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Trouble with 1976, 70hp Johnson motor.

Hello Lister's,
We have a 1976, 70hp Johnson outboard motor that has started causing problems. Last week it was running fine in the water and then it started losing power while on full throttle. After around 10 minutes it would only pull around 3800rpm on full throttle. Could some kind person please suggest where we should start checking for the fault?

TIA, Laurie.
 
The simple checks to do are a compression test first.-----If that is ok then check to see if there is spark at each cylinder that will jump a gap of 7/16" or more. -----If that is ok the fuel delivery systems need inspection.-----Post your result, without that information we can not help from afar.
 
In agreement with Racerone... and would add......

With engine NOT running BUT with the key in the ON/RUN position, find the TAN wire that is protruding from the cylinder head (Overheat Sensor). Ground that wire to the powerhead... this grounding action completes the warning horn circuit and the horn should sound. If the horn does not sound, find out why as that's the only warning you will receive should the engine start to overheat before it screeches to a seizing halt.
 
In case it matters, the model number of this engine is 70 EL 76D.

We Have tested the compressions which are: Top cylinder = 135psi
Centre cylinder = 130psi
Bottom cylinder = 130psi

We have tested the spark which appears ok on each plug wire and have replaced the plugs with new ones.

We have replaced the fuel with new fuel but have not run the engine with this yet.

We replaced the fuel hose and primer bulb last season.

The engine does not appear to have an overheat alarm but has a temperature guage and it appears to be pumping plenty of water to keep it cool while running.
 
There was an overheat buzzer in the control box in 1976.----Does primer bulb go hard when you squeeze it ??----Have you inspected the diaphragm in fuel pump ?--------Does any fuel leak out of carburetors when primer bulb is squeezed ??
 
In agreement with Racerone... and would add......

With engine NOT running BUT with the key in the ON/RUN position, find the TAN wire that is protruding from the cylinder head (Overheat Sensor). Ground that wire to the powerhead... this grounding action completes the warning horn circuit and the horn should sound. If the horn does not sound, find out why as that's the only warning you will receive should the engine start to overheat before it screeches to a seizing halt.

We had a go at checking the heat sensor circuit today. A tan wire comes off an external sensor unit but that appears to go straight to the temperature gauge which is working ok. However I tried grounding this wire but there was no sound. There is another tan coloured wire coming out of a box type structure near the head, maybe this is the one you referred to? but there was no bare wire to be able to attach a probe to. We then attempted to dismantle the control box and with one side panel off we could see a buzzer but could not access it adequately to check it out here. How do we dismantle the control box which has two halves ? With all the screws taken out we could not seem to be able to part the two sides easily.
 
No fuel is leaking from the carbs or the fuel line when the bulb is squeezed, however it was observed that oil was sitting in the external throat of two of the carbs. We did not know that one should disconnect the fuel hose and drain the carbs after flushing the motor so we had not previously done this. Also until now we have not had a problem with the motor and we have owned it for around 15 years.
 
The compressions seem ok and there appears to be plenty of spark.

We ran the motor with the muffs on today with new 96 octane fuel and it sounds ok so we will give it a run on a small lake and see if it runs better than before.
 
You should be able to slide the black rubber sleeve on the overheat ( tan wire ) to expose the connecter.---------Slide it back , turn key on , ground the exposed connecter to the block.----Horn should sound.
 
Here in New Zealand we have the choice of either 91 or 96 octane fuel. I have previously used 91 octane but have been advised that we should use 96 octane. I have refueled with 96 octane and given the motor a run in a lake but it is still lacking power.

We have purchased a repair manual for this motor but still can't fathom which octane rated fuel we should be using. Also I have been advised that the carburetors should be run out of fuel when the motor will not be used for a while but the manual advises against this as it will leave the pistons without lubrication. Can someone with more knowledge than us please advise.

Regards, Laurie.
 
There seems to be a lot of confusion about how a 2 stroke gets lubrication.-----The best advice I can give is to run the carburetors dry if you are not using the boat for a while,---------When one carburetor runs out of fuel then that cylinder is no longer doing any " work " , it is not producing heat either.-------So the oil that was coating all the parts to lubricate them will still be there and be there for quite a while.-----The oil simply does not disappear in an instant !!!-------Many folks will argue this because they are not well versed in how things actually work.
 
Simply remove the carburetor face plate and drain the carburetors.

(Running Engine Out Of Gas)
(J. Reeves)
If the engine has but one carburetor, it's unlikely that running the engine out of gas would do any harm. When the carburetor runs out of the fuel/oil mixture, the engine stops running.

However, a engine that has more than one carburetor should simply be shut off. The reasoning is that the top carburetor will run out of fuel first due to gravity and the engine will continue to run on because of the still existing fuel in the other lower carburetors.

This results in having at least one aluminum piston running up and down a steel cylinder wall with improper or no lubrication. Not a good idea!
 
This tan wire disappears into a component near the top of the motor. There is no rubber insulation outside the component that can be pulled back to expose the terminal. Would it be possible to test it at the horn terminals inside the remote control box?
 
I am getting close to having ago at stripping the carburetors so went to order the required parts from our local boat shop but they could not tell me what components are in the kit for a OMC Type 1 carburetor. Would it be best to just order the gaskets for the carb as it is possible the high speed jets may only be gummed up and need cleaning. Or is it better to replace components while the carbs are stripped down?
 
About 6 " ( 15 cm ) from the temperature switch in the head you should find a rubber sleeve. Slide if back to expose the knife switch to test the horn.----If that is not there then the motor has been altered from factory original !!!!!
 
You say that the spark "appears" to be okay... doesn't really do it. Does it jump the 7/16" gap as stated in a previous reply? Here's the location of the TAN heat sensor wire I spoke of.
3cyl_H-Sen_Loc.jpg
 
Laurie I would complete checking the overheating circuit before starting any repairs. At lease on my remote control to test the horn/buzzer there is a slot under the high rev. lever you can inset a key or thin strip of metal to complete the circuit and the horn should sound (at lease on mine it's a little newer 1987). And the way the guys are telling you to check the overheat switch is correct. This circuit is a ground circuit if you test the horn and it work and test the connector at the head and it dosen't it could be the temperature switch or a wire that has broken down. Anyhow check it all before you start just throwing parts and money at it.The warning horn test and the temperature shold be in your manual. I sorry I don't now how to copy and paste the pages from the manual page to here but if I believe the manual on the site is free for viewing. Good luck and let us know.
 
Hello and thanks to all who have responded and I will update you on my endeavors to sort the problem with my 70HP Johnson. Due to the valuable info forwarded by you all we have now been able to check the overheat horn and it is working fine. we took the cover off the thermostat and found it had disintegrated from corrosion so was not working at all. We ordered a new one but had to wait several days before we received it. The new one appears to be made out of a type of plastic so should not have a corrosion problem again.
We have purchased a repair manual that is also helping us understand the tasks at hand and we have borrowed a pressure gauge and have attached that in the fuel line from the petrol pump. We have run the engine in a barrel of water and it appears to be running very well at up to 2000 rpm. The fuel pressure from the pump at this rpm and idling is 4.5psi so we will take it for a run on a small lake tomorrow and see how it runs at full throttle. We will then report back to you all on our progress.

Regards, Laurie.
 
Hi folks, we have now been for a run with the pressure gauge attached in the fuel line from the pump. At 4000 rpm the pressure is 5 psi and at idle 4.5psi so we assume the fuel pump is operating OK. Is this right?

Regards, Laurie.
 
Hi folks, Today I have constructed a spark gap tester and report the spark on all three hi tension leads of my Johnson motor is indeed arcing across a 7/16 inch gap. We have OK compression, we have new spark plugs fitted,we have a new hose and bulb on the tank, we have new fuel and plenty of fuel pressure from the pump, we also have a new thermostat so it certainly looks like it is a carb issue.

Regards,
Laurie.
 
Hi folks, just to summarise on our 70hp 1976 Johnson.
We have checked the compressions.135,130 and 130.
We have fitted new spark plugs and the spark is arcing over 7/16 inch.
We have a new fuel line from the tank.
We have tested the overheat sensor and the horn sounds ok.
We have fitted a pressure gauge to the fuel pump and have 4.5 psi at idle and 5.0 psi at 4000 rpm.
We have fitted a new thermostat.
Today we took it for a ride in the sea. It started and idled well but as soon as we gave it full throttle it would only run until it seemed the carburetor bowls were empty. It would then idle away good. After a short time away it would go again but then cut out again to an idle. could it be a sticking needle valve that would cause this?

Regards, Laurie.
 
Good call kimcrwbr1 had a 4-wheeler this past year that the dirt dobbers had built a condo in the intake tube. Though I was going to get into a carb rebuilt, cleared the nest and all was well.

Laurie looking back you said the you changed the fuel lines what about the lines under the cowling, if you rebuild the carbs look for a fine black material as the lines from the fuel pump to the carbs may be coming apart on the inside on that 1976. Also when setting your float don't use the seat to adjust the it aginst you will do damage to the needle.

Good luck
 
What is in the body of a carburetor behind the soft plugs? I have stripped down two of the carbs and found no problem with the jets in either and have one to do. Maybe the other carb jets will also be clean and we may still be looking for a different problem. In a way I hope the high speed jet is gummed up there. Here in New Zealand we do not have ethanol in the fuel so may not have the same problem of gumming as you have in the USA.



Regards,
Laurie.
 
The Seloc Johnson/Evinrude repair manual has a reference to a check valve in the cylinder drain in the Fuel System section of the Manual. It is hard to see from the photos, so where do we find the cylinder drain?

Regards, Laurie.
 
If you have a portable tank, pull the fuel sender/connector and cut off the screen at the end of the tube. Doesn't take much to restrict flow, and it will be most apparent at high RPM ... :cool:
 
Hi again folks,
We have now stripped the carbs and everything is very clean. There is no sign of any debris or varnish in the orifaces which we did remove and cleaned all with carb cleaner anyway. As all parts came in the kits we purchased, we replaced the float, Neadle valve and seat in each carb and fitted them back on the motor. As it started and ran ok we took it for a run on a small lake where it performed ok. We then fitted the airbox and it was observed that the rubber fuel hoses could be being squashed by the box and obstructing the fuel supply. We shortened the hoses a tad and refitted them and then fitted the airbox. We took it for a run in the sea and it performed well until it was time to return to shore. The motor started ok and ran for about 5 minutes at good RPM but then ran out of power and would only run just above idle speed. Any more throttle and it would stall. After reading our previous threads and the answers to them, does anyone else have other ideas to help as I think we have about exhausted all suggestions gratefully received to date.

Regards,
Laurie Jorgensen
 
Mrs. Laurie have you checked the fuel pump, screen in the pump, diaphram in the pump, and the fuel hoses from the fuel pump to the carbs and don't forget the pulse hose from the motor to the fuel pump. and leaks or just cracks may give you problems. I've had several older outboards that the hose liners would come apart and you get small (very small) pieces of black hose in the fuel system stopping up a new carb rebuild. You may want to check fuel pressure while running. Hope this helps.
 
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