[dob]

I am looking through the service manual and try to tackle some of the electrical troubleshooting. Looks like I will need to learn about peak voltage testing and track down some test harnesses. Thanks for the input.

[Robin]

What is the compression in each cylinder after the engine has warmed up?
What is the fuel pressure at 4500 rpm within the first minute after starting vs after 5-10 minutes?
Did you do a real spark test (with a spark tester) on every plug wire with YDIS?

[Pegasus-Rider]

I have not tested the spark, although plugs, wires, and coils have been replaced. Would a spark test still be beneficial? [links]

[octopus]

What is the compression in each cylinder after the engine has warmed up?
What is the fuel pressure at 4500 rpm within the first minute after starting vs after 5-10 minutes?
Did you do a real spark test (with a spark tester) on every plug wire with YDIS? [links]

[vics5thor]

One interesting thing about this problem is that the engine doesn't just drop rpm when it's running rough. the tach will show it quickly increase rpm too. For example, if I accelerate to 4500 rpm and the misfiring starts occurring, the rpms may fluctuate quickly between 4300 and 4700 rpm. Does this sound like a symptom of an electrical or sensor issue? Thanks for the the help. [links]

[billy3321]

My compression test showed was anywhere from 125 to 130 psi across the 4 cylinders. I wasn't too impressed with the gauge I was using, or my technique for that matter. I put the fast idle lever all the way up but not sure if that opened the throttle enough to get accurate numbers for the test. [links]

[spa]

I think rejesterd summed it up, there could be a number things all of which need to be methodically checked (cleaned).

[grapescan]

If the fuel pressure is raised, say to 55 psi, while the air pressure stays at 15 psi, then more fuel will flow through. Can we agree on this? more

[bshimul93]

Boscoe you write well, seems a reasonable explanation to me.

[Beelzebuddy]

Boscoe you write well, seems a reasonable explanation to me.

[Marlo]

Hello fellow boaters. I am having some issues with my 2004 Yamaha F115TR (4-stroke) and could use some good advice. The motor came on a 2004 Sea-Pro 180cc that I bought last year. The only thing that was not stock on the motor was the prop (13 x 19 instead of the lower pitched 13 x 17).

[jessw1]

Last week I was headed off-shore with some buddies to slay some Amberjacks when the boat suddenly started to lose it's plane. The engine sounded like it was bogging a bit, the rpms dropped and the boat slowed to about 15 mph at full throttle. We turned around and started to idle back. Once in the harbor I wanted to see if the problem had gone away and to my suprise it had. for a minute. Then it happened again. more

[AndrewBuck]

She ran at about 5800RPM WOT and had a top speed of about 40mph. My only issue was that it took about 10 seconds to get up on a plane, but I figured that was probably caused by the prop pitch combined with the 4 stroke's weight in the back.

[reQunix]

Rodblot when you say test the rail pressure at the problem RPM what is my problem rpm? right now the engine tops out around 4500 RPM, but if i ppump the throttle i can get it to around 5000 RPM. what should i be looking for at that RPM? i've read up about it and should the pressure be around 40-45 psi? Thanks for your help

[Zeerover]

Re: Yamaha F115 2003 fuel pressure more

[lior]

dude read some on speed density open loop EFI.
the only thing "pumping" the trhottle does is rapidly move the TPS, as the TPS voltage increases rapidly the ECU opens injector on time and will fire injectors both synchroneously and asychroneously. its an electronic equivalent of a carbs accelerator pump.
rail pressure s CRITICAL and CONSTANT.
only time it spike is at rapid intake vacum loss and then only by about 5PSI and then only momentarily.
works in conjunction with the above TPS thing.
you max RPM with a normal load should be 5800 to 5900 RPM. if not you will post about making oil.
rail pressure testing is ONLY valid AT the problem RPM AND LOAD.