25 hp johnstone air lok in water pump

[chopz]

my 25 hp has had a new pump installed and the slightest bit of cavatation stops the water flow and it wont self bleed (water pump)-i have to stop the motor and wait awhile then its ok till the next time --its got to the stage that i cant trust the engine anymore always looking at the tell tail and not enjoying myself anymore any advise

 

[jimmbo]

The pump doesn't have to bleed the air out, it is self priming anytime the lower unit is under water. If the engine is mounted too high, once on plane it can be staved of water. Older 25s had the water intake on the front of the exhaust snout, with and additional one above the antiventilation plate. The old OMCs really needed to be mounted with antiventilation plate about 1 1/4 inches bolow the bottom of the keel, not flush. Newer, since mid 80s, the 25 had the water inlets on the side of the gearcase, about 1/2 way between the AV plate and the propshaft centerline. Who installed the pump? If it has been run starved of water, it is time to take the unit apart and replace the impeller again. When that is being done a thorough inspection of the pump housing, liner, grommets, and base should be done.

 

[GA_Boater]

Welcome to iboats, chopz.

This is one time a picture is worth 1000 words. As jimmbo said, the motor must be mounted too high because the water intakes should be submerged on plane and while tuirning.

You don't have a short shaft motor on a long shaft transom, do you?

 

[chopz]

not according to google all the measurements are ok for a short shaft i did lower the transome 60 mm which helped somewhat as it turned the old waterpump melted itself due to lack of water --it was my fault tho -i never realised the motor was out of the water and sucking air -lucky i checked regular and saved it -the intake is on the side of the gear box -i also wonder if the leg is maybe off another model -the id plate is missing of the motor so i dont know what year it is -most annoying as i got the wrong pump the first time round -i installed the pump myself and checked all the leg housing and surrounding castings all ok- i just dont trust it any more i spend all my time checking the tell tail and getting a sore kneck lol--i will post some piks of the unit mounted on the boat -if i can figure out how to post here im sure that piks would tell you guys a lot more --thanks heaps for your time

 

[chopz]

piks for you hope this helps -cheers chopz

 

[chopz]

the last pik-if you need more just yell

 

[jimmbo]

Looking at your pictures a few concerns; the anti ventilation plate looks like it has some modification done to it, the water intake above the anti ventilation plate looks like it has had a couple of extra holes drilled as well as being incorrectly installed, the cylinder head is not the original. Need a pic taken from the side at bottom/keel level. It is hard to ascertain if the engine is deep enough from the angle of the pics. The motor is a short shaft, it has two water inlets, the one with the messed up cover and the one directly behind the prop on the exhaust snout. There is a plug on the exhaust snout, if that plug falls out, water will not get to the pump once the boat is moving.

The plug I am referring to is # 42 on the diagram

 

[chopz]

i took the head off the engine to inspect the bores after the first over heat and they were good so i replaced the gasket and painted the head-the water inlet screws were locked in real tight and i was unable to extract them for inspection there fore i had to drill and tap new ones and replace the plate (homemade) everything was fine -no blockage- i have just checked the plug you speak of and its ok--the last time i took the old boat out there was a small log in the river that i ran over the motor kicked up and it wouldnt pump up water again untill i stopped it and restarted it again

 

[AlTn]

as stated...#42 ..plug and #43..screen...are in the main passageway for water intake...the cover on the side... #19 is a water bypass cover...I started using a small amount of sealant around the underside edge of the wear plate..#17..and the base of the pump housing #13...to help seal them from exhaust gases and it seems to help in this situation...a little skin of grease around the od tip of the water pipe.. #9.. and io of the grommet.. #10.. if slides into helps to align and seat the pipe into grommet correctly

 

[chopz]

interesting indeed -food for thought thanks

 

[chopz]

is the plug and screen servicable -should i remove it for cleaning?-i thought the plate was the main water intake--i have so much to learn

 

[racerone]

Install the proper plate on the side of the lower unit, get rid of that modified plate !!---Repair the cavitation plate too.-----The screen keeps junk on the outside so is technically easy to clean.----can be replaced easily as well.