Thread: Yamaha GP800 Engine Problems
04-05-2011, 04:42 PM #1
Yamaha GP800 Engine Problems
I have a 2000 Yamaha GP800 that I got from a friend. It had been sitting for about 4 years because the impeller housing bubbled and caused the impeller to keep jamming. So, I went ahead and replaced starter, spark plugs, battery, rectifier, impeller housing, and I rebuilt the carbs. Now it starts and runs fine, out of the water it revs to the full 8k RPM, however in the water it only goes to about 5k RPM and maxes out at about 25 mph, any ideas?
04-05-2011, 05:39 PM #2
- Join Date
- May 2006
Assuming the impeller spins freely now...
Start with a compression test. If you have a bad piston/dropped PV running it will just make things worse.
Since it has not been run in 4 years you will want to remove the PV caps and clean the power valves as well.
It could still be a fuel issue. If you feather the trigger will it rev more?
04-06-2011, 01:19 AM #3
I ran a compression test back when I was replacing the spark plugs and I know I had good compression in both cylinders but I don't remember exactly what it was. I have a feeling it may be the PVs but I can't exactly take the PV caps off because the exhaust manifold is in the way. And I can't fully remove the exhaust because of the bolt locations/lengths with the engine in the ski, any suggestions as to how to clean the power valves with the exhaust still in place, or how to fully remove the exhaust without taking the engine out of the ski? (and as for a fuel issue I don't think its case because the ski cranks just fine out of the water)
04-06-2011, 07:34 AM #4
- Join Date
- May 2006
I have the same ski. It is tight, but you can take the exhaust off. You can also get a wrench in to take off the PV caps just to look - but the exhaust will have to come off to pull the PVs if you have to. Don't even try to take the exhaust bolts out. Just loosen them and pull the assembly out with bolts. You should definitely replace the gaskets with that much age but I have used 1211 silicone on top of the gasket and reused for testing.
It really still can be fuel. As the boat comes under load it will use more gas. One of the "gut check" tests you can do is to run it on the water for a while and kill it. Spin the gas cap and see if you have any pressure in the tank. You could have a problem with the fuel system pressurizing. Look at the threads on jetting. There are lots of extra information in them.
You said you checked the compression. What exactly was it on each cylinder? Compression is one of the single most telling measurements. The exact compression number isn't as important as consistency. Make sure you have a fully charged battery.
04-07-2011, 01:21 AM #5
Before you go nuts cleaning powervalves disconnect the PV motor electrically and see if you can turn it by hand. Are they cycling after shutdown? Next leave it disconnected and take it on the water with them fully open. It will take sometime to get on step but if you get full rpm then you know its a pv motor issue or there just sticking. Also make sure they are moving freely. Clogged PV by itself itsnt going to loose 3K rpm. They may not be opening. If it still doest work then go through the carbs again.
04-07-2011, 07:13 PM #6
I had a very similar problem. The ski would run fine on the trailer but was a dog in the water, barely making 25 mph. Rebuilding the carbs cured the ski. I know you said that you rebuilt your carbs but if the valves are good you might reconsider a fuel issue. Is the ski very difficult to start?
04-07-2011, 08:02 PM #7
If the ski has the fuel selector switch, try bypassing it. My GP1200R was acting similar. I bypassed it and it worked like a charm.
04-07-2011, 11:45 PM #8
Definitly start by checking that the PV servo is opening the powervalves. Start it for a few seconds with the seat off and when you shut it down you should be able to see if the servo is cycling and opening-closing powervalves. Most of the time if a 800 will run good up to 5000 and not pull any further-it is the servo motor.
04-13-2011, 04:57 PM #9
Well today was my day off so I went ahead and took it apart to clean the PV valves, I got the whole thing apart however the actual valves don't wanna come out, the slide back and forth but they don't come all the way out and I didn't want to force them, any ideas? Also the PV motor does cycle after turning it on and off and the ski starts pretty easily, I will try disconnecting the PV motor and leaving them fully open if cleaning the PV valves doesn't work
04-13-2011, 11:12 PM #10
There is an allen key at the base that needs to come out. After that take a screw driver and pry up at the pin. IT does take a little
force to get them out.
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