Boat vibration at slow speed no wake zones. Engine overheats until speed increased.

[Fisnfool]

I have a 18' 1965 Starcraft Holiday with the 4 cylinder inline. I bought the boat used a few years ago and the owner had bill to show the engine was rebuilt in 2005. I bought it a few years after that.

When I bought it, The boat had a vibration at no wake speed, and starts to overheat. This goes away as speed is increased slightly, but then in narrow marina channel, wake was an issue. Points were worn. I put in new plugs and points.

I did install new gimble and U-joint bearings I did install new water pump and impeller in the outdrive. Adjusted carb idle screw to try and smooth it out.Did not help. Idle on water muffs is 800 rpm. If I adjust the timing so it lines up with the timing marks, engine idles very rough. and stalls. Adjusting timing until this smooths out, then the flywheel indicator is no where near the timing mark plate. Top end speed is good, boat in calm water can reach 30mph reading on the gps. I never run that fast, just a speed check. Can not run at trolling speed because engine overheats until about 6 mph.

I just found the Timing and Sync of Merc Inlines (to 1988) article in the FAQ. I'll try this and then reset the timing and see if that helps. I am thinking that the low speed overheat is due to engine vibration equals impeller not turning smoothly, causing poor water flow until speed increased.

I have 2 outdrives. The one on the boat is 1:84. This matches what the book says for the 1A easy shift. But, I will have to check the other drive. If 1:84 is not correct, what ratio should it be?

Any other hints on solving this is greatly appreciated.

 

[alldodge]

Being a 65 I'm thinking its a 120, correct?
Have you had the prop looked at by a shop?
As for overheating, I would lean toward the pump housing is sucking some air, or not making full contact. Have you replaced the pump housing?

 

[Fisnfool]

Yes it is the 120. New pump housing and impeller. I even pulled the onboard engine water pump. Nothing blocking the pump fins or feed to the pump. Changed the drive belt to make sure it was not slipping.

In regards to the prop, I have 2 out drives for the boat. I'll task both props to be checked and see if they are different in pitch. If wrong pitch is on the boat, maybe with the right pitch, rpms will be higher for same speed. This should then feed water at higher flow rate.

The strange thing about the vibration is that it goes away as speed is increased instead of getting worse.

 

[alldodge]

The pitch of the prop should not make any difference in the amount of water being pulled in by the pump. I'm just wondering if the there is something wrong with it. Probably not the issue just my first thought.

Going away with higher rpm puts more load on the components. Might have a bearing going out.

 

[Scott Danforth]

did you back flush from the thermostat housing to the outdrive? you could have sucked up some debris and pushed it up the hose/passages from the raw water pump in the drive to the t-stat housing.

did you re-align the motor

did you verify the dwell when you set the points?

 

[Fisnfool]

I'll do the back flush and check the dwell. Try that tomorrow.. The gimbal and u-joint bearings are brand new. So are all the bellows. No water getting to new bearings.