Merc 40 problems

[Ozzie]

I have a40 Merc twin that is causing problems ie it drops a cyl it has new coils,pick up,brain box,new plugs,compression in both cylds o/k it changes cyld drop from one to the other on an intermitten basis,not sure if its ignition or fuel related any suggestions would be most appreciated.motor is a l983 model year,

 

[12Footer]

Re: Merc 40 problems

Watch the timing advance linkage, while someone operates the throttle from idle to WOT,and back a few times (engine not running). Look for any play.<br />If you find play,see if you can eliminate it by tightening loose bolts or whatever.<br />Once you are happy with that, pull the plugs and look for one that looks brand new (clean), or much cleaner than the other.<br />This would indicate water jacket leaking into the cylinder via head gasket or exaust port plate.<br />Also, check the small hose on top of the carb,going to the block by the reed box.<br />It can drive you buggy,and it is hard to get to (requires removal of the carb).<br />But even a pinhole leak in that hose, and it'll miss and surge and sputter and...<br />Good luck , and letme know what you find.

 

[keiser]

Re: Merc 40 problems

One way you may be able to check to see if it is ignition, it should work. Pull your cover and plug a clamp-on type timing light to your battery. While running when it drops the cylinder connect the clamp-on to a plug wire to see if the timing light flashes. If it does, switch the clamp-on to the other plug wire to see if it makes the timing light flash. If the timing light flashes on both plug wires seperately then you are getting spark.<br />You may want to try it back and forth a few times to be sure.<br />If you happen to have or know someone with a timing light, <br />

<br /><br />If it is a fuel problem. To see which cylinder it is coming from. While running, when the cylinder drops pull one of the plug wires if it stays running you found the faulty cylinder. if it dies right away, shut off the motor and try it again with the other plug wire, if it stays running you found the faulty cylinder. After you pull the plug on the faulty cylinder let it run a short time to see if it will die, if it does it may be happening to both cylinders. Which would mean it would be something with the fuel before it seperates to each carb. If it keeps running it is just the one cylinder, then it would be something with the fuel after it is seperated to each carb. Don't let your motor get too hot when you are trying these tests. Take your time and it would be wise to let the motor cool between tests. Also you may want to use a pair of rubber handle pliers or rubber gloves to pull the plug wire if there are any cracks in the boot as the spark can jump and give you a good little jolt. Make sure your not grounded.<br /><br />Good luck,<br /><br />keiser