It sounds like its running ok on LPG?
This says to me, you have no major issue like cam timing, bad plug leads, or bad coil/igniter etc.
I think it's fair to assume the engine may have never been opened up for major work. Don't get distracted on maybes yet.
How many km has it done?
I think at this point here's where I'd start
1 - remove all plugs.
- Lay them out in order #1-#6 as you remove them. post a photo of all the plug tips.
- Do they all look the same?
- If any stand out as being different, it may indicate an isolated problem at one or more cylinders.
- If they are all fouled up, it may be worth replacing them even though they are new.
- were plug gaps set before fitting? Or fitted straight out of the box?
Gaps can vary a lot from the factory. It's best practice to check and set the gaps with feeler gauge.
- having plugs out makes it much easier to rotate the engine for following checks
2 - remove distributor cap.
3 -loosen the distributor and position it approximately centred on the adjustment bolt/slot
4 - rotate crank to TDC.
5 - check dizzy rotor button is pointing at #1 terminal. ( use sharpie etc to mark #1 terminal location on the distributor body before removing the cap)
- if it's 180⁰ wrong, rotate the crank another full rotation back to TDC.
- If its not quite pointing at #1 terminal, rotate the distributor body to see if you can point it at #1.
- If you run out of movement in the adjustment slot, the distributor drive gear may be 1 tooth out of position. (Its possible someone pulled the distributor at some point to replace the oring on the distributor housing. This is a common oil leak location.)
6 - refit dizzy cap, plugs, leads etc and start the engine
7 - if it started up ok, stop the engine.
Insert your paper clip in the diagnostic port. Restart the engine,
check engine light should be flashing.
8 - check timing with timing light.
9 - adjust diatributor position.
* If dizzy button is not pointing at #1 distibutor cap terminal with the distributor close to centred on the adjustment slot, remove the distributor assembly. (Clean it while it's out).
Reinstall the distributor assembly using the FSM procedure, and lining up the match marks on the distributor gear, and distributor housing.
Good guide here 'Timing Marks Won't Align'
Timing Marks Won't Align - https://forum.ih8mud.com/threads/timing-marks-wont-align.1331362/post-15339156
* When the crank is at TDC, can you see the top of the #1 piston right there? Now is the time to probe this.
As the crank approaches TDC put something straight ( pencil etc) into the spark plug hole. Watch it rise as you rotate the crank toward TDC. It should stop rising when you're a few degrees away from TDC.
There's a bit of guess work here, but if upward movement of a probe doesn't stop approximately corresponding with the crank pulley timing mark meeting the 0⁰ mark on the timing case, it suggest harmonic balancer
might be an issue.
You may have to rotate the crank several times to watch your probe rise, stall, drop a you rotate the crank through TDC. It takes a few go's to get a feel for when you're actually at TDC.
@Malleus if he's not seeing a consistent static timing advance with paper clip in place and CEL flashing, is there any checks for the ECU?
I personaly don't like a paperclip, I don't think it reliably makes good contact.
I have a piece of a stainless steel cable tie bent into a V shape I use. It's wide, flat, thin, springy and gets a reliable contact each time.