I have a 90 Yamaha, that starts up perfectly and idles great but when you put into gear it falls flat on its face. I have replaced plugs, drained gas tank and filled up with ethanol free gas with a can of Sea Foam. I've had all 3 carbs completely overhauled, replaced a coil, filters. It runs about 4800 rpms at 29mph for about 5 to 25 minutes then it shuts down and you can't throttle up to no more than 2500 rpm and only goes up to 5-6 mph. Has anybody had this problem before? Would like any type of help. Now, my mechanic wants to put a Head Gasket on it and says if that's not it I'll have to re drill and tap the brass tabs on the carbs for air flow. This is not making any sense. Any Ideas thanks
I think on those models there is a brass plug on the carbs that needs to be drilled to access to some adjustment screws. Other than that I don't like the idea of drilling and tapping anything on that motor. Those 90 Yamaha two strokes were some of the best 90s out there.
Stuff that comes to mind... collapsing fuel line/bulb/vaccum in tank... I'm drawing a blank on anything else that would be a delayed reaction like what you're describing.
Clogged pickup screen in tank. I would isolate the problem by putting a temporary fuel tank on the engine to bypass the permanent tank, fuel lines, fuel filter, etc. and see what happens.
Bad Power pack? I had them go out and have erratic loss of power like dropping a cylinder. I have also pulled the plug wires one at a time to isolate ignition problems. If you think it's fuel related you might try hitting the primer when it bogs down to see if it picks back up.
I doubt its a power pack issue or it would die. You could be losing a coil(but I doubt it), its not likely a compression issue or it would be there all the time. I would start with fuel delivery. either a fuel filter clogged or fuel pump/hose issue.
We had a 1999 70HP yamaha 2-stroke do something like this a while back. Started fine, idled fine, would run all RPM to wide open for a few minutes then almost completely shut down. The inside lining of the fuel lines were falling apart and clogging up at the motor plug, restricting fuel to the motor. Enough would pass to start and idle but once you get going fast enough it wouldn't flow fast enough and run the carbs dry until we slowed down. Replaced the fuel lines and never had another issue until we sold it in 2012. Very easy to check, remove the fuel plug that connects to your motor and look inside the fuel line.
I had a very similar issue with my old carbed Yamaha 150. After new fuel lines, primer bulb and carb cleaning I fixed it by replacing the manual fuel pumps. The sell a rebuild kit (new diaphragms etc.) but I did not feel like messing with that so I just replaced both pumps and she was good to go.
That was an Idea I was thinking about before, I've completely drained my fuel tank, replaced 16 gals of ethanol free gas from Bucee with a can of Sea Foam. I've even installed a water/fuel separator on as well.
Might check the spark on that bottom cylinder with a spark tester or ground it against the block to see if it has hot blue spark. Which coil was replaced? Might pull that plug wire once it starts acting up to see if your loosing fire there. Is this a new problem with a motor you've had a while that was running fine or is this motor new to you and trying to get it lined out? Your mechanic had recommended mods to the carbs. If the motor was previously running fine without these mods I don't see the need.
I had similiar problem last year with my Yama 70. It turned out to be the gas line pickup at the gas tank. It was corroded. Replaced and no problem since. About 20 years ago, I had the same problem with a Mercury 90. At that time it was rings getting blow by when trying to plane. Too much cost to fix, so I bought a new engine. Compression test should show this.
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