1968 6HP Johnson cooling problem

[Swell]

I'm new to the boating scene - I'm technically minded but struggling to work out what's going on with my motor (part no. CDL-25C). Seems to run ok - not totally consistent with starting, but always runs and pretty smoothly. It has a cooling problem which I've partly fixed. There was a good stream of water coming from the small hole on the port side of the lower unit, but nothing from the exhaust on the rear and almost level with that hole. I removed the gear casing and checked it all out. The impeller looks ok, but I wouldn't know what to look for in checking the impeller. Any tips? I did notice that the grommit at the top of the water tube was part way down the tube, and when I checked how it should fit noticed that the hole through which the cooling water was coming out of looked like it should really be a locator for one of the lugs on the grommit. Is this right? I also noticed a cut away where the other lug on the grommit should locate, but it's quite a big cutaway so not sure if this is normal of if a bit has broken off? Suspect the water tube was not properly sealed and was pumping water into the lower unit and out of that hole. Put it back together with water tube grommit in place. Now, no water coming through side hole - can see the rubber lug - and now there's a spray out of exhaust hole, but only at low revs. Any idea why this might be? As soon as revs are increased, water stream dies down which should be the opposite. Could this be an old impeller? Or do I still have a badly connected water tube? Or something else. There's no thermostat & water does seem to flow into the housing. Head gets hot but not sure what's normal?

Any help would be very much appreciated as it's driving me mad!

Many thanks,

Roger

 

[F_R]

Re: 1968 6HP Johnson cooling problem

Well obviously the water tube grommet was #1 problem. As for #2 problem, that might not be so easy. Or is it? Are you running that thing on a boat in the lake, or what?

 

[Swell]

Re: 1968 6HP Johnson cooling problem

In a wheelie bin!

Just bought the motor & don't want to take it out until I've sorted out the cooling issue.

Just found this actually which is quite interesting, and a very informative article:

http://www.sschapterpsa.com/ramblings/OMC_6hp.htm

Near the bottom is:

"Observation: One reader from Australia has found through experience that running these motors in a stationary test tank may not always produce a great flow of water through the cooling system -- they require the forward motion of the boat plus the prop rotation to help send water to the pump. However in my test tank I have no problem in this matter even at an idle if the motor is in forward gear. "

So could be that a moving boat helps the cooling, though suspect my problem is more than just this.

 

[F_R]

Re: 1968 6HP Johnson cooling problem

Biggest problem is not having it deep enough in the water. Make sure the water is several inches above the water pump. On a boat is a different deal because the prop wash force-feeds the pump. In a tub, it is full of air and turbulence.

There is a bleed hole in the water pump housing that might be plugged. It is a small hole leading from the water tube area to the drive shaft area. Its purpose is to keep a water seal around the drive shaft hole. Otherwise, it may suck air.

And of course, there is the possibility of a worn pump housing, an improper seal between the pump plate and gearcase, and a worn impeller. You should have caught those things while you were in there.

 

[1kruzer1]

Re: 1968 6HP Johnson cooling problem

"IObservation: One reader from Australia has found through experience that running these motors in a stationary test tank may not always produce a great flow of water through the cooling system -- they require the forward motion of the boat plus the prop rotation to help send water to the pump. However in my test tank I have no problem in this matter even at an idle if the motor is in forward gear. "

I didn't read the article, but in my opinion the statement above is bull. As long as the water in the test barrel is higher than the housing of the waterpump, there should be very good cooling system flow. The motor certainly does NOT depend on movement of the boat for proper flow. I run my 6HP in reverse all the time, and I'd be in big trouble if this theory were correct. Having coolant flow drop off as RPM increases is something I've seen before with old impellers. Don't go by looks, if you don't know how old it is replace it. You are correct in what you saw and corrected with the grommet. I suggest pulling it apart one more time and replace at least the impeller, and if the water pump housing doesn't look perfect and un-scored replace that too. Also, this motor does have a thermostat. The thermostat housing is the triangular shaped piece above and to one side of the cylinder head, with a thermostat inside. A bad or stuck thermostat won't affect the volume of water coming out the exhaust relief, so you are correct to address that issue first. Good luck!

 

[Swell]

Re: 1968 6HP Johnson cooling problem

I've had the thermostat housing cover off and there's no thermostat in there. I did run the motor briefly with the cover off to check that water was at least flowing and it was, but not sure if its enough - hard to tell really. I was considering retro fitting a pee tube as in the article I posted above. Worth it? I know a thermostat should be fitted, but is it ok to run without while testing?

Also, I notice from the diagram in the article nearly half way down titled Water passage thru this motor that water could also be exiting through the lower exhaust outlet. Could this be the case?

I understand that the best thing is to fit a new impeller which I will do!

I did clean the whole housing while in there, but didn't check the inside surface where the impeller spins. Will do though, and I'm pretty sure the bleed hole is clear.

Just to add, I made sure there was enough depth of water in the wheelie bin. I read a posting from someone who thought he had a cooling problem but didn't at all - just not enough water in the test tank!

Another note - there was no pin near the top of the driveshaft as in the drawings & descriptions. Impeller pin was there, but the pin supposedly keeping the driveshaft seal to the crank case wasn't there. Didn't note if there was a hole in the driveshaft where it should have been. Is this an issue? Could it be that it's not needed? Suspect it should be there.

Thanks for all the replies by the way! Much appreciated.

 

[Swell]

Re: 1968 6HP Johnson cooling problem

Here's an update on this one.

Bought a new impeller - old one's limp and distorted so pretty sure that's the cause of the lack of water flow at higher revs.

The big worry though is the driveshaft. There's no pin through the upper part of the shaft, the one which compresses the various components of the crankcase seal. There's not even a hole for the pin which suggests to me it might be a driveshaft from an earlier model than 1968 where my understanding is that they didn't have this and the whole lower unit had to be removed to access the impeller. I believe that the addition of the pin & slotted ledge is a feature added to later models. I can see the slot etc. at the top of the leg, so it does need the pin.

I'm happy to drill the driveshaft but need to know the approximate hole diameter and how far down the shaft from the top it needs to be located. Can anyone help me with this? If the drawings on the BRP website are to scale I can work it out pretty accurately from those, but only if they are to scale! An actual measurement would be best.

Just to add - I've worked out from 2 separate prints - one from BRP site and one from the service manual - that the distance between the impeller pin hole and top pin hole should be about 371mm (average of 369.5 and 372.2). Can anyone confirm if this is anywhere near?!

Many thanks,

Roger

 

[Vic.S]

Re: 1968 6HP Johnson cooling problem

The big worry though is the driveshaft. There's no pin through the upper part of the shaft, the one which compresses the various components of the crankcase seal.

I have seen your post on another forum too ..... traitor!

I would not rely on scaling off the parts diagrams.

 

[Swell]

Re: 1968 6HP Johnson cooling problem

Fair comment! I'm not sure how critical the position is thinking about it, as the pin is only compressing a spring which then holds everything in place, so a mm or 2 either way shouldn't make much difference. Or am I wrong in this?!

 

[raczekp1]

Re: 1968 6HP Johnson cooling problem

hi.i think the cooling problem is becouse of water pump housing.its an old construction pomp.
my advice is to buy water pump conwersion kit for models made in 1980 or later.its cost of around 40$.omc number is omc 0763758
instal this and for sure problem will gone

 

[Swell]

Re: 1968 6HP Johnson cooling problem

All sorted. Cooling's fine - just fitted new impeller and further cleaning of waterpump housing, including clearing the bleed hole. Also used chemical metal to rebuild some of the water tube grommet mount in the leg - a portion had come away - so good seal now and loads of flow out of the exhaust.

Fitted the driveshaft pin (2.4mm stainless roll pin into a 2.5mm hole, 19mm long) 60mm from the end of the driveshaft. Went into the crankcase ok so have to assume it's doing what it should be doing.

Will try engine on the water tomorrow - can't wait!

Thanks for all the help.

Roger