Mercury 850 with weak spark?

[zorak]

Hi all. I have searched this forum and I cannot find a definitive answer to my issue. I have a 1976 Merc 850 with ADI ignition. I got this motor with no spark. After testing the stator & trigger, I felt the switchbox was bad. I put the box off my parts motor 1977 ,same make/model and had spark. I started it on the test stand and It fired right up and idled perfectly. I put it on my boat and it now idles like crap. I feel the spark is weak (see attached pic) as I have seen ADI Mercs throw some seriously mean sparks. I have retested my stator with a DVA adapter. Blue & blue white (should be 180-400v, tests @240 volts) and red&red/white (should be 25-100v, tests @ 39volts) Compression is 140,140,135,145. All 4 plugs are firing. Of course I will link & sync it later as I feel there is definitely an issue that was not there when I originally had it running on the stand. I haven't retested the trigger since all 4 coils are firing. I have installed an aftermarket regulator and new ignition switch (magneto type that grounds kill switch wire) and have tested the stator both with and without it connected to the switchbox and tested with the kill wire disconnected and there is no change in stator output.I have had to patch up the trigger wires (corrosion) but the stator wires look good. Can I test the output side of the switchbox to the coils with a DVA and if so, will the voltage going to the coils be similar to whats coming from the stator? My gut tells me the switchbox may have a capacitor that's gone bad, provided both caps are used together (2 cyl at a time) instead of both firing simultaneously. Any info would be most appreciated. Thanks in advance!

 

[Chris1956]

Those motors will idle poorly if the carbs are out of adjustment or dirty. A spark gap test will tell you if the ign system is good or not. The simplest spark test is to listen for the spark snap. If she snaps on all the spark plugs, the ign system is likely good.

 

[racerone]

In the water there is more back pressure to change engine air behavior.-----Look into carburetor adjustment before blaming ignition spark.-----Checked bleed restrictors if that engine has them ?

 

[zorak]

I will be looking into the carb adjustment as I have not completely ruled them out. If you look at the pic, the spark seems to be a little weak. If you look closely, two are sparking in the pic, but there is no snap. I can get the spark to jump a 1/2" gap, but it Just does and its not hot blue, just orange. You can hear the arc, but i don't think its hot enough.

 

[Chris1956]

Ok, then get a spark plug tester and see if she will jump a 3/8" gap.

 

[racerone]

Spark jumping a gap of 1/2" could be considered good.----Other trouble shooting needs to be done.----Look into the bleed restrictors , often they are missing.

 

[zorak]

Good advice from all. In retrospect, I will check the bleed restrictors, but i am leaning toward a carb issue. The engine seemed to either be getting too much fuel or spark was weak. As I stated before, it ran great, the first time I ran it. It then sat for a month and a half, with gas in the carbs. I am thinking I might have an issue with my floats. I pulled them out and let them dry for a day or so and weighed them. I then let them soak in gas for a day and they do weigh a little more. I don't know if it's enough to affect the fuel level, but I am thinking about replacing them just for good measure.

 

[Chris1956]

Your original floats were foam. Today's alcohol/gasoline fuel can break them down, besides, floats do not last forever.

Replacement floats are hard white plastic. Replace them and their inlet needle and seats, plus all carb gaskets, after a good cleaning.

 

[zorak]

Well, I think the floats are ok, but I'm still stumped. I had no fuel in the carbs so I shot a little in both carb throats and it still just doesn't want to start. I do know I should do a link and sync, but what keeps getting me is it idled perfectly until I put new gaskets and diaphragm in the fuel pump. I just do not see what could have happened between the last time it ran good and now. It almost reminds me of a massive vacuum leak. I pulled the fuel pump off to see if there is a vacuum leak, but I don't see any issues. I have cleaned both carbs and installed new gaskets. I have placed the carb body in fuel while I blew on a hose connected to the fuel barb to verify the float valve will close. Compression is good, spark is good according to everyone I've checked with. I just don't get it. I know it cannot be anything serious as I have had it running great recently. ARRRRGH!

 

[jbuote]

but what keeps getting me is it idled perfectly until I put new gaskets and diaphragm in the fuel pump.

Nowhere near an expert here, and I haven't rebuilt a fuel pump yet, but based on that statement I think I'd revisit the fuel pump rebuild you did.
I do hear (rather, read) that while very simple, they can be tricky..

I'm most familiar with Johnson/Evinrude, but guessing it's similar?

I'm sure (and hoping actually) that the seasoned folks here will correct anything I'm about to say...

If you pull the fuel line off the carb and crank the engine. Do you get fuel coming out of the fuel hose?

I've heard another test is....
Can you blow into the fuel pump inlet? Then, can you blow into the pump outlet?
If you can blow through them both and air passes both ways it's a problem.
I think, if rebuilt right, you should be able to blow into the inlet ok, but blowing into the outlet should not work.

I may be way off in the details, but just off the cuff, I'd revisit the fuel pump rebuild and ensure it's correct and working.

My newbie $0.02..

 

[Chris1956]

Gee, remove the main jet cover on the bottom of the bowl. Pump the primer and verify fuel leaks there, proving fuel is getting to the carbs.

If the carbs fill with fuel, timing is correct and compression and spark are good, she should fire with fuel down the carb throat.

Float height is set correctly? Choke is working? Engine is cranking at least 300 RPM?

 

[zorak]

I do know a little something about how engines, even how outboard 2 strokes operate. I am quite sure that lack of fuel is not my problem. Thanks for all the great suggestions, but I think I have the problem figured out.

 

[racerone]

And what was that problem ?

 

[zorak]

Switchbox. 240 volts in, approx 70 out measured with a DVA. I checked with and without the box wired to the ignition coils. Ordered a new switchbox, plus a flywheel puller so if my trigger or stator ever go out i can replace them as I do have good used spares. My theory as to why the box went is that when i got the parts motor for practically nothing, i tested the compression with all plugs out and NOT grounded. I was amazed at the amount of lightning jumping out of the plug wires. I think I've read somewhere that cranking the engine without grounded plugs can pop one or both main caps in the switchbox. Since the switchbox consists of rectifiers ,SCR's and two caps there is nothing else present to step up the voltage. I will post my results after repairs are complete.

 

[racerone]

edited.

 

[zorak]

I do have something to add to this. I was in the process of checking and rechecking all wiring on my engine. I did check ohms on trigger and stator even though previously they have all passed the DVA test. I cranked to check for good spark with the same results I've been having. I then moved a bundle of wires close to the bottom of the switch box where one of the capacitors are located and I did see and hear a spark jump out of the epoxy covering the capacitor. I think I can guarantee that's not supposed to happen. I checked the wire for knicks, which there were none. It was the + wire for the new voltage regulator I recently installed, which was keeping the voltage between 13 and 14 volts. I do hope that spark didn't fry anything inside the module, but will just have to wait and see.

 

[Edbom]

Any update on your issue?

 

[Faztbullet]

The idea that cranking it with plugs not grounded is bovine scat...if this was true doing a spark test would damage it. Also spark jumping from switchbox epoxy to wiring that box has a problem.