Since the motor struggles to turn over with the plug in, makes me think the compression release isnt working like it should. Does it pull really hard with the pull rope by chance?

Or the starter can spin with no load against it( bench test you did) but it struggles to turn with a load ( such as cranking against compression, which you don't have when you removed the spark plug) I'd lean towards starter.

Put a new plug in it anyway, they are cheap but I don't think it will be your issue( especially since you stated the motor struggles to turn over) and like every other electrical issue, check connections, etc.,etc.etc.

 

Typically a starter relay is energized by starter button. It then closes big contacts that energize starter.

You can take big wire or channel locks and go across the big contacts. Better yet take a better and check for power on ons side, hold starter button in and check for power on other. Same with the iittle wires, hold starter button in, and see if power gets to relay.

You should have 12 volts across the coil with the button held in. I would probably just jumper it and see what happens.

Also need to make sure all the connections are good. Figure when bench testing you are not using the same cables, starter relay, or start button.

 

I have seen lots of stuff not start or click, with a 1/16 or less of a turn on battery bolt.

 

Ok so I've ruled out the plug, new one made no difference.
The pull rope is hit and miss, sometimes it's smooth, other times it jams about half way, but it is fairly stiff...I'm sure it's gotten worse over the years. Does this point to compression release? If so, it's getting beyond the realms of my abilities! Is there anything I can do to check the compression release?
Thanks

 

A compression test would tell you if the auto comp release is working correctly or not. My gut feeling right now is the release isnt the issue.

Just out of curiosity, is this the same issue that had when you replaced the starter to begin with?

 

Yeah it's the same issue, when I first replaced the starter it was fine for a while.

 

Did you just install the starter from the top, or did you by chance get into the recoil(pull rope) cover. Reasoning is the bendix has a washer behind it and most new starters come with a new washer. I have seen them doubled up before. One other thing I have seen is the recoil gasket gets torn and have seen them reassembled by removing the gasket and using only sealer. This can cause and interference on the bendix, causing it to turn harder than its suppose to, ending it excessive load on starter and early starter failure.

One other queston, when the starter worked good, did the motor act like it wanted to spin good(approx a second) and then act like it hit a load(compression) stop spinning briefly, and then turns over again?

 

I did not go into the recoil cover no.
I can't really remember how the motor was acting when I put the new starter in!!
I guess my next step is to get into the recoil starter and see what's going on!

 

Not yet. When you pull it over with the rope, when it hits compression, is it REALLY hard to rotate any farther? They can almost be too hard to turn over when the release is not working correctly. Are you interested in pulling the valve cover. I might be able to explain to you and see if the comp release is at least slightly working. No cam, rocker arm, etc disassembly at all. Just eight bolts holding the cover on and then visually try to verify it. If you live in the states I may even just send you my phone number and have you call when you get it apart. Easier to explain then type!!!!

 

Im not sure what you mean about "when it hits compression" I'm guessing it's when the recoil goes tight/stiff, like the rope pulls out easily at first, about 1/4 or a third of the way then goes tight...if that is the case then yes sometimes it's really hard! I'm happy to do whatever it takes, I just don't really know what I'm doing or looking for! Unfortunately I'm in the UK, but appreciate the thought...it is hard to explain with typing!