25XD 85 model........Fire but no combustion

[Bob Gilvary]

Mercury 25 XD 85' model Had hot spark on bottom cyl and weak spark on top cyl. New coil fixed that problem, but still no combustion and the compression is 115 at the top cyl.
Bottom cyl is 120 compression.

Bottom cyl fires fine and good combustion, top cyl fires fine and no combustion.

Swap out the lower plug which shows burn with the top plug that still looks new and the burned plug comes out clean, and the plug from the top shows burn.

Would I be looking for a water leak in that top cyl?

 

[Faztbullet]

Re: 25XD 85 model........Fire but no combustion

Yep...

 

[Laddies]

Re: 25XD 85 model........Fire but no combustion

Check the flywheel magnets it sounds like one is lose

 

[Bob Gilvary]

Re: 25XD 85 model........Fire but no combustion

I'll do the flywheel magnets first before wasting a head gasket. When I've found an omc with a bad head gasket, it would loose more than just a few lbs of compression. Not being as familiar with small mercs, I wasn't sure.

The weak spark at the top coil was evident when I pulled the plug wire. The spark jump wasn't near what the bottom was, and getting 220 volts to the both coils left me with thinking the coil was the problem. Might have been the loose ground wires from both coils that caused the one to go bad. Also tested the plug wire.

BTW, There is no backfire, just plenty of sludge from unburned fuel gas mixture.

I'll let you guys know what I find.

 

[Laddies]

Re: 25XD 85 model........Fire but no combustion

There is no head gasket on your mercury just a water jacket cover.

 

[Bob Gilvary]

Re: 25XD 85 model........Fire but no combustion

Popped the flywheel and found the trigger assy was broken off at one point to the plastic ring. You can see rub marks on it from the magnets. Could that've been the ticking sound I heard? Had I done a proper diagnostics, I would've found it earlier. In testing the resistance, I found the brown white wire that should show open had a resistance reading.

Magnets were still on solid.

A new trigger and we'll see how it goes.

 

[Bob Gilvary]

Re: 25XD 85 model........Fire but no combustion

Just wanted to post a thank you to Laddie for putting up that Rapair diagnostic paper that I found in an old post, that pertained to 25 Mercs..
and a reminder to all of us to not skip through proper diagnostics because we think we know so danged much.

 

[Bob Gilvary]

Re: 25XD 85 model........Fire but no combustion

I got in the new trigger assy, to replace the broken one. No change in operation. OEM trigger.
Getting 225V to the coils.
In testing the output now of the new trigger. When I test the white/brown wire for output, the engine speeds up.
When I test the yellow/brown wire it kills the engine. Disconnect either wire and it kills the engine.

Ran out of daylight, so now I'll go though the power pack testing in the morning.

I think I read somewhere in these posts that a bad trigger could ruin a switch box. Can anyone verify that?

 

[Laddies]

Re: 25XD 85 model........Fire but no combustion

Bob, while in some cases a shorted component may cause another component to fail, it's a good story for a inept mechanic to change out a complete ign. system. In your case it does sound like a diode in the switch box rectifier has failed. Here is a wiring schematic and description of how your system operates. Hope it helps.

 

[Bob Gilvary]

Re: 25XD 85 model........Fire but no combustion

Going by the sheet you posted in this thread, http://forums.iboats.com/showthread.php?t=238432, in testing the trigger circuit, using a peak reading CDI adapter, I can't get decent voltage reading from the two trigger wires. Testing the brow/white speeds up rpm and testing brown/yellow kills the engine.

Testing without the adapter, shows voltage in excess of 35v and has no affect on running rpm's.......... Damnedest thing I've ever seen.
Ohm readings are shown on wires where it's supposed to be open. Yeah sure sounds like a feed back through a bad diode.

I was hoping to find out I was smarter than this metal and plastic..........That's yet to be shown..........But I'm trying!!!

 

[Bob Gilvary]

Re: 25XD 85 model........Fire but no combustion

In a nutshell.

Replaced both coils because of weak spark on one of them. Fixed

Replaced broken trigger assy. Still up in the air on that one, but broken is fixed. In testing the new trigger, brown white to brown yellow, should be 4v or more. I'm getting 10v at very slow idle, and still can't test the two wires individually. It tests as posted before.

Replaced power pack that tested odd.

Stator is putting out 300v plus to the power pack on the black/yellow wire and 80v plus on the black/white wire at idle.
Pack is then putting out 225 plus to each coil. That voltage is quite a bit higher than specs also.

All grounds are cleaned and tightened.

Didn't ohm test the stator due to it putting out voltage I thought were in specs, yeah, kicking myself for that one.
Ohm testing the stator shows Black/yellow at just 3 plus ohms and black/white at over 300 ohms. Nothing in specs in this testing. More than backwards as posted in the Rapair diagnostic sheet.

It will be a complete ignition change over if a new stator is recommended. Still no change in running operation..........No combustion at the top cylinder. Even pulled the carb to check the reeds, and also see gas in the upper plug.

I'm not the first to work on this engine, and have seen errors by someone before me. Not that that's any excuse.

What say you Laddies?

 

[Faztbullet]

Re: 25XD 85 model........Fire but no combustion

My opinion is your electrical is ok and you have a mechanical failure. That engine is a chrome bore and piston can be gaulded and still have compression. I would perform a leakdown test on upper cylinder or remove the exhaust plate to check condition of pistons. If a plug goes in cylinder dirty and comes out clean,it either being washed with fuel or steam cleaned by water intrusion.

 

[Laddies]

Re: 25XD 85 model........Fire but no combustion

Bob, just returned home and read your post sounds like you now have good spark to both cyls. When you say that it is only running on 1 cyl. is that at idle only or are you trying to run it up, as a bad reed will not run at idle but will pickup at 1500 to 2000RPM. I hate to even bring this up but have you tryed a new plug in the cyl thats not running a few years back I beat myself to death on a new plug that was bad. Then comes Fastbullets suggestion, You can remove the port covers and check the chrome with a light from outside.

 

[Bob Gilvary]

Re: 25XD 85 model........Fire but no combustion

My opinion is your electrical is ok and you have a mechanical failure. That engine is a chrome bore and piston can be gaulded and still have compression. I would perform a leakdown test on upper cylinder or remove the exhaust plate to check condition of pistons. If a plug goes in cylinder dirty and comes out clean,it either being washed with fuel or steam cleaned by water intrusion.

My first post stated that about the plug being cleaned, and I did ask about it. I did look in the water jacket to look for a sign of a cracked water jacket. It all looked good.
Compression is at 115 on that top cyl. 120 on the bottom.

Why is it I can't test the trigger wires independently without causing an rpm fluctuation on one wire and killing the engine with the other wire being tested, testing one to the other is over 10V and climbs as I increase rpm's.

Totally off base numbers when testing stator resistance. High ohms where there should be low, and low ohms where there should be high, and no numbers close to specs. Yet it's putting out 300plus volts to the pack at idle and 225 plus to the coils

Didn't pull the exhaust cover yet. Didn't want to waste a gasket if I didn't have to.
You think all this electrical mumbo jumbo is for nothing and it could just be a gaulded piston? I guess anythings possible at this point.

 

[Bob Gilvary]

Re: 25XD 85 model........Fire but no combustion

Bob, just returned home and read your post sounds like you now have good spark to both cyls. When you say that it is only running on 1 cyl. is that at idle only or are you trying to run it up, as a bad reed will not run at idle but will pickup at 1500 to 2000RPM. I hate to even bring this up but have you tryed a new plug in the cyl thats not running a few years back I beat myself to death on a new plug that was bad. Then comes Fastbullets suggestion, You can remove the port covers and check the chrome with a light from outside.

I've had good spark to both cyl's since I changed the coils, and 225V plus to the coils, since before I changed out the broken trigger. Yes, with all this I sure did put in new plugs. Recommended electrode plugs rather than the surface gap. This motor runs like a swiss watch on one cylinder and can even tune the carb to lean and rich with one cyl working.Looking in at the reeds this morning, I saw nothing wrong, and lightly pushed on each one to be sure nothing was caught in them.

I did a fairly new 25 Yamaha carb, that had a mouse nest in the intake manifold. It was full of what looked like cotton and a couple kernels of corn in there to feed on. One of which got caught up in a reed plate when I tried to start it, before working on it, just to see how it ran. That's why I always look at the reeds.

This engine is hanging in a barrel so I can run up the rpm's, and in neutral it's a beast or sounds like one. I've got a good supply of unburned gas and oil floating in the barrel again though.

I know the trigger was doing what it does to get spark to both coils, but it had bad ohm reading which made me look at it and see that it was broke at one contact point and rubbing the magnets as they passed.

Those ohm readings at the stator wires are way out of range, and I suppose it couldn't hurt to ease our curiosity to look at the pistons through the exhaust cover, just didn't want to waste a gasket. Guess I could order it along with a stator though, if they look good.

I'll be back!!!

 

[Laddies]

Re: 25XD 85 model........Fire but no combustion

Pull the intake side it's got o rings instead of gaskets

 

[jbjennings]

Re: 25XD 85 model........Fire but no combustion

Just following this thread out of interest, and because I like to see reponses from Fastbullet and Laddies since they are so knowledgeable. I was just thinking that a bad exhaust gasket might spray water in the lower cylinder, killing spark and steam cleaning the plugs.
JBj

 

[Bob Gilvary]

Re: 25XD 85 model........Fire but no combustion

Top cylinder only with 115# of compression. I'll get to it tomorrow.

 

[Bob Gilvary]

Re: 25XD 85 model........Fire but no combustion

Told ya I'd be back.

I pulled the exhaust side, and the pistons are beautiful, cylinder walls have minor lines at or about the intake port areas. Both pistons are wet with fuel around the rings, rings are springy. Seem to be equally wet with fuel around the spring area. As seen in the barrel, plenty of unburned fuel oil mix is floating on top.Top cyl and piston looks better than the bottom. Both piston tops seem to be washed at the intake ports and carboned at the exhaust side. I inspected both exhaust and water jacket gaskets, they appeared to both be intact and not leaking anywhere. Some carbon build up at the exhaust gasket where exhaust first comes in contact. Equal build up at each port.

No blow back through the carb from crankcase pressure. No back fire from plug firing out of time.

Made up two new plug wires, just to eliminate that issue.

Put a timing light on the dead cyl, and it's getting fire. Question now is, where is the timing mark in these engines. The light illuminates the flywheel timing marks when the fly wheel is at the strbrd side position, but looking all over the engine, I can't seem to find a pointer or reference point to the flywheel marks.

No, I didn't think to put the light on the bottom cylinder before I took the exhaust cover off. That just now came to me.

Still have the issue of poor and almost opposite ohm readings on the stator. Low ohms on the black/yellow wire and high ohms on the black/white wire
Still not able to test each trigger wire without either killing the engine, or increasing rpms, plus 10v or more when testing trigger wire to trigger wire, and it should only be 4v or more, but that much more?

 

[Laddies]

Re: 25XD 85 model........Fire but no combustion

There should be a line cast into the starter housing above the dashpot on the starboard side of the engine and a decal on the flywheel if the decal is gone the should be a punch mark to mark TDC. 10 volts are not uncommon at the trigger leads, don't know why the system shuts down when your testing it I have not run in to that, your using a DVA when testing aren't you. You don't have exhaust pressure build from to much water in the tank, snuffing the top cyl. with exhaust pressure at the intake port do you?