'99 70 HP Johnson does not idle well

[HaywardBound]

I'm posting this for a friend.

His boat will not idle down very well. To keep it running he has to choke it to keep it running and has to constantly re-start the motor because it dies so often.

Give it some throtle and it runs great. We are going to change the plugs, but I think the air-mixture needs to be adjusted.

Is this something we can fix, or would it be better to take to the shop? We are paranoid about adjusting it so it doesn't get enough oil.

If we can do it, which screws to we mess with? Is it running too rich or too lean?

Last question, what spark plug do you recommend, and what gap does it need?

Thanks.

[Jim Almquist]

If your motor is a 2 stroke I would find out what NGK plug it takes. I have found that in a 2 stroke they can make a lot of difference.The oil you use can affect the idle as well. I had a 50 yammaha that liked only merc 2 stroke oil.I would try both of those and some premium gas and see if that helps.When it comes to adjusting carbs I would bring it to the shop.

[No_Net]

I had the same problem with the same moter. We trade it in and the frist thing the dealer said was that the gas hose was to small. Hope this helps.

[Surface Tension]

If hes constantly choking it to keep it running its most likely fuel shortage. Start by checking the condition of fuel lines and connections. Replace the fuel filter. Still have a problem then I'd be looking at dirty carbs or bad fuel pump.

Before bringing it in to a dealer you can try the miracle cure in a can. This will be a shock treatment, in a portable tank use 1 gallon gas to one bottle Sea Foam or carb cleaner. In the driveway with muffs run the mixture for 15 minutes and then shut off the engine and let set for a 15 minutes then repeat the process till your out of gas.

If the problem is still there bring it into the shop.

[VMS]

Couple of things to check before adjusting fuel air mix...

First, I would disagree on going to NGK plugs for a Johnson Motor. The recommended plug is a champion, and those will run best.

Second, If you have access to a timing light, check the timing of the engine. Somewhere on the side of the lower unit/mounting bracket there should be a plate that states what the timing should be at idle. If this is off by a few degrees, that will explain the problem. I had to adjust mine on a 99' 50hp Johnson. It was off by about 3 degrees, would not stay running at low idle, but then after adjusted, ran like a purring kitten. Top Cylinder should be the #1, hook timing light to that one, and test... The adjustment of timing should not be too hard to do either.

Also, I would do a deep cleaning with a product like sea-foam. Mercury sells the same type of cleaner called power tune. Take off the air box, get the engine as warm, spray into each carb quickly until the motor dies or almost dies. Spray in some more to each carb and let sit for about 15 minutes or so. Then, take it to the lake and open her up. It may just be you have some restriction in the jets, or carbon build up in the cylinders...

Only after doing those would I suggest changing the fuel air mixture on the carbs. They need to by syncronized so all cylinders are getting the same mixture of fuel.

Steve

[Valv]

Quote:

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Second, If you have access to a timing light, check the timing of the engine. Somewhere on the side of the lower unit/mounting bracket there should be a plate that states what the timing should be at idle

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Where is the "timing" on a 2 stroke ?

[PierBridge]

Great info guy's.........sounds like a plugged carb to me.

[marine_man]

Yep... sounds like a plugged low speed jet to me...

marine_man

[Jeremy airjer W]

I'll second the seafoam. This is a fantastic product! Its basically a fuel system cleaner, decarbonizer, and fuel stabalizer all in one. It wouldn't be a bad idea to run a tank mixed with seafoam the first time out anyways. It made a huge difference in the idle quality of my Mercury.

[VMS]

If you look at the top of the flywheel, it will be along the edge. There will be a pointer along side the flywheel. If I remember right, it was located to the port side of the engine just a little, and toward the front.

The idle adjusting rod should be just below it if memory serves me correctly.

Pretty easy to do. I just went slowly to figure out which way to go first, then adjusted as needed.

Steve

[HaywardBound]

Thanks for all the help. We'll start with the plugs and fuel issues first before adjusting anything.

HB