Tohatsu 25/30 Idle Mixture Adjustment

[HaMm3r]

I have a 2005 Mercury Sea Pro 25 (really a Tohatsu M25) that I've done the 30hp gasket/timing upgrade to. While doing the gasket swap, I went ahead and rebuilt the carb, fuel pump and replaced the fuel filter assembly and all the fuel lines. My question is about the idle mixture screw however. Before tearing the carb apart, I counted the turns of that screw (2 1/4) and put it back on reassembly. The motor runs fine, but I've noticed the plugs seem a bit oilier than with other motors I've had, so I was curious if anyone knows the proper idle mixture screw setting for this motor? The service manual I have doesn't list it, and internet searches have only provided one reference for racing these with a setting of 1 3/4 turns.

Thanks for any help!

 

[pvanv]

Re: Tohatsu 25/30 Idle Mixture Adjustment

There isn't any "correct" setting. Idle mixture adjustment must be done by trial and error. It requires a good ear, a very good shop tach, patience, and a willingness to repeat the adjustment until it is the best for your motor under your conditions, running your fuel and your oil at the mix you have selected. The carb must be impeccably clean, and the whole fuel system must be working 100% correctly before beginning.

1. Start with the gas you will be using. Make sure it is fresh.
2. Use the TCW-3 oil that you intend to use.
3. Mix the oil and fuel precisely and completely.
4. Warm-up the motor thoroughly. It is best to do this procedure on the boat, but you will get very close in a large-enough (over 150-gallon) test tank.
5. Start with an initial pilot screw setting of about 2-1/4 turns out.
6. Adjust the idle speed stop screw to specifications. (refer to the shop tach).
7. Adjust the pilot screw to increase idle speed.
8. Repeat items 6 and 7 several times until you have the maximum idle speed with the minimum idle stop setting.
9. Richen the pilot screw about 1/8 turn to improve idle stability.
10. Done.

 

[HaMm3r]

Re: Tohatsu 25/30 Idle Mixture Adjustment

There isn't any "correct" setting. Idle mixture adjustment must be done by trial and error. It requires a good ear, a very good shop tach, patience, and a willingness to repeat the adjustment until it is the best for your motor under your conditions, running your fuel and your oil at the mix you have selected. The carb must be impeccably clean, and the whole fuel system must be working 100% correctly before beginning.

1. Start with the gas you will be using. Make sure it is fresh.
2. Use the TCW-3 oil that you intend to use.
3. Mix the oil and fuel precisely and completely.
4. Warm-up the motor thoroughly. It is best to do this procedure on the boat, but you will get very close in a large-enough (over 150-gallon) test tank.
5. Start with an initial pilot screw setting of about 2-1/4 turns out.
6. Adjust the idle speed stop screw to specifications. (refer to the shop tach).
7. Adjust the pilot screw to increase idle speed.
8. Repeat items 6 and 7 several times until you have the maximum idle speed with the minimum idle stop setting.
9. Richen the pilot screw about 1/8 turn to improve idle stability.
10. Done.

Interesting... Thank you for the very helpful and complete response! I may attempt this tonight after some prop testing I have planned.