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Homelite 26cc hedge trimmer motor is starting backwards how to fix


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04-25-13, 08:44 AM   #1  
Homelite 26cc hedge trimmer motor is starting backwards how to fix

I have a homelite 26cc hedge trimmer. The motor apparently starts backwards frequently, and when it does the clutch unscrews. When that happens the flywheel loosens and the integral key then shears. What can be done to make sure it starts in the correct direction? It can be somewhat difficult to start, so I suspect it is kicking back causing it to fire in the wrong direction. It only has about 20 total hours on it, and I am on the 3rd flywheel.
I infer it is starting backwards because it unscrews under load, which can only happen if the shaft is turning in the wrong direction. Otherwise the clutch would have to turn faster than the engine shaft in order to unscrew. The fact that it unscrews when under load indicates it is in fact running backwards.
Examining the flywheel reveals the magnet passes by the magneto pretty close to top dead center, so the timing is nearly identical regardless of the direction of rotation. Just Answer tried to tell me there is no way it can run backwards and that there is an electronic spark advance, but the evidence shows both to be wrong. I know this is a popular motor with the RC crowd and that one reason they like it is that it can run in either direction. I also see that the same motor used in string trimmers has had problems with running backwards where it is more obvious what is happening. I hate to just throw this away and spend money on a different one after only using it about 20 total hours.

 
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04-25-13, 10:50 PM   #2  
Lol, that's a fair enough deduction I suppose. What is happening is during shutdown, the engine is coming to an abrupt enough halt that inertia is unscrewing the clutch. It won't run in reverse. Often a bad cable or oversized aftermarket trimmer head will cause this. Usually it's because of a big heavy trimmer head.


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04-26-13, 05:00 AM   #3  
Wouldn't the centrifugal clutch let go before allowing the trimmer head to drive the engine?

 
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04-26-13, 06:26 AM   #4  
In a ported engine I can't see why it couldn't run in reverse. A lot of times diesels would run the wrong direction and have to be shutdown and restarted. High compression causes the kickback and once they go through the cycles once or twice with fuel/air they run the wrong direction. The lighter the flywheel the more likely that is to happen. Compression releases on small engines would help prevent that as well as make starting easier.

In your case the only question is about the timing. There is a timing advance in the module, but I can't say how much. With the flywheel in place to provide the inertia to keep turning the engine, it would have everything it would take to start and run - in your case backwards. Because of the timing issue, performance would be compromised, but in a high rpm engine, like a two stroke, you probably wouldn't notice it as much as you would a fuel issue.

Preventing it with starting procedure is about all you have. Anything with the timing or compression (without a release) would require engine modification and the chance of messing it up to where it wouldn't run at all would be pretty good.

 
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04-26-13, 09:33 AM   #5  
The engine, by design, doesn't care which way it runs. The timing, however, would not let it run well, if at all. If the timing was set at exactly TDC, then it could run either direction. The main reason it can't be running backwards is the pull start only can turn the engine one way. It is starting in the direction the pull start turns it, and it can't switch direction while it is running.

Pilot Dane, I thought I read trimmer when I replied, now I see it says hedge trimmer. I guess there's no head to be worried about anyway. In the case where I have seen this happen, the oversized head starts to surge. The cable in the shaft winds up, then the once it has wound too far, the head goes flying faster than the engine, then it slows way down and the cable winds up again... all happening in very fast cycles, like every second or less. That's when it makes the stuff come loose. The clutch is still engaged the whole time. I've also seen them do it on shutdown though, but I'm not certain how it does it. If there is not enough friction holding the clutch to the shaft, just the weight of the clutch can provide enough inertia to unscrew it.


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04-26-13, 04:55 PM   #6  
I can see where a trimmer head would unscrew it. I just don't know if the rebound off compression could possibly have enough force to take the engine back the other way far enough to start the cycle.

If you pulled the engine up tight against the compression stroke and released it everything being equal between the two compression strokes, the engine shouldn't be able to go past the compression stroke the other way. If it could break over center, so to speak, it could start the cycle going backwards and start.

It's a matter of whether the compression stroke would have less air/fuel to compress as it tried to go backwards. Past that point it would be a matter of the way timing comes into play.

More of a brain teaser than anything.

 
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04-26-13, 05:23 PM   #7  
I suspect it is kicking back to get it going the wrong way. It seems almost 50/50 whether it goes the wrong way or not. It has always been a little difficult to start. Mechanically, the magnet is centered on the magneto at TDC, measured that pretty carefully. The flywheel is pretty lightweight, so when pulling for a start there is not a whole lot of inertia there.

I don't know what if any electronics are packaged in with the coil, but in order to be anything but passive they would have to be energized by something. In the case of starting, there wouldn't be anything to power a circuit, so the starting timing kind of has to be determined solely by the magnet position. I don't see any electronic modules on the coil, but there could be something there I don't see. If any one has seen the circuit for the internals of the coil assembly, I'd love to see that (I'm an electrical engineer).

 
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04-26-13, 06:38 PM   #8  
It's a transistor and capacitor in the coil. The passing magnet through the armature switches the transistor to ground the larger induced field and produce the spark out the secondary lead.

I don't know what's causing it to loosen, but I do know it's not running backwards.

If you could turn the engine backwards to start and the timing was perfect, the fuel/air mixture would be the same. It doesn't matter which way it runs being a piston ported engine. The ignition isn't dead TDC though, even though it may appear to be by looking at the magnet. The spark is produced just before TDC. Emissions, heat buildup, and performance won't allow the engines to be manufactured to run with timing at 0 degrees TDC. If you ran it backwards, it would have retarded timing, which would make it terribly hard to start, it would barely run or produce any power, and it would probably melt down in a short amount of time. But, since the starter only turns it one way, it can only start one way anyway. Compression cannot make it kick back hard enough to start backwards. It can't push it back with more force than you exerted to move it forward (that's getting into overunity).


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04-26-13, 07:07 PM   #9  
Yeah, that's just it. The physics of it would say it can't get started backwards just driven by the pull starter. However,,,,,, what would do it is the firing of of the compression stroke if the timing of the ignition is BTDC enough to drive the piston back down before it breaks past TDC. That would provide enough impetus to the flywheel to carry it the other direction with the force to start the engine backwards. I believe that is why some of the older diesels would do that.

In order for that to happen with this hedge trimmer, though, the kickback of the engine would have to get past the starter mechanism dogs. You'd have to notice what it's doing. In fact you would likely have the starter rope jerked out of your hand.

 
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04-28-13, 06:30 PM   #10  
OK, I'm still stuck on the unscrewing under load. The only way I can see that happening is if the load rotates faster than the motor if it is turning the correct direction. The hedge trimmer has a centrifugal clutch screwed onto the motor shaft, and that clutch holds the flywheel on the tapered shaft, engaging the key cast into the flywheel's arbor hole into the slot in the shaft. The load side of the clutch has a pinion gear, which engages a larger gear that has the crank pin on it. The crank pin engages a slot in the reciprocating blade. The combination of the crank gear and blade doesn't freewheel all that much, and doesn't have a lot of mass.

If the motor is turning in the correct direction, the crank gear would somehow have to turn even faster than the motor would turn it in order to cause the clutch to unscrew. With the friction and load on that , I can't think of a situation where there would be enough inertia to unscrew the clutch, especially considering that cutting hedges puts a load on the crank, which in turn would try to screw the clutch on tighter rather than loosen it. Because the blade reverses direction twice in each revolution of the crank gear, it loses the inertia it had twice per revolution.
Even if it were some sort of odd transient event where the gear maintained enough inertia while the motor suddenly decelerated (keep in mind the flywheel on the motor side of the clutch is far more massive than the crank gear), it would have to cause the load side of the clutch to freewheel in order to cause it to unscrew.
On the other hand, if the engine somehow is managing to get running in the wrong direction, then it would be easy to see that as soon as you started to cut into branches a the resulting load on the cutter side of the clutch would tend to unscrew the clutch from the shaft. The evidence supports this since the times it has unscrewed to the point where the flywheel can shear the key and rotate has occurred well after starting when I try to start cutting with the tool. Turning the wrong way is the simplest explanation for it unscrewing like that, and frankly the only explanation I can arrive at that is at all plausible.
I have not noticed the starter pull back on me, However one of the times it did this the starter dogs did catch on the pawl in the flywheel causing some breakage.
I am thinking about ways to instrument it to be able to observe the direction of rotation. One possibility would be to place pair of a hall effect sensors adjacent to the flywheel to pick up the magnet's passage and use that data to determine the rotation direction. It would take a fair amount of work to develop that however. I can't get at the shaft easily when it is assembled to get a visual on the shaft (i.e. painting a spiral on the face of the shaft, flywheel or clutch might give a visual, but one that can' be seen when it is assembled enough to try to start it.
I've tried tightening the clutch, and even put loctite on it. The only result is it takes a little longer before it unscrews.
Next time I put it back together, I'll try to determine where in the pull it is starting. I agree if it starts in mid-pull it would be very difficult to imagine how it could reverse direction. If instead it is kicking over at the very end of the pull, it could very well be hitting the compression peak with not quite enough inertia to get over the hump, firing causing it to kick backwards and then start going in the reverse direction. The pull cord would have to be near the end of its travel so that the dogs are already starting to retract when it kicks.

I guess I don't really care how it is happening as long as I can find a way to keep it from happening. So far, I haven't been able to prevent it. The most obvious solution to me would be to drill and tap the clutch in order to put a set screw in to hold the clutch in place once it is screwed onto the shaft, so that the clutch could not unscrew. That addresses the symptoms but doesn't get at the root problem.

 
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04-28-13, 09:12 PM   #11  
Does this happen while the engine is running, or when you shut it off, or the next time you start it?


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04-29-13, 04:17 AM   #12  
The clutch unscrews while the engine is running under load. It will sometimes run fine through several starts and runs, then one run while cutting it will unscrew. Once that happens it stops because the flywheel becomes loose. It has to be disassembled and repaired. Right now it can't be reassembled unless I get another flywheel because the key stripped again the last time it came apart. The flywheel has an integral key on it rather than a woodruff key. Once that strips, the flywheel is trash. I suppose I could cut a key slot in the flywheel and use a woodruff key if I had the right tools to cut a slot on the inside diameter of the flywheel arbor.

 
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04-29-13, 08:40 AM   #13  
Try using a locking nut or lock tight, or a lock washer.

 
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04-29-13, 10:45 AM   #14  
I did try loctite, it did not hold against the unscrewing force. The clutch itself is threaded. When threaded on, the end of the shaft is pretty much even with the surface of the clutch; no room for a locking nut. The shoulder on the shaft that the clutch sits on is not wide enough for a star type lock washer, and a split type is too thick, as eliminates the clearance on the top side of the clutch. I am considering drilling the clutch for a set screw. Need to look at it again to see if that is even feasible.

 
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04-29-13, 12:00 PM   #15  
Ok, my bad,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

 
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04-29-13, 12:05 PM   #16  
My only suggestion is that it is not tight enough, or the threads are sloppy and don't provide enough friction to hold it. How are you tightening the clutch?


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04-29-13, 06:45 PM   #17  
I would go back to the clutch vs engine rotation to begin with. You know which way the engine crank turns because of the way you pull start it. Does that clutch go on the opposite way?

Thinking you may have a clutch intended for the engine in the application where it runs the other way.

With the engine driving it you should be able to put it on finger tight and the turning of the engine against any load at all would tighten the clutch. The clutch drum has all the leverage against the crank threads.

You don't have to drill or glue that thing in place. Something else is doing it.

 
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04-29-13, 07:02 PM   #18  
Cheese, As a later poster said, with the motor turning the correct direction, the clutch will tighten up. Even finger tight will tighten it up to where you need tools to get it off when it turns correctly. That said, I did make up a tool to hold the fly wheel while I tightened the clutch with a pipe wrench last time. It still came unthreaded after a short time under load. The evidence strongly suggests that the motor starts up and turns the wrong direction sometimes, as I said before I cannot see any other way it would unscrew under load like it does. It isn't every start either, sometimes it will start and work fine, keeping it tight. Then other times as soon as you start cutting with it the clutch unscrews.

The clutch threads onto the crankshaft, and the threads are correct for the way the starter turns the shaft. This is all original except the flywheel which is an OEM replacement and the primer bulb and fuel pick-up hose which are Lowe's generic replacements. The originals succumbed to the ethanol in the gas.

I agree that if the hedge trimmer was working correctly it would tighten the clutch and there would be no need for a set screw or glue. Something is causing it to start up backwards some fairly significant percentage of the time, and when it does putting a load on it causes the clutch to unscrew. The reason I was considering a set screw is that I haven't figured out how to keep it from running backwards. It doesn't seem that running backwards would cause any harm as long as the clutch didn't come unscrewed, so I figured that would at least make it usable and fix the symptoms even though it does nothing for the root problem.

That gets back to my original question: what is wrong with it that is allowing it to start up running backwards like that. I imagine it must be that the motor is not quite making it over the compression hump at the end of pull and then firing causing it to kick back. Where I haven't noticed any grabbing of the dog and whipping the pull handle out of my hand, it kind of has to be at the end of the pull. Bad magneto? Improper carb settings? The motor really is pretty simple not much else that could go wrong. Maybe it is the way I am pulling it or something.

 
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04-29-13, 07:30 PM   #19  
Depending on how much you wanted to mess with... you can try retarding the timing (about 4 degrees) by moving the flywheel on the crank. Take the flywheel you said had the key trashed clean out the old molded key and position it about 4 degrees to the rear (of rotation) key in flywheel vs keyway in crank, then tighten the clutch down tight. See if that does it.

The two things that cause kickback in a two stroke are compression and timing. Retarding the timing may keep it running. the right direction.

 
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04-29-13, 11:42 PM   #20  
The engine tightening itself against the clutch is not enough. That little thing doesn't make as much torque as you think. That is why I asked.

You can tell if the engine is running one direction vs. the other by the way it torques when you rev it.


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04-30-13, 09:29 AM   #21  
Well, it makes enough torque to unscrew it when after tightening it down with a pipewrench on the clutch and my homemade tool holding the flywheel stationary. I don't know the torque I put on it, but it was pretty significant.

 
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04-30-13, 10:38 AM   #22  
If you can't find any other solution I would start thinking about a thread locking compound. I would start with a removable type (blue) and if that does not hold try the permanent (red). Keeping in mind that removing red Loctite often requires a torch so you may never get it apart again without damaging the machine.

 
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