1973 7.5 Mercury 75 Cooling Issues

[Grain Boat]

This motor has survived 25 years of abuse since being relegated to communal cabin use. Of course the impeller (water pump) disintegrated and the engine ran hot on more than a few occasions. The impeller was replaced by a one-man shop on my sister's dime. When I finally got to inspect the work, I ran it on the lake and it initially ran good but stumbled when hot and was obviously still not cooling properly. I'm suspecting broken bits of old impeller blocking the passages. My question is, where do I start? Should I try a little compressed air? Remove cooling jacket/exhaust cover? I read a lot about snapped bolts and such, so I'd like to avoid that. I think the motor is worth preserving for a few more years unless it becomes too costly. Any ideas?

 

[Laddies]

Re: 1973 7.5 Mercury 75 Cooling Issues

Air will not do anything, your best bet is to remove the powerhead soak the bolts before trying to remove them, there is a plastic grommet under the powerhead on the water tube that melts when the engine overheats

 

[Grain Boat]

Re: 1973 7.5 Mercury 75 Cooling Issues

Air will not do anything, your best bet is to remove the powerhead soak the bolts before trying to remove them, there is a plastic grommet under the powerhead on the water tube that melts when the engine overheats

Thanks Laddies,
I guess I'm willing to pull the power head if necessary. I've got a Clymers manual in hand. Is that the power head-to-exhaust plate gasket that you are referring to? Is there any possibility of clogged passages, or am I barking up the wrong tree? Any tips on bolt soaking would be greatly appreciated as well.

 

[Laddies]

Re: 1973 7.5 Mercury 75 Cooling Issues

Item 10 :: iShopMarine.com :: I use mercury powertune as a penetrate and remove the exhaust covers to check for debrie, if there is none there remove the water jacket cover and check there also.

 

[Grain Boat]

Re: 1973 7.5 Mercury 75 Cooling Issues

Thanks for the tips Laddies, but the link you sent is a later model diagram. I have a serial # 3729885 which is also pictured on iShopMarine. I can't seem to create a hyperlink, but here is the link:
:: iShopMarine.com :: Part Lists == evinrude parts, mercruiser engine part, mercruiser outdrive part, mercruiser part, mercruiser sterndrive part, mercury marine, mercury marine engine part, mercury marine outboard part, mercury marine paint, mercury

I couldn't find the nylon bearing/seal you referred to in these diagrams, but maybe I still have it on the engine. In any case,shall I attempt to remove the powerhead,or just remove the covers? I just want to avoid any unnecessary dis-assembly.

 

[Laddies]

Re: 1973 7.5 Mercury 75 Cooling Issues

I think there all the same, item#2. :: iShopMarine.com ::

 

[Grain Boat]

Re: 1973 7.5 Mercury 75 Cooling Issues

It's only been 8 months since my initial post, so I thought an update was in order. Cabin Fever has compelled me to get this outboard cooling properly. I was able to get the exhaust port cover off without much difficulty. No salt water here. I blew some compressed air through the small passages and they seem to be clear with the exception of some whitish corrosion dust. Ditto with the telltale line. As per Laddies suggestion, I'm going to attempt the power head removal armed with a really vague Clymer manual. My goal is to get at that pesky plastic bushing/seal on the water tube. It would appear that I can pull the entire assembly (recoil, coils, carb, and block as one piece. Correct so far? Also I'm looking at the throttle linkage and wondering, what is the best way to separate it from the advance assembly? Thanks, and stay tuned...

 

[Gibbles]

Re: 1973 7.5 Mercury 75 Cooling Issues

I noticed on my 1978 (if memory serves me) 9.8hp merc... the impeller is so small and thin.

I had burnt one up just trying to get the water flowing from the water tank and it was done...

I found I had to install it with dish soap to lube it as it first started up, but even then I can end up loosing it in minutes if water flow is interrupted.

Don't rule out the idea that the water pump needs some attention.

 

[racerone]

Re: 1973 7.5 Mercury 75 Cooling Issues

The powerhead is usually easy to take off.-Just 6 nuts on the bottom.-Fuel line.-Throttle is splined so just pulls out..-Wires from stop switch come off and water telltale hose comes off.

 

[Grain Boat]

Re: 1973 7.5 Mercury 75 Cooling Issues

The powerhead is usually easy to take off.-Just 6 nuts on the bottom.-Fuel line.-Throttle is splined so just pulls out..-Wires from stop switch come off and water telltale hose comes off.

Hi racerone, Got the powerhead off without too much difficulty. Thanks for the tips. I also have the new rubber seal and nylon washer, yet I'm stuck at removing the old rubber seal as it broke off below the recess around the water tube. Any tips for a quick extraction without doing any damage? Incidentally, there were obvious signs of overheating and blockage with the old seal and washer, so I'm hopeful replacement will get the water flowing again. Here's what remained of the old seal:

 

[racerone]

Re: 1973 7.5 Mercury 75 Cooling Issues

I have seen that washer melted more than once.----One motor had been to a shop 3 times for overheat issue and they could not find this fault.

 

[Grain Boat]

Re: 1973 7.5 Mercury 75 Cooling Issues

221980[/ATTACH]

I quoted myself. Turns out, aggressive use of a stout dental tool, WD-40, and a propane torch will do the job. The rubber is likely to be dry and hard. WD-40 is not flammable by itself. Soak area liberally, heat with torch for 45 seconds or so, and pick away with your dental tool. Bits and pieces will come free and then repeat as necessary. Eventually the seal will come out as one piece. Try not to damage the copper tube or the surrounding aluminum casting. If you do, don't sweat it because it will likely seal anyway.