100 HP yamaha 4 stroke driving me crazy


New Member
This is driving me crazy yamaha 100 hp 4 stroke
Yesterday, 11:14 AM

I recently acquired a new to me boat and am trying to trace down some engine problems I am having with it. 1999 Model #F100 TLRX. Test drove the boat and it ran good before I completed the purchase. Drove it home (About 300 miles) and took it out on the water a few days later. Ran ok at low speed but would bog down badly and cut out at about half throttle and above. The longer I ran it the worse it got. Barely limped back to the boat dock when I finally took it in.

What I know about the motor: The previous owner had recently rebuilt the motor and it had less than 2 hours run time on the rebuild. The motor started running poorly so he took it into the marine mechanic who put new plugs in it and completely rebuilt and synced the carbs. Boat ran good shortly but he had to take it back in again because it was doing exactly what it is now according to the info written on the receipt he gave me of work done by the shop.
They told him that somehow the carbs had gotten out of sync after they were synced previously. They resynced the carbs, did not charge him, and sent him on his way. This is where I come into the picture. Test drove the boat, bought the boat, now it is doing exactly what it was for the previous owner prior to him taking it to the marine mechanic to have carbs rebuilt and synced.

I messaged him to get some more info and he told me it ran great after they resynced the carbs (which I can attest to). IT didn't make sense to me but I thought perhaps the carbs got out of sync and if I resync them then it should run fine, right? I purchased a Carbtune II carburetor sync tool and proceeded to sync the carbs. Engine still bogging down at full throttle.
Checked the tank and found that it was missing an O-ring on the fuel cap and had allowed water into the fuel. I replaced the fuel separator, drained the tank, and replaced the fuel. I thought this had to be the cause of the bad running motor. Deduced that the road trip home had mixed the water up into the fuel from the bottom of the tank and that is why it ran well on the test run and poorly when I got home. I was 100% wrong. The boat still ran poorly.

I removed the suction tube from the onboard tank and it was clean and unobstructed. I did note that there was a check valve on the fitting where the suction tube attached to the tank. IT seemed to work properly and I was able to depress the ball in the valve and it would return to position without issue when I released it. I assume the vent to the tank is unobstructed because I removed the fuel cap when the motor was running and it had no effect on the poor running motor. I thought it could perhaps be sucking air somewhere in the fuel line but all Fuel line clamps were tight.

I even replaced the fuel line on the motor and used new clamps to make sure the fuel system was airtight and unable to suck air. Even hooked a portable fuel tank to the engine just to be sure it wasn't a problem somewhere in the onboard tank or fuel lines or primer bulb. Same problem.

Removed and checked carbs. They were pristine inside and out and no issues. Removed fuel pumps and checked. Diaphragms were intact and no apparent issues with fuel pumps.
It just seems that the fuel delivery system is not keeping up with the demand. Maybe electrical??

I purchased a leak down tester and will do a leak down test on it this weekend to check for internal issues but don't expect to find any.

Any other thoughts or suggestions here on what could be the problem would be much appreciated.