GregP
05-30-2005, 12:27 PM
Motors: 1993 Yamaha L250TXRR carb motors with micro-processor controled ignition.
Stbd Motor:
Ran fine on the way out, surging if try to hold RPM between 3000 and 3500 (it's did that last year too, I've only run the boat about 4 times since I got it), runs fine above 4000. Idled around (looking at what's new in waterfront houses) for about an hour. Headed home, ran up to power fine. Port motor started acting up (see below). Stbd motor ran fine, able to pull RPM, but started to idle home (fast idle, about 1500). Had to slow for a cruiser. Pulled back to idle, then put in neutral. Engine quit. Would restart but not idle below 1500-1700 RPM.
When starting the motor it starts fine, and idles while on the "warmup" fast idle. When it starts to drop to idle it idles for a few seconds then quits again.
At port pulled the "emergency" plug, which manual says disconnects the advance micro-processor. Idles fine with plug disconnected but has obvious intermittant misfire. Put a timing light on the motor and I can see the advance with RPM at higher idle, but can't get an idle reading as it quits.
Port Motor:
Ran fine on the way out, no surge like stbd motor, pulled RPM fine. Same idle around then turn toward home (obviously). Came up to speed fine, then started to drop a couple hundred RPM, then would only pull to 2500 RPM (even though stbd was still running 4000). Droped off plane to check the filters for water (?), a little found in the bottom of both racor bowls but nothing in the motor one. Decided to give one short try, came up to 2500, jumped for a few seconds to 4000+, then back to 2500. Gave up and idled home. Didn't die like stbd (luckily).
At port put a timing light on this motor too. With computer connected the idle advance is about 4* ATDC!!, moves irradically to TDC with increasing RPM, but never anything BTDC (like stbd shows). Pull the emergency plug and timing goes to about 4* BTDC and stays there.
Diagnosis?
Port motor could be fuel starvation? All 6 carbs pull lots of fuel if I put my hand over the intake to increase the local vacuume. Motor filter shows nothing in it. Racors are new. Primer bulbs are (both) new. Lack of timing advance makes me think it is the timing computer, also I know from prior tests that if you disconnect the timing computer plug the motors run fine up to 2500 RPM, but that's all they pull, much like the port motor when it dropped power. Also, the sudden 10-15 second surge when I made the second tray sounds more electrical an fuel to me (but what do I know about outboards). Timing computers are a $500 pop so I'd like to be really sure I need one before I buy one (or two?).
Stbd motor is more confusing. More likely to be a coil pack or several than the timing computer? It seems to be advancing fine, but I don't know what it does at idle. This motor also shows 16VDC on the voltmeter, so could a bad regulator with overvoltage be confusing the timing computer?
We did carb overhauls last year and it ran fine last fall and the prior time I had it out this spring.
-Greg
Stbd Motor:
Ran fine on the way out, surging if try to hold RPM between 3000 and 3500 (it's did that last year too, I've only run the boat about 4 times since I got it), runs fine above 4000. Idled around (looking at what's new in waterfront houses) for about an hour. Headed home, ran up to power fine. Port motor started acting up (see below). Stbd motor ran fine, able to pull RPM, but started to idle home (fast idle, about 1500). Had to slow for a cruiser. Pulled back to idle, then put in neutral. Engine quit. Would restart but not idle below 1500-1700 RPM.
When starting the motor it starts fine, and idles while on the "warmup" fast idle. When it starts to drop to idle it idles for a few seconds then quits again.
At port pulled the "emergency" plug, which manual says disconnects the advance micro-processor. Idles fine with plug disconnected but has obvious intermittant misfire. Put a timing light on the motor and I can see the advance with RPM at higher idle, but can't get an idle reading as it quits.
Port Motor:
Ran fine on the way out, no surge like stbd motor, pulled RPM fine. Same idle around then turn toward home (obviously). Came up to speed fine, then started to drop a couple hundred RPM, then would only pull to 2500 RPM (even though stbd was still running 4000). Droped off plane to check the filters for water (?), a little found in the bottom of both racor bowls but nothing in the motor one. Decided to give one short try, came up to 2500, jumped for a few seconds to 4000+, then back to 2500. Gave up and idled home. Didn't die like stbd (luckily).
At port put a timing light on this motor too. With computer connected the idle advance is about 4* ATDC!!, moves irradically to TDC with increasing RPM, but never anything BTDC (like stbd shows). Pull the emergency plug and timing goes to about 4* BTDC and stays there.
Diagnosis?
Port motor could be fuel starvation? All 6 carbs pull lots of fuel if I put my hand over the intake to increase the local vacuume. Motor filter shows nothing in it. Racors are new. Primer bulbs are (both) new. Lack of timing advance makes me think it is the timing computer, also I know from prior tests that if you disconnect the timing computer plug the motors run fine up to 2500 RPM, but that's all they pull, much like the port motor when it dropped power. Also, the sudden 10-15 second surge when I made the second tray sounds more electrical an fuel to me (but what do I know about outboards). Timing computers are a $500 pop so I'd like to be really sure I need one before I buy one (or two?).
Stbd motor is more confusing. More likely to be a coil pack or several than the timing computer? It seems to be advancing fine, but I don't know what it does at idle. This motor also shows 16VDC on the voltmeter, so could a bad regulator with overvoltage be confusing the timing computer?
We did carb overhauls last year and it ran fine last fall and the prior time I had it out this spring.
-Greg