Slow Crank 280se 4.5
 

I've got a 1973 280SE W108 that cranks, kinda. Its battery is ancient and we hooked it up to a running jumper car, 96 Land Cruiser, and it cranks, but SUPER slow. Theres a strong click when the starter engages, then it barely rolls the pulleys over. Its pumping fuel, and I checked one of the plugs and its got spark.
I am going to plug the battery in to charge overnight, although I think it should be tossed, then Im gonna run it with a jumper car and try to get it to hopefully sputter start.
Am I correct to assume the slow starter is just a weak battery?
what else would cause a refusal to start? Ive added fresh fuel to the empty tank btw.
Thanks!

When was the last time this car was running?

Be sure to check the ground path from the battery to the starter / engine block. ( As in remove the connections and clean , just because its bolted on does not mean it has electrical conductivity. )

You could have a crappy connection with the jumper cables. Been there, done that. Let the battery charge and try again. If it keeps up, you may need a new battery.

If it's been a while since the car ran (as in years), the engine could be tight. If it has been sitting for years, it may be a good idea to pull the spark plugs and spray some fogging oil in each bore and let it sit overnight.

Its sat for 2 years. I scrubbed the terminal connections with a wire brush, I will pull the ground to frame connection tomorrow and scrub it good as well.

There is likely very stale gas in the lines from the tank to the injectors. Been there done that. The 4.5 will actually start as long as it can compress the first cylinder it hits - so if your engine is turning, even at a snail's pace, it'll start (also been there done that).

The thing I did was I pulled the line off the cold start valve, drained it into a gas can - key cycling from accessory to run a few times to cycle the pump. Do it until the fuel comes out clear and not yellow. It may take 10 to 20 key cycles. Then it should start easier.

My 300D is slow to start every Spring. Seems engine and probably starter are tight from sitting for 5 months. Once car starts and gets driven a bit, everything is back to normal. This with all good grounds and battery connections.

When jumping, use heavy duty cables. I recently had to jump start two cars that had been sitting for a year or two from my 300D. I didn't have anyone to keep revs up on 300D. Jumping didn't help much. I then found that owner of cars had a heavy duty set of cables (thicker gauge wires). Put those on and cars started up fine!