1985 Merc 90 (inline 6) trouble shooting help needed

[redsput]

While out on the lake last week (boat running fine to this point) while throttling up (punched it) the engine "seemed" to foul a plug. Changed plugs, no difference. Carbs were wet, so I ordered some kits and replaced the needle-seat and cleaned them. (Though, they were ridiculously clean to begin with, and I know they have not been touched in at least 10 years.)

Checked the carb/butterfly's - all are visually in synch, I didn't mess with any of the settings - and made sure I put the idle screws right back where they were.

Engine still stumbling, like it's not hitting on all. Re-checked plugs - wet... Checked spark, all 6 appear strong (I have one of those gizmo's that allows you to see how far the spark will jump.) Checked compression, all 6 are right around 140 on my gauge.

I have always run 40:1 with Pennzoil premium lube, 89 or better octane, and when I cannot find ethanol free gas, I treat with lucas or stabil marine ethanol treatment.

I'm guessing something electrical/timing related possibly just based on what I've read, but certainly no expert. Also, all the "testing procedures" I find on the old inline mercs seem to be for the older distributor models (mine is electronic ign, no distributor.)

Guide me oh' outboard wizards, a boatless 4th of July is not something I look forward to.

 

[canoemang]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

Even though you didn't mess with any of the settings you still should do a linc n sync after carb removal..

 

[CharlieB]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

Couple things to check.

Carb fuel inlets may have screens, check to see if any are obstructed causing that carb to run low on fuel.

Fuel pump flow, pull the hose to the carbs crank the motor to check for a strong pulsed stream.

Pull the crankcase hose to the fuel pump, check for wet fuel and pump the primer up and hold pressure watching for fuel leaking indicating a pinhole in the pump diaphragm.

Fuel hoses, aging can cause cracking dislodging chunks of the clear liner clogging the carb inlets or actually creating a 'flap' that can restrict flow to a carb.

Quick disconnect fuel fitting, O-ring can age, sealing pressure from the primer yet leaking air IN when the fuel pump sucks on it. Replace a hose to the carbs with a temporary clear hose, run the motor and watch for air bubbles passing thru indicating an air leak. REMEMBER to remove the clear hose once finished testing as clear hose is NOT APPROVED for fuel use.

Lastly, check the timer lever for free movement, sometimes the pivot or trigger plate gets sticky and the timing fails to advance when the throttle opens.

 

[redsput]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

Excellent suggestions - thanks!

*No screens, clean as a whistle. I replaced every fuel line from tank to carbs including a new bulb and filter... sierra line.

Tried the clear hose thing in the line that goes to the pump (inside the engine cover), saw a few bubbles when I squeezed the bulb the first time, but once running - seems clear. I have a fuel pump rebuild kit that will be going in this afternoon.

Note on the comment above about the linc-sync. I read some posts, even watched utube videos showing how, but everything seems to be about the older - distributor models. Can you direct me to the elec ign process for this? Or is the same process, but I just turn and adjust the timer thing vs distributor?

*One other note. It starts at the touch of the key, no issue there. But smokes like crazy - and still "stumbly" like it's not hitting on all consistently... seems like its drowning in fuel.

 

[canoemang]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

link to link n sync..

http://forums.iboats.com/engine-fre...q/timing-sync-merc-inlines-1988-a-168855.html

 

[redsput]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

I did have bubbles in the fuel line going to the pump, more new parts installed including a fuel pump rebuild. Now zero bubbles, still starts at the touch of the key, yet zero improvement. I double checked compression with someone else' "srew in type of gauge. All are between 125-130. I seem to have good spark on all, carbs are just cleaned with new needle and seat.

The person I borrowed the comp tester from suggested a possible bad reed based on when it started running poorly (at wot, kind of popped and started sputtering like a fouled plug...) Do the symptoms fit, and how would I be able to tell for sure?

I also ran through the lync-sync instructions and set everything as noted (which is right where it was at.)

 

[CharlieB]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

What is your WOT RPM or will the motor pull enough to get up on plane?

Do you have a DVA meter or a DVA adaptor to use with a Volt/Ohm meter?

The reason I ask is you need to make sure that the stator is generating sufficient voltage from the high speed windings to both switchboxes, if either box does not receive sufficient voltage them the ignition will not make an adequately hot spark at speed.

Once you are sure the stator is proven good/bad you can move on to checking spark at speed, either using a DVA meter at the switchbox/coil wire, a timing light, or an inductive tach to test each plug wire at speed. If the timing light goes out then that cyl is not arcing/firing. This would cast suspicion on the switchbox.

 

[daveswaves]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

Lots of good suggestions here, one that is missing is to re check your float drop settings. You changed your needle and seats so the height will be off. If the float level is too high fuel will overflow into the carb mouth via a tiny hole on the inside left of the carb throat. Your engine will drown in fuel.

 

[redsput]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

Found the issue finally. Connected an old Sun engine diagnostic box that has tach lead an build in gauge. Some cylinders are getting spark cross feed etc giving a drastically different reading (rpm) from one cyl to the next. Bad timer. Ordered one, hopefully I'll be back on the water by the weekend. Thanks all for the great suggestions and insight.

 

[Chris1956]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

Gee, What the heck is a "timer" on an IL6?

 

[redsput]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

I guess trigger would have been the more appropriate description...

Mercury / Mariner Trigger: 134-6456

 

[redsput]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

Well, so much for my mechanic pal and his fancy tester. New trigger installed, same stinkin issue, no difference at all. Debating now if I go invest in some diagnostic tools or just take it somewhere...

 

[CharlieB]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

The only ignition diagnostic tool needed is a DVA meter or a DVA Adapter to use with any VOM.

An inductive tach is also nice to have.

If an inductive tach register one or more cyl's different than others AND the trigger output is stabile THEN you have a switchbox developing a bad Bias circuit which blocks the other switchbox from firing a cyl.

A bad Bias will totally block the other box from firing and 3 cyls will fail to spark.

Verifiy RPM on every cyl, if differences exist then order switchboxes. Many of us prefer CDI boxes, much sturdier internals AND can be replaced one at a time where Merc boxes should be replaced in pairs.

Any box with a bad Bias can still be used on a 3 cyl motor, the Bias is not used. List it on ebay, someone else may need it.

Always remember that NO RETURN thing on electrical parts. Test, test again, and re-test again just to be sure. Positively PROVE the fault.

 

[redsput]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

DVA adapter ordered. If I have a bad switch box, and they are both OE type, do I need to replace both if I go CDI ? (Assuming 1 cdi, 1 merc would not be good?) Also, I read a post somewhere I think that suggested replacing the stator at the same time. Is this necessary? If so - ouch!

 

[jimmbo]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

Try richening up the carbs a tiny bit, 1/8 - 1/4 turn from best idle. the 84 and 85 115 were really finicky about the carb/timing sync. I know mine was

Use 50:1. Adding more oil actually leans the air to fuel ratio since the carbs have to meter a mix of more oil less gas.

 

[CharlieB]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

If the switchboxes are Merc then yes, you need to replace both. If they are CDI then you can replace only the one bad box. The trick is to determine which one is bad. If it is a bad Bias circuit then it is the one that is firing all cyls fine. However if it is only one cyl misfiring then it is a bad SCR in the box so the misfiring box is the one needing replacement.

If the stator is generating voltage within the specified ranges then no, do not replace it.

 

[jimmbo]

Re: 1985 Merc 90 (inline 6) trouble shooting help needed

I recall the dealer telling me they would change both switchboxes when one was bad. One day I had a switchbox fail. My friend at the dealer ship gave me a box with 8 or 9 used switchboxes. He said they were from the 'changed both switchbox' inventory, and that they might be good or might not be. I took one, put it one and it worked. It was still working 9 yrs later when I traded it in.