Good advice here except for one thing....
... Good luck, except you won't need it, you have us.
Well ... he had stated in his first post that he;
... Replaced standard starter, battery, cables, and checked timing. ...
... was fine until about 2 years ago.
So I didn't feel it was worth going over again with him.
Maybe an 'assume' mess up on my end.
He has me quite
with this reply though ...
The engine in question is a stock 350, 300 HP with an in line bolt mounting pattern on the starter. ...
A "stock" RPO-L48 350/300 'SHOULD
' have a 168 tooth ring gear - for either the manual 11" clutch or the auto options ...
I guess someone could have installed a dual pattern aftermarket starter, or something in the drivetrain has been changed since the vehicle was built by GM.
I guess my question at this point would be - what is suffix of engine and what exactly is the trans & clutch/flywheel installed?
There must be a reason 153 tooth components (as indicated by a non-staggered starter pattern) rest there now ...
I guess you have answered the question that the stock big block starter will not work on the small block engine because the bolt pattern is different. ...
... Bugs me why after all these years(I have owned the car 15 years) the engine suddenly turns over slow when hot.
... I, nor the others, have answered that - we stated that it WILL work, as long as the ring gear tooth count is the same for that given set-up.
I routinely use starters on both SBC's and BBC's that are meant for another application as I build the drivetrain and match the components to my needs. GM did the same thing
While one would 'assume'
that all the HP factory offerings all used the larger 11" clutch, with the 168 tooth ring gear, many didn't ... they instead used a HD version of the smaller 10.4" clutch and the smaller 153 tooth ring gear flywheel.
BTW - the L48 came standard from the factory with a 'high-torque' designated starter, as did most of the GM offerings with 'Higher Compression' engine offerings.
No telling when you're getting a replacement if it was rebuilt to the GM spec.s or not.
That's why I now replace them with an newer design GM "high-torque" spec. starter that is listed above. I use the same starter for most higher compression/performance/marine applications, they have yet to let me down after many years.
The one last item I would be checking is the engine grounding system at this point.
Is the negative cable from the battery mounted to the stud at the alternator bracket at the thermostat housing, as original, and is it making good contact? This an area of high corrosion and the contact integrity can be compromised as the system is serviced over the years.
Make sure you have a very good path from the battery Neg. to the engine block as well as correctly sized Pos. feed cables.
I often find these pieces changed on these older vehicles to sub-standard parts ...