I have a mechanically injected Deere 4045 in one of our compressors. This is probably a 2005 or so machine. No computers just a mechanical injection pump. So the engine fires right up and runs clean as can be. Issue is when I load the compressor air end and that puts a load on the engine it can't put out the power. The entire machine only has about 200 hours (really!). So to give you an idea...this is what happens...compressor is running at 50psi no problem...I turn the switch to load up the compressor to 100psi and the throttle on the engine is pulled to full throttle...however the engine starts vibrating and running rough but without any smoke...just feels like its gonna stall. My first thought is that the original 16 year old lift pump isn't putting enough fuel to the injection pump. There is a new lift pump in the mail. If that doesn't fix it what else would you think it is? The air end spins freely as we separated it from the engine to test and it turns by hand no issue and when we had the engine disconnected from the air end we started the engine and it does free rev without any issues. Also there is no smoke when the throttle is pulled to full throttle which again makes me feel like there isn't enough diesel being delivered to the engine. Sorry for the long winded message but any thoughts would be amazing.