Stihl BR550 blower no spark

[bertsmobile1]

Magnets slowly die over time
Heat & mechanical damage will hasten this .
Over time I have found that if it throw a spark without the plug but won't throw a spark with the plug, then the magnets are gone
The caveat to that is trying the coil slightly too close and making sure the plug is the right grade, resistor type & gapped correctly .
To verify I have a sliding gap tester so if It can be started I just keep on increasing the gap till the spark stops
1/3" or more is OK
1/2 " or more is great
Less and it wont throw a spark inside the engine.

 

[StarTech]

.333333333" (1/3") is way over the min spark gap spec of .0787" (2 mm) using a spark gap tester. The min cranking spec for testing the magneto is 1000 rpm. This is per the Stihl SM.

 

[bertsmobile1]

Thanks Star,
Good to know,
Iooks like I may have consigned some mags & flywheels to an undeserved early grave.
Those are the numbers used for anchient motorcycles and I just applied the same to hand held equipment.
Just so we are clear, plug is set at the specified gap & tester sits on the end of the plug with no gap to start then would out till the spark can no longer jump the points gap in air .
I would have thought that modern blue smokes would have a bigger free air gap as they run a lot higher compression ratios than my old side valves and it is the pressure inside the cylinder that increases the resistance the spark plug spark has to overcome in order to make the jump.
I don't know if you could get a blower down as low as 1000 rpm so I gather that it is done on a test rig of some sort .

 

[StarTech]

You're looking at a total gap with the spark plug installed of around .107". (2.7 mm). So if the tester is grounded to the cylinder instead of the plug you're looking at an air gap of .107" (2.7 mm) plus. Now of course the plug resistor is no longer in the circuit so a larger gap is possible with the grounded version of the test. Plug used is a known good NGK CMR6H. New don't always mean good.

Now your test setup maybe the grounded version but I was speaking of the Stihl setup using the spark plug as part of test setup. I should thought of that but with 10 hrs in shop I was very tired, plus I tend to test things per the SM anyways.

And as you said the plug gap is effective larger under compress fuel/air load which would make the tester air gap smaller which why Stihl specs the test at 2 mm.

Also I have seem these CMR plugs to be defective straight out the box which why I keep a box of them on hand. I had one of these plugs last year that was causing a HS56 hedger to diesel at full throttle. Basically a runaway engine speed.

 

[glentre]

This morning, I reset the coil closer to the flywheel at 0.006". Got a very weak spark with plug grounded to the head and a strong looking spark with a socket extension stuck into the spark plug boot and held close to the head. Basically no change from before. I then put everything back together again including the handle and pull start but the engine would not start.

The only parts needed to get a spark are a plug, a coil and a flywheel with magnets. Of course, all must be calibrated per factory tolerances which I have done. The kill switch is disconnected. I hate to keep throwing parts at this blower, but I have a new OEM coil and a new plug. The only thing left is the flywheel but it seems to have decent magnetism, although how does one determine how much is enough for the coil to develop a good spark. The engine didn't slow down or falter in any way.......it just stopped. I appreciate the input from others so far but what would be suggested at this point?

Glen

 

[Fish]

Pull the muffler and look at the piston condition.

 

[glentre]

Fish,
Thanks for your input but what would I learn by pulling the muffler and checking the piston? Wouldn't it be true that the piston has nothing to do with whether a magneto engine fires a spark or not?

Glen

 

[Fish]

It doesn't sound like you have an ignition problem.
These newer style coils produce a dim spark, and a drill usually doesn't turn the engine fast enough.
I suggest that you look at your piston, because doing so is free. And it will save time and money if the piston is in fact damaged. I have this problem a lot with buyers buying coils, then saying that it is defective when their unit won't start.

 

[glentre]

The speed necessary to get one of these new coils to fire is confusing. Would pulling the start rope spin the engine faster than a drill at high speed? Is that possible? If I pull the muffler off and look at the piston, what kind of damage should I look for? Since it is an engine with valves, I guess the only thing I could see would be the top of the piston. A hole in the piston seems like the only thing I might see. Anything else I should look for?

Do these engines have compression start unloaders such that I could not get a meaningful reading if I did a compression test? If not, what would a normal compression reading be?

Glen

 

[Hammermechanicman]

The BR550 is a 4 mix engine. You can't see the piston by removing the exhaust.
Pulling the rope starter provides enough speed for the ignition to work. The biggest problem with the 4 mix engines is valves going out of adjustment. If you adjust the valves install a new valve cover gasket as the cover is part of the primary compression and if not sealed engine won't run. Spark can be hard to see but should get spark to jump a 2mm gap.