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

Hi,

I have just bought a J.A.P.80 Rotovator, but having taken the flywheel off to change the Condenser, because the spark was weak, I can't remember how to time it.

The problem is that the points cam has 2X key-ways and can be put on reversed, giving 4X choices. The flywheel then goes on and has 2X key-ways.

One question is: Does the spark always happen as the points open, or can it be as they close?

Cheers, Camerart

Hi Camerart, welcome to Outdoorking.

With a simple magneto ignition, the spark always occurs as the points open. If the ignition timing is fixed rather than variable, this always happens before top dead center (BTDC). Note that the size of the maximum points gap affects the ignition timing fairly substantially. The correct maximum gap is usually between 0.012" and 0.020". I think the JAP 80 has the correct gap stamped on the flywheel - it might be 0.018 - 0.020".

The engine will produce its best power output with somewhere in the region of 20 to 30 degrees of advance, but if it has an aluminium flywheel (unlikely for a JAP rotavator engine) it will be limited to around 5-8 degrees BTDC because of severe kickback when starting. With a substantial cast iron flywheel, somewhat more advance may be safe, and advantageous. I have read elsewhere on the internet that the JAP 80 timing was specified at 25 degrees BTDC - see this report:
http://www.tractorbox.co.uk/forum/forum_posts.asp?TID=7407

Remember, kickback is dangerous. Back in the days when cars were started with crank handles, people were occasionally killed by the handle when kickback occurred, and broken arms were fairly commonplace (usually due to not having retarded the ignition before cranking).

OK, thank you, I'll try those settings.

Have you any idea why there are two keyways in the points cam and the flywheel?

Cheers, Camerart.

The usual reason to have two keyways that are not precisely at 90 degrees to each other, is so that the part can be used in two different models. That could apply to an ignition cam. Two keyways at right angles are sometimes used in high-stress devices like flywheels to provide extra shearing strength - it provides twice the torque capacity without weakening the shaft very much.

Ok, the two keyways, are approx 25Degrees to each other, so I assume this is for different models.

If you look at the PNG I've attached, looking at the Blade part. You will see which way they turn, and imagine I am to the left, so they will be pushing me backwards. This is if I follow the direction the starter string. It also makes the points cam go in the direction shown in the sketch. I assume the points follow the cam till it 'slowly' closes, then hits the sharp opening shoulder.

If this seem correct, then there are only two keyway choices. I have chosen the points opening 2-3mm BTDC, this is producing no firing at all. The other choice of keyway would bring it 25 degrees more retarded.

Cheers, Camerart.

There are some mysteries here. Normally a points cam is symmetrical, rather like a valve-operating camshaft but with very much less lift. Please post a picture of the cam, so we can see what is happening.

It is conventional for internal combustion engines to rotate clockwise at the end opposite to the output shaft, so if you face the output shaft, it rotates anticlockwise. This is in line with what you have reported.

Your points, when set to open to 0.018" maximum, should begin to open when the crankshaft is 25 degrees BTDC. Can you flip the cam over, so it goes on the shaft the other way up?

Here's a photo as requested. Assuming the Flywheel rotates clockwise looking at it, then the rotors rotate towards me.

Cheers, Camerart.

The magneto has a notch for a pull string, on top of that is a modification of pullstring with return spring. Both of these pull the flywheel clockwise as you look at it.

So it definitely goes clockwise.

Cheers, Camerart.

Clockwise rotation when viewed from the flywheel/ignition system end of a full-crank mower engine is normal. It gets a bit tricky with half-crank engines, and with car/truck engines, which have the flywheel on the output shaft end for obvious reasons, but they still rotate clockwise when viewed from the opposite end to the output shaft.

If you hold the crankshaft stationary with the keyway on top, anything that moves the ignition cam clockwise then locks it to the crankshaft, is advancing the ignition (because it results in the points opening sooner, relative to the crankshaft).

I can't see from the picture whether the ignition cam is on a parallel section of the crankshaft, or is on a taper. If it is on a parallel section, you appear to have a choice of 4 different ignition timings. Compared with the timing as it's set up in your photograph, you can retard the ignition about 30 degrees by using the other keyway in the ignition cam. Alternatively, if the cam is on a parallel section of shaft, you can pull it off and put it on flipped over. That would give you two more ignition timing options (one for each keyway position), both of them considerably retarded from the position you have it in at the moment, because they move the active part of the ignition cam anticlockwise relative to the crankshaft. However, because I don't know where the piston is relative to Top Dead Center when your picture was taken, I don't know which of the four positions coincides with 25-30 degrees before TDC. If the keyway is at TDC (which is fairly usual) the setup you have in the picture would give you ignition timing of about 25-30 degrees BTDC.

Note that to get the ignition to operate, you have to choose whichever of the two flywheel keyways results in the magnet in the flywheel passing over the ignition coil at the moment the points break.

The two key positions in that flywheel are not at right angles to each other, so they are alternative timing positions, not a provision for using two keys simultaneously. The angle between the two flywheel keyways appears to be about 60 degrees. After choosing a keyway for the cam to give you the required ignition timing, you should then choose the flywheel keyway that results in the magnet passing over the coil simultaneously with the points opening. There is probably only one combination of key positions that will result in a spark happening at all, and it will be about 25-30 degrees before top dead center, depending on the size of the breaker point gap.

I checked what you said,and chose the best out of 4X (parallel shaft)keyway positions, which is also the same keyway to line up the magnets as they over the coil as the point just open.

It looks as though the spark is advanced about the correct amount BTDC. So it ready to go back in the frame.

Cheers, Camerart.

Thanks for the update, Camerart. Please let us know how it starts and runs, if possible with photographs. We don't see many JAP engine projects at Outdoorking, so this will be interesting for our archives.

It appears that I can only post one photo at a time, here's 1.



Cheers, Camerart

Here is photo 2.



Having timed the points as best I can, and checking a good spark from the 'removed' spark plug, I put the engine back into the frame. It still doesn't fire. I removed the plug and it looks wet, and smells of petrol.

When I dismantled the engine after it not starting before, there was quite a bit of fuel in the crank case, so i am pretty sure it is getting through. I am starting with an electric drill. At first no choke and no throttle, then while turning added choke, then throttle. Nothing!!!

Any ideas?

Cheers, Camerart.

Thank you for posting the pictures, Camerart, they are excellent. Before we get into the tricky stuff, just one question: are you quite certain the points are opening at about 25-30 degrees BTDC, when you rotate the flywheel clockwise, and your electric drill is also rotating the engine in that direction?

The elements required to make the engine go are fuel, spark, and compression. Let's look at those one at a time.

First, fuel. Believe it or not, it is possible to make an air/fuel mixture that is too rich even for a 2 stroke. I suggest you turn off the fuel, drain the float bowl, remove the spark plug, spin the engine with the electric drill for about 5 seconds to expell all the fuel from the crankcase and cylinder (don't extend it much beyond 5 seconds, or it may run out of oil), then stop, put a small spoonful of fuel into the cylinder through the spark plug hole, re-install the spark plug and connect it, then with choke open and a completely dry carburetor float bowl and engine crankcase, spin the engine with the drill and see if it starts and runs briefly, then stops. If it does, your problem is fuel mixture, probably too rich. Tell us, and we can work through the possible specific causes.

Second, ignition. Remove the spark plug, and connect any old spark plug that works to the plug lead, with its gap increased to 0.060". Put a few drops of engine oil into the engine through the spark plug hole, to ensure the rings are lubricated. Clamp the over-gapped spark plug to an exposed metal part of the engine, and spin it up with the electric drill, just for 5 seconds or less. Watch the spark plug gap for what looks like a virtually continuous blue arc. If you get one, substitute the actual spark plug for the dummy one, and repeat the test. If it sparks properly, increase its gap to 0.060", and test again. If all of these tests show good blue spark, chances are the ignition system is OK.

Third, compression. Remove the muffler and take a look at the rings and the side of the piston, through the exhaust port. If possible, photograph what you see. If the rings are stuck or the piston is substantially scored, chances are the engine isn't going to run without new or at least better rings and piston. If you have or can borrow one, apply a compression test gauge to the spark plug hole and spin the engine briefly with the electric drill. Tell us what the pressure gauge reads.

Essentially, it sounds as if you may have been flooding the engine, wetting the spark plug with liquid fuel/oil mix, and thus putting out the fire. However if you go through all three test procedures, we should be able to establish what is going on, whatever it is.

First/ choosing the correct key-way (only one on the shaft) As you noticed, if the parallel points cam is fitted reversed, both key-ways retards it too much. Using the points cam 'correctly' (the lift shoulder on the cam is now going clockwise)and choosing the most advanced Key-way, and turning the points frame anti-clocwise, making the most advanced of all choices, I think it is about 25Degrees.

I tried holding the 'hot' lead and turning the flywheel backward and forward, trying to feel when it sparked, while watching the piston/flywheel, so see if I could get some accuracy, but not too good. With the flywheel closed in as photo, is there a way to make a 'simple' timing light from the spark lead?

I think I might have a compression tester somewhere!

Confession time!! When I replaced the barrel I broke the top ring 50 years of experience and now I forget the ring locator pin!! I removed it and moved number two to the top of the piston.)

I'll report back in a couple of days after your tests.

Cheers, Camerart.

To check the actual ignition timing you can use an ordinary old-fashioned neon automotive timing light attached in series with the plug lead (gives red flashes, not very bright, no external power source required) or you can use a later xenon automotive timing light (attach power leads to a 12 Volt supply and just clamp a sensor around the plug lead, gives very bright blue-white flashes). Either way you'd need to locate TDC accurately though, then set up an external rotating protractor on the output shaft, unless there is some other rotating feature you prefer to use, such as the starting rope pulley. Note that the output shaft rotates anti-clockwise of course. I don't know if you happen to have a keyway on the output shaft, to use for a position reference to simplify locating TDC.

With only one piston ring, if it turns out to be fairly worn, it may not give enough compression to run. However, let's wait for the test data.

TIMING: If we accept that the timing might be correct.

FUEL: I'm also guessing that the fuel might be ok too, when I said the plug was wet, the gap was dry. I tried the dry test, no fire!

IGNITION: I changed the capacitor, but it's 0.12uF not 0.18uF as the one I took off. The spark isn't the 'fattest' I've ever seen, it's a bit white and thin.

COMPRESSION: Found the compression gauge and it is about 100P.S.I.

Cheers, Camerart.

Did it fire when you dried it out then put a spoonful of fuel directly into the cylinder, through the spark plug hole? If it didn't, there isn't much point in going further until you get it to do so.

Can we see pictures of the ring and the side of the piston, taken through the exhaust port, please? If the piston is scored the ports will not function adequately, despite it having compression.

No it didn't fire with a spoon full of fuel.

The piston is a bit scored, I am looking for a new one with rings.

Does 100 P.S.I.compression seem ok?

Cheers, Camerart.

100 psi should be quite enough for it to start and run. You can forget about the carburetor, and intake air leaks, if it won't start when the chamber is primed with fuel. We are left with ignition or compression faults. I take it you are using the old spark plug that was already in the engine? Did you check whether that same spark plug would spark with its gap increased to 0.060"? Have you tried to start it with the muffler removed, in case it is blocked?

The spark plug might work at atmospheric pressure, but not work at 100 psi pressure. Hence the need to check it with a much larger gap.

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.