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New to the forum......hope someone reads this and helps with a solution.

While operating with my 6 yr old Stihl BR550 backpack blower, the motor suddenly stopped. There was no sputtering or coughing so I suspected an electrical rather than a carburetion problem. I removed the plug and grounded it out on the head but got no spark. Suspecting a bad plug, I used a spark tester inserted in the plug boot and grounded out on the head...still no spark. I removed the kill switch connection and cranked it using an electric drill to get up some speed with both the plug and tester...no spark with either. The gap between the flywheel and coil was 0.010". I cleaned off a little surface rust on the magnets and coil contacts but still no spark. The magnets were not discolored or damaged and both had strong magnetic pull.

After these tests, I felt the coil was bad. Neither of my local Stihl dealers had a coil in stock so I went on line and found a bunch of cheap Chinese coils with poor user reviews so I opted for a more expensive OEM model.

I mounted the coil when it arrived, making sure the insulating washers were installed properly between the new coil and head and the gap set correctly. The specs call for a gap between 0.006" and 0.012". I set it at 0.009" and the plug at 0.028". Cranked it up again using the drill and still no spark.

I removed the kill switch connector from the coil and again cranked it up...no spark. I ohm tested the ground wire and kill wire for continuity of the wires to the handle switch and found no breaks or shorts.

Please, can anybody help me with this no-spark problem? Unless the old coil and the new one are both bad, I can't think of anything else to do to get this blower firing.

Thanks for the help anyone can give me.

Glen

Try actually firing it up after pouring some gas into the cylinder.
I say this because with my BR600 I don't see any spark, tester or not yet it ran(i was testing hard to start which was from a bad plug).

Blowers spin at silly speeds
The spark from a magneto is speed dependent so at cranking speeds can be very hard to detect .
Spinning with a drill is not as fast as one would think .
To test I mount these things on the lathe and have them spinning around 4000 rpm and use a variable gap tester to check the strength of the spark.
A lot of stuffing around.
For general use I have the red flashing spark testers because the 3 point gap or white globe testers are impossible to see on hand held engines .

A drill doesn't spin it fast enough with these newer coils.

Wow! I'm really surprised that a drill is not fast enough to produce a spark. Does this mean pulling the starter cord produces a faster speed than the drill can? Thank you all for replying so quickly. Do you feel lack of start impulse speed is the reason I have no spark? Anything else I might have done incorrectly? Should I disconnect the kill wire from the coil, install the shroud and pull start mechanism and give it another try at starting by pulling the rope? Disconnecting the kill wire should take the handle mechanism out of the equation in case there is something wrong in the handle, shouldn't it?

Glen

In a workshop pulling the wire off the magneto is SOP for diagnosing .
Another trick is to ground the plug lead directly to the block and see is you get a spark.
Weak magnets will give a spark with no pug resistance but will not spark through a plug
And the exact right plug grade & gap is very important.
There were a pile of lightning flywheels around a while back where the magnets were just under spec
So it fired outside the cylinder but not under cylinder pressure.

This morning, I tried the old plug using high speed on the drill with the plug resting against the head. At very high drill speed, there was a weak spark at the plug. So then I bought a new plug and tested it the same way after setting the gap to 0.028". With high speed on the drill and the plug against the head, I could not get a spark. I then replaced the plug with a 3/8 socket extension and held it close to the head at high speed and got a good spark. So, at least I know the coil is capable of generating a spark, although a weak one that apparently can't pass through a plug. Any suggestions on what I should do now would be appreciated. I have the shroud and pull start off the blower plus the handle is apart. With still no good spark at the plug, I am reluctant to put everything back together if I know the spark is not strong enough to fire up the motor.

Glen

Pull the muffler and look at the piston condition.

That says weak magnets in the flywheel
Try setting the coil closer to the flywheel and see what happens.

Thanks for the suggestion. I'll reset the coil closer to the magnets in the morning and give it a try. Should have thought of that. Can magnets go bad all a sudden and the engine just stops when it's been running fine for the previous half hour and then not be able to generate a spark?

Glen

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.

.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.

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 .

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.

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

Pull the muffler and look at the piston condition.

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

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.

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

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.

Hammermechanicman said:

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.

Click to expand...

You're right, I wasn't thinking.
Well then, remove the valve cover and take a peek at the rocker arm movement.
That could explain it suddenly shutting down too.

So, I think I finally have a decent spark. The problem was that the coil mounts on two bolts into the engine. Even with the correct thickness feeler gauge between the flywheel and the coil, when tightening one bolt before the other, the coil moves in the adjustment slots so the gap at one end of the coil is too wide and the gap at the other is too narrow, resulting in no spark or, in my case, a weak one. With that resolved, I moved on the adjusting the valves per hammermechanic man's suggestion.

I found the valves to be a little loose and had to close the gap a little to the 0.005" specs. Unfortunately, the engine still won't start. While turning the flywheel slowly by hand, I noticed some clicking noises in the engine that seemed unusual. I have done nothing that I know of to mess with the timing and think it's still ok but it is acting like it is out of timing. The "S" on the flywheel magnet is at the right side of the coil at the top of the compression stroke and both valves are closed. On the power stroke, the exhaust valve starts to open just before 6:00 o'clock at the bottom of the stroke but the intake valve opens slightly starting at about 4:00 but closes again a little after 6:00. It seems strange that the intake valve would open at all during the power stroke as that would blow back through the carb.

On the exhaust stroke, the exhaust valve starts to close at 10:00 with a noticeable "click" sound and the intake valve is fully closed.

On the intake stroke, the intake valve starts to open at 12:00 TDC but the exhaust valve is not fully closed until 2:00. Just before 6:00 or at the bottom of the stroke, the intake valve starts to close but also with a noticeable "click".

On the compression stroke, the intake valve is fully closed at 9:00 but the exhaust valve opens slightly starting at about 9:00 and closes again at 11:00 with a "click". Why would the exhaust valve open at all during the compression stroke. Is this a designed unloader to reduce compression to more easily start the engine?

At those spots where the intake and exhaust valves open momentarily, if I hand turn the flywheel back and forth, the clicking gets louder and if I press down on the rocker arm while doing that, it feels like the pushrod is falling into a depression or a small raised bump on the cam, I can't tell which. Also, at those spots, sometimes but not always, as I slowly move the flywheel, the valve will slam shut with a more pronounced "clack" sound.

All these internal noises concern me but they may be normal if they are from the compression unloading mechanism. And, the opening of the intake and exhaust valves seem to overlap each other. But that would be in the design of the cam and not anything you could adjust. More importantly, the engine will not start. I'm anxious for some reply with suggestions on how to proceed from here to get this blower running. Anyone with ideas, please let me know.

Thanks..........Glen

The clicking may be the compression release. Try spraying a small amout of carb cleaner in the carb and with choke open try to start and see if it fires up for a second or two. Are you trying to start it with the valve cover off? If so it won't start. You must have the valve cover on and air tight. The rocker chamber is part of the crankcase primary compression.

glentre said:

I found the valves to be a little loose and had to close the gap a little to the 0.005" specs.

Click to expand...

Specs are actually a little under .004" (.0039) per SM.

Those coils have 2 little plastic washers that the coil sits on, It might affect things if they are not there.

I can e-mail you the repair manual if you need it. The file is to large to send it here.

Thanks for the several good suggestions above, addressed in order.

First, I tried spraying carb cleaner into the carb after removing the air filter and cranking with the choke open but got no firing. I did have the valve cover on and tight but did not have a new gasket. But even if this caused a slight loss of pressure, the engine should have at least shown some sign of life.

Second, I don't recall where I got the 0.005" valve setting and will re-do that setting to 0.004". With that slight difference, it should fire anyway. The rockers were at a far wider setting before it suddenly quit.

Third, when I first researched possible problems with the coil, I read where not putting those little plastic washers would short out the coil resulting in no spark. So, I was sure to put them on when mounting the new coil.

Forth. Thank you Fish for your kind offer to send me the service manual but that won't be necessary since I was able to find the manual for the BR blowers on line last night. At this point, I need to read it, paying particular attention to timing and how to run a compression test on this engine. I tried that test yesterday but couldn't get any reading, I thought because of the unloader mechanism. If that wasn't the reason for not getting a reading, then this engine may have some serious internal problems.

Thanks again for all who are trying to help me get this thing fired up. I'll keep you all posted on what I am doing and, hopefully, find the problem myself or get some further input.

Glen

Double check the flywheel key. The clicking noise could be an issue problem is looking inside the engine requires pretty much complete disassembly of the engine. You can try a bore scope if you have one. Or a leakdown tester.

Yesterday I adjusted the valves and while doing it, verified that both valves were opening and closing properly and appeared to be in time with the flywheel and coil. Today I did a compression test using a high speed drill. All I got was the needle flickering and basically, no compression. To verify the lack of compression, I then tried to do a leakdown test. Before doing that, I pulled the cap off (and then replaced it) the rocker chamber to verify the piston was at the top of the compression stroke and that both valves were closed with play between the rocker arms and the valve stems. Also, the flywheel was locked in place at that point. On applying air to the cylinder, it came out of the muffler. As higher pressure air was applied, it escaped through the muffler in whooshing volume, not a hiss like you would hear from a valve that had not seated well. I found that odd because while looking at the rocker arms, springs and push rods, even at high rpm, everything seemed to be working normally. This is other than the intake and exhaust valves opening briefly during the power and compression strokes which may be part of the cylinder decompression for easy starting.....or not.

I also used a boroscope in the cylinder to try to see if there was any damage to the piston. The resolution was a little fuzzy and while not being able to verify it for sure, the top of the piston seemed undamaged. It was impossible to turn the scope to see the condition of the cylinder walls. I'm at a loss to guess where the pressure is escaping. It doesn't appear to be a hung up exhaust valve because before adjusting the valves, both were closed and the tappet gaps were both about the same. With a hung up valve, the gap on that one would have been much wider. On the other hand, with such a small cylinder volume, a valve that is not seating might let more air escape than I would think. Anybody have an idea what is wrong inside the engine? It seems it must be something in the exhaust valve area. Sure would like to get some input from anybody who has run into this problem before I start tearing the engine apart so I know what I'm getting into.

Glen

Excess carbon and sticky valves were a problem, so they came out with this stuff. Might be worth a try...

Fish is probably right. Things the 4 mix engines don't like are more oil than 50:1 and running at partial throttle and long periods of idiling. It tends to carbon them up. If you get it decarboned and running try Red Armor oil. It leaves less carbon than any other oil i have run. The worst is Stihl Ultra synthetic. Never run it more than 50:1 or it will carbon things up. If you can't find the stihl stuff use Sea Foam.

Fish,
Interesting that Stihl came out with a liquid to help resolve sticky valve problems in their 4-mix engines. Seems like an admission that the engine is prone to valve stem carbon buildup. I found a Stihl service bulletin on the liquid that says to put the piston on valve overlap where both valves are open at the same time and then fill the cylinder with the liquid and let it soak for an hour. I assume this will also coat the valves, stems and guides of both partially open valves and hopefully free them up. Thanks for the heads up on this liquid as it's certainly worth trying before tearing into the engine. Also interesting is the fact that there is a short time when both valves are open as I found in my analysis of the valve working sequence.

I am now away from home and will not be back to work on the blower until late this week or on the weekend. There are Stihl dealers where I'm at now so I'll try to pick up a bottle of the "magic liquid" so I'll have it when I get back. Let's hope the problem is no more serious than a carbonized hung up valve.

I'll post again as soon as I get back to work on the blower.

Glen

I know this thread is now 2 years old but I am having the same problem with my BR550. No spark. Did you ever figure out what was wrong with your machine?