1987 75 HP Merc won't idle

[deh1]

I recently bought a boat and motor from my girlfriend's Dad, who was the original owner. The motor is a 1987 (maybe 86) 4-cylinder 75 HP Mercury S/N A995919. It has at most 50 hours on it and was mostly garage-kept. It starts fine and seems to have full power, but will not idle at all. I suspected a minor fuel problem since it first occurred about the time that Dad replaced a leaking fuel tank last fall. He had tried adjusting the idle screws but it didn't help. <br /><br />I've removed the Autoblend oiler, and replaced the fuel line and primer bulb. There is a clear in-line fuel filter between the fuel pump and carburetors, which doesn't show any indication of clogs, an air leak, or lack of fuel. Tank venting is OK. Pumping the primer bulb has no effect whatsoever so it doesn't appear starved for fuel. I disassembled the bottom carburetor (except for the welch plugs) after finding green scale around the main jet. It was clean except for the bottom of the bowl and I cleaned the bowl and all the orifices and passages. I left the top carburetor alone since it looked spotless when I removed the idle screw and jet plug.<br /><br />After reading other posts I suspected timing may be the problem instead. I checked all the wiring and found a deteriorated white wire from the trigger to the switch box. The insulation has crumbled off and the copper is corroded. From other Mercurys I've seen, this appears common and unique to the white wires. I can't tell if it is corroded completely through. I have a manual but about all it says is I need a DVA meter (not just a multimeter) to check anything. The motor is not misfiring and I don't see any indications that one cylinder isn't firing. Is there any way to check if that wire is good other than disconnect it and see if anything changes?<br /><br />Any suggestions on where to go from here would be appreciated.

 

[andy6374]

Re: 1987 75 HP Merc won't idle

Well the if the white wire is shorted then no voltage is getting from the trigger to switchbox (at that terminal) and hence the SCR switch is getting closed and thus you aren't getting fire on one of the cylinders. That would cause a nasty idle.<br /><br />Get a spark tester to unsure all cylinders are firing.<br /><br />As far as the carbs are concerned. If it was laid up for a while, which I imgaine is the case (87' and only 50 hours), the carbs should be disassembled, soaked in carb cleaner, blow and dry out every orfice with compressed air, and reassemble with new gaskets. Trust me, just because they appear clean, even spotless, they can be very dirty.<br /><br />-andy

 

[deh1]

Re: 1987 75 HP Merc won't idle

Thanks for your reply.<br /><br />The boat is parked down by the river an hour or so away so I won't be able to check it until this weekend, but I do have a couple of additional questions:<br /><br />If the wire is either open-circuit or shorted to ground, does this typically damage the circuit or do I just have to replace the wire?<br /><br />If spark is present, is there anything else I should check before going back to the carburetors?<br />I would like to be ready to check all possibilities since once I go I'll be offline until Monday.<br /><br />Also, once I remove the top carburetor, will the throttle pickup settings stay or should I be prepared to readjust everything?<br /><br />Thanks again<br />D1ck

 

[andy6374]

Re: 1987 75 HP Merc won't idle

As for the carbs, just remove the linage that connects all the carbs and keep the springs attached to each carb. The only adjustment needed when reinstalling them is to make sure the throttle shutters in the carb throat are shut all the way when the engine is in neutral and that at full throttle fully open.<br /><br />Replace the wire and see if you get spark to all cylinders. If you don't, first check to see if "no spark" is from the white lead (i.e. check spark on the cylinder in which the wire that runs from the coil to the switchbox is next to the white wire on the switchbox.).. If one cylinder isn't firing, DO this:<br /><br />1.)You have four wires running from the switchbox and each wire services one coil. Swap a wire from a coil that has spark, to a coil that does not. Don't swap the coils, just the wires that run from the switchbox to the coil!! If the "no spark" jumps from one coil to the next, then your switchbox is a fault and needs to be replaced. If not it is the ignition coil.<br /><br /><br />-andy

 

[Rock Hall]

Re: 1987 75 HP Merc won't idle

I am having a similar problem with a 1985 25hp Mariner. I replaced the switch box and stator because it would not start at all. I replaced them one at a time and it turned out to be the stator at fault. Now I have an extra switchbox. The motor starts easily now but no spark on one of the cylinders. I do get a spark every now and then, but the motor runs on only one cylinder.<br /><br />The only ignition part I didnt replace is the trigger and I have one ordered. One thing I noticed it that the trigger doesnt seem to sit firmly. I think the plastic ring may have warped over time. <br /><br />I'll let you know how things turn out. I should have the new trigger by Thurs.<br /><br />Mark

 

[andy6374]

Re: 1987 75 HP Merc won't idle

Rock Hall-<br /><br />If the trigger was damaged, you wouldn't get spark at all. Try swaping the leads that run from the switchbox to the ignition coils (not the coils themselves, just the wires (green) from the switchbox to coil). If the no spark condition jumps from one coil to the next, then your switchbox is bad. If the no spark condition stays on the same coil, then the coil is probably bad. Make sure the coil has a good ground as this is a classic reason for no spark. Is the switchbox grounded properly, a poor ground on the switchbox can damage it. <br /><br />Just to make sure that it isn't the trigger. Switch the trigger leads at the switchbox and see if the spark jumps, if so then the trigger is probably bad (But I doubt, usually a bad trigger is no spark at all anywhere). Just remember to switch the trigger wires back to their original place on the switchbox.<br /><br />-andy<br />-andy

 

[deh1]

Re: 1987 75 HP Merc won't idle

I have just gotten this motor back together after the winter. I cleaned the fuel pump and carbs (but didn’t soak them) and replaced the needles and seats and all gaskets. I also replaced the corroded wires by removing the bad sections and soldering in new wire.<br /><br />It will now fast idle for maybe 5 to 10 seconds before it stalls, which is enough that I can now detect that it has a definite miss. It still stalls immediately when I reduce it to neutral.<br /><br />Squeezing the primer bulb still has no effect.<br /><br />Compression is good and uniform. The in-line spark tester shows spark on all cylinders. The miss was not visible with the spark tester. The spark plugs are all uniform with the typical amount of black soot and oil sludge.<br /><br />The manual says to check for “weak spark”. How is that done? How can the spark plug wires and plugs be tested?<br /><br />According to the manual, the only timing adjustments seem to be throttle pickup, maximum advance, etc; nothing that should affect it at idle.<br /><br />Does anyone have any suggestions before getting the DVA meter?