Update on Merc inline 6 piston burning!!!

[200valeroyt]

Today I dropped the boat in the water on the trailer to check all 6 cylinders under load and I found that all 6 cyl's were advancing to 27-28 degrees @ roughly 5k rpm! I can't imagine this being right since its suppose to retard 2-3 degrees? I assume I have at least 1 bad switch box? One other thing that should be noted, when I first started the motor idle timing was 5 degrees atdc, which is where I set it and close to factory recommendations. After running the motor up on all the cylinders for 5 minutes or so it was showing 6 degrees btdc, which is 11 degrees advanced an idling high. The plan is to replace both switch boxes, unless someone can shed further light on this problem? Thanks, Sean

[JWTjr.]

you're on it.

[200valeroyt]

Is there any chance this could have something to do with the idle stabilizer being bad?

[daveswaves]

Is there any chance this could have something to do with the idle stabilizer being bad?

Sean, it is common practice to disconnect the idle stabilizer. I should have asked you if you disconnected it before. I still think you have one bad switch box but the stabilizer can screw up the troubleshooting big time. The stabilizer retards the timing and holds it in reserve to boost the idle if it drops too low. At idle to 2000 rpm all your speed adjustment is timing based. If the stabilizer fails it will dump that timing back into the equation.
I would disconnect the stabilizer and re set your timing (again) at cranking speed. Kudos to you for noticing the timing increase.:thumbsup:

[200valeroyt]

I would love to believe that's the whole problem, but that would be too easy and cheap. Ha... What's weird is that all 6 look very close with it running under load, just very high! I feel sorry for what the pistons have been going through! Ill try again today with stabilizer removed.

[200valeroyt]

I headed back to the ramp today with the idle/advance module removed. The timing stayed in check without advancing to 27 degrees like it was doing yesterday. Apparently, this was the problem. Now that i can actually see my marks better on the flywheel, i notice 2,4 &6 are still a couple degrees more advanced. Different switchboxes? I ran the piss out of it for 30 minutes on the water without issue. #4 piston is looking much better and looks to have a bit of carbon deposits forming were the burning was taking place.

[CharlieB53]

If you are willing to invest a little more time, switch the switchboxes and see if the odd cyls are then advanced that couple of degrees.

OR....

Disconnect the Bias wire linking the boxes, while cranking check individual timing to see if the even cyls are advanced. If not, reconnect the Bias and confirm the advance.

Looking to confirm if it is the even switchbox or the Bias on the odd box causing the advance.

[200valeroyt]

Yea, I was planning on switching the boxes to see if it changes to the odd cylinders.

[daveswaves]

Yea, I was planning on switching the boxes to see if it changes to the odd cylinders.

Good idea, but also follow charlie's advice and check the timing with the bias wire disconnected and connected after you swap the boxes. Post your results and we can interpret them for you.

[200valeroyt]

Well, I first tried it with the bias wire disconnect and it appeared to show a bit more timing, maybe a couple degrees? I then switched boxes and it didn't seem to change, with or without the bias wire connected? Is this maybe a combination of the crank being out a degree or so from stoke to stroke and having 2 different boxes? A factory mercury and a cdi? Kinda strange I guess? I have had a couple v6's that showed a degree or so off?

[200valeroyt]

Or the magnets in the flywheel being off a bit?

[daveswaves]

Well, I first tried it with the bias wire disconnect and it appeared to show a bit more timing, maybe a couple degrees? I then switched boxes and it didn't seem to change, with or without the bias wire connected? Is this maybe a combination of the crank being out a degree or so from stoke to stroke and having 2 different boxes? A factory mercury and a cdi? Kinda strange I guess? I have had a couple v6's that showed a degree or so off?
A degree off is no big deal and the bias circuit usually brings it in line. The fact that it does not change when you connect the bias wire implies one box has a defective(open) bias circuit. With the bias wire disconnected measure the DC voltage on the terminals where the bias wire connects. Measure both to ground. One of them will be zero volts, thats the bad one. I think the idle stabilizer was making it worse for you and likely was the cause of the switchbox failure in the first place.
Not the magnets, guaranteed.

[200valeroyt]

Good info, I don't thk it's the magnets either, just thinking it could contribute somewhat to the difference? I will go through the voltage check your saying.

[200valeroyt]

The bottom box, which is the even cylinders shows a steady 10-12 volts cranking. Not the same every time, but steady. The top cdi box will not take a steady reading, it just flashes crazy on my cheap volt meter? I assume it does not have voltage and this is the bad box from what your saying? Thank for your help! Sean

[200valeroyt]

Also on my meter it showing negative voltage with the positive lead on meter to the terminal and the negative to ground. Switching the volt meter leads of course shows straight voltage.

[200valeroyt]

What's even more strange is that all 6 seem To advance a couple degrees when bias wire is disconnected?

[daveswaves]

What's even more strange is that all 6 seem To advance a couple degrees when bias wire is disconnected?

That makes sense, in order to balance the two banks of 3 you have to pull back a few degrees from max so you have something to work with. When you disconnect the bias wire they default to the max. Before I answer your question on the voltage measurement I need to think on it a bit, let you know tomorrow. It is difficult to diagnose when you are not there.

[200valeroyt]

Yea, it's a bit frustrating. It's almost like its working as it should other than the even cylinders firing more advanced? I double checked my marks on the flywheel to be sure I didn't lay them out off and they look very close. # 6, which has not been a problem, is getting the most at around 23 deg. 2&4 are 20-21 and 1,3&5 are 17-18. I have another flywheel off a v6 that I may try just to confirm that's not causing it.

[daveswaves]

Ok I will explain the details later cause I am on my I pphone the switch box that has
the steady voltage is the good one. The jumpy voltage is the bad one.

[milkdud]

I have heard that the later distributor model L6's retarded #3 a bit from the rest to help the issues #3 had over the years. So #1 and #3 usually are the worst offenders. Would not surprise me if the odd cyls were retarded. 1, 3 and 5. on the adi motors. 1 and 3 could use it 5 would just be along for the ride.

Just a thought . Glad you took the stabilizer off !
C

[daveswaves]

OK, lets try this again,Conrad, you are correct there is one window in the distributor wheel that is narrower than the rest, just one though. Number 3 runs hotter and leaner than the rest and so it gets retarded.
Back to the switchboxes. The switch box that had the jumpy voltage is bad. The steady voltage is the good one.
There are 3 scrs, biased by 3 diodes, that get their bias voltage from the bias circuit. The bias circuit is negative voltage, you noticed that when you were measuring. The negative bias retards the ignition until a positive voltage from the trigger circuit is high enough to overcome the negative bias. The trigger generates both positive and negative swings. The positive swings trigger the scr, the negative swings charge the bias capacitor through a diode. The diode in your BAD switch box is shorted. That is why you are seeing the jumpy voltage on the bias terminal of the bad switch box. The bias cap in that box is getting both plus and minus. This is why that box is also too advanced. The bias voltage is not negative enough due to the plus getting through the diode.

Lastly, your v6 flywheel will not work on the inline since the trigger magnet is different for the v6 than the inline 6. Your flywheel ring is likely the wrong tooth count as well.

[200valeroyt]

Good deal! Thanks a lot for your help on this, I look forward to having the problem corrected. Sean

[daveswaves]

Good deal! Thanks a lot for your help on this, I look forward to having the problem corrected. Sean

My pleasure, you gave me good troubleshooting feedback which made it easier to track down. Hopefully number 4 survived the process.:thumbsup: