Yamaha Blaster misfiring problem

As it only occurred at the higher revs I would have thought the first place to check would be the auto advance / retard mechanism on the points - it could have just been sticking?

Thanks Electroman,

The Blaster is full electronic ignition with no adjustment possible as standard.

Wow, it sounds like you covered all bases. There is one other thing that will cause top end miss that you did not mention.

If your intake is restrictive (dirty air filter or just design) it can limit top end air too much and starve out max RPM running. I'm glad it's running good now though.

I applaud you helping out one of the younger folks. Keep up the good work.

Eddie

Thanks Eddie,

I should have mentioned we went right through all the carburetor and air filter and breathing issues in case any of it was restricted or choked but it was all fine.

I should also have said that the thing we are trying to figure out is this;

how can two flywheels go haywire enough to cause a misfire and seemingly damage the low tension coil and/or the pick-up as well?

Our magneto specialist friend is 100% certain that only extreme heat or extreme hammering can damage magnets, and magnets are all that there is in the flywheel.

So how could we damage 2 flywheels to the extent they cause a misfire or can they themselves damage the low tension coil or the pick-up when we were running the bike exactly as it has always been run?

There has never been any heat or any hammering applied to any of the flywheels other than under normal running conditions and the engine has never seized through lack of oil or through running too lean.

All the magnets on both the (now presumed faulty) original flywheels are as solid as if they were new and they look the same as new.

All the crank bearings are well within Yamaha'a quoted tolerances too so we are not dealing with a runout or an imbalance issue.

The engine has got probably less than 2000 miles on it as it's only run 3 or 4 times a year on track days and it's 3 years old.

Anyone have any idea how a magnetic flywheel can cause a misfire?

Does anyone know of a dynamic test that can be done on magnetic flywheels. and what would any such test be measuring?

I think we would need a dynamic test coz this malfunction was only appearing at the very top of the rev range, guessing here, but probably in the top 500-1000 rpm of an approx 7500 rpm rev range.

Thanks if you can help.

I'm thinking of the problems I had with my old Villiers 200 cc engine back in 68.

That used a similar magneto but the magnets in the flywheel were useless (very low power) and so for reliability I converted the ignition to battery operated coil with the breaker...

But you say yours doesn't have a breaker and is all electronic??

So I'm stumped....

Thanks Electroman,

That is exactly our dilemma.

We're totally stumped too, because although we have cured the misfire now, we don't actually know what cured it so if it re-occurs again we won't know how to fix it again.

Have you checked to make sure that all wiring is in good condition and properly connected including the spark plug wire / boot?

You should also make sure that the keyway is not partialy sheared and that the magneto nut is properly torqued.

Have you changed the spark plug?

Make sure that the gas petcock / flow to the carb is not restricted.

Thanks Guest,

We swapped the entire wiring loom including CDI and HT coil for a known good replacement set and it made no difference.

That included a known good HT lead and known good plug boot.

We have had some instances of keys shearing in the past, even when the flywheel is properly torqued but that actual problem was not present when we had the misfire problem.

When the flywheel key has sheared in the past it tends to make the whole rev range completely flat and it's immediately noticeable.

The gas flow is AOK. From wide open running we have chopped the gas tap and the ignition at the same time and then checked the fuel level in the carb bowl and it's exactly where it should be.

We've also tried several new and used plugs in there and that made no difference.

check to see if the key that aligns the fly wheel to the shaft isn't partially sheared. Yamaha at one time sold keys that you could use to advance / retard the timing and looked like a key that had not been sheared thru.

Thanks Guest,

We have tried one of the staggered keys in the past but it wasn't fitted when we had the problem and the flywheel was secure throughout the time we had the problem.

I can't explain what you saw, but for high speed misfire, the very first supect is always a fouled spark plug.

.....maybe caused by a plug that is one type "too cold" for that particular engine.....try the next "warmer" plug and see if that helps. That will usually stay cleaner due to the slightly higher temperature.

Thanks Andy Germany,

We have tried several new and used plugs including hot and cold ones but they all come out clean and dry.

There was a point though at the worst of the misfiring when the engine was getting oiled up and we fitted a hotter plug and that did actually stop the cylinder oiling up but it didn't cure the misfire.

We speculated that what was happening was that because the full ignition burn was not taking place at the very top of the rev range, it was therefore also not burning all the oil off as it normally would, and while the gas would evaporate before we got the cylinder removed, the unburnt oil was still present when we stripped it.

Thanks StandardsGuy,

We have tried several new and used plugs and they all come out clean and dry.

This is probably because the engine has only ever run at full revs except when this misfiring occured right at the top of the rev range.

However, at one point, when the misfire had been present for quite a few runs and while we had got no clue as to what it was we took off the head and cylinder and found excess oil on both the piston top and on the head. The exhaust manifold was also leaking oil.

When we put it back together we used a hotter plug and that stopped the oil from collecting but it didn't cure the misfire.

The only thing we found that cured the misfire was fitting a new LT coil, a new pick-up and a new flywheel all at the same time, but since we had previously fitted (another) new LT coil and (another) new pick-up without making any improvement, the only conclusion we can draw from fitting the set of 3 items together (LT coil, pick-up and flywheel) is that it was either the set of 3 items together in sync that was causing the fault or it was the flywheel itself but we don't know which, and now that the misfire has gone we don't really feel like risking the new status quo by messing around testing different combos again, at least not until we can get a plausible explanation of what might be happening.

Hi annaesthetised,

Flywheels don't go bad, and then get good again. Your problem is electronic. You have an intermittent, and your changing things may or may not have made a difference. It just happened to get good when you changed all three. Cross the flywheel off your suspect list.

S

Thanks StandardsGuy,

I think you're right, it's not likely that a flywheel can go wrong and especially not 2 of them, one after the other.

We'll just have to wait till it happens again and see what we can find then.

hey I don't know if this will help you at all but it may help you eliminate some false causes of the problem. my 00 blaster had the same problem and i was actually just rebuilding it because the compression was bad. but all I did was put a new piston, rings and a new spark plug in and it revs all the way up nice and smooth now where before it would only go like half that far. like you said it sounded like it had a rev limiter. one thing i noticed when i pulled it apart is that my piston had rub marks on it like it was slapping up against the side of the cylinder. so I just rebuilt the top end. bore and put in new piston and rings. I don't know if i could of accidentally fixed something while i had it apart. but it runs like a champ now. so i'm guessing your misfiring problem doesn't have anything to do with electrical unless its just spark plug. I know this isn't an answer but i hope it helps