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Impeller issues merc 7.5 (narrowed down)

b521

Regular Contributor
Replaced Impeller with the correct size,I only get water flow at Idle. The old impeller almost did the same thing, but it got worse as I was starting the motor as I was fixing other neglected things. Someone replaced this impeller years ago and motor was then moth balled. That person don’t remember too much about his process. So there is the possibility that he left some parts out. (maybe not as a non related item to use as an example, as follows , the drawing which is attached shows an o ring for the tip of the drive shaft, this one did not have one and the merc mechanic confirmed my unit does not have one. ( so as usual I guess the parts diagram can vary in accuracy for a lot of reasons. )
MY QUESTION WILL BE, IS THERE ANYTHING ELSE THAT GOES IN THIS WATER PUMPS PLASTIC GUIDE TUBE.
I HAVE TAKEN IT APART TWICE THE IMPELLER IS GOOD AND IS PUMPING ENOUGH WATER TO KEEP IT FROM BURNING UP. I HAVE DRIVEN HIGH PRESSURE AIR THROUGH FLUSH HOLE AND UP WATER PICK UP TUBE INSIDE THE UNIT AND BACK DOWN THROUGH THE PEE HOSE AND REVERSED THIS PROCESS AND CLOSED OFF ONE OPENING AT A TIME TO FORCE MORE CONCENTRATED FLOW THROUGH ISOLATED AREAS, COVERED AREAS WITH A WIRE STRAINER, FOUND NO RESTRICTIONS, THEN RAN MOTOR WITH A HOSE HOOK UP THROUGH GARDEN HOSE WHILE MOTOR WAS STILL IN A WATER BARREL Checked for a exhaust cavitation from exhaust interfering with water pick up intake.
Etc etc . same problem still remains. Also The impeller is not rotating on a worn key way.
Here is what my problem may be, so if someone can shed some light on this I would appreciate it. This comes from me watching you tube videos on installing impellers. This one fellow had a damaged water pick up guide tube that was bad and he was going to reuse it stating that it is really not needed as some people leave them out all together when they install impellers. Now I did not go that far but it did cause me the mind set that there does not have to be a tight fit from the plastic pick up tube to the copper tubing up to the power head. There is some type of a seal in there on the attached drawing, mine does not seem to have one and my guide tube fits down in the water pump housing where it seems it is suppose to by verifying the discoloration of the exposed part of the tube. I do not see in my guide tube opening anything like part number 38. The drawing shows 2 part number 38”s
So do I need these parts to stop a pressure loss or is it something my motor does not have, or the last guy left it out. What does the part look like? I am thinking that if it is just an o ring and not some type of a seal with a catch lip on it . then it would just be forced up the copper tube and blow out of the way and be useless. Or is it just a general type o ring that I can take off the shelf at a hardware store.
Thx
SORRY CAN'T PASTE IN THE ENGINE DIAGRAM. BUT THIS IS A 1973 7.5 HP SER NO 3731914






 
Parts of the old impeller is probably stuck in water passages in the power head. The best way to remove them is to blast compressed air and water mixture backwards through the entire system, with the LU off.

Jeff

PS: I bucket test those LUs before going through the hassle of assembling them. Here's what the test set up looks like
 

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thx Jeff
That is an awsome bucket test. You should patent it and go on the TV show Shark Tank. Try to get the Blond to sponser you, Laurie, she has great legs and is a looker. I already Did exactly what you suggested, as I stated in the first post, I know the post was long and a lil confusing, But I was trying to save you hard working contributing coordinators the time and trouble to have to type a lot like starting me off on this problem from scratch 1. I just pulled lower unit again and am in the process of trying to remove the power head to get at the top of the copper water pick up tube where I guess it enters the engine block. I was blowing some H.P. (100 psi) air up through tube and I pinched the pee tube and it lets a lot of air come out from that area. (the top of copper tube) maybe a lot should come out there, don't know, u tell me
) ( I hooked up my smoke test fixture, which is a piece of soft vacuum hose and me smoking a ciger and blowing the smoke thru the tube. (Somehow I can't see me getting any shark tank investors willing to sponser that invention, again I would pick Laurie, as maybe she could identify with it. )on that invention I think I ruled out restrictions and have a leak , somewhere. I am working on it now and will keep you posted. I am going to search for cliff notes on how to get that power head off.
 
ON severe overheat there is a plastic washer at top of the water tube it melts and blocks the water going to the powerhead. The pump is very simple and i
I never feel a need for testing them before assembly.
 
No there may not be an all the time need to test a pump before you reassemble, but there are circumstances , as In my case where a problem that you are looking for may mimic two different scenerio's. That and is happening to me now ..so when I get a couple steps completed I will run that pump on an air drill and if it pumps well at high speed then I can rule it out of my thinking. Do not take anything out of context, Your perspective is nothing but a help, and so is Jeff's bucket test when needed.
ANY HOW HERE IS WHERE I AM AT.
POWER HEAD IS OFF Some air leakage from top of copper tube. Most air is coming from out of the exhaust ports from cylinders.
BUT I want to take that tube out and check for the melted seal and I can't get it out, which may be because its is melted etc. But I have not got serious yet, I have to stop for awhile and then I will mount it on a work mate, grab a stool and get serious but carefull
My question #1 is .DOES THAT COPPER TUBE JUST PULL OUT?? or is it flared ,or a set screw, did not see a set screw.
#2 If I get to the point of reassembly can I put the power head back on without using a new gasket, Like temporarily or leave it as is for now.(I know it is best to replace for now, but I need to cheat on this one. I want to temp mothball this motor, and it will be an emergency spare for the rest of the year, next year it will go into service and i will replace it then.
 
ON severe overheat there is a plastic washer at top of the water tube it melts and blocks the water going to the powerhead. The pump is very simple and i
I never feel a need for testing them before assembly.
I got the tube out, Can't see any plastic washer, nor ny residue of one being there. Don't know if I am suppose to have one as it is not on the engine diagrem, 1973 merc 7.5 3731914.
QUESTION IS DO SOME OF THESE MODELS NOT HAVE THAT NYLON WASHER ?
 
racerone says the washer is shown for your motor.

And you are correct. Thank you. I was looking in the drive shaft housing diagram. It stops at the tube. So I took your advice looked further and found it in the bottom cowl adapter plate diagram. The nylon bushing seems to be ok if I look from the bottom, but there may be something going on in there with the o ring which may be a problem in the upper part of the nylon bushing. How do I get that adapter plate off the bottom of the motor. It came off with the powerhead when I pulled it. I disconnected a lot of stuff but failed to completely separate the adapter plate. Its dark and raining out so I can’t get at it till the morning. I am assuming that the nylon bushing and the o ring can be pushed down from the top after I separate it from power head. What else do I have to unbolt to separate it. I removed a lot of stuff, some of which was not necessary, but I did not yet stumble upon something toward the front that should be the final thing to separate it. (it’s hanging there like a loose tooth.
I did find another restriction, and it may be the remnants of some of that nylon seal. Now keep in mind I have been blowing air and water every which way but loose in this thing, A rubber tipped blow gun up the copper water tube and I was still not satisfied with the pressure from the top tube, when I got the copper tube out I found a wad of like dry clay that fell down the tube as it must have got stuck in the top and dislodged when I got it separated, poked it out with a wire and could crumble it like dried earth clay it was dry, I do not know how anything could be dry in that motor being I have ran so much water through it. AND SO PLASTIC NYLON DON’T SUCK UP WATER SO IT COULD BE RESIDUE FROM A PARTIALLY MELTED BUSHING.
I WILL DO MY BEST TOMORROW BUT IF ANYBODY CAN TELL ME HOW TO GET AT THAT NYLON BUSHING IN CASE Raycerone don’t get back on in time, pls chime in.
 
nice work Kim.
how do i get the spacer plate seperateb from the bottom of the motor, (powerdhad/ motor is sitting on a bench).
 
The adapter plate is off and the plastic bushing is not melted.
I found more of that dry clay like material in the water jackets. It appears to be the oxidation of the aluminum and probably a lot more of it due to the engine being sitting for years. I am going to go try and get some adapter plate gaskets and inspection plate cover gaskets for up by the cylinders. Did not pull them yet,(outatime)
QUESTION IS,
DOES ANYONE WANT TO MAKE A SUGGESTION ON WHAT ALL ELSE i CAN TAKE APART TO DO SOME PICKING AND SCRATCHING OUT OF THAT JUNK.
 
It sounds to me like you need to put in a new set of gaskets, grommets and bushings. Just get a gasket set and pull the powerhead and put a new exhaust cover gasket, water jacket cover. The hard part is pulling all the bolts without breaking one or five. The brown stuff that concerns me also blowing 100 psi air pressure almost for sure did more harm than good. Take some pics of the shift and throttle linkage and just start taking it apart so you can properly clean the cooling circuits sounds like it sucked up some mud your gonna need to ice pick it out. I use permatex aviation grade gasket maker the brown stuff you brush on. Just brush a very light coat on both sides of the gaskets and quickly snug them down. Just take your time use lots of chemicals and heat to remove the bolts I prefer power steering fluid as a liquid wrench it dont dry out and dissolves corrosion real well. You can inspect the pistons and rings with the exhaust cover off. Pick the carbon buildup out and carefully push on the rings with a phillips screwdriver they should spring righ back in place. If the cylinders are scored it is almost allways on the exhaust side.

Hey Kim how are you,
I am curious as to what damage you are talking about could occur by blowing compressed air through the water passages. (I understand there is exhaust gas cooling on a marine engine so theoretically there is a way to get from water passage to exhaust relief, but it is indirect and well open so it would not back pressure the exhaust side of a 2 cycle, (and then if it could which i don't know how it could ,,follow its way back through a 2 cycle internal combustion motor. to crankcase, reeds and lastly the carb.
now that is my honest opinion, so please enlighten me cause I am no marine mechanic. (I am just the dangerous auto, motorcycle ,outdoor power equipment, inboard boat mechanic). so I probably am quite dangerous in the outboard marine tech world.
So your perspective is always appreciated, give me the lecture (compressed air, 1 oh 1)
 
Oh forgot to ask I bought new inspection cover gaskets and adapter plate (bottom and top ) gaskets.
Do I need any permatex on these, I usually use the brush on stuff.
 
AND AGAIN i WANT TO ASK, IS THERE ANYWHERE ELSE I NEED TO BLOW OUT AND OR SCRAPE WATER PASSABES WHILE DIS-ASSEMBLED, CAUSE i HAVE TO START REASSEMBLY FAST .
 
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