2-stroke timing

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2-stroke timing

This topic has 19 replies, 7 voices, and was last updated 8 May 2010 at 19:13 by Keith Wardill 1.

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6 May 2010 at 19:55 #51464
Keith Wardill 1
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@keithwardill1
I would like to get some advice if possible – I was asked to find out why a weedtrimmer will not start – I know – not model engineering, but it is only a 30cc motor, so model engineers have to be the best source of knowledge.

I was told that water had been put in the fuel tank by accident, but it was dry when I was given it. I turned it over without a spark plug fitted – all seemed OK, and a good spark was available at the plug when connected. Filled it with the correct fuel, and tried to start it, with absolutely no result. Cleaned and checked the carburettor (very simple device), and tried again. After much heaving and pulling at athe starter, still nothing, not even a hiccup.

Using a piece of dowel as a probe, I tried to check when the spark was occurring – it seemed not to be anywhere near TDC. AFter removing the magneto cover, I found that the magnet poles (visible on the ‘flywheel’), are as near as makes no difference at 180 degs from where I would expect them. In other words, the piston is at TDC, but the magnet poles are 180 degs away from the magneto coil.

I can’t believe this is correct, and is completely unlike any other 2-stroke engine I have (or have ever seen). I am beginning to suspect that the magneto fly wheel has moved on the shaft, because there is no sign of a key between the shaft and magneto. Before I try to get it apart, I thought perhaps someone might have a better suggestion – please?
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6 May 2010 at 19:55 #2270
Keith Wardill 1
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@keithwardill1
6 May 2010 at 21:39 #51466
Terryd
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@terryd72465
Try a new spark plug first – that solved my similar problem. Why should the magneto have moved on the shaft? The motor presumably was ok before the water got in the fuel – so try the obvious first.

I had a friend who was university trained auto engineer and when his cars wouldn’t work he always tried the most esoteric solutions but it always turned ot to be the most simple solution as I predicted. He used to get really pissed off with me

Terry
6 May 2010 at 22:30 #51468
Jeff Dayman
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@jeffdayman43397
If the flywheel shaft key is intact and the magneto plate bolts are tight, the problem is not likely the magnet/ignition timing. You can NOT set the spark timing of weed trimmer engines using the flywheel magnets. You need to time it with the freshly cleaned points, or if electronic ignition, change the module. If there is an ignition “off” switch on the trimmer check that to make sure it is in the “on ” position while cranking, and that its wire is not shorted anywhere. Check that the plug wire is intact, and put a new plug in. Check for spark by using a gap type tester with .080-.090 gap and pulling the engine over with the recoil starter, with the plug out of the engine. If there is not a healthy blue spark in the gap the points or ignition module likely need work. Before doing anything radical on the ignition, I would check several other things to deal with the water in the fuel situation.

–disassemble the carburetor and blow out all fuel lines and the fuel tank with compressed air, and wash all parts with ethanol or methanol.

-inject raw gasoline into the crankcase down the inlet hole, slosh around and drain it. Discard this gas. Inject an ounce or so of 2 stroke oil into the crankcase and slosh it also, and then drain. This will get rid of any water in the crankcase waiting to enter the engine.

-reassemble carb and fuel system and try starting.

If it still won’t go, the engine can be mechanically timed. This is done by physically holding the piston at the manufacturer’s specified distance BTDC and setting the ignition plate to open the points (or position the ignition module at the specified location in the manual). Distance BTDC for firing will be in the service manual.

Good luck Jeff
7 May 2010 at 04:15 #51471
Terryd
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@terryd72465
Well said Jeff,

I was trying not to confuse the issue with too much info. But all the steps you suggest are the right ones, However Keith I would try them one at a time and test the starting each time. This may be a bit of a pain but will isolate the problem and you will know what to look for next time.

Always go for the simplest first and test a stage at a time.Ensuring the removal of water and checking loose, broken wiring for open/short circuits are the most likely if the engine worked perfectly before but I would start with a new Spark plug – cheap and high on the likely list if water has been in the system.

Terry
7 May 2010 at 09:00 #51473
Keith Wardill 1
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@keithwardill1
Thanks for the comments – I had tried all the obvious things – new plug (even tested the old one successfully in another machine) – no visible cabling faults. Fuel supply appears OK. (difficult to actually confirm this in the engine – the plug ends up wet if I pull it over with the ignition switched off). There is compression.The ‘On-Off’ switch must be OK, because I get a spark – all I could find (to my great surprise) was the timing error.

As you have noted – I dismissed this at first, because it seemed so unlikely, and I know it was not normally possible to adjust the magneto. However, As I noted, there is no sign of a key for the magneto flywheel – even after I removed the securing nut, so now I am suspicious of this, unlikely as it seems.

I like Jeffs comment about the cleaning (i already did this) and mechanical timing – unfortunately there is no means to move anything related to the mechanical timing – the magneto coil is mounted rigidly on a casting extension on the cylinder head (not even slotted boltholes). The other problem is that this is (apparently) a cheapo chinaman from a a box-shifter – technical details on the instruction leaflet(!) are conspicuously absent, and nothing on the engine itself (no plates or timing marks) so I am back to the magneto flywheel.

Thanks to all for the interest. If I get a solution, I’ll post it for interest (and its such a frustratingly simple machine!)

Keith
7 May 2010 at 10:06 #51475
Gordon W
Participant
@gordonw
I agree with all the above, but from past experience;- double check the carb. esp. pilot jet /passage ,corrosion and or bits of dirt can hide there. How did the water get in the fuel? It is possible for the motor to hydraulic lock, if this happens at speed the flywheel might well move, but very unlikely.
7 May 2010 at 11:31 #51481
Ian S C
Participant
@iansc
These weedeaters,even non Chinese can be a bit stubborn, they suffer from 2 stroke disease, theres so little to go wrong, if the fuel and ignition systems are OK theres not much else. My only 2 stroke is a little Villiers that I’v got driving an alternator. When I first got it, it would not go, tried every thing I could think of, then a small engine mechanic told me to put grease around the crankshaft by the oil seals, if it starts it needs new oil seals because there is air getting into the crankcase. Ian S C
7 May 2010 at 12:00 #51482
Jeff Dayman
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@jeffdayman43397
quote -“However, As I noted, there is no sign of a key for the magneto flywheel – even after I removed the securing nut”

Keith, I understand there was no key, but were there keyway slots for one in the flywheel and crankshaft? If not, that is the first time I’ve heard of an engine without them. If so, find and fit a new key. If a manufacturer makes an engine without key/keyway features they are flying in the face of fate for service, or it is a “disposable” unit ie not meant to be serviced. I have seen this approach on small snowblowers where they even rivet the carburetor to the crankcase!

(Not my cup of tea though, such a waste of an otherwise serviceable unit)

Back to your issue – I think if there is no key or keyways you will need to go trial and error positioning the flywheel to time it if the flywheel has slipped.

As a first guess you might try positioning the flywheel so the magnets LEAD the points or coil position by about 20 to 30 degrees. Logic for saying this is that the magnets must fully pass the coil poles to generate the primary voltage and it takes a little time (milliseconds) after that for either the points to open/secondary current to flow to plug or in electronic ignition for the trigger coil to induce the secondary current.

Jeff

Edited By Jeff Dayman on 07/05/2010 12:02:03
7 May 2010 at 12:55 #51484
Pat Bravery
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@patbravery
In my experience with two stroke motor bikes and lawnmowers I have found that there is always a timing reference somewhere. Have you looked on the edge of the flywheel and the back of the back plate, there could be a very feignt arrow head type mark. Should you find such a mark then set the piston to 3/16 inch BTDC and line the mark up and fingers crossed. The fact that there is no keyway suggests that timing is adjusted by this method. Good luck anyway. Pat
7 May 2010 at 13:45 #51485
Versaboss
Participant
@versaboss
I seem to remember that on such motors the flywheel could sit on a conical part of the crankshaft, and there is a Woodruffe key on that conical area. So maybe you don’t see it, unless you remove the flywheel!

As there is something wrong anyway, you cannot make it worse if you do.

Greetings, Hansrudolf
7 May 2010 at 15:16 #51489
Ian S C
Participant
@iansc
I would agree with Hansrudolf that the flywheel is proberbly on a tapered part of the shaft and located with a Woodruffe key, depending on direction of rotation the retaining nut may have a left hand thread. When I replaced the oil seals on my Villiers I did 2 motors (4 seals) for about ?5 five yrs ago wouldn’t be much different now. Check for crankcase leaks if your 2 stroke won’t go. Ian S C
By the way these little motors are used by model boat and aicraft builders, so it is relevant.

Edited By Ian S C on 07/05/2010 15:21:15
7 May 2010 at 15:35 #51490
Jeff Dayman
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@jeffdayman43397
Agreed Ian and Hansrudolf, most engines of this type that I have seen have a conical taper and a key. However Keith is saying he has not found a key. I’ve asked if there is a keyway for one, however, as he doesn’t say if there is or not.

I’ve never seen such a trimmer engine without cone, key and keyway, and I have serviced many, but that doesn’t mean there isn’t someone making them that has taken out the key and keyway to save costs. In the past few years I’ve noticed a lot of makers taking off good features to save a few pennies. A couple of examples – fuel shutoff valves under the tanks, and carburetor bowl drain valves – these both save a lot of headaches on mowers snowblowers and tillers. Another good example is grease fittings on blower and tiller low speed high torque shafts being discontinued. If they aren’t present, a simple greasing job becomes a major disassembly and if not greased, life of the unit is short. But I digress…

Good luck to Keith in any case. Jeff
7 May 2010 at 15:43 #51491
Jeff Dayman
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@jeffdayman43397
Forgot to say that the starting problem could be crank seals as Ian suggests, heaven knows what quality they are. Also it could be a bad coil. A coil test would also be on my troubleshooting list after the items I already mentioned. If you don’t have one Keith, a mower service shop will.

Jeff
7 May 2010 at 16:56 #51493
Terryd
Participant
@terryd72465
Villiers small 4 stroke engines never had flywheel keyway. The tapered crankshaft holds the flywheel perfectly as does a properly fitted Morse taper in the workshop, . this allows for minor adjustment of the timing if it needs it through wear.

If there were a woodruff key it would be impossible to lose unless the thing had been disassembled which does not appear to be the problem in this case. That’s one of the main reasons for using them, they cannot be removed (or slip out accidentally) unless the flywheel is removed.

Ian SC, the only way I could get my previous Villiers to run was to substitute an electronic ignition system in place of the notorious magneto coil. These were Lucas types and the enamel insulation on the wires was always breaking down.

I would still go with Gordon’s suggestion of water perhaps remaining in the system, It is terrible to get rid of short of a complete strip-down or a good soak in a warm oven (not advisable just before cooking the Sunday roast ). Oh, an electric oven not Gas!!.
7 May 2010 at 21:05 #51501
Keith Wardill 1
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@keithwardill1
Thanks for all the inputs – Sorry I didn’t make the keyway issue clearer, Jeff, but I did mention I could see no sign of a keyway, and until now, I had not removed the magneto flywheel until I had explored all the other possiblities – most of them covered in this thread. Anyway, I finally bit the bullet and pulled off the magneto flywheel, and found the problem.

Under the flywheel the shaft reduced in diameter by about 30%, the transition being a short taper, as mentioned by a couple of people. This cone did not extend the full depth of the flywheel, only about 50% of the depth. Cut into the slope of the cone was a Woodruff key slot, but there was no sign of the expected steel Woodruff key, just some very small ‘monkey metal’ particles (same material as the flywheel). In the hole in the flywheel were two narrow (~ 20 or 30 thou) slots, spaced apart about the same width as the Woodruff key slot (Sorry if this is unclear, it is not easy to describe without a diagram). Between the two slots the metal was clearly broken up and sheared. I think that the key was originally cast as part of the aluminium alloy magneto wheel, probably as a cost saving.

This cast-in key would have simply slid into the slot in the tapered part of the shaft when the flywheel was fitted. Since the flywheel had never been off the machine (according to its owner, and it certainly appeared that way), the key must have broken up, and significantly, the flywheel key was not located in line with the key slot – it clearly had moved, hence the spark timing was completely wrong as I originally noted. Possibly the key feature was defective – it wasn’t possible to say due to the condition of the thing.

It seem possible to me that somehow the engine became ‘locked’, possibly hydraulically due to the water, but it is difficult to see how this could be present in enough quantities (it was in the petrol, according to the owner), to lock it solidly. A firm pull on the starter cord (integral with the flywheel), could then possibly cause the wheel to move relative to the shaft – the key was obviously very insubstantial, and had slots on both sides – a failure waiting to happen.

Anyway, I made a new close fiting Woodruff key, slotted the flywheel ‘right through’, and fitted it all back together. It started and ran fine after a couple of priming pulls. I guess the new key is slightly bigger and heavier than the original, but no additional detectable vibration seems to be there (it is a 2-stroke putt-putt after all).

Many thanks for the various ideas – lots of ideas in 24 hours.

Keith.

Edited By Keith Wardill on 07/05/2010 21:11:03
7 May 2010 at 21:58 #51504
Jeff Dayman
Participant
@jeffdayman43397
Glad you got it sorted Keith.

There were several Briggs and Stratton engine models on mowers a few years ago that liked to shear their flywheel keys when they first came out with the flywheel safety brake band. When these engaged, the heavy blade inertia kept the crank turning while the flywheel on top of the engine was braked. This overstressed the key, and the flywheel would shift on the shaft, throwing the timing off. I was given several mowers that people said were “blown up” or “scrap – take it away” and after just a few minutes work and a new key they ran great.

I have seen a few trimmer engines and chainsaws with the key cast into the soft zinc alloy flywheel. It’s a poor design but it is cheap to do for the makers.

Lots of fun for us.

Cheers Jeff
8 May 2010 at 10:07 #51507
Gordon W
Participant
@gordonw
Glad you got it working finally. Just a by -the-way, the Woodruff key is not meant to transmit any torque, just for location, the taper should hold the flywheel.The little Villiers, 2 and 4 stroke, didn’t have keys, but did have clear timing marks(and self extracting flywheels). The motor I mentioned earlier that had hydraulic locked and sheared the key;- well the water got into the petrol after the man had dropped his strimmer into the pond.
8 May 2010 at 11:19 #51508
Ian S C
Participant
@iansc
Sounds as though the key was designed to break if the weedtrimmer hits something, sometimes if a motor is shocked by hitting something solid the flywheel can be shorn off. At the museum that I assist at with mechanical bits we have a 5ft Tallus chainsaw with a 250cc Villiers 2 stroke, Many yrs ago when it was half way through a log it stopped suddenly, shearing off the flywheel, bending the conrod (that took a chunk out of the piston), and broke the bigend bearing. It still makes a good museum exhibit. Ian S C
Jeff better a broken key than a stuffed motor.
8 May 2010 at 19:13 #51516
Keith Wardill 1
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@keithwardill1
Thanks for the additional info (Jeff, Gordon and Ian). It was interesting enough fixing it, and the fault was a bit uncommon I suppose, but onto something new now. I have a new workshop to finish, all the tools to move (from the garage) and a Minnie to start – who said retirement was lazing away your time.

All the best, Keith
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