25hp Merc won't pull and rev under load.

[sailquik]

Hi Guys, I hope someone can help me here cause I'm stumped!

My 25hp mercury circa 1992 had been going great. Left for a couple of months over winter. Started OK. Ran fine with flushing ears. When put in the water it would not rev hard enough to get the tinny onto the plane.
It was running great the last time it was used.

Did these things:

Checked and changed Fuel
Had mechanic check electrics. One coil tested weak so it was replaced.
New spark plugs
Sand found in filter and fuel tank - Carby cleaned with new Kit put in. - tank cleaned.
External filter between engine and tank fitted.

After all this the motor started and ran exceptionally sweet at idle and free revving with flushing ears.

Put it in the water and it did exactly the same thing - It ran sweet at low revs but rough at higher revs and would not rev enough to pull onto the plane.

Next steps: Checked prop. - Fine
Checked spark advance and timing. - Perfect

While revving under load in test tank, pumped primer on fuel hose - not difference
While revving under load in test tank, pinched fuel hose between filter and carby -

Bingo! The motor revved up and pulled like mad to full power!!

The diagnoses was thought to be running too rich on the high speed jet phase but we can't work out why. The jet has not been changed other than a new needle. (it just got a new kit in it but did the same thing before the kit was installed.)

There are no obvious leaks in the carby (it just got a new kit in it but did the same thing before the kit was installed.)

Any suggestions? We are at a loss........

Thanks. Andrew

 

[oldman570]

Re: 25hp Merc won't pull and rev under load.

Did you rebuild the fuel pump when you did the carb rebuild? It might have a bad diaphragm or the float setting in the carb needs to be checked because the angle of the motor changes when you give it gas, when on the water and stays the same when in a tank. Motors when running on muffs should never be revved to high RPM's, and final setting of the carb should be done on the water for correct setting.
JMO
Oldman570

 

[racerone]

Re: 25hp Merc won't pull and rev under load.

Were the reed valves checked when carburetor was off ?----Has the fuel pump diaphragm been replaced ?--Does spark jump a gap of 7/16" ( 1 cm ) on both plug leads.

 

[sailquik]

Re: 25hp Merc won't pull and rev under load.

Did you rebuild the fuel pump when you did the carb rebuild? It might have a bad diaphragm or the float setting in the carb needs to be checked because the angle of the motor changes when you give it gas, when on the water and stays the same when in a tank. Motors when running on muffs should never be revved to high RPM's, and final setting of the carb should be done on the water for correct setting.
JMO
Oldman570

Thanks for you reply.

No. Fuel pump not rebuilt. Have checked the diaphragm for leaks. Not detected any so far.

Motor would not pull under load in test tank either (until the fuel line was restricted) where the angle is static.

The motor was only revved up and put under load in test tank.

 

[sailquik]

Re: 25hp Merc won't pull and rev under load.

Were the reed valves checked when carburetor was off ?----Has the fuel pump diaphragm been replaced ?--Does spark jump a gap of 7/16" ( 1 cm ) on both plug leads.

Thanks for you reply.

Reed valves were not checked AFAIK. I will follow up on that. Is the signs consistent with a reed valve issue?

Will see if I can get a new fuel pump diaphragm anyhow.

Spark jumps the gap very strongly!!:joyous:

 

[racerone]

Re: 25hp Merc won't pull and rev under load.

Need to revisit carburetor cleaning.-Inspect enricher circuit.

 

[Mi duckdown]

Re: 25hp Merc won't pull and rev under load.

Fuel pump. reeds rarely Go Bad. Don't KNO what AFAIK Means???

 

[racerone]

Re: 25hp Merc won't pull and rev under load.

Maybe -----as far as I know----Young folks comunicate like that.

 

[sailquik]

Re: 25hp Merc won't pull and rev under load.

Fuel pump. reeds rarely Go Bad. Don't KNO what AFAIK Means???

LOL! (Laughing out loud) :lol:

AFAIK (As far as I know).

I'm just a young 62 but being a school teacher keeps me up with the lingo! ;-)

 

[sailquik]

Re: 25hp Merc won't pull and rev under load.

Need to revisit carburetor cleaning.-Inspect enricher circuit.

Thanks. Will do.

 

[aussieflash]

Re: 25hp Merc won't pull and rev under load.

Have you checked stator/trigger/switch box?

 

[sailquik]

Re: 25hp Merc won't pull and rev under load.

Have you checked stator/trigger/switch box?

Thanks for your reply.

Yes. Had mechanic test all the electrics. Replaced dodgy coil. Spark great. Spark advance to spec.

 

[racerone]

Re: 25hp Merc won't pull and rev under load.

I believe that pinching off the fuel supply stopped the flooding.--Revisit carburetor cleaning / fuel pump diaphragm.

 

[CharlieB]

Re: 25hp Merc won't pull and rev under load.

I believe that pinching off the fuel supply stopped the flooding.--Revisit carburetor cleaning / fuel pump diaphragm.

Drop the float in a coffes cup of fuel to see if it sinks.

Carb kits come with EITHER a viton tipped needle

OR

a steel needle and a sort of O ring that has to be installed for the needle seat.

Either will work fine, BUT...... you have to use the seat with the steel needle, and you cannot use a rubber seat with a viton tipped needle.

 

[sailquik]

Re: 25hp Merc won't pull and rev under load.

Thanks for the help people. We think we have found the problem. Looks like an air bleed hole in the carby was blocked. Seems to run fine now. Will report more details after testing in the water.

 

[belray]

Re: 25hp Merc won't pull and rev under load.

I hate to sound stupid but what is wrong with revving the engine with no load? I understand you wouldn't want to crank it up to wot but I have read several times here not to rev but whats wrong with some rpm to clean of a fouled plug?

 

[CharlieB]

Re: 25hp Merc won't pull and rev under load.

These motors gain RPM sooooo fast it is possible for a condition we call 'thermal runaway' to occur, which is, it will not slow down, the rods stretch, the pistons either starts banging against the top of the cylinder OR, the rods break and pieces start flying out of the cases at you.

Nither is anything you want to happen unless you are in the market to buy a NEW motor.