Only running on two cylinders

[fluidjuice]

Hi
i have a 40 hp classic mercury 1991 4 cylinder ,
It idles fine on top two but when you put in gear and give it some revs it stalls every time, this is done in the habour so it has enough back pressure.
The bottom two cylinders dont seem to be running at all so i took it to have carbs stripped down and cleaned thinking it was a stuck jet.


the carbs have been cleaned, and the engineer said that the compression on all 4 was good, and was sparking on all 4 cylinders ,but the bottom two cylinders dont run, so it must be fuel related ???
But for him to strip engine to trace problem at ?40 per hour was going to be a huge bill and not worth the effort???
I have read many trheads on iboats but none have any conclusions to point me in the right direction, any info would be truely helpfull.

 

[akbar]

Re: Only running on two cylinders

I'll be watching for some more posts on your problem as I have a similar problem. My 1985 50HP is making a pinging noise and trys to kickback. If it is idleing low enough, no not idling but running at a low enough rpm it will kickback and die. I've swaped the top two plugs with the bottom two plugs with no change. I should add it is the bottom two cylinders not producing power. I've cleaned the carbs with no change. I've even swappped the topcarb with the bottom carb and still no change. It's got a hot spark on all cylinders. I've got it in my mind that the bottom two aren't firing at the right time since it seems to be labor knocking. Hope a guru can help us.

 

[Laddies]

Re: Only running on two cylinders

Heres the test from CDI if you don't have a DVA or OHM meter we can give you some simpler tests.

 

[fluidjuice]

Re: Only running on two cylinders

Some simpler tests at this stage would be very helpfull

 

[Laddies]

Re: Only running on two cylinders

The simple test for a 4 cyl. that is running on 2 cyl (upper or lower 2) with the black/yellow disconnected.
Stator - Swap the red with the red/white wires and the blue with the blue/white wires - if the spark moves to the other 2 cyls the stator is bad.

Trigger - Swap the trigger wires from the 1&2 cyl (purple&white) with the wires to 3&4 cyl (brown&white/black) if the spark changes cyls it is a bad trigger. If nothing changes it's probably a bad box.

REMEMBER THERE IS NO SUBSTITUTE FOR TESTING WITH A DVA.

 

[akbar]

Re: Only running on two cylinders

Laddie I appreciate your response. I have good spark on all cylinders, it will jump an inch to ground. Have you experienced such a condition? Can there be a good spark but just at the wrong time? Are we checking for AC or DC voltages? Thanks

 

[Laddies]

Re: Only running on two cylinders

My responses were for the original question.

 

[akbar]

Re: Only running on two cylinders

Excuse me Laddie. I guess I'll need to post my own original question.

 

[akbar]

Re: Only running on two cylinders

I got mine fixed. It was a bad trigger. I had suspected it was firing out of time so I put some home made timing marks around the flywheel and sure enough the two cylinders not hitting were firing on #2. After disassembly the wire insulation was crumbly on one wire and it read high resistance according to the chart posted in this thread. Hope this helps someone else.

 

[fluidjuice]

Re: Only running on two cylinders

Ended up finding a local marine engineer to look at it,
Hes ruled out compression,
and electrics as they spark real good on all four.
the lower carb has been removed again now 4th time to be cleaned at his workshop , he is pretty certain its carburation , may be low idle curcuit, but if not then it could be crankcase seals.He seems to think. fingers crossed its only the carb, since last post its had new fuel lines new inline filter, new pump diapram and new primber bulb, weve noticed that when you look in top carb fuel is coming from jet and getting sucked into engine but on bottom carb fuel seems to be ristricted ie not as much fuel leaving jet, but if you put a bit of clear plastic over lower carb throat to ristrict air flow you can see the fuel amount increase and get sucked in to bottom cylinders and the engine starts to pick up as normal untill plastic removed and only top cylinders agian,
What does this sound like to you guys as hes not working on engine again till next wednesday and im intrested in any ideas.

 

[Boatleech]

Re: Only running on two cylinders

I'm no expert, but by restricting the airflow you're choking the carb, resulting in a richer mixture. If it runs OK then, the carburetor isn't providing a proper mixture on its own. For the WMA-7 carb (is that what you've got?) I think there are only two adjustments -- the idle mixture and the float level and drop -- but if either of these is out of whack it might cause the problem. If all your guy did was clean things, but didn't check these items, he owes you...

 

[fluidjuice]

Re: Only running on two cylinders

mmmmmmmm you got a point but there is no adjustment on the idle screw and floats although no measured looked level with carb gasket will check float acuratly now.

 

[Boatleech]

Re: Only running on two cylinders

What model carburetor do you have?

 

[fluidjuice]

Re: Only running on two cylinders

I have WMA 7 carbs.

Ok update time, marina came over and brought back carb he cleaned out in his workshop along with a new gasket, fitted carb, did comp test again .
cyl1=125
cyl2=125
cyl3=125
cyl4=120 so all is ok.

ran her up and now we have all 4 cylinders running,he also did a temp check on cylinders and all ran at around 40c except for bottom one which ran at around 32c also tested with portable tach and was able to set idle around 650 which he said was still fluctuating alittle and was due to the carbs needing synching hes bringing his vacume gage with him on thursday to set up carbs properly.with all this done i really really really hope that the engine will pull out of the hole with out stalling under load.?