Idle mixture adjustment problem

[TwoFish]

I tried to set the idle mixture screw on my 1998 75 ph Force. Only one carbie so it should be easy.

I set the screw to 1 1/8 turns out, boat in the water, in neutral about 1000rpm. The engine ran OK. ? turn in the motor ran rough. All good. When I turn the screw out past 1 1/8 it didn’t seen to make a lot of difference. I stopped at about 2 3/4. The engine ran slightly rougher but not much. I have been running the engine with the screw at about 2 turns out for the couple of hours as I had this trouble when I serviced it. Tried to set it again last night but had the same problem.

A bit of history. The fuel is about 10 weeks old (no E10). I have just finished servicing it. Timed, new plugs, new fuel lines, new inline filter and water separator, new wiring loom, removed carbie and cleaned. I posted some photos of my plugs on another thread and fucawi thought I may be getting some water into the motor. I still need to check this although I would have thought it would make the engine run even rougher.

Latest photo of plugs after about 3 hours of running. A fair amount of time at idle looking at the sounder and trolling.

Is this normal or should I try to clean the carbie again.

Thanks

Dean

Left to right, cylinders 1 to 3

 

[ratherbeboating]

Re: Idle mixture adjustment problem

i agree with fucawi, from you picture it looks like you have water on one and two. you could try a compression test to see if everything is even. pulling the head and checking/changing the gasket is relatively easy and in-expensive. and thats hpoefully where the water could be coming from. did the engine ever overheat??? you may need to check your head surface for warpage and it may need to be milled.

 

[pnwboat]

Re: Idle mixture adjustment problem

Make sure the fuel recirculation system is working correctly. This burns off any puddled fuel in the by-bass passages that build up a low RPMs. Item 3 and 5 in the diagram are one way check valves. Make sure they are working correctly. Sometimes they get clogged. Also check the hose and the other fittings items 1, 6 and 8.

http://www.crowleymarine.com/mercury-outboard/parts/2534_5.cfm

 

[TwoFish]

Re: Idle mixture adjustment problem

@ ratherbeboating

140 on all cylinders. Water could still be getting in through the side plates though. The plugs have only 3 hours running since new. I have put it down to a single carbie running 3 cylinders. The top plug runs leaner than the bottom so the bottom plug gets dirtier. I run in salt water and would have thought the salt water would have caused more problems if it was getting in. It has done this for the three years I’ve owned it. I’ve put it off as the block needs to be removed before you can get at the side plates. May still need to do it though. Just trying to eliminate easier things first.

@ pnwboat

Thanks for that one mate. I haven’t checked these before. They looked like something that would get brittle with time and have left them alone. I’ll wait till after the holiday season and give them a go in case I happen to snap one off. They look like a fairly standard part and hopefully easy to get hold of.

 

[jerryjerry05]

Re: Idle mixture adjustment problem

From the looks of the white stuff on the 2 plugs I'd say it's possible there is water getting in at the space between the cylinders(headgasket).
I too run in salt water and have had to replace a gasket even though the comp was 145 on all 3 cyl. The head had a small spot where the salt collected and wormed it's way into the gasket.
Good compression but the plugs looked like that.
I had to get the head welded in that spot.The salt and heat ate a small hole in the alum.J

 

[TwoFish]

Re: Idle mixture adjustment problem

Thanks for the advice Jerry. Not what I really wanted to hear. As far as I know the head gasket & side plates have never been touched. For my own piece of mind I am going to have to check them. I don’t fancy having to get the bolts out of the side plates.

To get to the side plates on this motor the engine block needs to be lifted off. One of the bolts on the port side near the solenoid has a hexagon socket on it. Do you need a special fitting for a sockets wrench set to get this out?

Thanks

Dean

 

[Jiggz]

Re: Idle mixture adjustment problem

You mentioned using 10 week old fuel for this test. How about cleaning the plugs first and then get a fresh gallon of fuel mix and do a test and visual check of the plugs again. This is to make sure the water source is not from the fuel itself. If the same symptoms persists then it's time to check on the head gasket. As for the original question of idle mixture setting, 1 1/8 is the minimum opening for idle and start position for tuning. Since this is like the minimum setting, when turned or screwed in the engine idle roughs due to a very lean mixture (mark this point of write it down). So you screw it back out until it smooths again usually around 1 1/8 or 1 1/2 turn. If you screw it out, you have to listen carefully, for initially there will be very little change in engine idle from this point on until about 2 turns or 2 1/8 turn out where the idle will start to increase slowly. Continue turning the screw out which will eventually raise the idle high and then will start to rough again due to very rich mixture. The manual recommends setting the idle between these two points (rough idle at lean and at rich mixture). The idle rpm could be low or high depending where the idle screw is set. After setting the idle mixture screw midpoint of the rough idle points, then set the engine idle screw (not the idle mixture but IDLE screw) to set engine rpm at 1000~1200 in the water neutral which will be around 750~800 RPM in gear idle in the water. The midpoint setting is not set in stone per se but this is the point where you can fine tune the engine performance also. Do a "shot in a hole" test and if the engine is initially sluggish but eventually picks up, you might want to increase the idle mixture screw a little bit like maybe 1/8 turn. Or if the engine has a very good initial start but smokes heavy, you might want to decrease the idle mixture screw a bit.

 

[TwoFish]

Re: Idle mixture adjustment problem

Thanks Jiggz!

That description of setting the mixture clears things up for me. I will reset the mixture and see how I go.

I have a water separator with a clear bowl and have not noticed any water collecting in it. I have had the boat for three years and check the plugs regularly and they have always looked the same. In that time I have drained the main tank and had fresh fuel. I have a portable tank which I can fill with new fuel so I’ll give your suggestion a try. As I said earlier the plugs have only been in the motor for about 3 hours. If I leave then in for longer the top two plugs darken up but there is still a difference between them and the bottom plug.

Thanks for your help

 

[Frank Acampora]

Re: Idle mixture adjustment problem

Is this an engine you have just bought or an engine you have had for a while? Reason: Previous owner may have modified or removed parts. The manifold does NOT have a completely circular opening in it to match the carb body. Part (about 1/3) is blanked off by the casting itself, and it was designed this way. The carb itself has a stainless blanking plate bolted to the front. Now, I don't know what these parts do but I suspect it has something to do with the way the air/fuel mixture is directed to the three reed vee blocks since there are also baffles cast into the manifold internally. Removal of either or one MAY affect the idle mixture setting in that one or more cylinders may not get enough air/fuel mix at the correct ratio.

 

[TwoFish]

Re: Idle mixture adjustment problem

Hi Frank, Thanks for your help.

I have had the boat for 3 years. Only one previous owner. Only driven to church on Sundays. Usual story.

The boat was in immaculate condition and the motor had 70 hours on it. Used in fresh water. The owner didn’t look like the tinkering type but it is hard to tell. He had receipts for services done at a shop near him.

Plate on the carbie is there as per photo. From memory the manifold was blanked off as you say. I removed and cleaned the carbie when I serviced it about 10 weeks ago. I had to wait for a new engine wiring loom to come from the States. Weather and work have prevented me from using the boat until last week hence the in-water setting of idle and mixture. It leaned off fine but I was expecting to start running rough quicker and more noticeably than it did when turning the screw out. I set it at 1 1/8 turns and expected it to start running rough by the time it was out to 2 turns. I will set it again the next trip out and see what happens.

I’m more concerned now about the possibility that water may be getting into the engine. The motor is 13 years old now and as far as I can tell has never had the head or side plates checked. The words "Can of worms" keeps popping into my head.