1995 Yamaha P50 Heat Soak Problem

dpidd

Recruit
Joined
Sep 4, 2016
Messages
3
Hi there,

I have a nagging issue with my 1995 P50TLRT that has me quite vexed. In cold weather, it will take off right away and run very strong. When I stop and troll or idle, then try to take off again, it just falls on its face. If I cut the motor and let it cool, it will eventually take off again.

In warm weather ( 75 deg this morning) it will take off right away when cold, but it isn't all the way there. I can feel what seems to be a cylinder cut in and out on the holeshot, but once on plane it isn't as strong as it should be. Once running for a while and trying to take off again, it falls on its face. The YouTube link below shows this condition. If I keep the throttle in it, she just dies. It idles just fine though.

https://youtu.be/mDXDJsG9IXA


What I've done so far:

- Gone through the carbs three times
- Used the OEM Yamaha kit
- Last time I soaked for a week in carb cleaner, blew out every orifice possible with air then aerosol carb cleaner
- Made sure all the orifices in the jets were clear

- New fuel pump
- Tested/verified thermo switches
- Tested oil level sensor
- Replaced CDI Iginition Module with a unit from CDI Electronics
- Swapped out trigger coils
- Put some ice on the rectifier for the hell of it

Questions:

How could the carbs work when cold and not when hot? I struggle with this question.

Can the coils cause this? If so, how could all of them fail in the manner?

There really isn't much I haven't touched on this thing. I'm sick of throwing parts at it that I thought would fix the problem. Compression when cold is consistent in the low 90's.


Hopefully the video paints a picture. Any help would be greatly appreciated.


Dan
 

dpidd

Recruit
Joined
Sep 4, 2016
Messages
3
I need to add that it won't die like this when the lever on the prime start assembly is in the emergency position. So, dumping a ton of fuel seems to help. It seems to coincide with a discussion I had that it might have a leaking crankcase seal that manifests itself when it is warm. Not sure how common this is. This thing sat for eight years before it saw limited use stating two years ago.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
have you checked to see if the overheat alarm works?
maybe it is going into RPM reduction because of overheat or oil problem

low 90s may indicate rings are not seating well.
sitting that long may have caused rusting cylinder walls and other shiny surfaces.

if dumping the extra fuel in helps you may still have carb problems or like you said air leak

Did it run good 2 years ago when it was brought back into use?
 

dpidd

Recruit
Joined
Sep 4, 2016
Messages
3
I disconnected the thermo switches and oil level float to see if they were causing the CDI unit to go into protection mode. No change. From what I can tell from the FSM, protection mode means limiting the RPM's to 1800-2400. This thing just dies after heat soaking and giving it throttle.


At 6.0:1 compression, a basic calculation of compressing atmospheric pressure of 14.7 psi gives me 88.2 psi. Not sure how directly that correlates and I could be way off, but I keep digging through the FSM looking for that spec and I'm not finding it. If anyone knows what spec is I'd like to hear it.

When brought back into use two years ago, it ran great. It would haul arse on a '76 Mr. Pike. Then, after sitting for a year without a proper winterizing, it started this issue. It has also gotten worse. I immediately thought carbs so that's why I went through them. The problem didn't go away so I progressively got more detailed in cleaning the carbs in the two more times in went through them. Not sure what else to do with the carbs beyond ultrasonic cleaning of the main bodies. I have some ultrasonic tanks at work, but they are too integrated into our DI water system to fill with carb cleaner.


I need to get it back into some water to test the extra fuel hypothesis, but I got on here to see if anyone has other ideas. I appreciate the feedback and will continue to try an diagnose and update with what I find.
 
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