Sudden RPM drop

[grog337]

My 1997 50 hp 4-stroke Yamaha (T50TLRV) has shown some odd behavior the last 2 times I tried taking it out. It ran just fine for the first 5 to 10 minutes, then the RPMs dropped suddenly from near 5000 to 4000 and will not go past 4000. The first time it happened, I thought it may be bad gas or clogged line, but the second time I went out, it did the same thing. This is after changing the fuel filter (which actually looked pristine), fuel-water separator filter, and used a new portable gas tank with brand new ethanol-free fuel (after purging the fuel from the line from the tank. I actually cleaned out the onboard fuel tank as well, but just wasn't using it. Also, I don't know if it's relevant to the problem, but I noticed a fair amount of black residue on the prop (might just be that its a brand new white one). I don't think it's the temp thing, as the light/alarm didn't come on and that will drop the RPM back to 2000 (water pump seems to be working great, too).

Anyone got any ideas?

 

[99yam40]

Re: Sudden RPM drop

if compression is good and good spark on all cylinders and timing is in spec, you are left with fuel/air mix out of wack in one or more cylinders.

Fuel pump leaking, carb problem, or air leak are all possible

 

[grog337]

Re: Sudden RPM drop

Thanks for the direction 99yam40.

Compression looks very good. I haven't checked the spark just yet, but that was one of the next things I was going to check (need to get a new spark tester, as mine seems to have joined the witness protection program). Carbs were very recently broken down and cleaned, and reassembled per the service manual, so they may be out of adjustment a bit, but should still be clean. I was going to try and sync them this week. I had also planned to try checking the fuel pump with a fuel pump tester to see if that's it. The old squeeze the bulb trick didn't seem to reveal anything.

When you say air leak, where/how were you thinking?

I also had another thought last night. Could it have something to do with the engine getting warmed up? Like something to do with the choke (maybe something being way out of adjustment that starves the fuel when the choke turns off)?

 

[99yam40]

Re: Sudden RPM drop

being this is a 4 stroke 50 with 4 carbs, a solenoid, a diaphragm assembly, and prime start (not a choke)

You need to find someone that knows more than me about that motor and its systems

Prime start system on my C40 2 stroke advances the timing on a cold motor, and adds extra fuel until things warm up.
Not sure what all yours does, but once things warm up the system should turn that stuff off.
could be that the carbs are running too lean when the extra fuel is turned off after warm up.
But I do not know enough about it