3 cyl. outboard issue

[rd9952]

I have a mercury 1979 70hp 3cylinder outboard that will start and idle with or without the # 2 plug wire attached. We found what appears to be either or both a bad coil and wire on the # 2. Would that cause it to run on muffs ok but bog completly out in the water? Will they idle on 2 cylinders like this one does ?

 

[ajgraz]

Re: 3 cyl. outboard issue

Yes that motor will start and idle on muffs with one cylinder out, but will have little or no power when run on the water.

 

[rd9952]

Re: 3 cyl. outboard issue

Thanks. We may have finally found our problem. Will let everyone know the results.

 

[CharlieB]

Re: 3 cyl. outboard issue

Follow the easy directions in the CDI Ignition Guide to figure out the cause of the failure.

ISSUU - CDI Electronics Practical Outboard Ignition Troubleshooting by CDI Electronics

 

[rd9952]

Re: 3 cyl. outboard issue

Thanks for all the insight. I believe now the coil is good but the connection between it and the plug wire is bad. Getting a new wire today and will let you know.

 

[rd9952]

Re: 3 cyl. outboard issue

The coil was good but the plug wire was bad. We changed that out and it appears we may have a slight fuel issue. When running at low rpm it runs ok but at WOT it spits and sputters and dies out. I can take my hand and cover the upper carb and it stalls out very quickly but when I do that with the lower carb nothing happens. Could that be because the lower carb is not letting out as much or no fuel??? Could that be the cause of the stalling when trying to increase throttle under load in the water???

 

[danamul1977]

Re: 3 cyl. outboard issue

Do you have any leaking from the carbs, could it be a stuck float/needle or something suck in the carb jets. do you feel any less air sucking when you put your hand over the carbs? You can run with one carb getting to much or little fuel. Would be my thought but someone might know more.

 

[rd9952]

Re: 3 cyl. outboard issue

Very little suction on the lower carb. But when you remove your hand there is fuel that runs out the front of the carb. Could it be the reed valves sticking?? Also we have good suction on the top carb. And again it shuts the motor down when you cover the top carb. We are going to check the lower float setting this weekend again.

 

[danamul1977]

Re: 3 cyl. outboard issue

What I would think is happening is, you have a stuck float/needle, or could be something stuck in one of the jets (prob the main jet) in the bottom carb. So you should take it apart and check it out (clean it). I think the motor is running on the top cylinders and when you shut off its only air it shuts down. Unless you get a better suggestion that sounds like it

 

[CharlieB]

Re: 3 cyl. outboard issue

You need to check the carb linkage to ensure it is 'sync'd' You should look to see how all three carbs are linked so as to open and close together. With the motor off and the throttle at idle the roller should not be touching the throttle cam, all carbs should be fully closed. Loosen the adjustment screws at each carb so each can be opened a little without opening the others. With all the adjustment screws loosened all carbs are free to fully close. Now very carefully begin to gently snug each screw making SURE that the linkage bar and the carb you are tightening do NOT MOVE, repeat on all carbs, now go back and finish tightening those screws. Done.

Now repeat the idling tests you were describing before.

 

[rd9952]

Re: 3 cyl. outboard issue

We removed the carbs and rechecked the floats. We must have had an issue because after cleaning again and re setting the floats we were getting alot of fuel into the carbs but it seems it is not sucking it into the cylinders. You can feel the suction when you put your hand close to the top carb but nothing on the bottom. Could it be bad reeds?? If so how hard is those to change?? Thanks.

 

[CharlieB]

Re: 3 cyl. outboard issue

The reed blocks are in the intake manifold behind the carb mounting plate.

With the carbs off you should be able to look in and see the reeds, you may see an obvious broken reed but otherwise you will have to remove the plate and pull the reed block out and see if each reed is sealing. They are allow a very small gap, like two or three sheets of paper at most as crankcase pressure 'pushes' them closed. Ideally you would not see any gap.

Check spark and timing on each cyl. If timing on this cyl is later than normal this cyl is lagging and will not have the intake vacuum of the others.

With the motor off and trim set level. open the throttle and squeeze the primer bulb, do you see any fuel in the carb throat? If so the needle and seat are faulty.

 

[rd9952]

Re: 3 cyl. outboard issue

So the Timing could be off causing lower intake vacuum ?? Also this motor has two carbs...does the upper carb supply all the cylinders along with the lower carb ? Or do they supply to specific cylinders??? We have wondered about the damn timing for a while now. I will have it checked to make sure. Thanks.

 

[CharlieB]

Re: 3 cyl. outboard issue

3 cyl w/ twin carbs, the mounting plate has a open passage behind the carbs so all cyls draw off of all carbs.

The actual airflow pattern is the top cyl draws most all its air from the top carb, the center cyl draws mostly equallly from both carbs, and the bottom draws mostlly from the bottom carb. The path of least resistance thing.

This is why is it absolutely critical that the carbs are 'synched' correctly. if either carb is even slightly open at idle, then most all air is drawn thru that carb.

I suspect that your top carb is slightly ajar/open, just only a hair, and the bottom is closed.

You can 'feel'' the airflow thru the upper and virtually nothing thru the lower.

Timing. The triggers are spaced 'equally around the trigger ring. Each gets 'triggered' by a magnet in the hub of the flywheel. If one trigger coil is misplaced during manufacture then the timing for that cyl will be 'off' by however how many degrees the coil is misplaced. It may be either advanced or retarded up to three degrees depending on how wore out the mold is and how observant the operator was. Usually this isn't a big problem but on some motors it can make a big difference.

My money is still on your carb synch. Critical for idle, but once throttle is opened no longer a problem.

 

[quicktach]

Re: 3 cyl. outboard issue

You may want to get a manual for the motor to learn about troubleshooting for it and how to do some minor repairs and adjustments. I suspect you should do a link & sync on the motor.