Ideas?

[kgibbo]

Reassembled a non-running 1925 engine. Rebuilt coil box, new wiring harness, rebuilt carburetor… Starts right up, idles great…runs in low gear and high on level. After heated up completely loses power - stalls trying to pull in low gear. I have to let it cool off to even start it again? Any ideas where to begin?

[Norman Kling]

Have you checked carburetor needle valve setting? It could be running rich. Also, did you put in new pistons or rings? it could be tight and needs to be broken in .
Norm

[RajoRacer]

Sounds as though the pistons are getting "tight" !

[kgibbo]

Someone before me had installed what looked like new pistons. 0.030 over…good compression. Do I continue running on level to “break them in”? Or is there a process for doing this?

[tdump]

Use 1 of those cameras that can sneak in the spark plug hole and look for scaring on the cylinder walls, my pickup engine did the same thing and it had those 'egg" pistons in it and someone had not bored it out loose enough for them to expand,makes a mess of the engine.

[speedytinc]

Well, test/confirm your tight pistol theory. When it dies after getting hot, can you crank the motor over?
(Not to say your bores are not too small. The instructions with new pistons suggest .002" fit. Thats way too tight. )
If so, it could be as simple as being too lean. You can also test this by pulling on some choke as it wants to quit.
Nest question would be are you running too retard?

[Norman Kling]

Even an engine with the proper clearances can seize up if it gets too hot. If it still turns over with the crank, when hot, it is not too hot. Steam would indicate it is too hot.
Norm

[Kerry]

A big problem of re-builds dating back into the pre 90's was the pistons did not have oil holes for the wrist pins and slowly locking up when hot.

[speedytinc]

A big problem of re-builds dating back into the pre 90's was the pistons did not have oil holes for the wrist pins and slowly locking up when hot.

True.
The wrist pin fit is typically too tight also, even in todays new pistons.
The wrist pin should easily fall thru when the piston is turned in its side.

[kgibbo]

Will “tight” pistons and/or wrist pins break in with time and patience? Is there a process I should follow? Or are these reasons to pull an engine and work on internally immediately?

[TXGOAT2]

First, confirm what the problem is. If the engine gets stiff when warmed up, you may have a fit issue. Confirm as discussed above. You could have brake or transmission band issues. Both the spark timing and the carburetor mixture can be and should be adjusted on the go for best performance. It might be helpful to add 6 ounces of Marvel Mystery Oil to a full tank of gasoline. What kind of oil do you have in the engine? Model Ts do NOT like thick oil. Use a multi-grade oil like 5W30 or 10W30. If the engine proves to be tight when warmed up, you can try starting and running the car for 5 to 10 minutes, then stop and let it cool down for 10 to 15 minutes, then do it again. Do this a dozen times or more. Each time the engine is warmed up and stopped, check for excess friction by using the crank. Don't confuse friction with compression. Be sure the transmission bands are not too tight and that the parking brakes are not too tight. Note that the Model T does not have an actual neutral. Tight bands or clutch will drag any time the rear wheels are on the ground.

[speedytinc]

Will “tight” pistons and/or wrist pins break in with time and patience? Is there a process I should follow? Or are these reasons to pull an engine and work on internally immediately?

No.
Tight pistons will gall, depositing aluminum in the bores. There is a maybe on breaking them in.
Tight wrist pins are more likely to seize & do serious damage. Not to mention the strain added to the crank.
If either are the actual problem, the head comes off & the pistons removed.
Do the diagnostics first to be sure & not waste your time.
Pull the dipper cover & look in from the bottom side before pulling the head.