Yamaha 225 2 stroke - Weak fuel pump output

[networker]

'96 Yamaha 225 2 stroke. Model L225TXRU
Problem: Engine stalls if idled for very long. Fuel pumps won't deliver adequate fuel at idle. Good fuel flow at higher rpms. Engine runs rough and has knocking noise when *****ing at idle speeds. Runs great at all other speeds.

I've changed out all 3 fuel pumps. I have replaced primer bulb, and fuel line from tank to bulb. I have bypassed fuel line altogether and run from separate portable tank.

I used clear tubing at the output of each fuel pump. Noticed lots of bubbles on the top two pumps but the bottom one looked pretty good. Switched top and bottom pumps and the bubbles still existed on top pump so it doesn't look like it's a pump problem. Also, all 3 pumps work well at any speed higher than idle.

Is there a way to test vacuum/pressure pulse levels that drives the pumps? I wonder if there just isn't enough vacuum (or a leak somewhere) to adequately drive the pumps. Are these pulses the same place for all three pumps or are they coming from different chambers in the crankcase? Could a leaky seal or reed valve cause weak pump output?

Am I barking up the wrong tree? Any other ideas of what could cause this problem?

[rodbolt17]

anything that would cause poor primary compression could affect pump operation.
that engine is basically 6 engines sharing a common crankshaft.

[networker]

Thanks for the quick reply. I recently checked compression and found 105-110 psi all cylinders.

[rodbolt17]

secondary compression has nothing to do with primary compression.
secondary compression is from the rings to the head.
primary is from the rings to the reeds and the crank seal rings.
as well as crankcase flange sealing.

[networker]

Thanks for the explanation. I understand.
Can primary compression be measured?

[networker]

Well, thought I would share the solution in case someone has similar problem. I took the thing to a Yamaha master mechanic in my area and this is what he discovered:

• Aftermarket Mallory fuel pumps I bought weren't strong enough to bring fuel from tank. Replaced with OEM pumps.
• Removed anti-syphon fitting at tank. He tells me Yamaha indicates they are not needed.
• Replaced fuel inlet valves at floats. He says several were flooding carbs. Floats were also misadjusted by mechanic who previously cleaned carbs.
• Carb #1 enrichment valve was stuck open, replaced. That one took a while to figure out.
• Knocking noise was due to missing bushings in water pump assembly. Previous mechanic replaced pump assembly and didn't install correctly.

He had the thing for 5 weeks but it's finally running great. I don't feel so bad about not being able to figure it out myself.

[rodbolt17]

on the anti-siphon valve.
as Yamaha did not make the tank,lines, valve or hull they CANNOT say it needs it or not.
the USCG and the BIA say whether it needs it or not.
basically if ANY part of the fuel system drops below the top of the fuel tank the USCG requires anti siphon protection.
Yamaha cannot trump USCG regulations and rules.