1984 Mercury 50hp tiller problem?

Working on a friends 50hp merc tiller, it was his dads boat. Starts great, but doesn’t reach full rpm’s in forward. Other than that it starts and runs great. Full rpm’s in neutral and reverse.

It’s had at least one other person work on the carbs, possibly 2 and he did some small stuff to it as well. The one guy did a full carb clean/rebuild. But there was a leak on the one bowl gasket. Replaced that and no more leak.

The hose on the 6 gallon fuel tank is up front and was on top of the hose on 2 sections of it pinching off the fuel line, restricting fuel flow. Hose was old and was concerned about the fuel’s freshness, so replaced the fuel lines. Emptied the tank completely and put fresh non oxy in and made sure the hose wasn’t pinched or kinked anywhere else now.

Through all this it’s been in the lake 5 or 6 times now.

Decarbed it with some mercury power tune and got an incredible amount of motor fecal matter out of the exhaust. Initially it wasn’t exhausting well at all.

Both props are good and not spun.

It runs great on muffs and doesn’t seem to have any issues with any blockages from the exhaust anymore. No hesitations at all, no rough idle, no nothing problematic even kind of.

It shifts flawlessly.

It starts better than most older merc’s I’ve worked on. Spark is good on all cylinders.

It has 120 psi +/- 1/2lb on all cylinders.

Way more familiar with omc 6-35hp motors than the older mercury tillers in this yr range. So not sure if there’s something easy in the linkage I’m missing or if the carbs aren’t adjusted properly for full rpm’s in the forward gear? Is there an rpm limiter I’m missing?

Can’t believe how good it starts and runs, yet can’t get it to reach full rpm’s in forward?

Any thoughts or help would be greatly appreciated if someone has any insight.

Went thru this exact scenario last year on my BIL’s 50 Merc tiller. After pulling the carbs and cleaning, with no difference, took it to the wrench in town, who cut-off the quick-connect at the motor, and starting shaking the bits of degraded fuel line out of it. Just how much fuel line did you replace, bulb, under-cowl lines, under-cowl filter?

HRG

With the cowl off I’d watch the throttle when in reverse, neutral and forward. Maybe something is jacked up with the linkage and it’s not allowing it full throttle???

If you have good spark and compression, it’s a fuel problem. Carbs were either not cleaned properly or there was still contaminants in the fuel system that plugged the jets after the carbs were cleaned.

What pitch prop is on it? Almost sounds like its pitched too high and can’t get fully up to speed.

Does this one have coil packs? Sounds to me like its dropping a cylinder under load. These motors can idle just fine then drop one under load, which will restrict RPMs. Been awhile since I monkeyed with one, but something to YouTube.

Since you say you can achieve full RPMs running it off the garden hose I suspect a fuel constriction somewhere. Since there is no load on the engine when running it on muffs it is a easy to be fooled into thinking everything’s okay but when the engine is under load things go south.

As each HRG wrote above, I would look at all the under cowling fuel lines and see if there’s debris from a degraded fuel hose in one of them.

I would take off the under cowling hoses and blow them out one by one. my first suspect would be the main hose between the quick connect fitting and the carb.

If at all possible rig up a way to tank test the outboard at home by somehow submerging the lower unit in water with enough room to actually run the prop. I bought a big tub at Menards that holds about 30 gallons of water. I’ve used this a number of times to be able to tank test an outboard that could not be taken off the transom.

Grouse