slow running on start, then slowing down again

[tyssniffen]

ok, more jackass questions from someone who obviously doesn't know what's going on.

After a batch of cleaning up carbon (reset valves, cleaned piston top), putting all back together, attaching a different sort of alternator, I've got a weird thing where it won't actually *get going*.  check out this short video of it firing, but not getting up to speed:
https://www.youtube.com/shorts/V7vGNtFD4kE

then, after maybe 5 minutes, and I hold open the fuel line (hold back the gov) it gets going, but then randomly slows down??
here's that video:
https://www.youtube.com/shorts/O0mSJEJDqcU

couple other facts:  I haven't run veg oil in this thing in a year or two, the water tank was disconnected, I haven't changed the fuel filter in years, but it's only had good diesel going through it... and that new alternator is an experiment, I don't know, could it be doing some sort of extra drag?

Open to being told obvious things.

[veggie]

It doesn't sound like it's knocking or heavily loaded.
Could it be starving for fuel or air? Check the cleaner to make sure its not clogged with dust and debris.

A more complicated possibility would be a binding governor linkage.

When it slows down, can you physically see the fuel pump rack rod moving to a slower position?
If the rack is not closing, but the engine is slowing, then I suggest air or fuel starvation.
However, one would think that the rack would open further to try an speed up the engine.
( I'm scratching my head on this one ) 

[AdeV]

So, watching the 2nd video, there doesn't seem to be any rack movement as it slows up, so I think we can rule out a problem with the governor. I very much doubt the air filter is the problem, as it looks to be an oil bath air filter - if that were acting up I'd expect it to over-speed on ingested oil. Which just leaves the fuel system....

Listening to your first video, the combustion sounds kind of weak, and after a few pops, it seems to stop entirely, before trying to strike up again. So for me, that's definitely a fueling problem.

First thing I'd do is check out the filter. You say it hasn't run veg for a while; but unless you've thoroughly cleaned the lines, there might be a gob of the stuff blocking up the injector. I'd be tempted to start by disconnecting the injector fuel line: Crank her over a few times & make sure you're getting a dollop of fuel every other revolution. If that's happening, then you're definitely in injector territory: Pull the injector, re-connect the fuel line - then (with the injector safely pointed into a catch pot of some kind, and definitely nowhere near your skin!), crank her over. You should get a nice fine spray of fuel every other rotation. If it's looking a bit dribbly, or inconsistent in volume, or it's not injecting every time - then that's your problem...

If, on the other hand, you're not getting your dollop of fuel out of the disconnected feed line; then either there's air in it (keep cranking & see if it steadies up), or there's a problem with (most likely) the fuel pump: Either it's worn (and letting fuel past the piston instead of out into the high pressure line), or there's something preventing enough fuel getting into it. If it ran OK on veg & not so much on diesel, it COULD be wear, since veg is thicker than diesel. Another possibility is it's not shimmed correctly (so is under-fuelling), or the cam lobe is worn (= under fuelling). Both could be fixed with more shims, although if it's a worn cam lobe, you're only putting off the inevitable replacement...

However... my money's on the injector.

[AdeV]

...If that's happening, then you're definitely in injector territory...

...and by "definitely" I mean "probably"... It could still be a worn fuel pump, which would appear to deliver a decent gob of fuel when there's no pressure behind it; but equally it could be leaking away down the sides of the piston when it tries to build pressure. A feeble squirt of fuel out of the injector when reconnected would also point to that being the issue, especially if there was a good dose of fuel coming out of the disconnected line.

[tyssniffen]

...and by "definitely" I mean "probably"...

So, I should have mentioned - I put a NEW injector on there not that long ago (sometime last year) and I always check the spray, as I have had carbon build up trouble.   I set it up upside down and crank her over, making sure I see and hear the spray.   even in the video, you can hear the injector creak...

and I pulled the fuel filter and the canister was gunky, so I've cleaned that up and am shopping for replacement filter this morning.

But, again, it's been maybe 150-200 hours since I ran veg through this thing (and that never went through the 'regular' filter, it went through a different one)

Could this be some sort of loss of compression from leaky/loose/bad/scratched piston or liner?  that'd be bad news.

[AdeV]

Low compression would make it hard to start from cold, but once running, it shouldn't suddenly slow up like it does. Anything loose enough to cause that, should also be causing all kinds of other noises, I feel.

Does it do this whole "only just running" thing every time you start it, or does it sometimes reach operating speed without assistance? Are you seeing grey/blue smoke when it's struggling or slowing?

[tyssniffen]

I'm sad to report that there's not enough consistency to answer your questions well.

I'm struggling to get it started at all at the moment (I don't work on this every day, all day. too tiring and disappointing) but since I got my valves properly spaced, I've started it a few times, and it has done the 'run super slow' thing, and not reached operating speed.   

No, I've not seen blue smoke.  (I am familiar with it, just not in this instance)

[38ac]

If that second video is after you set the valves then you need to double check your work because the intake is barely opening due to excess lash and the lock nut is loose.

[tyssniffen]

ok, will do.

[cujet]

Air in the fuel lines, preventing a full injection squirt. That's my guess. Maybe you have a leak introducing just enough air on the suction side to cause this and it won't ever bleed out.

[horsefly76]

ok, more jackass questions from someone who obviously doesn't know what's going on.

After a batch of cleaning up carbon (reset valves, cleaned piston top), putting all back together, attaching a different sort of alternator, I've got a weird thing where it won't actually *get going*.  check out this short video of it firing, but not getting up to speed:
https://www.youtube.com/shorts/V7vGNtFD4kE

then, after maybe 5 minutes, and I hold open the fuel line (hold back the gov) it gets going, but then randomly slows down??
here's that video:
https://www.youtube.com/shorts/O0mSJEJDqcU

couple other facts:  I haven't run veg oil in this thing in a year or two, the water tank was disconnected, I haven't changed the fuel filter in years, but it's only had good diesel going through it... and that new alternator is an experiment, I don't know, could it be doing some sort of extra drag?

Open to being told obvious things.

"Open to being told obvious things"  "I haven't the fuel filter in years" Change the filter bud especially if it has ever had veggie oil in it at any point

[tyssniffen]

UPDATE:

AMAZINGLY STUPID AND LUCKY

so, I'm pretty sure this is what it was, and I can't believe it worked at all before, and that I figured it out. 

I was just checking through the whole thing, wondering if my fuel pump was shot or what, and in taking off the injector from the brass fuel line, I noticed the fuel running out continuously, when the engine wasn't on.  Now, I have the fuel tank a bit higher than standard mount, but that's not normal.
so I go further down, taking out the brass fuel line and then the whatsit brass knob above the pump.  As I carry the knob over to my tool bench, I see that there's NO SPRING INSIDE!  I spend 15 minutes on my knees in the dirt looking for where I might have dropped it, no luck.  Could it be I am remembering wrong and there is no spring??  I check to see if I have any replacements in my supply. nope.   then, as I'm standing there scratching my head, I notice - half buried in the gravel around the base of my mount - a bit of spring.   I put it back in, put everything together, and it runs like a charm.

AND, as a bonus, after all the struggles with a gift alternator (cutting off the pulley : https://listerengine.com/smf/index.php?topic=9713.msg111256#msg111256 )  and getting a 4" pulley onto it - it runs a perfect 110ac out.   I'm back in business baby! 

thanks again for all the help.