Fuel Supply Sleuthing - C40TLRX, 2-Stroke

[a387673]

Engine has developed a starvation habit, runs maybe 30 - 45 seconds at or near full throttle before laying down. I replaced fuel/water separator filter, and cleaned/rebuilt carbs. At idle, pressure test after fuel pump showed about 2.5 psi, and vacuum test before pump showed about 3 in.

I know I need to test at WOT, but I'm hoping that these numbers would be low enough to warrant replacing the fuel pump, and save me the potential inconvenience of going into the fuel tank, under the floor, with access under the console. Anymore, when I get down there, it's questionable if I'll ever get back up. Can someone please tell me it's obviously the fuel pump? Specs for the psi and vacuum numbers aren't that easy to find. Thanks.

[robert graham]

Maybe check your fuel tank vent...if closed or clogged it can run great wide open and then bog down as you describe....any filters or seperators that may require cleaning/checking/replacement?....they could cause a restriction and bogging...I replaced my fuel pump after the first 8 or 10 years whether it was warn out or not....keep the old one on the boat for an emergency spare....you know those little diaphragms are only going to last so long!....good luck!

[99yam40]

most problem with fuel pump is the diaphragm getting a hole/tear leak in it.

I do not remember what my C40 vacuum ran at idle when I tested it, but 3 " at idle sound high off hand.
restricted vent, pick up tube, anti siphon valve, or a pinched or collapsed line
are all possible

Run the motor with a portable tank to see what it acts like would be the easiest test.

[boscoe99]

Any test at less than the RPM at which the problem occurs is meaningless. Best to do a WOT test.

3 "Hg at idle seems excessive. It will only go up from there. 4 "Hg is the max for that motor.

[rodbolt17]

I agree. 3"Hg at the fuel pump intake at idle is very high.
even for a 3.1L that idle number is on the high side.
max fuel pressure out should be in the 5 PSI range at or near WOT.

[a387673]

You guys were right (of course) that the vacuum reading was high. It still took some poking around with the gauge to isolate, but apparently the bulb itself was restricting flow. In hindsight, I might should have changed the bulb simply because it was aging, but the Yamaha OEM bulb was pricey, since I wasn't patient enough to wait for an on-line order to arrive.

- Yamaha Primer Bulb: $29
- T-fittings, Diagnostic fuel connectors: $11
- Not having to go under console and boat floor to repair: Priceless

With new bulb, vacuum reading at fuel pump is about 1" (vs 3" before). Thanks for the help.

[throrope]

Run the motor with a portable tank to see what it acts like would be the easiest test.

Ditto.

I know you cleaned the carbs, but you may repeat my mistake which was contaminated fuel. As soon as I fired it up, I undid all my work.

Luckily, My tank is under the center console. I replaced with a smaller one in line with my habits including all lines to the engine for less than a failed trip.