No telltale at WOT

[TAF]

I noticed the other day that the telltale stopped peeing after a little while of running at WOT.. When I slowed down it came back. What is the cause of this? Its a 1991 30hp rope/tiller.

 

[Silvertip]

Re: No telltale at WOT

When was the water pump impeller replaced last?

 

[redjmp]

Re: No telltale at WOT

The impellor is slipping around its hub at higher speeds and the extra heat in the top of the engine is boiling the water and the pressure of the resultant steam is forcing it back down. You should see steam come out of the telltale.
Change your impellor before you roast your #1 piston.

 

[Texasmark]

Re: No telltale at WOT

Very unusual (to me) question/occurrence.

Don't know your background Red, so I will direct this to Silvertip.

Plugs are still doing great!
---------------------------------
Question on what you said. If the impeller is slipping around the hub at WOT wouldn't you think that the ram water pressure would be adequate to help it around so that you wouldn't know that it's broken loose? Then wouldn't you suppose that it wouldn't take long for it to completely strip out and be dis-functional at low speeds also?

I have no idea what kind of water pressure the impeller is capable of alone at WOT.....like a test you would run at a repair shop where the engine was submerged in a tank and a test prop was installed for engine loading.

Nor do I have any idea what the ram water pressure alone would be at say 35 mph for any given engine/setup.

Would be interested if you happened to have a general idea. Been curious about that for a long time.

Course I am at a total loss as to what it would be other than what you guys said.

Thanks,

Mark

 

[Silvertip]

Re: No telltale at WOT

If the impeller is free wheeling so to speak, I suspect ram water pressure would probably be severely restricted since the impeller is acting more like a dam. Keen observation Red. A head gasket leak could probably cause the same effect. A quick compression check would show that to be the case (or not).

 

[throwback2332]

Re: No telltale at WOT

The impeller has a key that holds it to the shaft. The impeller is placed in the housing in a half s-shape so the tight friction acts as a vacuum. If it was spinning on the shaft it would not pump at idle and pump at wot ( kinda like a torque convertor). Next what you described is maybe the engine height. When you get on plane you trim up correct (yes) well when you do you pull the lower unit closer to the surface causing a drop in water pressure. Next time your out try this, when it stop (peeing) trim down a bump or two, it should come back. Also need to check your engine height on the boat.

 

[gejandsons]

Re: No telltale at WOT

Had a friend who installed the little plastic strainers (at the water intake) backwards. I'm not sure if your can be removed,but if applicable in your situation, they will divert the water away from the intake causing cavitation & sucking of air @ WOT.

 

[TAF]

Re: No telltale at WOT

Wow!
I didn't expect to see so many replies. Two of my theories were suggested in the above replies. 1. slipping at WOT, 2. not enough water getting to the intake at WOT. Of couse you probably know the one I want to believe it is: 2.
I'm not sure when the w/p was last replace, but it works great at all but WOT. I'm not one to dig into something that seems to be working good, the old "can of worms", comes to mind.
I think I will try to lower the motor, but I'm not sure I know how. It is a manual tilt. Do I need to remove that bar that goes across the bracket and put it in a lower hole?

 

[dsinger]

Re: No telltale at WOT

Yes to lower the motor.

 

[throwback2332]

Re: No telltale at WOT

Sorry I thought you have automatic tilt and trim. Try either lowering your motor on the transom or pull the pin in the transom and lower the motor a hole at a time.

 

[Silvertip]

Re: No telltale at WOT

A tell tale that "appears" to be working fine may indeed not be. Only a water pressure gauge can determine that. It would also help to know if this problem is something new, or whether it has acted this way since you acquired the boat. If it began acting up just recently, then engine height is probably not the cause. If it has behaved this way since you acquired the boat, it may very well be an engine height issue. If the anti-ventilation plate on the motor is flush with the bottom of the boat, height should not be the issue. A short (15-inch) shaft motor on a boat that requires a long (20-inch)shaft could also be the issue.