Logo
FULL SITE MENU

7.5hp merc running on one cylinder

F

fvee91

Member
Hi all
I have a 7.5hp Merc, serial no. is 9442197 made in Belgium. Firstly, does anyone know the year model? The motor appears to run on only one cylinder, here is what I have done:

- New spark plugs, no difference
- compression test 115psi on each cylinder (exactly the same)
- Tried to start with the top spark plug lead off, will not start, occasional back fire through exhaust
- Try to start with bottom spark plug lead off, starts up and runs the same as when the lead is on
- There are two seperate red coils. Each has two colored input wires comming from the CDI box. I swaped them around along with the spark plug wires. After trying the same test as above, the bottom cylinder is not running, however now the other coil is running the bottom cylinder.

At all times there was healthy and regular spark on both spark plugs regardless of how I wired up the coils. I beleive that the problem could be one of two things:
1) the CDI box p/n 239 6222A4 may be firing the bottom cylinder out of phase, or perhaps with not enough voltage. The cheapest price I can find is $175 so considering I also need to rebuild the water pump I question if the motor is worth rebuilding!
2) Is there one reed valve for both cylinders or two sets? If two sets than perhaps the bottom cylinder is damaged in some way?

Can anyone help with this problem?

Thanks
Daniel
 
There are 2 sets of reed valves.--------One set for the top crankcase and one for the bottom.---Could be caused by a broken / open valve.---------Could be a failure of the bottom crankcase seal as well.
 
You probably have a bad coil. Switch the two coils and see if the problem follows to the other cylinder.

Jeff

PS: Make a diagram of the wiring color codes before you switch them!
 
As mentioned above, I already swithed the coils to no avail.

Racerone, I like your thinking regarding a broken/stuck reed valve. Not knowing a whole lot about 2 strokes, can you tell me how to get to them? I asume I pull the exhaust plate?

Thanks
Daniel
 
As others have suggested, Compression test both cyclinders first, with engine switch off and other plug removed. 100-115 is usuaql for a MAerc, but no more that 10% variance in pouds between both. eg #1 reads (dry test): 110; #2 reads 100 -- OK. Or, eg #1 reads 110; #2 reads 96 psi -- Not OK and engine requires professional service.

I'd further consider pulling the high tension lead (one at a time) while motor's running using an insulated leather glove or proper insulated plug-wire puller pull one wire at a time: lf no difference--other plug/coil/cylinder problem in idle or if stalls engine, it's that plug/cylinder/coil giving the trouble. In both cases it may also be a faulty high tension plug wire problem.
If yes, switch the two coils and do the same running test taking notice if the problem now exists on the other plug/cylinder.
If yes, you have a faulty coil. You can also look up the ohm reading and check each coil while engine off to see if the reading's out of spec.
 
Cylinder compression does not show broken / bad reed valves !!------------The reed valves are internal in the engine.------The engine must be taken apart for reed valve service.-----Pulling the exhaust plate does not allow for service of reed valves.-----Reed valves have nothing to do with exhaust.
 
Nightfisher, please re-read my first post, everything you have suggested I covered before posting to this forum.

Racerone, I found a parts diagram a while ago that shows how diffuclt it is to get to the reed valves. Incidently I shot some compressed air down the throat of the carbie and it seemed to greatly improve the engine, so perhaps I am lucky

Daniel
 
fvee91 said:
Nightfisher, please re-read my first post, everything you have suggested I covered before posting to this forum.

Racerone, I found a parts diagram a while ago that shows how diffuclt it is to get to the reed valves. Incidently I shot some compressed air down the throat of the carbie and it seemed to greatly improve the engine, so perhaps I am lucky

Daniel
Click to expand...

After a lot of googeling I found this thread. I'm having the exact same problems (good Sparks on both, good compression on both, lower cylinder not running, ...) I am really at a loss. Reeds is also the only thing I can imagine, although I would have expected that that would show up on a compression test. Maybe not. I also swapped carbs, which didn't change a thing. Help anyone? I have also shot compressed air into the intake and have revved the engine up on one cylinder but so far no luck. I am not going to tear apart that powerhead, I tried it before and its just not worth it...
 
Back
Top