Starter motor won't work

[Dutchyankee]

Hi, my 1977 911S won't start. When I turn the key it just buzzes. I have replaced the Starter motor assembly and battery. Verified good ground and 12V connection to the starter.
I don't know what else to do to troubleshoot this issue. Some additional information: The last year it would occasionally not start. I wo6put it in 2nd gear and rock the car back and forth. It would then start. It was very random at first but occured more often. I expected the battery which was 3 years old but even after an overnight slow charge it still occurred. So I replaced the battery. Still no start. I replaced the starter motor assembly and it worked for a few days. Now just buzzing when the key is turned.
any advice would be appreciated. Thank you for any ideas that will help me bring this back to life.
Stephen

other information
It is a Californian Car with all the smog equipment removed since I now live in The Netherlands.
I changed the 2.7 litre motor(with magnesium block) with a 1976 Aluminum block and 3 litre Carrera heads motor. All upgrades were done and It has been extremely reliable and no more top end rebuilds due to leaking head gasket. Once a daily driver now a weekend warrior. I have owned this car for 40 years and what to get it back in pristine condition to hand it down to my son. I do most of the work myself except when it requires special tools or a lift.

[ant7]

The fact that you have to rock the car in gear to unstick the starter should be the first clue, i.e.; the starter is some how getting jammed when the solenoid throws out the start gear into the flywheel ring gear, not sure why this might be happening, unless something in that area has changed, ie' flywheel ring gear has become damaged etc.
Ant.

Originally Posted by Dutchyankee

Hi, my 1977 911S won't start. When I turn the key it just buzzes. I have replaced the Starter motor assembly and battery. Verified good ground and 12V connection to the starter.
I don't know what else to do to troubleshoot this issue. Some additional information: The last year it would occasionally not start. I wo6put it in 2nd gear and rock the car back and forth. It would then start. It was very random at first but occured more often. I expected the battery which was 3 years old but even after an overnight slow charge it still occurred. So I replaced the battery. Still no start. I replaced the starter motor assembly and it worked for a few days. Now just buzzing when the key is turned.
any advice would be appreciated. Thank you for any ideas that will help me bring this back to life.
Stephen

other information
It is a Californian Car with all the smog equipment removed since I now live in The Netherlands.
I changed the 2.7 litre motor(with magnesium block) with a 1976 Aluminum block and 3 litre Carrera heads motor. All upgrades were done and It has been extremely reliable and no more top end rebuilds due to leaking head gasket. Once a daily driver now a weekend warrior. I have owned this car for 40 years and what to get it back in pristine condition to hand it down to my son. I do most of the work myself except when it requires special tools or a lift.

[usera]

Total shot in the dark, but I just replaced a starter and the new starters solenoid was bad right out of the box.

[yelcab1]

New battery, checked.
New starter (not a rebuilt starter) ....?
Read the voltage at the starter when you son is trying to start the car. Run extension wires from starter to outside the car to your volt meter. What voltage do you get?

[Dutchyankee]

Thanks for your feedback and tips.
The rocking worked on the old starter but not the new one. I’m going to verify the voltage at the starter self once I feel better ( been under the weather the last week).
The original one had to issues on the teeth and the new one work thee times before the “buzzing “ issue.
I can’t imagine an 10 year old starter AND a brand new one both have the same root cause. I’m determined to get to the root cause and will share it once discovered. Keep those comments coming, I appreciate your support.
Stephen