Not sure how a "leaking" diode might behave, since I have absolutely no clue how that system operates and no schematic to try to learn from.

Things I would do are-

Compression test. You may not get "proper" readings if the compression release (if equipped) is functioning, but the readings SHOULD be similar to each other.

Remove the valve covers.
Observe if both valves seem to be opening and closing the same amount.

I can't say if these are all spec'd the same, but the ones I am familiar with have a small square box which contain the diode. The diode will be mounted in the kill harness that runs between the 2 ignition coils- other end to the ignition switch or whatever switch that supplies the kill signal. Single Hondas can have them too. Twins are under the shroud I believe. I suppose in theory, you could unhook the harness from the coils and give it try. It would be nice to have the series #s. Choke the engine to kill it while testing.

P.S. also tried running it without the exhaust box attached (just in case there was a mouse nest blocking it) - no change, still guttless :(

While you are waiting for your other stuff, rotate the engine by hand and observe how much your valves actually move up and down.
Measure the distance by marking a pencil or a dowel and compare all valves to see if you get the same figure. This would give a clue as to whether the camshaft lobes might be worn.

Thanks - I didn't measure the movement but turned the motor over by hand several times whilst observing and it looks like the valves move the same amount both sides for the pairs (inlet pair and exhaust pair).
I even observed the valve stem movement of the inlet valve whilst the manifold was off and measured air flow from the exhaust downpipes whilst the engine was running whilst the muffler box was off. Measured it by sticking my hand under the downpipe - not an exact science but no noticeable difference in the amount of hot exhaust air exiting the pipes. Possible that the cam is worn but there certainly seemed to be a good amount of movement of the valve stems (about 1 cm if I was to guess an amount).

Aren't there spec #s that could ID this? The tractor model # even? I assume yours is spec'd w/ a 2 barrel carb? I see where the carb has been cleaned several times- but what about carb parts? Would you have part #s on a kit maybe? Or maybe one main jet loose or anything else suspicious there? I've not ever split one, but I assume it had plastic cam gears and two timing belts. Jumped a tooth on one side? I'm guessing it needs to be split to know. BTW what I have shows Int .10 exh .15 on the quick spec chart.

Though it appears that there are hash marks on the gears that should be visible at the top of the head for comparison sake. Here is more info on that:

Here is a link that might be useful: Honda QR

Thanks again for your help.

I've checked the timing marks as per the diagram (very useful) and they're spot on.

The serial number on the engine is: GJAMM-1002637
EXA1 Honda 530cm3

The carb is a Mikuni

It's a French sit on lawn tractor Model no: VETRS LOISIRS 16H42

I've located the "owner's manual" for the engine and it confirms that the valve clearances are as "tomplum" stated at cold - they were'nt far out and are now within the tolerances.

Off to take the carb apart (again) just in case I missed something ?????

Fixed it - yippeeeeeeeeeeeee

Not sure which bit was not playing ball but this afternoon I've:

-Adjusted the valve clearances (they weren't out by much but are now spot on)
-Cleaned completely the petrol pump and blown through with an airgun.
-Added a new in-line petrol filter
-Re-cleaned the carb - took everything apart - washed and cleaned in neat petrol, used an airgun and blow dried everything and pushed fishing line wire into every hole I could find.

I've completed a test cut of 40cm high grass in one of my paddocks (way higher than I usually cut with this tractor) and power was definately back to as it should have been and managed to wade through the grass cutting as we went without the engine dying on me. Yesterday it would have struggled to cut 10cm high grass and would have probably died several times in the process.

Many thanks to all for your help and advice and the links to the papers (all printed out and in the file) and hope this post is useful to someone else later on.

Off to fix the 4x4 now....

If that carb had 2 Main Jets, maybe one of them was restricted?

Glad it's running in any case!

Yes the carb has two main jets - so I made sure that the fishing wire passed through both of them. Given the way everything else was I'm inclined to think that it was a fuel starvation issue. Changing the coil packs definately improved the quality of the spark but didn't resolve the problem however the last carb clean + new in-line filter + pump clean has solved it so logically it was a fuel issue.

Interesting (and frustrating) past few weeks but learnt a lot - think I'll stick to motorbikes and cars though ;)

This is the French version of say, a Craftsman LT2000. Craftsman (AYP) produced this model 2003-2005 with the Honda twin, nice engine. I ran the GCV-530 on a Gravely Commercial 36" walk behind for years. I too had carb problems, the key was to remove the metering jets beneath the main jets, and clean all eight holes in each tube.

After carb problems with so many of my small engines, including the motorcycles, I too have gone to a completely different fuel.

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