2009 30hp Tohatsu 2 stroke occasional power loss

[bobbyroyal]

Hi there,

wonder if anyone has any more ideas on this,

Starts great, runs great but if run on full throttle after 8 - 10 mins the revs will drop 300 to 500 rpm like its being fuel starved, wind the throttle off then straight back on again and its fine for another few mins.

carb is clean, rebuilt fuel pump with new seals etc,

plugs?
coil?
sticky float?

i get the hunch its either fuel or temperature related.

regards

Rob.

 

[pvanv]

What is your actual model? Verify the fuel primer hose is OK and the vent is open. O rings can dry out and leak. If an overheat, try running with the cowl off for better cooling.

 

[Sea Rider]

To know :

-Is it a M30-A4 model or a previous one ?

-Which pitch number is stamped on prop body or blade, check ?

-You could have already a prop that started going into a spun hub territory. Will need to make a hub slip test to rule that out.

-Have you tested with a new set of spark plugs say NGK B7HS-10 or BR7HS-10 with electrode gaped tight to 1.0 mm ?

-Is the motor using 50:1 fuel/oil ratio, not a drop more ?

-Was the motor used in fresh or salt water, now used same ?

Have found several 30's being factory delivered with slight less timing advance than the usual 25º advance.

Happy Boating

 

[bobbyroyal]

Hi guys,

its an ME 25M, merc sea pro, which by all accounts is a tohatsu. did the 30 hp upgrade mod approx 2 years ago.

its got a 9.9 x 13 prop on it , again had it on for years so thats not changed, plugs are quite new, stringent 50 -1 ratio with the same oil for the past 3 years, has been used a lot in salt water but been in the fresh for the past 2 years.

Checked the compression, 80psi in both, low!! but like i said starts great and runs great for 5 - 10 mins.

all hoses and connections are tip top.

new rings and a hone?


aarhrgrhgr!!

rob

ps, cheers for the help.

 

[Sea Rider]

For 30 HP motors factory specs calls for 100 PSI, is the motor difficult for cold starts ? Runs good except the stated rpm drop when at full throttle. Personally would try a new set of spark plugs, if same, perform a hub slip test. Have had issues with relatively new spark plugs or with motors stopped for long time while resting on humid places.

With motor ON and sufficiently warmed up at idle, put your hand at back of middle leg, check the water temp fluctuation while discharging, if can feel warm and cold cycles going on, thermo is working good. If not, change thermo and gasket as well. Which was the max wot rpm achieved at full throttle as loaded on flat calm water cond ? Find that all 30's are being factory delivered over pitched.

All are basic troubleshooting procedures, check and report back.

Happy Boating

 

[bobbyroyal]

Thanks Sea Rider, will try new plugs.

Runs at 4900rpm flat out, always has. seems a bit over pitched id say.

Motor starts 1st pull from cold runs like a dream. Took thermostat out and tested it and that works fine. May also try another pressure guage just in case.

Rob.

 

[Sea Rider]

Rob,

When the motor experiments stated rpm drops at full throtte, can you feel the rpm drop, say screems bit less or it's entirely a tach rpm fluctuation issue ? What's the tach brand ? Which type of boat is that motor powering it, lenght, number of passengers/load on board. Assuming the tach is reading right, at 4900 wot revs the motor is running on the extreme lugging side of ithe wot rpm range which is 5200-5800.

So running 500 rpm less from middle range and 1,100 rpm from max range. That motor needs a prop maximization urgently...LOL!!

Happy Boating

 

[bobbyroyal]

Hi guys,

it's a definite engine drop not just a tacho problem. It's on a smartwave 3500. Normally 2 on board sometimes 3. Boat is about 100kg motor 55kg plus people 140kg and gear 20kg. Can't really tell any difference between having 1 or 2 on board.

just got new plugs and will be heading out over the weekend.

maybe I should try find a 9.9 x 11

regards

Rob.

 

[Sea Rider]

Rob, to get the max out of that nice 30 will need to be perfectly transom height matched along well propped. With a 11 OEM prop motor will be able to gain as much as 1K wot revs with 2 up and slight less with 3 up, for that to happen motor needs to be trimmed fully vertical (90 deg) on transom.

Tip : with deck weight evenly distributed, motor trimmed to 90 deg go for a wot spin on flat water cond, once on plane pull head side of motor..



check at which lower leg is water flow passing by.



Those 30's with a maximized 11 pitch prop runs top at this precise lower leg height setting, try to duplicate it by rising/lowering the motor on transom if needed to achieve that spot on working height. Report your findings on current motor height and if new plugs and 11 prop cured the rpm reduction issue. Good Luck!!

Happy Boating

 

[bobbyroyal]

Good info cheers. Prop won't be here for week sup will report back then.


regards

Rob.

 

[Sea Rider]

Rob, meanwhile new prop arrives, was wondering being a 10 year oldie if the motor has few or hundreds of hours use. When you flush your motor with fresh water after use if like doing so, you use ear muffs, barrel flush adapter ? Meanwhile check motor height, new plugs and current installed prop.

Happy Boating

 

[bobbyroyal]

so tried the new plugs, made no difference, after 7 or 8 mins still did it wierd power loss thing.

New prop arrived, literally launches itself onto the plane, revs up to 5400 - 5500 rpm and had it running flat out for 10mins and didnt lose any power. slight increase in top speed too.

Maybe with the higher pitch prop it was just working too hard and getting too hot maybe??

 

[pvanv]

so tried the new plugs, made no difference, after 7 or 8 mins still did it wierd power loss thing.

New prop arrived, literally launches itself onto the plane, revs up to 5400 - 5500 rpm and had it running flat out for 10mins and didnt lose any power. slight increase in top speed too.

Maybe with the higher pitch prop it was just working too hard and getting too hot maybe??

Possible. Might also have a lazy ignition part that was overheating when lugging. Sounds like you have it on the run.

 

[Sea Rider]

That's why newer 2 strokes 30 HP motors are being factory requested with 0EM 11 prop pitches instead of current 13 pitch props factory delivered with. Would like to know who was the insane guy at the factory who thought that a 13 was a proper pitch. On a light boat a 13 pitch will only rev up to 4800 wot revs with 2 up under ideal flat water conditions.

Interesting your motor achieved 700 wot revs going 2 less in pitch, so the idiotic prop rule of thumb didin't worked out here as the motor achieved +350 wot revs per one pitch down instead of the ad nauseum cast on stone + 200 wot revs.

Bobbyroyal, the new performance description is superb : Literally launches itself into the plane, used to call that scenario jumps on plane, way to go, enjoy your new boating experience with a maximized OEM prop.

Happy Boating