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1975 9.9 a few questions

J

jim18611865

Regular Contributor
Second season on this engine. My 15 yr. old easily put 20 gallons through it this year, but the low idle situation became worse.
http://www.marineengine.com/boat-fo...75-9-9HP-should-I-fix-this&highlight=1975+9.9
So I have the flywheel and armature plate off.
Looks OK under there.

My plans are new plugs, new Delrin ring (armature plate ring as suggested in other post), and impeller.
I will check timing and points gap.
Should I replace or do anything else?
Oiler wick? (Don't really want ot because the driver coil would need to come out.)

Thanks
 
Gap the points to 0.020". Add a drop of motor oil to the oiler wick. Just a drop. Most idle problems are from the carb so cleaning it with a rebuild kit may help a lot. Make sure you put in a new slow speed bearing and clean under the welch plug. Lastly, make sure the carb and throttle cam are sync'd up. Arrow in middle of roller just as the roller touches the throttle cam.

New spark plugs can help as well. I know my '75 9.9 really hated champion plugs so NGK B7HS plugs have proven to work much better for me.
 
Two years ago I bought it from a guy who seemed really knowledgeable. He went over the carb twice with a new OMC kit.
He is a retired engineer who hunts down all types of OMC's and refurbs them for something to do.

If this doesn't work, I will go after the carb, but I am fairly sure it is missing on spark, and not fuel.
I have the suggested new Champion plugs, but I will order the NGK just in case.
I was told the 74-76' engines were poor trollers, but they shouldn't be as bad as mine.
Thanks all.
I will report when work is complete.

What is the function for that Delrin ring? Insulator or something else? There was a small crack in it, and it was sort of loose.

It really runs great from mid range and up.
 
When the motor is idling is there exhaust coming out the idle relief ???------------These motors have been know to plug idle relief ports with carbon.
 
racerone,

Assuming you are talking about the exhaust relief ports, yes, a fine spray of water comes with it at idle.
At WOT it is pouring water like a teapot.

Jim
 
If you troll a lot, go for the NGK B6HS plugs. Missing and fouling is what happens with the champion plugs, at least it did on mine and quite a few others on these forums. Have you done an external spark test to see if the ignition can spark a 3/8" gap? Not sure about the Delrin ring. I think it creates a friction for the timing plate above it but cannot remember why it is needed.
 
With a 15 yr. old at the helm 90% of the time, it is WOT 89% of the time. We don't troll, but we do have to wait for traffic to ease sometimes. (Navesink and Shrewsbury River/Sandy Hook Bay)
Mostly I am concerned about safety.
Stalling in heavy traffic, a ripping current, and waves in a 12' boat is no fun.
Don't ask me how I know.

Does it matter how that Delrin ring is installed, ie. the gap located in a specific place?
The manual doesn't say, but in the pic the opening in the ring is port side.
 
OK going back together, points are a little wide, would this have caused my symptoms?
Book says .020, but they are more like .025.
 
As ignition point gap changes so does the timing.-------------If motor refuses to idle properly then one of the things to check is blocked holes in the exhaust extension.-----They plug up with carbon.---------Sounds like a big job to pull the powerhead to check this, but the motor will not fix itself.-----Elbow grease is required.
 
OK tank and boat tested.
90% improvement. Only place it still misses, is about 1500 RPM (a guess) under load about every 20-30 seconds. Otherwise it ran perfectly.
Only things replaced were plugs, Delrin ring, and adjusted the one point to .020. It was .024.
Low idle under load was perfect, no misses. We could have trolled forever, and we did for quite a while, as we sneaked up the Farmington River from the Rainbow Reservoir.

I don't know about pulling the powerhead to fix a minor annoyance at this point, but I will consider it if it gets worse again.

Thanks again all!
 
You never really said what the exact problem was with your low idle but a point set at 0.024" would not have helped it. If you look closely at your throttle system you will see a piece under the flywheel that contacts your roller that controls the throttle butterfly in your carb. As you turn the throttle, this makes the carb open more. What is interesting here is the idle system. When you idle down, the roller actually moves off of the throttle cam, which means your carb is not closing anymore. So why does your idle decrease when you reduce the throttle more? It is because the timing plate is still moving. The timing plate is just under the flywheel and it advances the spark timing at the different RPMs of the motor. As you reduce the throttle twist more, all you are doing is moving the spark timing a little more past top dead center (TDC). This makes the pistons move even slower for the low idle.

Now when your point gaps are adjusted to 0.020", we know it positions everything into perfect timing so the spark fires at TDC at every throttle range except idle, as mentioned above. At idle the timing is moved in the other direction and this is what gives the smooth very low idle. When you increase your point gap, you are also advancing the timing in the other direction. At high RPMs you may not notice the difference from 0.020" to 0.024", but at very low idle you just might. I will point out that it isn't much of a difference and when installing new points, OMC usually recommends a gap of 0.022" anyway, to account for the quick wear that happens to the rub bar on brand new points. My point is that a wider point gap will probably become more noticeable at low idle and a narrower point gap will be noticeable by a reduced top end performance.

The other thing you will want to do is ensure your carb is sync'd properly to this throttle cam. This is done by ensuring that the little arrow indented on the throttle cam is in the center of the carb roller when it "just" hits the throttle cam. This also, is very important to ensure proper spark timing.
 
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A statement is made here,----------" spark fires at TDC at every throttle range except idle "----Are you 100% sure about this and can you explain that again.
 
That's the theory anyways. The timing plate advances the timing (spark fires before TDC) as the throttle grip is turned to a faster throttle. The opening carb butterfly produces higher RPMs and this advance is designed to get the main energy of the blast as close to TDC as possible (since there is a time delay between spark and maximum combustion energy). This is done by making the ignition fire the spark more "in advance" at higher RPMs, positioning the high energy of the combustion at TDC.

When idling, however, as one turns the twist grip in the slower direction, the carb roller eventually moves off of the throttle cam. Since the carb butterfly does not close anymore, the only thing that can be causing the continued reduction in RPMs as the twist grip is turned even slower, is the timing changing from TDC to something after TDC, produced by the timing plate movement. Eventually if you keep turning the grip in the slower direction this timing reduces to a point that the motor will die.

All that being said, the timing plate is designed to position the maximum energy of the spark at TDC at all RPMs ranges except idle where the design allows for spark after TDC to produce an even slower idle RPM.
 
First you say that " spark fires at TDC except at idle "----Now you talk about highest pressure at TDC and that is totally different !!
 
Sorry about that. Yes, they advance the spark to get the maximum energy at TDC over all the RPM ranges, except idle where they have designed it to provide that energy at positions after TDC, to produce a lower, smoother idle.

That is what I am trying to say.
 
At idle on a 2 stroke engine only so much air and fuel / oil is metered to go in with throttle plates closed.-------In order to slow the RPM down the timing must be retarded.
 
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