Mercury 1986 V6 135 rough idle issues - Please help

[UKMike]

Hi everyone,

I've been told there are some real Gods on here that may be able to help me identify the problem I have and I was hoping one of those Gods would be so kind as to advise?

I have a Mercury Black Max V6 135 serial 0A961186 that has started to have an idle issue. The engine was running fine after changing the following parts but has just started to run rough on idle so that I can't park up or start up and put it in gear without using the throttle first. The engine has all original parts still attached including the self oil mixing devices.

I recently de-winterised her and have changed the following parts:
Lower gear lube
Water pump
Impeller
Poppet Valve
Thermostats
Petrol pipes through out
Increased petrol pipe diameter from front tank to water separator
New water separator filter
New cable ties across the whole engine pipes
New coils
New spark leads
new plugs
New water pressure sender and pipe
New Fuel tank sender unit
New Fuel Pump Diaphragm

The only filter I have not changed this season is the engine fuel filter, that was new at the close of last season and only had 2 weeks usage!

All of the above was done 4 weeks ago and the engine has been running great since but last week I started her and she just stalled with a bit of a splutter. I started her back up again and she starts but the revs go up and down for approximately 10 seconds and then the revs drop and she stalls. If I start her up and increase the revs using the throttle she runs fine but with a little bit of roughness, she seems to be firing on all 6 and she doesn't seem to be pissing unspent fuel out of the carbs or casing cover.

I have sprayed carb cleaner into her while running and I have also used a whole tin of power tune on her, this did improve the running a little bit but I have ended up increasing the idle throttle screw to bring the revs up enough for her to not stall. I don't like doing this as there is something wrong and I have been told that if the idle stabilizer has gone it is possible to pop/crack a piston on WOT and that is not only expensive but it's very dangerous as I only use the boat at sea, normally 12 miles out....

If I take the revs over 1200 I loose all the roughness so I was wondering if it could be the idle jets or something else?

Please could someone advise? I have today purchased a spark tester and a comp tester as I believe that the compression should be around 110-120 per cylinder.

Hope someone will help, I just can't bear the thought of my engine being sick, I love her

Br
UKMike

 

[Faztbullet]

You listed everything but cleaning and rebuilding carbs...

 

[UKMike]

Thanks Faztbullet, I have the kit's for the carbs but I'm not 100% sure on the linc and sync process and also I cant find any carb float replacements. Do you know of anywhere to get them from? I checked Iboats but couldn't see any..

 

[UKMike]

Can someone advise that If I use the carb kits, why do I need to do the timing? I understand the 3 carbs are linked by the throttle rod but I don't understand why the timing would be affected by servicing the carbs?

Is it just a good idea to do the timing regardless or is this required? the link and sync documentation I have from solec online says you must do the timing..... Now i'm confused...

 

[Chris1956]

It makes sense to check the timing. It will take a couple of minutes to check idle pickup and max spark advance timing. Hopefully your Seloc is up to it.

You can probably find a Merc Service manual online, if you poke around a bit.

 

[mumfrumson]

Mike I have that exact Mercury on my sons boat. The fact that you call it "her" is cute. The problem your having at first glance is that your spraying quick fix solutions into your motor that you can buy at the local mini- mart. You need to remove the Carbs and them cleaned professionally if your not comfortable with this. All you have done with the "spray cleaner" is wash salt and grim into the Jets. Once that is complete, you have a base line.
I was able to Google the appropriate rebuild kit.

Even without the rebuild kit, carefully remove the Carbs so save the Gaskets. Soak the carbs an parts in Acetone over night. Carefully clean with a toothbrush, wash and soak a little more. visually inspect the jets, carefully clear with a pin if necessary.


It's not the timing so dismiss that post. Trimming the boat certainly will not help with idle problems, throw that post away. I'm not sure what you mean by lync and sync, I cannot comment on that.

Also tread lightly with screwing with valve, throttle and carb. adjustments. These have been in place for 20 years and are probably not your problem.

One thing I have found with this motor is that it is essentially a gravity feed / vacuum fuel system. Mine started experiencing similar issues over time. What i noticed was air in the fuel filter (get a clear one). I was able to track down the air leak to the mechanical fitting between my inboard tank and the motor. I replaced that and added an external fuel pump (mounted in the boat itself) with a check valve. The motor runs amazing. if you have inboard tank, purchase a separate portable tank with new hose and pump ball to eliminate on board hose leaks etc. once less variable. I have rebuilt the fuel pump and replaced other fuel line etc. The addition of the pump turned out not to be 100 percent necessary after the leak was found, but I left it in place since i could notice the engine loved the extra fuel. No more idle or rev issues.

You also might want to take a hard look at ignition wires and plug wires, they tend to crack over time. Try running the motor at night with the cover off. Look for arcs, this can also be causing your problems.

Check the fuel screen if you have an built in fuel tank. Tread lightly removing the cover from the Tank. Dont use electric drills for this, and smoking is not a good idea.

Good luck, beware of backyard mechanics. once you start screwing with timing, idle adjustments your going down a wormhole. There are no quick fix chemicals at the mini mart that will help.

 

[GA_Boater]

mumfrumson

UKMike has not visited since starting this thread. We appreciate the helpful advice for Mike, but he has left the building. Take a look at the Help Tip on top of the page.

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