1985 90hp 6cyl Merc won't stay running when it is brought down to idol it only runs at 1000 rpm or better?

Question

the motor must start out of gear (safety feature) and when i back down into neutral to engage gear the motor dies again and again i believe a couple of the (new) plugs might not be getting much spark could it be 1 of the 2 power packs or 1 of the 6 coils and is this something i can replace myself ?

Answer 1

check the idle stop s***w, sounds like its out of adjustment,  or the throttle cable is loose on one end or the other. I've owned a 90 merc like yours, was one of the finest motors made back in the day.

Answer 2

First, unlike 80's kid said, there is no air filter on your motor so that would not be the problem.  Also to check the ignition system is a lengthy process with many tests.  Generally, however if there is an ignition problem the plugs in question will get no spark, so there is probably nothing wrong with the ignition.  And just info, coils are the absolute last thing that should suspected as they RARELY go bad.

As for the motor dying, check the compression in each cylinder.  There should be no more than a 10psi difference in any of the cylinders.  If this is ok, then you need to clean and rebuild the carbs.  

If you try to just adjust the idle mixture and not fix the problem you will end up with a powerhead that needs to be rebuilt.

Hope this helps.

Answer 3

It sounds like you have dirty carbs. Has it been a while since the engine has run? If it runs fine over 1000 rpm I doubt you have ignition problems. Check compression, looking for at least 90 psi(preferrably 110) and all cylinders within 10 psi. If that checks, fuel is next in line. Ignition is the most difficult to check and the most expensive to fix.  Always check fuel systems first. A good way to verify carb issues is to put some gas in a spray bottle and spray into individual carbs while trying to run it to see if the engine picks up. If you spray all the carbs and nothing changes, then start looking at ignition.

Answer 4

i would try this... pull the choke out when you feel like it is going to stall... if it remains running, then you are having carburetor problems... if it still stalls then i would definitely check spark plugs next...

Answer 5

I would double check the spark plugs, especially if this happened once you put new plugs in. Make sure they're gapped right. If you have a carburator, I would make sure air intake filter is clean, or try running it with air filter/flame arrestor off. I would also check fuel/water separator if you have one, maybe you have some water in the fuel.