Idle issues on my FD-11

[opposedtwin]

So I have an FD model (1957) that runs like a chanp at WOT, but at about 1/3 throttle, it will buck or jerk momentarily. It doesn’t die but it will come real close. Needless to say, the motor will die if I bring the idle way down. I never have to shut off the motor when coming into the dock. I have gone through the carb and the motor has less than 3 hours on the new rings. Excellent compression. Starts on the first pull without fail.

So what causes it and how do I fix it? Is it simply dialing in the low speed needle? I’ve tried that and it doesn’t seem to help.
Thanks!
Scott

[frankr]

What you describe sounds like running too lean. The low speed needle doesn’t fix it? Perhaps the carb sync with the mag plate is off. Or, an air leak.

[opposedtwin]

Thanks Frank hadn’t thought about the magneto or an air leak but I will look into them. the jerking is more like a thumping. it is different than the sneezing that I normally get when it’s too lean.

Any tips for me regarding carburetor magneto synchronization?

[opposedtwin]

An air leak would most likely come from the tank correct?

[Mumbles]

You might want to check the top crank seal for leaks, especially if it has never been changed.

https://www.youtube.com/watch?v=cXt6fV6Z5rc

[opposedtwin]

I remember changing that when I put in rings. I will check the synchro and try it again.

[chris-p]

Advance the throttle, and watch the carb linkage. It should just start to move when the roller is at the hash mark on the cam. That is easy enough to check.

Then try turning low speed needle, CCW, or out 1/4 to 1/2 a turn.

Another thing to check is to run it at night with cowl off. I wonder if at that throttle setting, the plug wire is chafed and shorting out to the powerhead somewhere. You should see and hear the arcing. Neon spark testers wired inline while you throttle down would show inconsistent spark as well, showing you it is spark related.

The air leak could be from many places, from the connectors at tank or motor, to the fuel line itself, to the crank seal Mumbles mentioned, to the carb (welch plug, throttle shaft,etc) It can sometimes be a bear to pinpoint, sometimes easily found.

[wiscoboater]

Did you replace all fuel and air lines? Good place to start.

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