OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions on Other Engines JAP80 timing Problem

I have the original spark plug set to 0.025" and a spare set to 0.060" and it sparks ok, but not blue. I haven't done, the large gap on the original, so I'll try that.

I have never taken the muffler off, so that might be a possibility.

I'll also see if I can find a spark l=lug tested in a garage, they test them while pressurized.

I got the price of a new piston and rings, 50uk pounds, too much to justify, so let's hope the muffler is blocked.

Cheers, Camerart.

Remember, 2 stroke spark plugs usually stop working properly after not very many hours, due to the effects of oil deposits on the insulators. It's best to replace them, but if you can't (for example, 18 mm spark plugs are a bit hard to get) I've been known to scrape the insulators carefully right down to the root, then clean with carburetor cleaner - an awesomely powerful solvent.

In the old days most garages had Champion spark plug testers, which apply air pressure of anything up to the shop air supply pressure of 80-100 psi. However the machines also had a sand-blaster built-in, to clean the spark plug insulator. Do not let them sand blast your plug - it removes the glaze from the insulator, and makes it foul very quickly. Inspect your plug, when you've cleaned it properly with solvent. If the insulator is rough and unglazed, get rid of the plug.

I suggest you go hunting for a piston ring or two, of the correct width and depth, and which gives an appropriate ring gap in your cylinder bore. If necessary adjustments can be made to the peg recess in the rings by careful use of a needle file. However if the piston is scored significantly, this isn't going to work, a better piston will be required.

I've used those sand blaster/testers in the old days. I doubt the garage would still use the same method if it does damage, but even if they did, it might get the engine started, then I could get another plug later.

Do you think pistons from other engines will be the same? I've never thought of this, but perhaps makers could use pistons off the shelf. I'll look around.

Cheers, Camerart.

There is a long history of using pistons from a different type of engine, for some particular purpose. At one time there was something of a practice in Australia of fitting Ferguson tractor cylinder barrels to Triumph TR2 and TR3 vehicles, together with ordinary Chevrolet pistons, to increase the displacement from 1.6 to 2.6 litres, while raising the compression ratio to something approaching 10:1. One of the attractions of this change was that it was sometimes cheaper than fitting new standard-size barrels and pistons. (This odd-seeming transformation was only practicable because the 4-cylinder TR Triumphs used the same engine as the Ferguson tractor of the same time period, of course. The engine was also used, in much larger numbers, in a rather unlovable car called the Standard Vanguard.)

Essentially though, in this case I doubt that you want to over-bore your engine to the size of some other piston, or alter your compression ratio by using a piston which has a different compression height, or grapple with problems relating to differences in gudgeon pin diameters. You also obviously face some extra restrictions due to your engine being a port-controlled 2 stroke, so you require piston ring pegs, and a skirt height identical to that of the original piston, since the piston skirt height controls the intake port timing.

If you can contact a group of JAP engine afficionados there is a good chance that they will have already investigated whether some other piston can be used satisfactorily. That does not mean they will have found a feasible solution, though.

Hi,

As I am only trying to get it working to dig my plot, and not for a museum piece, I don't want to spend much more on it. So I will stick to the existing piston, and try to get 1 or 2 rings.

I have attached 2x images of the piston, looking through the exhaust port, where you can just see the top of the piston and the top of the first ring. To me it looks fairly good. The second photo is of the worst score on the piston.



I'm sure that once the vital ingredients are applied it will work. Now I have to test the plug or buy a new one.

Cheers, Camerart. Image 2 to follow.

No 2.

That piston is ruined, Camerart, but it should still run for a while, in a fashion. I don't think those scores will be the only, or even the main, reason for it failing to start.

The score seems to line up with the upper ring locating peg, so there's a good chance someone installed the top ring badly then turned it over a few times. After that it either settled into place, or was torn down again to fix the problem.

Also, something unlovely has happened to the end of the gudgeon pin bore, and the circlip is missing. Do not run it without both circlips correctly installed, or it will ruin the bore beyond repair.