1981 2.5 over heats

[19611starcraft]

Hi everyone I have a 81 tempest boat with a 2.5 mercuriser 120.Probelm iam having is after about 20 min at anything close to half throttle the gauge gose towards the red.if I slow down to 10 kph it gose jnto the green but still too hot.my shop pulled one of the hose off it it's getting good water flow but they did say there's lots of back pressure coming from the engine outlite not allowing enough water in too cool engine.the shop said maybe it's the exhaust manifold gaskets they changed those and then said it has the wrong manifold bolt holes don't match up is this possible

 

[alldodge]

Hi everyone I have a 81 tempest boat with a 2.5 mercuriser 120.Probelm iam having is after about 20 min at anything close to half throttle the gauge gose towards the red.if I slow down to 10 kph it gose jnto the green but still too hot.my shop pulled one of the hose off it it's getting good water flow but they did say there's lots of back pressure coming from the engine outlite not allowing enough water in too cool engine.the shop said maybe it's the exhaust manifold gaskets they changed those and then said it has the wrong manifold bolt holes don't match up is this possible

Howdy

Anything is possible but I would be hesitant to say that is were the problem is unless it was leaking somewhere. Your manifold is in the link below
http://www.mercruiserparts.com/Show...MANIFOLD+ASSEMBLY,+END+CAP+AND+ELBOW+ASSEMBLY

I would start with the impeller and housing. If this is in good shape then go to the exhaust and check to see if the shutter (item 7) is in place and has not broken off and fallen into the pipe.
http://www.mercruiserparts.com/Show...nbr=1057&bnbr=20&bdesc=EXHAUST+ELBOW+ASSEMBLY

 

[19611starcraft]

Ok thanks the water impeller was changed I'm spring I just bought the boat ,I was just going to replace the manifold and riser next spring

 

[Bt Doctur]

a quick way to test for a restriction in the manifold/riser is the large hose from the circ pump to the therm housing should NOT be rock hard or have any pressure on it,.