1978 Mercury 402 will not crank fast enough to start??? Please help!

[1978 Mercury 402]

Hello all,

Engine will only crank 1/2 revolution and will not crank fast enough to start with each start attempt. I have seen similar threads on this issue however never seen the issue resolved. Now for the details:

Old (1978) motor that my father in law owned. Parked outdoors for 20+years and never ran. Best of my knowledge it was winterized and it ran fine when parked.

I have installed new exterior wiring harness, new starter, new plugs and plug wires (I do have good spark from both plugs), new fuel tank with new fuel 50:1. Fresh oil in lower unit with new impeller.

Engine rotates freely with spark plugs out and starter engages properly etc. When I install plugs there seems to be so much compression that the starter cannot rotate the motor fast enough to get a start. Yesterday I tried wiring 3 batteries in parallel in order to maintain power throughout the start. This was also unsuccessful.

I was going to try and replace the heavy gauge wires going to the starter next, however, I don't believe this will help either. The core problem is that then engine is very difficult to rotate when the plugs are installed?

Any help would be greatly appreciated!!

Thank you,
Chad

 

[Scott Danforth]

you should be able to pull start the motor.

clean all the connections to shiny metal - they need to be clean enough your kids could eat off of them.

if any of your cables even look a bit green, replace them.

 

[1978 Mercury 402]

Thank you so much for your response. That is my point it is also too hard to crank with a pull rope...I nearly pull the boat off the trailer....

 

[Mercurylips]

Just a thought to eliminate a possible problem. Take the lower unit off and see what it does about turn over then. Just try the pull start without the key in the on position.

 

[1978 Mercury 402]

Hi and thank you for your response. I just had the lower unit off to replace the impeller and it is in good condition and rotates freely. I am really stumped, just seems like there is so much compression in the cylinders that is unable to escape that neither elbow grease or the electric starter can overcome it enough to rotate the engine fast enough to induce a start...

 

[jimmbo]

You say it sat 20+ yrs. When you had the lower unit off, did you look up/into the midsection? Perhaps some rodents built a nest and plugged up the exhaust tube

 

[1978 Mercury 402]

Hello and thank you for your response. Yes, it sat un-used for 20+years. When I had the lower unit off I saw no signs of the exhaust being restricted. I have tried installing just one plug at a time and got the motor to fire once on each cylinder (with the other plug removed) and a puff of blue smoke came out of the exhaust each time.

 

[GA_Boater]

Welcome aboard.

Winterized 20 years ago, is 20 winters ago and doesn't mean much today 2 decades down the road.

I bet your rings/cylinder walls are dry as a bone or possibly rusted. Squirt some 2 cycle oil in the plug holes, let it sit a while and then start rotating the flywheel to lube rings and walls. See if that makes a difference and if it does, do a compression test.

 

[1978 Mercury 402]

Hi there, thanks for your response! The engine rotates nicely with the plugs removed and the electric starter will crank it quickly all day long. I don't have much knowledge on the inside of the engine but one of my thoughts was that the exhaust must be plugged?? Does this engine have exhaust valves that could be sticky? But I did get a puff of smoke when attempting to start with one plug installed?? I have put oil and fogging spray and fuel etc. into the cylinders and keep trying however as I have mentioned above; with both plugs installed it is VERY difficult to rotate which makes me believe that the compression must still be very good??

I REALLY do appreciate everyone's ideas and input....

 

[racerone]

There are no exhaust valves on this motor.-------Test the starter , cables and battery !!!

 

[1978 Mercury 402]

Okay thanks! Where does exhaust escape? New starter, new exterior wiring harness, 3 batteries wired in parallel that do charge up to 100% according to my charger?

The large wires from the solenoid to the starter and the ground were very weathered and the insulation was green so I wrapped them good with black tape. Could it be that those wires are corroded internally and unable to ground properly or carry the required current? If so, I will replace them. I still wonder how that little starter can overcome the force of rotating the fly wheel when I cannot turn it a full revolution let alone several revolutions by hand cranking it?

Thanks again, I will replace those large wires however everyone please keep the ideas coming; I really appreciate you all responding!

Thanks,
Chad

 

[roscoe]

Yes, the battery and starter cables can corrode internally, to the point that they are junk.

Do any of the cables get hot when cranking the motor?


Could also be starter itself.

Try using jumper cable from battery to starter directly.

 

[Scott Danforth]

The large wires from the solenoid to the starter and the ground were very weathered and the insulation was green so I wrapped them good with black tape. Could it be that those wires are corroded internally and unable to ground properly or carry the required current? If so, I will replace them.

Replace them. If the motor is in top top shape, a set of jumper cables is all you need to verify

 

[merc850]

Two things that can make it not start are excessive carbon build-up in the cylinders, (this raises the compression) and the wrong plugs; that motor uses surface gap plugs L77V and regular plugs will stick into the cylinder(s) too far and the top(s) of the piston(s) will hit them. I've seen this on a motor I just bought.

 

[1978 Mercury 402]

Thank you!

When I get time again I will check the plugs...I'm fairly sure that I have the correct ones. I suppose if my plugs were too long then the engine would just seize at TDC? This engine will rotate entirely however it is just very hard to overcome using the electric starter or pull rope.

I will also try jumper cables directly to the starter to eliminate possible corroded wiring from the solenoid to the starter.

I am curious about the excessive carbon build-up in the cylinders; if this is the culprit what is the fix?

Thanks again!
Chad

 

[racerone]

????----It says it " ran fine " when parked , so the problem is not excessive carbon in the cylinders !!----

 

[jimmbo]

That is a relatively small engine, 33.3 cu in, it shouldn?t be too hard to rope start it(unless the place you wind the rope is a small diameter). If the cables and connections are not in good shape/condition the starter motor can?t get enough current

It is a two stroke so the induction and exhaust is handled by ports in the cylinder walls.