18hp murcury runs erratic

[gobydon]

First let me say that this is my first boat, a 12' aluminium jon with an 18hp murcury (18XD 1984 year i think). i do not have alot of experience with 2 cycle boat motors but have a lot of mchanical knowlage of internal combustion engines, 4 cycles mostly.<br /><br />Here is my problem, the motor is slightly hard to start 4 pulls when cold idles at low rpm for a few moment then stalls, 1 pull with a little throtle and will stay running but erratically, once warm will idle without throtle but still erratic (gets power in burts in one second intervals) this behavior is the same throughout the rpm range, even at full throtle, feels like one cylinder is getting power in boosts. <br /><br />this i what i know so far, <br />compression = 130 psi top 125 bottom<br />idles best when idle screw is all the way in<br />will not run cold with choke(half or full)<br /><br />thx for any help gobydon@hotmail.com

 

[ODDD1]

Re: 18hp murcury runs erratic

Adam, you may be dropping spark on one cylinder.....these switch boxes usually fail one cylinder as a common prob....I have seen them do this type of intermittent thing......the only way to troubleshoot it with a degree of certainty is to run it with a DVA meter hooked to the coils...you will see input voltage to coils dropping off when the motor falters.....

 

[Boatman30]

Re: 18hp murcury runs erratic

Adam,<br />I would agree with ODDD1. I would bet you have an intermittent ignition problem. You can get a cheap spark tester for around 10.00 at an auto parts store to hook in line with your plug wire and plug. This of course would indicate an erratic spark problem<br /><br />I would start with the basics and try a couple of fresh (correct) plugs and hope for the best.<br /><br />You may also check your kill switch wires for a possible short?????<br /><br />Best of luck!

 

[gobydon]

Re: 18hp murcury runs erratic

Thanx for your advice. An ignition problem sounds about right. <br /><br />So how do I go about this test, what is this dva meter you speak of? Will a multimeter work the same? If so what should the input voltage be? If I find a low reading can I be for certain that the switch box is failing or could another component of the elec. system cause this to happen?<br /><br />sorry for so many questions but I have one more, is there a way to bench test the switch box so that I don't have to put my hand into a running engine?<br /><br />Once again I really appreciate all of your input into this problem .... thanx.

 

[Boatman30]

Re: 18hp murcury runs erratic

One way you might be able to check this problem, would be to switch the leads coming off the switch box going to the coils. If the problem follows to the other cylinder. You know its the box. If it dont follow, its a coil, but coils dont USUALLY go bad.<br />I know this works on OMC outboards but not positive on a Merc.<br />Boat

 

[ODDD1]

Re: 18hp murcury runs erratic

Adam, heres the deal..the meter i speak of is different than a regular multimeter...is a peak reading dc voltmeter.....if you can verify that one cyl is losing spark, then only 2 things in this system can cause this...a switch box or a coil...everything else is common to both cylinders....an boatman is right, coils are usually reliable, moreso than the switchbox....