Sinkingfast
Well-known member
- Joined
- Oct 5, 2014
- Messages
- 262
- Reaction score
- 17
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...
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...