978 Merc retards at full trim

[gonguana]

Hi All,
I have a 90 Merc 1978 flavour and it seems to have a problem if I am over half throttle and raising the trim. The trim seems to get all the way up and instead of stopping or moving to the tilt function. The motor cuts and retards. In the way of possible choking or dropping a cylinder or timing. makes a real beeeerrrr deep sound. and looses all power. and I have to drop back to neutral and then put it in to gear again before the motor will operate correctly again. If I trim at under half throttle or just on the plane then its fine.

Does anyone have the faintest idea what might be going one and how to fix it?

Thank you all so much for taking the time just to read this post.

 

[Chris1956]

A 1978 Merc 90 will have two trim/tilt cylinders. These 2 cylinders both trim and tilt the motor, so there is no difference in function between the two.

I am not sure what your issue is. If you overtrim the motor, the motor would rev up and speed would drop as the prop starts to ventilate. If that happens, trim her down a bit to allow the prop to bite.

There is a mercury switch that kills the ignition if the motor is trimmed too much. They usually do not cause any issue, unless the motor is trimmed all the way up. Maybe that is becoming activated?

 

[Georgesalmon]

There is a micro switch on the side that limits the trim and you have to activate two switches to continue into tilt. Many of these old motors have had that switch bypassed because you can't buy it anymore. You can buy a generic micro switch and replace it if you want to, I did that to my 1979 115. I agree you may be activating the mercury switch by tilting the motor to high.

 

[Faztbullet]

It does sound more like your overtrimming and prop is cavitating...its not mercury switch as it will shut engine off.

 

[GA_Boater]

I think you are over trimming and the motor is moving into the tilt range, The " beeeerrrr deep sound" is the exhaust not being muffled by water. At lower speeds the motor sits deeper in the water, so it can be trimmed higher. But I think you are still raising it too high.

If you have a trim limit switch, which all of them don't have, it's faulty or bypassed. On the trailer see how high the motor lifts just using the trim switch. It should stop trimming up no more than in the pic.

 

[gonguana]

First of all thank you everyone for your valued replies.
Yes there are 2 buttons for trim & tilt. One on the throttle handle for trim and one on the control box for tilt. Thank you GA_Boater, I will have a look at the trim setting on the tilt rail/bracket and make sure it is not over trimming. As for over trimming the motor, I have this 90 on a 14ft Haines Hunter. So the trim is quite high to get the prop up at a good angle where it's carving the top 300mil of the surface. At about max trim i am getting a nice wash out the back. The problem is 1 tiny press of the tilt button to far and the thing retards. I would rather it cavitated out so i can drop it a little to grip again. not "by the sound of it" hit the mercury switch. I'll check it out anyway and see what i can find. Also one quick side note question. Does this model have a thermostat or do they just run cold?

Thanks again to everyone in advance.

 

[Chris1956]

There is no thermostat on that motor. It will run dead cold at idle and warmer at cruise.

 

[gonguana]

Thanks Chris for the reply. I know everyone so far is stating that they think i am over trimming the motor and causing the issue but i don't think so. If that was the case. When the motor is "retarded" i should be able to drop it a little and gain traction and throttle. but this is not the case. If i drop it and use throttle it stays retarded. It stays retarded until i go back to neutral and then put it in to gear again. so even if i come completely off the plain but remain in gear the motor will remain in the same state. If i move the lever to neutral and then back in to gear. Everything is fine again. I checked the clearance as discussed before and at full trim the plate is just above the highest trim pin.
Please note this only happens when i am over half throttle. I know it is a wild thing to state but im almost at the stage of creating a video for you guys on it......

 

[flyingscott]

Check the wiring harness see if it pinches or gets pulled when you start trimming it out