Finally got my Tohatsu 4 stroke 4 hp in the water, installed a Tiny-Tac and used 1-2 hrs a day of my vacation to break it in for the factory recommended 10 hr cycle. My only problem was idle adjustment; after the first 2-3 hrs it was dying at low idle, the adjustment screw is VERY finicky and I had trouble dialing it in better than +/- 50 RPM. I believe the hot (90+) temps and humidity did not help, by the end of the week temps had moderated to low-mid 80s and I had it dialed back in to spec.
Motor runs like a champ, by the 8-9 hr mark I was running it up to WOT for 1-2 min per spec; I hit the top end right on the nose +/- 150 RPM due to headwind/tailwind and related wave and current action.
Strongly recommend the TinyTac for any small engine or outboard, what a help to have real RPM readouts for break in and idle adjustment. I am going to semi-permanantly mount the Tach to the front of the motor housing.
Now - for the infamous 4 to 6 hp carb conversion:
I had purchased the jets and nozzle for the 6 hp carb spec, wanted to break in the motor before trying these out. I understand that the pilot jet may cause trouble due to the 'fixed air jet;' I studied the service manual carb diagram and am thinking this:
1) You should be able to change out only the main jet and nozzle, leaving the 4hp spec pilot jet in place. Pilot circuit usually dominates up to ~1/3 throttle, main jet dominates over 1/2 - 2/3 throttle. I see no reason why you cannot get the 6 hp top end from the main jet while leaving the 4 hp pilot jet in place [although there may be some issues in the 'transition' zone where the bigger main jet kicks in over the choked back pilot.] Another issue may be overreving due to prop pitch; a higher pitch prop may likewise struggle at low speeds with the smaller pilot jetting.
2) The 'fixed' jet is not really fixed - it is a mixture screw adjustment that is tamper-proofed via a pressed in soft metal plug. I have dealt with these on several motorcycle carbs, usually pretty easy to drill out the plug and pull it with a sheet metal screw or similar tapping tool. If I can get the plug out it should be simple trial and error to adjust the 'fixed' mixture to accomodate the larger pilot jet.
I need 1 more hr of top end break in and then I will try to experiment with the carb jetting. Spark plug color / fouling is a good indicator of low-mid speed mixture, top end RPM will validate main jet power improvements.
When time allows I will post results.
Motor runs like a champ, by the 8-9 hr mark I was running it up to WOT for 1-2 min per spec; I hit the top end right on the nose +/- 150 RPM due to headwind/tailwind and related wave and current action.
Strongly recommend the TinyTac for any small engine or outboard, what a help to have real RPM readouts for break in and idle adjustment. I am going to semi-permanantly mount the Tach to the front of the motor housing.
Now - for the infamous 4 to 6 hp carb conversion:
I had purchased the jets and nozzle for the 6 hp carb spec, wanted to break in the motor before trying these out. I understand that the pilot jet may cause trouble due to the 'fixed air jet;' I studied the service manual carb diagram and am thinking this:
1) You should be able to change out only the main jet and nozzle, leaving the 4hp spec pilot jet in place. Pilot circuit usually dominates up to ~1/3 throttle, main jet dominates over 1/2 - 2/3 throttle. I see no reason why you cannot get the 6 hp top end from the main jet while leaving the 4 hp pilot jet in place [although there may be some issues in the 'transition' zone where the bigger main jet kicks in over the choked back pilot.] Another issue may be overreving due to prop pitch; a higher pitch prop may likewise struggle at low speeds with the smaller pilot jetting.
2) The 'fixed' jet is not really fixed - it is a mixture screw adjustment that is tamper-proofed via a pressed in soft metal plug. I have dealt with these on several motorcycle carbs, usually pretty easy to drill out the plug and pull it with a sheet metal screw or similar tapping tool. If I can get the plug out it should be simple trial and error to adjust the 'fixed' mixture to accomodate the larger pilot jet.
I need 1 more hr of top end break in and then I will try to experiment with the carb jetting. Spark plug color / fouling is a good indicator of low-mid speed mixture, top end RPM will validate main jet power improvements.
When time allows I will post results.
