75 H.P Optimax runs rough

[CNT]

I have a 75 hp. Optimax and it runs rough at idle then stalls. Sometimes hard to trart. Compression is good, plugs fireing hot, and it's getting gas. Could it be a Throtle Possitiion Sensor or something else? Any help would be appreciated.

Thanks

[merctech5591]

I have a 75 hp. Optimax and it runs rough at idle then stalls. Sometimes hard to trart. Compression is good, plugs fireing hot, and it's getting gas. Could it be a Throtle Possitiion Sensor or something else? Any help would be appreciated.

Thanks

there are a couple issues we have seen with the 3 cyl opti. one make sure u have spark on all 3 cyl. coils are a big issue on those motors. if it does not have the updated coils have them replaced there is a bulletin on them. second check you air fuel pressure and make sure u have he 10psi diff between the two. if you see fuel will only build to 35-40 psi take the vst apart and check the hight pressure pump grommet. i have changed more grommets than i care to admit due to ethanol.

[CNT]

Thanks for the info. What should the pressure from the elec. fuel pump be ? Also, where is the high pressure pump grommet lacated, in the elec. fuel pump canister or on the fuel rail ?

[merctech5591]

Thanks for the info. What should the pressure from the elec. fuel pump be ? Also, where is the high pressure pump grommet lacated, in the elec. fuel pump canister or on the fuel rail ?

fuel pressure should be 108 and air should be 94. this is checked on the fuel rail. the grommet is located inside the alum canister.

[CNT]

I checked fuel pressure at canister ( Svt) and sure enough it was only around 40 psi. I then checked pressure at rail and the top shrader valve read around 38 and bottom shrader valve read around 35. Can you tell me what valve is for fuel and what is for air? Also, what kind of pressure is the air pump on top of motor supossed to produce and how do i check it?

[CNT]

Sorry, forgot to ask were can i purchase the replacement grommet for high pressure pump , and diaphram (s) on fuel rail? Thanks for all your help.

[merctech5591]

Sorry, forgot to ask were can i purchase the replacement grommet for high pressure pump , and diaphram (s) on fuel rail? Thanks for all your help.

Valve on the top of the rail is fuel and bottom is air. U cannot buy any of the diaphragms on the rail. . I would start by taking apart the vst and inspect the high pressure grommet.