OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters Two Stroke Trimmers & Brushcutters makita line trimmer shagged flywheel key

hey guys,
i picked up this makita rst 250 line trimmer a wile ago, and after striping it down three times,cleaning the spark arestor, putting a kit through the carby then banging my head against the wall "all the normal stuff" cause it was still a no go untill i was reading another post on this forum, that some times line trimmers can sheer flywheel keys so i thought it was worth another go and surely anough there it was, the flywheel was more than 140 degrees out of TDC then looking at the setup, the clucth sits in the middle of the flywheel or magnito i should say and it drives a small outer hub that sits in the front of the plastic cover , obviously all that load driveing the gears and bump head is put on the magneto/flywheel and that skinny little key that tore throug the soft alloy, im not sure how other trimmers are setup but i thought this was not a very good design and costing around $300 new i expected a bit more from makita,
so i pulled an old flywheel key from an old victa and gave it quick run over with a file by hand to make it fit, knocked it in there was a little bit of the flywheel missing but the oversized key made up for it
now the only problem i have is the cluctch is realy noisy
sounds like a bad ringing sound untill it engage's im pretty sure its all balanced cause at over 60% revs it all works fine is it posibly loose springs on the clutch shoes and there just rattleing around?
you can just hear the ring at the end of the pull from the starter cord should they be engageing in a higher rev range? any ideas would be great just to make sure its sorted next time
thanks guys regards steptoe

steptoe, if you didn't touch the clutch it probably isn't the problem. Did you by any chance, replace an aluminium key with a steel one? And is this key substantially bigger than the original one? Depending on the diameter of the shaft the key is in, you may have created a big enough unbalance for it to be a problem. Remember, steel weighs three times as much as the same volume of aluminium.

It is normal with forced vibrations that the amplitude varies as you increase the speed, due to various parts of the system resonating. Having it feel smooth at high speed does not mean it isn't a problem.

Most likely the problem is in some part of the system that you have altered. It may not be the key - but it isn't likely to be a part you haven't touched.

yes the key is much bigger than the original,
i would suspect but i dont know how big the old one was
i was thinking about flywheel ballance, and you saying that steel weighs 3 times more than alloy makes a bit more sense, but i would not have thought a gram or two that close to the crank would make much of a diference, i might try adding some weight to the opposite side or removeing a little from the side the pin is on...... a small hole from a drill maybe going off your calculations of 3:1 and placement on flywheel or maybe drop it off to a shop for a ballance job,, but maybe if i add some weight to begin with and see if it operates any difrently might give more of a clue.. its hard because i dont know the history of the trimmer or how it was stuffed,, ill i know is someone gave it the toss in the rubbish bin
id love to get a new flywheel but in australia these parts are very hard to come by

I agree that if it is on a very small diameter shaft, that makes it less likely it is the key. Why not file up an aluminium key to fit the new dimensions, and see if that makes a difference? No cost, and you probably need the filing practice. Besides that, it had an aluminium key for a reason - the fact that it was sheared proves it needed to be aluminium.

no probs i will try anything to sort this thing out
as you say it had a aluminium key to begin with and i would be a bit lighter than the old victa flywheel key but i would have thought the aluminium key would be softer creating a weaker point
there is no damage to the crank only the flywheel and judgeing by the damage id say it probly happend once before aswell it is a fairly new modell trimmer and i recon its just had short hard life being trashed by a cractor or a big kick back from an edger blade atachment or something but apart from that it still has whopping crompreshion and the piston dosent have a single line on it inspecting through the exhust port... if i can get it right it might be a pretty good trimmer much better than el chepo homelites, ryobi ect.

Engine manufacturers, including Briggs and Stratton, use aluminium keys between the crankshaft and flywheel so it will act as a shearable key: when the blade hits something massive, the key shears so that the momentum of the flywheel will not shear the crankshaft instead. The aluminium key is a protective feature: the engine manufacturer is not concerned about the weight of the key, since they can compensate for it when they balance the flywheel. If your crankshaft is undamaged, by far the best way for you to proceed is to get hold of an undamaged flywheel, and put it back exactly the way it was when it was new.