1996 Yamaha 9.9 idle surging

awc31563

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I'll try to keep this short but I also want to give as much info as I can. I bought a 1996 Yamaha 9.9 MSHU 2 stroke that had been sitting in storage for over ten years. Before I put it on the boat I did the normal maintenance, rebuilt the carb/ fuel pump, changed the water pump and gear oil, cleaned the fuel filter and lubed the grease fittings. I had a hard time starting it the first time and when it finally did fire off, it had a terrible sneeze/ spit at idle and would die without some throttle. I ran a can of engine tuner through the airbox, spraying until the engine bogged down and then letting it soak for 15 mins. Did this several times and it helped a little. I adjusted the pilot screw and throttle stop, this made the sneezing subside some but brought on an erratic idle, it would vary between 800 to 1200 r's, never staying stable. It also jumps around a little bit, not unusual for a 2 stroke but more than you would expect. I ran the motor like this all last summer/fall because at mid and full throttle it has no issues and I hardly ever idle or troll. At WOT it is an absolute screamer with excellent throttle response and it jumps the boat on plane as soon as you open it up.

So last week I pulled it off and put it in my shop. Recleaned the carb, pulled the reeds and inspected them, replaced the fuel tank hose, cleaned out the fuel lines, pulled the cylinder head and removed the carbon buildup from it and the piston crowns (quite a bit of build up.) Replaced the thermostat, cleaned, as much as I could, the water passages (it had been run in brackish water.) The plugs were almost fouled wet, so I went to the next higher heat range plug. Adjusted the timing and all the controls. To get the timing on the flywheel marks I had to use up every bit of the adjustment length of the magneto control lever. I don't know if this could be a part of the problem because it is still in range of the adjustment but I wonder if the flywheel may have jumped a notch or something like that?

I put it in the test tank, it fired right up and the sneeze/ spit were gone. But the erratic idle was still there, running between 800 and 1200. I played with the pilot/ throttle screws, no change. Sprayed all the gaskets with WD-40 and then soap/ water, saw nothing (an air leak would be a constant lean condition, would it not?) Top plug slightly wet/ oily, bottom plug clean and dry.

While the engine was running, I noticed that the exhaust wasn't blowing out the hub but instead coming out the relief holes, I tilted the motor up to inspect this and when I did it revved up on it's own to about 1500 r's and the exhaust started coming out the hub. When I tilted it back down, the idle smoothed out to 850 and held for about 3 minutes before it started to hunt again. This made me think that something in the carb is building up pressure and then spraying gas when it gets to a certain point.

Cleaned the carb one more time, paying special attention to the idle passages, put the motor back on the boat, much better. Screamed across the lake, put it in neutral, let it idle, nice and smooth at 850 but after 3 or so minutes it would start to rev up a little before going back down, less severe hunting as before. Also figured out that if you rev it up in neutral it seems to "clear out" the condition and you get another 3 minutes of stable idle before it revs up. It still jumps/ shakes at idle but not much more than you would expect from a 2 stroke.

So, after all that, my question is - what could "load up" and release fuel after 3 minutes? Is the float bowl fuel leaking up into the idle passages? What about unburned scavenge gasses recirculating into the intake and the higher revs burning them off, returning the fuel/air back to normal? I don't think the CDI has anything to do with it, it doesn't have a "start/ advance circuit," does it?
 

awc31563

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Dec 29, 2011
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Re: 1996 Yamaha 9.9 idle surging

I replaced the float and the needle valve, measured the level for spec height, it was good with no adjustment necessary. The needle valve seat is not replaceable but it didn't look wore out and the new needle valve has a plastic tip so it should get a good seal. The action of the float was free with no binding.

Are you thinking the level of the fuel in the float bowl may be too high?
 

raczekp1

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Re: 1996 Yamaha 9.9 idle surging

if you hold carb in hand in operating position(as it is moaunted to engin block)and you should blow air into fuel inlet without any resistance,
but
if you turn carb upside down, you can not blow air into inlet.there should be reistant all the time.

float shoud be paralel to carb body when you hold carb in up side down position

also
compresion test is good step if you have idle problem, but i think there is something with fuel delivery,
slow speed circuit
 

awc31563

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Dec 29, 2011
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Re: 1996 Yamaha 9.9 idle surging

I forgot to add the compression numbers in my first post, it's kinda hard getting exact numbers holding the compression gauge in one hand and pulling the starter rope with the other, but both cylinders came close to 130.

I'll try the air through the fuel inlet tomorrow a.m. Thanks.
 

raczekp1

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Re: 1996 Yamaha 9.9 idle surging

keep track by story when you tilted motor he gets idle fine.something in this should be.
 

awc31563

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Dec 29, 2011
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Re: 1996 Yamaha 9.9 idle surging

Pulled the carb being careful not to spill any fuel from the bowl, set it level and upright on the bench and blew into the fuel pump/ inlet. Tight as a drum with the needle valve closed from the normal fuel level. Emptied the fuel out, put the carb back level and upright, air blew through it with no resistance. Turned the carb upside down, no air would pass through at all.

Removed the float bowl and held the carb upright and level, checked the operation of the float, it is smooth with no binding. Remeasured the float height, it is at 13mm, with the high end of the spec being 12.5mm. The fuel level is high but it is within spec. If I could I would adjust the height to 15mm lowering the fuel level some but keeping it in spec, just to eliminate this being a possibility. But the float is all plastic with no adjustment.
 

raczekp1

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Re: 1996 Yamaha 9.9 idle surging

i see.
so i clean my carbs in acid used to clean any restriction in kitchen sink outlet blockage.
just let it lay in this acid for few minutes and later on to wash in pure fuel.

when you tilted motor so symptoms just gone so my thinking is in blockage in the carb
or powerhead gasket is bad-to check this-take out upper covling and run engin and put back this covling and see if it changes
 

99yam40

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Sep 7, 2008
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8,851
Re: 1996 Yamaha 9.9 idle surging

never put aluminum parts into a drain cleaner, it is lye (caustic) solution not an acid and it eats aluminum.
There are carb cleaners on the market for soaking and cleaning carbs
 

raczekp1

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Mar 30, 2010
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1,327
Re: 1996 Yamaha 9.9 idle surging

i cleaned several carbs in this solution.all of them works fine.carb cleaners are also good but not strong as this solution
 
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