser and there is no alarm sounding. I'm not sure what the previous owner did as far as disconnecting the VRO system but, could they have disconnected a wire that would fool the motor into going into limp mode as if it were overheating. Any suggestions where else to look or what else to check? Also, the tachometer is not working so I can't get a good measurement of the RPM.
 
If you have a spark gap tester, you should be able to jump a 7/16" gap. When you have this issue, have you tried pumping the primer bulb. If that helps it stay at WOT, then you have a fuel delivery problem. When you cleaned the carbs, did you adjust the float levels according to specs? I'd start by removing the fuel pump and taking it apart to see if you have a hole/tear in the diaphram. I'm guessing either you need to adjust your floats in your carbs so they have enough gas in them, or your fuel pump isn't working well enough to to keep up with the fuel demands at WOT. Have a buddy pump the primer bulb when you're attempting to run at WOT and if it is able to maintain speed, you have a fuel delivery problem.
 
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