I thought I'd share my story after lot of bad and misleading information.
Our 1999 Yamaha 225 ran rough since we got the boat last fall. Many including us suspected it was from old gas. Running the old gas out and replacing it with new did not seem to make much of a difference.
After only a few hours of operation the engine died at idle and would not restart. We had to have it towed. We found one bad low pressure fuel pump and replaced all 3 with Mallory pumps from wholesalemarine.com (after my return, I can no longer find the part on their site?!). The boat would not run for more than a few minutes at idle, then die. It was not getting fuel. We checked filters, air leaks, anti-siphon valve, primer bulb etc. and came up with nothing conclusive. We replaced 2 of the new Mallory pumps with 2 of the old Yamaha pumps and the stalling at idle went away. The Mallory pumps would not pump at low RPM. Mid and up, they worked fine. We returned all 3 Mallory pumps, bought one new Yamaha pump and 2 new diaphragms.
The engine ran again, was much smoother at idle (thanks to the new LP pump) but missed terribly from ~1400-2200 rpm's. After 2200 it ran like a champ. Many commented (from other sites) that the OX66's all ran like this because they drop 2 cylinders below 1800 rpm's and the condition was "normal." Only one person claimed BS and said the engines should run smoothly regardless of rpm's.
I had suspected that the TPS might be a little off causing the rough transition condition. According to the books this required a special jumper wire assembly to make the adjustment. Tired of chasing for parts, I used my intuition and SLIGHTLY rotated the TPS in the direction that I thought would correct the issue. Sure enough the "rough" spot is entirely gone and the engine now runs smoothly from idle to WOT.
If your OX66 has never had the TPS adjusted, you aren't doing your engine any favors. If you have this "rough" spot and people have told you it is normal, tell them otherwise bc it isn't. Even if you need to pay someone else to do this for you, you will be glad you did it.
I hope this helps someone else avoid a headache!
Our 1999 Yamaha 225 ran rough since we got the boat last fall. Many including us suspected it was from old gas. Running the old gas out and replacing it with new did not seem to make much of a difference.
After only a few hours of operation the engine died at idle and would not restart. We had to have it towed. We found one bad low pressure fuel pump and replaced all 3 with Mallory pumps from wholesalemarine.com (after my return, I can no longer find the part on their site?!). The boat would not run for more than a few minutes at idle, then die. It was not getting fuel. We checked filters, air leaks, anti-siphon valve, primer bulb etc. and came up with nothing conclusive. We replaced 2 of the new Mallory pumps with 2 of the old Yamaha pumps and the stalling at idle went away. The Mallory pumps would not pump at low RPM. Mid and up, they worked fine. We returned all 3 Mallory pumps, bought one new Yamaha pump and 2 new diaphragms.
The engine ran again, was much smoother at idle (thanks to the new LP pump) but missed terribly from ~1400-2200 rpm's. After 2200 it ran like a champ. Many commented (from other sites) that the OX66's all ran like this because they drop 2 cylinders below 1800 rpm's and the condition was "normal." Only one person claimed BS and said the engines should run smoothly regardless of rpm's.
I had suspected that the TPS might be a little off causing the rough transition condition. According to the books this required a special jumper wire assembly to make the adjustment. Tired of chasing for parts, I used my intuition and SLIGHTLY rotated the TPS in the direction that I thought would correct the issue. Sure enough the "rough" spot is entirely gone and the engine now runs smoothly from idle to WOT.
If your OX66 has never had the TPS adjusted, you aren't doing your engine any favors. If you have this "rough" spot and people have told you it is normal, tell them otherwise bc it isn't. Even if you need to pay someone else to do this for you, you will be glad you did it.
I hope this helps someone else avoid a headache!