Detroit Diesel 6-71ti starting issue

Are you saying the starter is turning the motor over slower than the stbd? If that is the case, you're going to need to diagnose why it's turning over slower. Check batteries first (most likely culprit), cables second, starter third. If those all check out, it could be a leaking injector filling a cylinder or something else. I had an 8v92 low hour boat but sat on a river with a lot of current. Used to have to roll both motors over 30-45 seconds before they'd fire, then smoke a ton of grey smoke for several minutes then ran clean (DDECs). They seemed to perform ok also. Turns out each motor had a dead cylinder from sitting with the exhaust valves open and the current pushing salty air up there. This would be the extreme scenario.

I've been running 671s since the early 1960s. Usually when twins crank differently, it's electrical. Besides the battery and cabling, the solenoid can have contacts that are making a poor connection. There are two lugs where the battery cable and the starter motor connect. They contact a copper disk the rotates slightly each start. Eventually, long starts cause burning and pits on the lugs and disk. If you're ambitious, you can disassemble the solenoid and file ends of the lugs and disk to like new and restore the ability to transmit full amps to the start motor.
With the engines warm and running under power, you can use a hand temperature measuring gun to read the temps at the exhaust ports and get a good idea of the cylinder conditions. One cylinder running cold could indicate a problem with the rings, valves or injector in that cylinder.

Had the same issue on another motor..rack was out of tune

Vroom vroom!!

Will the rack being out of tune/adjustment cause slow rotation and hard start in summertime weather conditions ? mine are 471 TI J and T 1981.

Ok, well 55 degrees will do it, and I'm sure the motors were colder than that. It takes a day or three of warmer weather for them to come up to current ambient temperature ,such as 65 degrees etc. Those engine blocks are thick and heavy! and hold the cold.

You need to get a good Detroit diesel mechanic out who can put his ears, eyes, and hands on these engines and figure out what exactly is going on with them.

While not a Detroit specific Shop, call N+G engineering in Hollywood, FL. Otherwise the dealer FDDA and RPM diesel are both good. They are good with them. I haven't messed with anything major with a Detroit in over 5 years.

Do a compression test. With engine warm via block heaters it will start right up. If you have no heaters, Crank engine over a dozen times while holding the stop button in. This will heat cylinders without adding fuel - hence very little white smoke upon start.