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Tony Outboard

alan

Contributing Member
Tony the serial number of the

Tony the serial number of the 70hp Mercury is 8045578
 
" Alan,

That should be a 19


" Alan,

That should be a 1983 engine.

The ignition system on these engines can be very tricky. Do you have a multimeter of some type?

You should try to determine if a reed valve is broken first, and I would take a look at the fuel pump. The diaphragms on these pumps are very susceptible to damage from crankcase pressure.

If those things seem okay, and you are sure that there is no problem inside one or more carbs, you should thoroughly test the ignition.

You said that the engine loses power. Does this happen at high rpms, or would you say that the trouble starts around 1200-1500 rpms?

Topny "
 
Tony
The engine just has no


Tony
The engine just has no power at all will rev freely under no load starts first time everytime but as soon as you put it into forward it gets to about 1500 2000 rpm and thats it every now and again you can feel it pickup then die off but only for an instant
it has heeaps of spark and fuel all electics have been swapped over and the problem is still there
 
"Alan,

The stator, which su


"Alan,

The stator, which supplies electricity for the ignition, has two windings. One winding is for low rpms, and the other is for high rpms. If the high speed winding is not working properly, it can be tough to diagnose. The spark will appear to be good, especially at cranking speed, but the engine will not rev above 1500-2000 rpms.

I guess you have already checked the wires to the switchbox, but if you haven't, you should make sure the wires are connected properly. The terminals on the switchbox are labeled with the color of the wire that should be attatched to them.

It could also be a reed valve, as you mentioned. You can perform the test I told you about, but you can also make a more positive observation if you want to. If you remove the carbs, you will be able to see the collars that fit around the crankshaft. These collars hold the reed valves. It is possible to use a probe, such as a length of stiff wire, which you can insert through the intake flanges, and feel if the individual reeds are broken or not. This requires some patience and time, but it can be done. A bright flashlight will help. You have to bend the wire differently for each reed, so that you can reach in and press on the reed to see if it is there.

It helps to look at these valves on a parts diagrahm, so you can see exactly how they are positioned.

One other thing I would mention is to turn the flywheel in a clockwise direction by hand and see if you notice any difference in the resistance that you feel as each cylinder develops compression. If you feel no resistance for one or more cylinders, you probably need rings and or a piston. Sometimes, even though the compression test seems good, the rings or pistons are worn. This test is useful when that is the case.

There are a number of conditions that can cause this to happen. If there is a lot of carbon built up, the rings can be frozen and worn, but the carbon causes the compression to read high.

If the rings and or pistons are just worn a little bit, they might hold compression, but will not produce enough vacuum.

Tony"
 
" Tony
The rings etc where re


" Tony
The rings etc where replaced about 50 hrs ago so they should be ok The outboard Mechanic has replace the stato, trigger and switch box over the summer and they have also been tested and are ok so ill take your advise i think and have a look at the reed valve does it matter which way the reed block holder is fitted when reassembling? "
 
" Alan,

I don't think y


" Alan,

I don't think you can assemble those incorrectly. The crank would not set properly.

Tony "
 
" tony
Well i decided to pull


" tony
Well i decided to pull the whole engine down to the point i removed the crank, pistons etc the reed are fine and so is everthing else the only thing that was out of place was that 2 pistons had 3 rings and one has 2 rings which will not really upset anything except the balance a bit( all rings etc are fine compression is fine reeds are fine so this is crazy
I have reassembled the engine and it fired first pop set the timming and mixture etc and will test run tomorrow it sounds quiet pirky but the load factor will be the test
Im nearly at the stage that im ready to admit defeat which is a traite that does not sit with me very well
Ill let you know how it all goes
ps i have checked all the wires and they are clour coded to the correct terminal
by the way thanks for all the help so far its been great "
 
" Alan,

If your throttle pl


" Alan,

If your throttle plates are opening, and the timing arm is moving properly, I would bet that there is an ignition problem. I have worked on engines like this one, where everything checked out okay. I have all the test equipment that you are supposed to have and could not find the problem. On one, the stator turned out to be defective; on another, the trigger coil was bad. Both had perfect resistance, and output was fine at idle. Both ran just like your engine. I am reluctant to test ouput at wide open throttle on boats that are in the water as these were. I think the problem might have shown up if I could have done that, but I can't picture myself on the transom with a $300 tester telling the owner to open the throttle all the way.

If I can think of anything else I will post it later

Tony "
 
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