Increase 9.9 OMC idle speed by drilling larger hole in throttle plate?

[islander9]

I would like to see if there are opinions out there on drilling the holes slightly larger on the throttle plate to try and get a faster idle speed?

A bit of background... I am talking about the electronic ignition 9.9 & 15 hp OMC's built from approx 1978-2007. I have worked on these little gems since they came out in 1974 and now that they are pushing 40 years old I occasionally come across one that I would like to see a higher idle speed in neutral to stabilize cold running.

Right now I have a 1991, 9.9 hp that I have been all over and although it idles great, I would like to be able to increase it further.
To eliminate the obvious the carb has been soaked/cleaned with an updated carb kit, head gasket has been replaced and have 110 psi compression, replaced the lower crank seal, reeds look great. As is suppose to be, the new throttle cam follower roller is not touching the advance cam (link & sync) at slowest speed with approx .060 overthrow at full throttle.

Restricted by the throttle linkage contacting the shift detent bracket, in neutral, I am not able to advance the throttle linkage far enough to bring the throttle advance cam in contact with the throttle advance roller to in turn open the throttle plate. Hope this explanation makes sense to others who work on these engines.
I often see where the throttle roller adjustment is turned way in to make this happen, but it is not the right thing to do in that it screws up the link & sync and puts too much pressure on the roller/bracket at full throttle. Determined to find another way to increase idle speed I am running out of ideas.

I have been wondering if drilling the two small holes in the throttle plate (inside the carb), very slightly larger would increase the air flow at idle and in turn increase the speed.
Anyone have an opinion if this might work?

 

[flyingscott]

Idle speed is usually set on OMCs with the timing not the carb that is why the roller does not touch the timing cam. You may increase the idle speed but it will be lean if you drill the holes, there is already a flat spot on the butterfly to allow air flow through. Use the idle adjustment that these motors come with not to mention you can use throttle for cold starts. On the 91 model I think there is a screw under the flywheel to adjust the idle. I guess I would ask if it idles good what's the problem.

 

[islander9]

Thanks flyingscott, it idles great, but a bit too slowly even with the throttle turned up as far as the mechanisms will allow it. This motor will be running in near zero temps in the fall/spring and I anticipate it will need more idle speed to be stable and for easier starting.

I was thinking (without knowing for sure), if I drilled the holes in the throttle butterfly out slightly larger I could compensate the lean condition by enriching the idle jet adjustment.

You make a good point that the idle speed is controlled more by advancing the timing. If I could get the stator plate to rotate further counterclockwise I could get more timing, but it is being limited by the stator plate contacting shift detent plate. On others in the past I have shaved a bit off of the plastic shift detent plate to allow for more stator rotation, but I always feel like I am compensating for something wrong when I do so.

I am not aware of an adjustment under the flywheel unless you are referring to the older style throttle advance cams that had a screw to move it out towards the throttle roller. This 1991 doesn't have the adjustable cam. Are you aware of another way to gain more timing?
I'll take a look under the flywheel to see what there is.

 

[jimmbo]

Perhaps a little Dremel work on the shift bracket?