Power trim not working

[SkaterRace]

My power trim on my 1977 Mercury 850 is not working right. It 'falls' down slowly while on land until it is vertical and also when on the water under way it tends to not stay where it is put. This would not be a worry if it was safe for towing and driving it but I am not so sure. I do have a manual but am not sure where to start other than to bleed the lines and see if it is low on fluid or not. What should my next steps be?

The images are taken from about the same place and are 30 min apart to give an idea of how much it moves.

 

[merc850]

If there are no visible leaks then follow the instr. in the manual especially the pump fluid level must be kept up during procedure - I use a LU gear lube pump - it screws into the fill hole. It looks like low fluid in the UP hoses. Is the reverse lock knob on the pump body turned full left?
There are two cylinder bleed screws Allen type and slot head with O-rings, I have the Allen type and I had to put Loctite on the screws to stop leakage.

If you want to do restoration of the motor graphics you should contact these people, this is my motor after decaling which cost $60.

 

[fhhuber]

#1... for towing the boat on the trailer you should be using a transom saver to support the weight of the motor and keep it from bouncing... with the hydraulic not holding firm, add a strap to keep the motor tightly in the transom saver.

The motor will try to rip the transom out of the boat every bump you go over without that brace.

 

[SkaterRace]

#1... for towing the boat on the trailer you should be using a transom saver to support the weight of the motor and keep it from bouncing... with the hydraulic not holding firm, add a strap to keep the motor tightly in the transom saver.

The motor will try to rip the transom out of the boat every bump you go over without that brace.

If it has lasted 39 years without one I think it is fine. They are overpriced chunks of metal that are useless. Would not even consider using one if it were given to me.

 

[GA_Boater]

If it has lasted 39 years without one I think it is fine. They are overpriced chunks of metal that are useless. Would not even consider using one if it were given to me.

That over priced piece of metal won't save your transom as the name implies. But with your funky TnT it can save your lower unit or especially after someone fools with your boat and lowers the motor. Yeah, it happens because so many have no respect other peoples property.

As you say "As long as it does the job who cares..

"

 

[Chris1956]

You can use the "dog eared" shaped manual tilt lock or a 2X6 for the meantime.

Your issue is likely caused by either leaking piston seals or by a bad control valve. Make sure the control valve knurled knob is turned full left, and the PTT fluid is full. The proper PTT fluid is either 5W-30 motor oil or Dextron ATF. See what you have and top it off in the reservoir.

When I had that issue with my '77 Merc, it was always bad piston seals. Cylinder rebuild kits may still be available, but the teflon seal on the piston needs a special tool to install it. I had a dealer rebuild the cylinder for me. If you remove and plug the cylinders in turn, you can isolate the issue to the proper cylinder, as it is likely only one is leaking.

 

[SkaterRace]

You can use the "dog eared" shaped manual tilt lock or a 2X6 for the meantime.

Your issue is likely caused by either leaking piston seals or by a bad control valve. Make sure the control valve knurled knob is turned full left, and the PTT fluid is full. The proper PTT fluid is either 5W-30 motor oil or Dextron ATF. See what you have and top it off in the reservoir.

When I had that issue with my '77 Merc, it was always bad piston seals. Cylinder rebuild kits may still be available, but the teflon seal on the piston needs a special tool to install it. I had a dealer rebuild the cylinder for me. If you remove and plug the cylinders in turn, you can isolate the issue to the proper cylinder, as it is likely only one is leaking.

How exactly would I isolate the cylinder? I will try the rest and then see first though.

 

[Chris1956]

The cylinders are connected together. If either one leaks, the motor will settle. So remove the two hoses on the control valve that control one cylinder, and plug them. Run the motor up on PT and see if she settles. If so, the piston seal in the cylinder you are using, or the control valve is bad. Repeat for the other cylinder. Don't forget to bleed the trim cylinders.