72 Merc dies under load - at wits end!!

[ladycate7]

I have a 1972 65 hp Merc outboard that stalls when you put it in gear and apply throttle. I have flushed both fuel tanks, replaced fuel, replaced fuel pump dismantled and cleaned both carbs, reset float, changed the spark plugs. It starts great, idles well and runs and revs great on the muffs, but when i put it in the lake and put it in gear, it stalls out...if i manually work the throttle, it revs right up, soon as i put it under load, it sputters and stalls... Please, I am at my wits end and don't know what to try next...can anybody out there help me???

 

[P 0 P E Y E]

Re: 72 Merc dies under load - at wits end!!

When you put the gear and advance the throttle, does the ignition timing advance?

Check the physical connection between the throttle and the spark advance

Is the problem new or has it done it since you got the engine?

 

[Moody Blue]

Re: 72 Merc dies under load - at wits end!!

I've been working on the same problem with my 1971 Merc 800. I can "goose" the throttle just a little when going into gear and it will then stay running at idle forever. Hole shot performance is awesome and I can run 5200 rpm no problem.

As mentioned by Popeye, I think the key is the ignition timing. I have setup the carbs as best I can (good idle, flawless hole shot and able to run WOT for extended times) so I think the timing is the issue. Currently running about 4* BTDC at throttle pickup but want to change it to 6* or 7* and try it again.

Have you performed a sync and link on your motor? If not, there is a great post in the FAQ section.

 

[ladycate7]

Re: 72 Merc dies under load - at wits end!!

Thanks guys... I have tried adjusting the timing...it looks like its moving to advance under throttle, but it doesn't seem to make a difference. When i first shift into gear it will run up to about 1/4 throttle, after that it sputters and dies...can't get a whole shot out of it.

I bought the motor used, so not sure how long it has done this, but definitely since i have had it.

 

[CharlieB]

Re: 72 Merc dies under load - at wits end!!

Need the serial # to look up your ignition.

A weak high speed winding will run out of spark and allow the engine to die, but will NOT show up with no load on the garden hose.

A simple test with the boat in the water , connect a timing light and watch the spark fail as the RPM and load increases and the motor dies.

 

[Moody Blue]

Re: 72 Merc dies under load - at wits end!!

If you are able to shift into gear but it dies as you throttle up, then I would turn the idle mixture adjustment screws out 1/8 turn at a time then try again until it no longer stalls. Sounds like you have a lean fuel mixture.

There is a good procedure to follow as part of the sync & link FAQ to set up the carbs.

 

[ladycate7]

Re: 72 Merc dies under load - at wits end!!

The serial # on the motor is 7030172. Its a 1972 65 HP Mercury.
I have tried the 1/8 turn to a maximum of about 2 1/2 complete turns...seems to run best at about 1 full turn out, BUT it still stalls at about 1/4 throttle. The tanks were dumped and cleaned ...The mixture is 50:1 the timing has been adjusted and back again, still no difference.
Could it be a gearbox problem or a leg problem??
The water pump works great, prop turns ... no cracks or chips in the prop... if I have it running in gear - just before it stalls - if we lift the leg, it revs right up... soon as I put it back under load, it dies.

Thanks

 

[ladycate7]

Re: 72 Merc dies under load - at wits end!!

Thanks for the replies... Any OTHER suggestions???

 

[AMD Rules]

Re: 72 Merc dies under load - at wits end!!

Sounds like you might have a clogged exhaust. Carbon builds up over time, and eventually chokes the exhaust flow which is aggravated when in the water due to increased back pressure.

 

[ladycate7]

Re: 72 Merc dies under load - at wits end!!

Hmmm...exhaust...thanks AMD... will look at that and post back

 

[hkeiner]

Re: 72 Merc dies under load - at wits end!!

As previously suggested, check for a good spark at all RPM levels using a timing light or DVA meter. Until you confirm that the spark stays strong when the motor bogs, you may spend a lot of time trying to find other possible problems that do not exist. A bad spark is generally easier to detect (with the correct tools) than many other possible causes of engine bog. If the spark checks out OK, I would next suggest that you confirm all the link and sync settings. Just my thoughts...

 

[Af4408]

Re: 72 Merc dies under load - at wits end!!

I once owned a Johnson 70 that did exactly the same thing. I was told that the prop needed to be vented. I took it to a local prop shop the guy drilled a small hole (1/2" I believe) near each blade and it worked for me. Good luck