omc 9.9/15 owners who want better low speed operation

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Sinkingfast
Posts: 185
Joined: 05 Oct 2014, 13:19

omc 9.9/15 owners who want better low speed operation

Post by Sinkingfast » 06 Aug 2016, 23:19

I've had my '79 omc15 for couple of years. The blubbery, crappy low speed operation is a known issue with them. The plug angle, unequal intake tract length, and yes.. a non aireated low speed circuit when fully flowing are some causes. The head design was to blame but was just a fix for a design issue.

40 years ago I used to save dead snowmobiles, work them over and ride-sell. Most used the Tillotson HR or HD regulator style carbs. They use the same design low speed circuit as the metal top float carbs on the 9.9/15. If one got the sled stuck in the snow they were prone to fouling plugs. Long idle while lifting the back, then full throttle for a second then back to idle. Couldn't use a hot plug as these motors were air cooled and ran very hot. Getting the sled stuck then having fouled plugs..ugh.

Okay..I got a Mikuni carb and boot and installed on a single cyl Rotax. The mikuni carb has a fully aireated low speed circuit. I could not believe the improved low speed, no plug fouling issues and better throttle response. This was 40 years ago.

Fast forward to last week. I have spent almost 2 years on this motor as a project. I'll cut to the chase. I had remembered the carb swap to the Mik's on my sleds. I then looked over my 15hp carb and thought about a way to add aireation to the low speed. So I took a #75 drill and drilled a hole in the welsh plug right next to where the fuel is drawn in by the adjustment needle. Easy plug if no worky. Well let me tell you the improvement is terrific. Worked up to a #60 hole. Put a piece of foam over the welsh plug..clamped the foam under airbox. Interesting thing is I still use the same low speed needle setting...

My theory...when low speed is fully flowing it is 3 solid streams of fuel, not bubbled or aireated hit the plugs and quench the insulators. The blubbery low speed operation was caused by raw fuel droplets . I know that when there is partial flow there is some aireation in the low speed but none when fully flowing. Plugs are now clean and will need to go up from the ngk5's I'm using. I also notice better fuel economy. Even the wifey asked what did you do to the motor. This was not a simple mixture adjustment that it needed. The air being drawn in under the welsh plug emulsifys the fuel making vaporization more complete from that point on.

Not sure about the plastic topped carbs but this mod was easy..did it right on the motor. I can idle in gear so slow one can count the bangs. And when throttled up from long idle periods it revs cleanly. I'm tellin you it is a great mod..

...results may vary...
2 stroke every time I can
4 stroke only when I have to..I don't like boating with my lawn mower

Xtremeboats
timsmcm
Posts: 112
Joined: 25 Jan 2009, 15:19

omc 9.9/15 owners who want better low speed operation

Post by timsmcm » 08 Aug 2016, 02:06

You got picts?

Sinkingfast
Posts: 185
Joined: 05 Oct 2014, 13:19

omc 9.9/15 owners who want better low speed operation

Post by Sinkingfast » 08 Aug 2016, 21:33

Well..motor is in service now..must pull airbox..find camera..charge camera..find cord to puter..remember how to do pics..actually take pics that one can see..you know the drill.

Just got back from the second run and I can't be happier with the air jet. #60 drill, .040in, through welsh plug right next to the inlet/needle opening. If unfamiliar with spot order new welsh plug before drilling. I drilled mine while on the motor in the water.

Oh..back out the needle adjuster before drilling hole. End of needle might stick past inside carb body
2 stroke every time I can
4 stroke only when I have to..I don't like boating with my lawn mower

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wmk0002
Posts: 618
Joined: 17 Apr 2014, 15:39
Location: Cullman, Alabama

omc 9.9/15 owners who want better low speed operation

Post by wmk0002 » 09 Aug 2016, 11:37

Sinkingfast wrote:Well..motor is in service now..must pull airbox..find camera..charge camera..find cord to puter..remember how to do pics..actually take pics that one can see..you know the drill.

Just got back from the second run and I can't be happier with the air jet. #60 drill, .040in, through welsh plug right next to the inlet/needle opening. If unfamiliar with spot order new welsh plug before drilling. I drilled mine while on the motor in the water.

Oh..back out the needle adjuster before drilling hole. End of needle might stick past inside carb body
I can post the pics for you. Shoot me a pm and I can give you my number or email and you can send them to me and I will post them.
2016 Alumacraft 1648 NCS - 1992 Johnson 9.9 HP Tiller
http://www.tinboats.net/forum/viewtopic ... 21&t=40764

VinTin
Posts: 66
Joined: 20 Dec 2014, 13:23
Location: Atanta GA area

omc 9.9/15 owners who want better low speed operation

Post by VinTin » 09 Aug 2016, 15:40

I thought I'd read where it was the fuel recovered in the crankcase drain recovery system that was causing idling problems on these motors.

Sinkingfast
Posts: 185
Joined: 05 Oct 2014, 13:19

omc 9.9/15 owners who want better low speed operation

Post by Sinkingfast » 09 Aug 2016, 21:23

Yes,but if there is a lot of saturated vapor, raw fuel and not vapor that will exacerbate the problem. It will puddle at the bottom during low speed operation. Just look at the head dam to deflect the charge away from the plugs. Those dams are not for vapor deflection but to deflect unvaporized droplets away from the plugs. And yes the operating temp is correct on my motor. Exactly the issue 40 years ago with low speed operation of many of the vintage sleds at the time. Just the change to Mikuni carbs was like night and day. Dry up the fuel charge and things get better. I can run at low speed for couple of minutes..1000rpm or so and when I throttle up no clearing its throat.

If one looks at later year heads there is no cooling in the center like in past heads. I figure that helps with fuel vaporization, to keep the plugs cleaner.
2 stroke every time I can
4 stroke only when I have to..I don't like boating with my lawn mower

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