1988 Mercury 9.9 2 stroke, won't run without choke, might be running on 1 cyl?

[vipermagic]

Per title, I have a 1988 Mercury 9.9 2 stroke, 2 cylinder.

It starts up great- turn the idle speed knob all the way to fast, pull out the choke, one pull and she's running. It runs okay, nothing to write home about, but decent. It will shake around a bunch on the stand sometimes, if you turn the throttle up it will respond immediately, but continue to build RPM for several seconds. If you push the choke in, it'll drop down to a lower idle speed, and almost always die. If you push the choke in and turn the idle speed knob down, it will stall every time.

If you pull the bottom cylinder plug wire off, the motor dies immediately. If you pull the top off, there is no change in how its running. Both cylinders ARE sparking - nice, strong spark, too, and both have the same compression.

We've cleaned the carb and installed carb kit, and replaced the plugs. I'm not sure where to go from here. Could it be the reeds? I feel like it runs too good at fast idle to be only running on 1 cylinder... but its pretty damning evidence when I pull the plug wire off.

I don't have a whole lot of experience with two strokes - any help would be much appreciated!

 

[gm280]

Re: 1988 Mercury 9.9 2 stroke, won't run without choke, might be running on 1 cyl?

First of all welcome to iboats :welcome: good to have you aboard...

I don't see how you can have great spark AND compression and no change when pulling the plug wire on the top cylinder... That sounds like a dead cylinder to me. So you need to look over why it doesn't change the RPMs when you pull that plug wire. It is not contributing to the power output of your engine in any way... It has to be your problem. You just need to find out why... Maybe under a load it doesn’t spark...?

 

[vipermagic]

Re: 1988 Mercury 9.9 2 stroke, won't run without choke, might be running on 1 cyl?

First of all welcome to iboats :welcome: good to have you aboard...

I don't see how you can have great spark AND compression and no change when pulling the plug wire on the top cylinder... That sounds like a dead cylinder to me. So you need to look over why it doesn't change the RPMs when you pull that plug wire. It is not contributing to the power output of your engine in any way... It has to be your problem. You just need to find out why... Maybe under a load it doesn?t spark...?

Thanks for the welcome! I've been a lurker for a while, but finally signed up for my own account - hopefully I can benefit the community and not just benefit FROM the community!

As far as the motor, that is my thought too - the cylinder is dead, but why? Of course I can't directly test the spark under load, but the little blinking light pen that is supposed to detect spark, still sparks under load, and the plugs are brand new, so I am reasonably confident that its sparking under load.

What I've been finding leads me more and more to believe its the reeds. I took the reed block off, though, and they look fine- not bent, no chips. I destroyed the gasket getting it off, though, so now I need to get another one of those - any other ideas?

 

[Bosunsmate]

Re: 1988 Mercury 9.9 2 stroke, won't run without choke, might be running on 1 cyl?

Compared your plugs?

 

[Laddies]

Re: 1988 Mercury 9.9 2 stroke, won't run without choke, might be running on 1 cyl?

Use a light and look under the flywheel to see if the magnets are 180 degrees apart they will come unglued and one will move over against the other, thats a common problem and the engine will still have spark on both cyls.

 

[gm280]

Re: 1988 Mercury 9.9 2 stroke, won't run without choke, might be running on 1 cyl?

You could also swap the coils and see what happens too...

 

[vipermagic]

Re: 1988 Mercury 9.9 2 stroke, won't run without choke, might be running on 1 cyl?

Sorry for going AWOL - worked called, and its been a few days. I do appreciate the help, though!

I took some pictures, of the hole where the reed block goes, the reed block, and the spark plugs. Look at those plugs! The dirty plug is from the bottom cylinder - the one thats working! Definitely looks like no fuel getting to the top, doesn't it?

outboard pics - Imgur

The engine starts up REALLY easy, which baffles me also.

I checked the magnets - they're 180 degrees opposed. I'll try swapping the coils, as soon as my reed block gasket gets here and I can put the thing back together (hopefully friday!)

Thanks again for the help!

 

[Bosunsmate]

Re: 1988 Mercury 9.9 2 stroke, won't run without choke, might be running on 1 cyl?

Yep the top looks to be dry for sure.
You might want to run another compression test on it too after dropping in some oil in the cyl for a few cranks.
Check those reeds are clean and intact

 

[CharlieB]

Re: 1988 Mercury 9.9 2 stroke, won't run without choke, might be running on 1 cyl?

Possibly upper crank seal leaking enough insufficient upper crankcase vacuum/pressure to move enough fuel to fire the cyl.

Remove the flywheel, inspect the seal.

If it were mine, I'd be tempted to drill and tap a port into the intake before the reeds to plug in a vacuum gauge, if the seal was bad you'd see a definite 'pulse' when the lower cyl pulls and a very reduced needle movement when the upper is supposed to pull. My Wheel Horse garden tractor with an Onan came this way, it is a constant check of cyl/valve health.