Yamaha 250 OX66 - Will not idle

[Patsy Mac]

Have a 1998 Yamaha 250 two stroke, generally runs well. After a 10 mile run out into the LI Sound last week, when we got to our location, it would not idle. Limped it home, and it will not maintain idle speed below 1300 RPM. Changed filters, ran a slave fuel tank. Mech says fuel pressure drops to zero under 1300 RPM, causing the stall. He recommends the High Pressure fuel pump be replaced. (Very expensive part). Any thought's on what else it might be? How do the low pressure pumps function? I should note that at top end, sometimes the RPM would drop a couple hundred, but would recover when I trimmed motor up or down a little.

 

[seasick]

Have a 1998 Yamaha 250 two stroke, generally runs well. After a 10 mile run out into the LI Sound last week, when we got to our location, it would not idle. Limped it home, and it will not maintain idle speed below 1300 RPM. Changed filters, ran a slave fuel tank. Mech says fuel pressure drops to zero under 1300 RPM, causing the stall. He recommends the High Pressure fuel pump be replaced. (Very expensive part). Any thought's on what else it might be? How do the low pressure pumps function? I should note that at top end, sometimes the RPM would drop a couple hundred, but would recover when I trimmed motor up or down a little.

I would think that the low pressure pumps are more likely to be an issue than the high pressure VST. If the VST were bad or even clogged, you would have more issues at higher revs and load not at lower revs.
You didn't mention the motor model and what I said applies to SX motors not carbed ones. of course if carbed, you don't have a VST

 

[seasick]

Have a 1998 Yamaha 250 two stroke, generally runs well. After a 10 mile run out into the LI Sound last week, when we got to our location, it would not idle. Limped it home, and it will not maintain idle speed below 1300 RPM. Changed filters, ran a slave fuel tank. Mech says fuel pressure drops to zero under 1300 RPM, causing the stall. He recommends the High Pressure fuel pump be replaced. (Very expensive part). Any thought's on what else it might be? How do the low pressure pumps function? I should note that at top end, sometimes the RPM would drop a couple hundred, but would recover when I trimmed motor up or down a little.

I don't think it is VST since that usually affect higher revs first. Could be low pressure pumps or a simple air leak. Take a look at the onboard motor gas filter and see what the fuel level looks like. The bowl should be about a 1/2 full. If empty, there is no fuel getting to it. If full you probably have an air leak.

 

[CleanupRequired]

Low pressure fuel pumps will cause this exact symptom. One of them probably has a hole in the diaphragm. Very easy fix.

 

[grady33]

When I had OX66's, we routinely changed the low pressure pumps - every few years and replaced the VST every other year or so. Always used non-ethanol fuel. Are you using ethanol or non-ethanol? I'm guessing change the low pressure first and then see. Might want to tackle the VST filter also at the same time.

 

[seasick]

When I had OX66's, we routinely changed the low pressure pumps - every few years and replaced the VST every other year or so. Always used non-ethanol fuel. Are you using ethanol or non-ethanol? I'm guessing change the low pressure first and then see. Might want to tackle the VST filter also at the same time.

If there are no issues with high speed operation or surging, I wouldn't touch the VST tank.