Merc 260 riser question

[Ddickie]

In trying to track down the cause of a high speed overheat, I removed the cooling hose from my T-stat housing to one of the risers. My question is, should I be getting exhaust blowing back through the fitting from the riser at idle? This is with the boat in the water and drive down.
Thanks in advance

 

[Bondo]

Ayuh,.... Welcome Aboard,..... Ya that would be typical results,.... less resistance goin' backwards, than out at that point,....

Tell Us more 'bout yer high speed over heatin' issue,.....

Have ya changed the impeller yet,..??

 

[Ddickie]

The boat gets on plane great, but anything over 3100 RPM and the temps rise. It will do anywhere from idle to 3100 all day long. I replaced the impeller and housing late last season. Old impeller was fully intact. Replaced circulating pump as well. Risers and manifolds are clean, thermostat housing is new. Power steering cooler seems clear as well.
Are the Alpha drives known for the restriction of the inlet pipe due to corrosion like the Bravos are?

 

[NHGuy]

What is your wide open rpm? Can it run to 4500 or so? If it has a tall prop, that can make it work too hard. Is it timed to 8 degrees BTDC at idle?

 

[Ddickie]

I can only get about 4100 out of it. 19p 3 blade prop on a 23 ft Doral cuddy. I checked timing last year, and all was well

 

[Ddickie]

Still having issues. Did an impeller flow test. Lots of water. I put a clear hose from t-stat housing to a riser and noticed that when up on plane, there is a steady stream of tiny air bubbles in the flowing water. I'm going to re-do the impeller and housing just to eliminate the possibility of a leaking impeller housing. Am I on to something here? Possibly water pocket cover?

 

[NHGuy]

I'd try the original plan first. While doing them have a look at the water pocket cover. If it's melted, change it.

 

[Bondo]

I can only get about 4100 out of it. 19p 3 blade prop on a 23 ft Doral cuddy.

Ayuh,..... Yer over propped, 'n luggin' the motor,....

Drop down to a 17" prop, 'n try it,....

 

[Ddickie]

Just an update. It turns out the outdrive impeller was the culprit. My first trip out on this boat with the new impeller last year, the boat overheated and I traced it to the circulating pump on the front of the motor. Looks like that overheat was enough to cook a new impeller. Runs cool all day now. Will try a 17p prop and see if I can get a little more RPM out of it. Thanks for the help. Now I need to figure out why I was left stranded on Sunday! New topic.