Merc 2 stroke operation Question

This is a TECH question concerning specific 2 stroke operation as it applies to a 3 cylinder Mercury OB. I am familiar with basic 2 stoke operation of a single cylinder engine.

I have a 1989 3cyl 90hp 2 stroker Merc. For many months I have been experiencing "periods" of power loss. The problem usually begins after running at WOT for 3-5 minutes whereas the engine losses power from one cylinder. RPM's drop from 4,800 WOT to 2,000 WOT. After chugging along on 2 cylinders for a while the 3 rd cylinder will begin to fire intermittently and ultimately come back on line again only to shut down again after running for 3-5 minutes.
This has really turned into a head scratcher?
My question is this.......are EACH of the cylinders charged with fuel/air/oil mixture from a common crankcase area or is the crankcase compartmentalized with a space under each piston for individualized cylinder air/ fuel/oil mixtures with internal crankcase seals seperating them?
Does the "looping scavenge term" mean that air/fuel/oil mixtures from ALL carburetors combine within a common crankase from which each cylinder will be charged as the piston moves through it's intake portion at the bottom of each power stroke?
Can anyone clarify?
Thanks a lot.
Will

Re: Merc 2 stroke operation Question

You have three, one cylinder two strokes powering a common crankshaft. Each cylinder has its own carb and each crank cavity is sealed from the adjacent cylinders.

Re: Merc 2 stroke operation Question

First look at your engine as 3 one cyl engines that share a common crank and electical system, each cyl has it's own crankcase cavitity.
Looper means the the fuel from the crankcase not only travel to the cyl aruond the sides of the piston as well as through standard transfer port.. A looper has no defflecter on the top of the piston, it is flat on the top.
I would think you have a bad ign componet, also a shorted rectifier will cause intermittent spark. If you need test spec we can furnish them

Re: Merc 2 stroke operation Question

Laddies said:

First look at your engine as 3 one cyl engines that share a common crank and electical system, each cyl has it's own crankcase cavitity.
Looper means the the fuel from the crankcase not only travel to the cyl aruond the sides of the piston as well as through standard transfer port.. A looper has no defflecter on the top of the piston, it is flat on the top.
I would think you have a bad ign componet, also a shorted rectifier will cause intermittent spark. If you need test spec we can furnish them

Click to expand...

Re: Merc 2 stroke operation Question

Thanks a LOT for your reply concerning the 2 stroke operation, and the looper term.
I too, have suspected a bad electrical component, however, I cannot isolate the problem as per the Clymer Manual. The STATOR, TRIGGER, and VOLTAGE REGULATOR (RAPAR kit), and plugs and plug wires. Coils test good on resistance checks and HOT spark is visable at each plug wire when removed from each plug when running.
I am now suspecting insufficent fuel supply and have cleaned the tank, new hose and bulb, filter and fuel pump kit goes in tomorrow.
By the way, you mentioned a shorted rectifier could cause an intermittent spark...? I do not understand how this is possible. The rectifier changes AC from a separate stator coil winding to DC for the charging system. If this is a possibility, disconnecting the rectifier from the charding stator coil should eliminate the intermittent spark... correct?
Could you elaborate on this possiblitiy please?

Thanks again
Will

Re: Merc 2 stroke operation Question

If the rectifier has a intermitent sort to grd it will heat the stator till it breaks down, If I were guessing I would think it's the swich box I have seen them fail on heating up and also drop a cyl when cold and pick it up hot. With out a DVA to test voltage when the problem is happening it's tough to find
Yes, you can test the rectifier as you said

Re: Merc 2 stroke operation Question

Thanks for your help. I read your profile, and I am inpressed by your Merc Master Mech experience, maybe you are the one that help me pinpoint my ongoing problem.
As of my last posting, I have rebuilt the fuel pump, checked for fuel flow through the lines, filter and carbs by removing the drain plugs on each float bowl. I get excellent fuel flow from each drain plug, and threw the carb main jets (on all 3 carbs) if I restrict the air horn with a rag momentarily when idling.
Here is what I do not get.....At idle or at 2,800 rpm with engine running on flusher muff.
I do not get any change in rpms/runnability if I manually spray a mist of fuel/oil mix in Number 1 or Number 3 carburator throat. Only number 2 richens out and begins to bog down.
Number 1 cylinder...Minimal change in runability (200-3 rpm's drop) with plug wire disconnected.
Number 3 cylinder...No change in runability (0 rpm change) with plug wire disconnected.
This engine seems to be creating about 90% rpm and power on the center Number 2 cylinder, 10% rpm and power on Number 1 cylinder and zero on Number 3.
Shouldn't I get approximately the same rpm/power drop with each cylinder being disconnected one at a time? (cylinder power balance test)
Even though, I am getting a crisp blue spark at EACH plug wire to plug when running and disconnected I have switched out the position of the ignition coils and the problem is unchanged.
My last effort involved attempting to isolate the problem to the switch box. I switched the wires around from the trigger and coil on number 1 terminals to number 2 and vice versa to see if the transistor/capacitor circuitry had an effect on the problems cylinder location.
No change.
Is it likely that the switch box is firing only Number 2 at the correct time?
If the transistor within the switch box for number 2 was changed to fire number 1 wouldn't this cylinder begin to operate then?
I am truly at a loss on this one, it seems to defy logic and physics. I always understood AIR+FUEL+IGNITION = BANG.
Is there anything I am overlooking?
Thanks
Will

Re: Merc 2 stroke operation Question

Will, I have the advantage of a DVA so I can test the system for voltage a it operates, I have never tryed the carb trick so I don't know how they act. In the old days we used to use a jumper and short the trigger terminals out to make sure it was running on a cyls.but then merc said it could hurt the system so we stop (I never had a problem) Also you should never run a engine over about 1200 on a hose (merc says idle. If you have a induction tach try to establish if the probem is ign and if so which cyl--Bob

Re: Merc 2 stroke operation Question

As I was eating dinner I was wondering if you were running on a built in tank if so you need to ck the anti-syphen valve, the will stick causing erratic running at higher speeds

Re: Merc 2 stroke operation Question

There are several things in the boats fuel system (Not engine fuel system) that will cause your problem. Along with a plugged antisiphon valve Bob mentions, you might want to check this thread out. It may have been meant for IO's, but the same things happen on outboards regarding leaks and plugs.

http://www.iboats.com/bbboard/bbBoard.cgi?a=viewthread;fid=24;gtid=1107009

Re: Merc 2 stroke operation Question

Thanks guys.
I have already removed and cleaned the onboard fuel tank.
I have replaced the fuel line and primer bulb.
I am not sure about the presence of or location of an anti syphon valve. The tank is vented via a seperate (thru the hull) vent line.
From all indications, fuel is present at all three carburators under all power settings.
Without question, Number 1 and 3 cylinders are along for the ride and not producing power at idle or partially opened throttle settings.
On the water, as previously described, only on occasion do the 1 or/and 3 cylinders begin to fire.
I am operating the engine on a hose/muff and trying to keep rpms under 2K. Periodically one of the other cylinders will kick in and push the rpms up but then cut out again allowing revs to drop back.
I can actually disconnect BOTH the 1 and 3 plug wires and the engine performance changes minutely (maybe 200 rpms lag) at idle/POT.
I do not have a DVA or a scope so my troubleshooting "electrical issues" is limited to visual and digital multimeter readings.
Any input on the cylinder balance testing issue mentioned earlier?
Shouldn't each cylinder be contributing equally to the total engine power and rpms being produced regardless of the power setting?

Thanks again

Re: Merc 2 stroke operation Question

Will, on one occasion I ran into a 3 cyl that the trigger wires on 1 and 3 were the wrong colors and switched them and the engine ran fine. The anti syphen vave shoulb be right on the outlet of the tank where the fuel line attaches on most outboard ther have been knocked out becaust the do stick alot and the O/B fuel pumps won't overcome them like a I/O pump, It is alway better to change them

Re: Merc 2 stroke operation Question

Well this morning has revealed some strange findings, but has convinced me that the switchbox is the culprit.
This morning when I started my engine, I immediately noticed a different runability so I started removing spark plug leads 1 at a time from each plug to check spark. Yesterday the engine was running on number 2, with 1 and 3 producing no power or rpm.....whereas today it is running on number 1 with numbers 2 and 3 not producing power or rpms.?????
I disconnected 2 and 3 trigger wires from the Switch box and while the engine was running on number 1, touched the 2 trigger wire to the switch box connection....to my surprise number 2 began firing momentarily! Same thing happens with number 3 trigger wire and number 3 cylinder. I am guessing, that with all 3 input voltages coming from the trigger into the switchbox that it's circuitry is breaking down and scrambling the switchboxes ability to operate all 3 coils simultaneously.
Currently waiting on UPS to deliver the new component to FINALLY confirm and correct this elusive, intermittent problem.
I will post the results later
Thanks
Will

Re: Merc 2 stroke operation Question

It sounds like the bias circuit is bad on your switch box. ohm out the connection that goes to the white black stripe terminal, it should be between 13 to 15000 ohms.

Re: Merc 2 stroke operation Question

i have had a similar problem with my motor and it ended up being the primer bulb had a leak in it. Just a thought to check

Re: Merc 2 stroke operation Question

opps didnt see where you said replaced the primer bulb.. soryy

Re: Merc 2 stroke operation Question

For what it's worth, all of the electrical component replacements and the rebuild of the fuel pump had no effect on the runability of my Merc. The performance (or lack there-of) problem still existed. It wasn't until I disconnected and removed the fuel line to engine "quick disconnect" connector from the engine and replaced it with a $1.09 plastic double male ended "fuel line nipple splice" did the problem resolve. The engine was either..
a. Starving for fuel due to a sticking valve check valve in this connector at high RPM's...or.
b. Drawing in air at this unit when pump suction was high and constant.
HOPEFULLY, this will once and FOR ALL eliminate this elusive gremlin.
Thanks
Will

Re: Merc 2 stroke operation Question

Well, that was worth a lot.

Recently I had asked the guy at my local authorized Mercury repair shop about my wanting to install a quick connect between my built in tank and the motor. Currently the fuel hose from the built in tank runs directly to the fuel pump without any connectors. I explained that I wanted to connect a small 3 gallon fuel tank to the motor on occassion in order to run a high concentration of Seafoam/fuel mix through the carbs from time to time. I thought that using quick connects would be easier than disconnecting the fuel lines from barb connectors each time I wanted to switch tanks while having the boat out on the water. I didn't want to put the high concentration of Seafoam in my 20 gallon built in tank for various reasons.

The Merc guy said that it was OK to use the quick connect fittings on a temporarary basis ONLY (i.e.,during the days I wanted to do the switching) and that I should remove the quick connects afterwards. He said that the quick connects would unnecessarily risk the problems that you highlighted above. Drawing air into the fuel line and/or restricted fuel flow is, as we all know, a bad thing.

I followed his advice.

Re: Merc 2 stroke operation Question

This is funny since mercury used a quick disconnect for years and still are useing in some of the small motors, could it be they are saving money on these attachments. a lot of things are discontinued,adjustable trim tabs, centrifugal slinger,sound proofing inside of cowling. The only thing that I can guess is that they could have been discontinued because of running out of gas while the oil pump was still running. Small things I suppose,I am seeing too many of these oil gears going bad to try to keep them, when the motors run great without them and save the motor down the road.