1970 merc 9.8.... bit of a puzzler here boys

[green4themoney]

hey guys, alright i need your input on this problem, i had my 1970 merc 9.8 hp out 2nite after everything was fixed and ive had it running awesome the past few weeks, but tonight for some reason it only wanted to run on one cylinder ONLY at low rpm, when i opened it up it ran fine, i was with my dad and hs gf and wanted to give them a tour of the lake and felt a little embarassed of how the engine wouldnt idle..... the only thing i have changed since was i used some gas from one of my other cans that i have purchased in june and it was already mixed 50/1 everything else was fine with my fresh can of gas now i use the old gas then this happens, so i take the engine home, pull the plugs and im getting GREAT spark on both cylinders, so at this point i asume there is water getting into one cylinder or one cylinder isnt getting gas at low rpm's... so i pull the exaust cover off to see if im leaking at i see the motor side of the baffle plate and its got like a light light green milky colour to it, which is telling me im not getting fire, but this colour itself im not overly sure if it indicates water is getting into one of the cylinders?

so i think i have it narrowed down to a few things

-for somereason the reeds have frozen and will only open at higher RPM's
-im getting water into a cylinder at low RPM's and at high RPM's the exaust pressure is keeping it out
- i have an air leak somewhere that is leaning out one of the cylinders
- maybe the gas was just bad and for some reason only one cylinder would burn it

i have a new powerhead gasket set on the way

the problem is fine at WOT

it only really starts to cut out and cant decide if it wants to run on one or 2 cylinders at MID throttle and at idle it is most def running on one cylinder

anybody have any insight on this problem? what are your thoughts? any help would be appreciated

 

[Unbalanced]

Re: 1970 merc 9.8.... bit of a puzzler here boys

clogged idle jet?

 

[d.boat]

Re: 1970 merc 9.8.... bit of a puzzler here boys

My '74 9.8 had similar symptoms: would intermittently drop a cylinder at idle, but the cylinder would always kick in at higher RPM.

My engine has coils and a switch box. I'm sure yours has coils, but not sure of the switchbox. Anyway.... I solved the problem the first time by tracing it to a bad coil and replacing the coil. It started happening again after a few more months. I thought it was a bad coil again, but it turned out to be a bad switchbox.

By the way, it's possible to think you're getting "great spark" but unless you actually test it with an adjustable gap spark tester, you really don't know how strong it is or if it's strong enough to actually fire the cylinder. Mine "looked great" when I just watched the spark jump the spark plug gap. But the adjustable spark tester showed the spark was in fact weak.

In my (limited) experience, this would indeed fit with your symptoms of a bad/weak spark at idle but getting stronger when RPMs go up.

Another thing to do is check out the coils. It's pretty easy:

when idling (rough, one one cylinder we presume), remove wire from one of the spark plugs:
- if engine quits, it's the OTHER cylinder that is not firing properly. By removing the wire, you've stopped the only good cylinder, now you have no cylinders firing.
- if engine stays the same, it's THAT cylinder that is not firing properly. By removing the wire, you have only removed the non-firing cylinder therefore not changing the situation at all.
- if by chance the engine stays running but gets rougher, that means that both cylinders were firing but now only one is. I doubt if this will happen in your case.

After you figure out which cylinder is not firing at idle, switch the coils and see if the problem follows the coil. If it does, replace the bad coil and see if it solves the problem. If it does not follow the coil, you have a different problem - the switchbox (if you have one) or the parts undr the flywheel.

This worked for me the first time. The second time it did not. Upon further assessment, I found that the same cylinder would fail to fire regardless of which coil was hooked to it. That told me the switch box was failing.

If yours doesn't have a switchbox, the second thing doesn't relate to solving your problem, but might help you figure out which parts to look at. I don't have any experience beyond this!