Intermittent misfire under load.

[Guest Gimlet]

My Husky 365 SP has started stumbling and misfiring under load. If you let the saw rev freely in fresh air with the throttle against the stop it revs clean a a whistle, but put it into a tree and put some pressure on and it stumbles and misses. 

 

It doesn't do it every time though. And it starts cleanly and runs well on a light throttle and a light load. It's high revs under load when it starts playing up. 

It does have a habit of burning out plugs if it's left idling too long, so I've fitted a new plug and a clean air filter but it's still doing it. Any ideas?

[spudulike]

Interesting one...burning out plugs.....are you saying they are melting? If so, has the spark plug hole been helicoiled?

If the saw is lacking when flat out and I had the saw in front of me, I would get a feel for the compression and if that was good, I would strip, clean and service the carb, check and clean the fuel tank breather, clean the air filter with fluid and COMPRESSOR.....not a wash in warm water and that will probably do it.

My thinking is that the saw is probably lacking fuel at high speed and that comes down to fuel filters, fuel lines, tank breather and carb condition.

The other side is HT electrics but am thinking it is more likely fuel related.

The gauze strainer in the carb is always a favourite issue on chainsaws.

[Guest Gimlet]

It is slightly down on compression with slight scoring in the bore. But that was diagnosed a year ago and it's run fine up to now. 

The plugs aren't physically burning out, they get oiled up, carbonised and therefore presumably too hot, and start breaking down inside. The mechanic who serviced it last time and found the compression drop thought a piece of carbon dropping into the bore from the plug may have caused the scoring.  

 

I did wonder about a coil related issue after the 550XP thread. (Got one of those too and that isn't working either). But if it was coil related I wouldn't have thought it would be randomly intermittent. 

[spudulike]

What is the compression and how is it being measured? Does the running issue get worse as the machine warms up? 

If you have greater than 155psi cold then it is unlikely to be a piston/bore wear but worth checking when hot and sub 140 is getting passed it.

Coils, the usual faults are: -

1) They fail completely

2) They fail once hot

3) They cause instant failure similar to pressing the kill button

4) The saw will only idle and not rev out - this happens on coils with a variable ignition advance and happens when the max speed advance circuit fails.

5) Misfires or lack of spark - due to bad  HT connections.

Coils can fail when the HT wire, connection or plug connection is broken and the coil has to jump the spark across a much larger gap.

Carb issues are much more common than coil issues.

 

The plug getting "oiled up"....Has the saws carb been set correctly for high revs? I believe 12,500 rpm is about right on these saws. If the saw is running rich, this could be the reason the saw is cutting out. 

 

As I said earlier, I would replace the fuel filter, check the fuel line - on this model, the fuel line is pushed through the wall of the fuel tank and the pipe can split on this join causing your issues. I use a Mityvac to test the line. Do a carb service and tach it and see if this has a positive effect. I don't see how this would oil up the plug but just listing some of the issues I have found over time.

 

[Mark_Skyland]

Have you checked to make sure that it isnt something simple like the HT lead shorting out? Ive seen it on a few of the big husky saws, and some of the smaller ones, and some of the strimmers.

 

[Guest Gimlet]

Well it gets weirder. Still struggling on with my misfiring 365 while my 550 XP gets fixed..

 

Realised today that the misfire is happening when the saw is used in certain positions. If I'm doing an upper cut with the top of the bar, it runs clean. It's when I'm doing a downward cut and especially exerting pressure on the end of the bar that it's worst. If I get stuck in deep right up to the bump spikes, it's fine. Pull back so there's pressure on the end of the bar and it coughs. 

 

TRied it several times and I can get it to miss to order. Does the fuel line run somewhere where it could get pinched in the body of the saw when the damper springs flex?

 

 

[adw]

I would not expect a miss fire due to a fuelling  issue, more likely  the earth cut out wire shorting out, or the ht lead shorting against the cylinder.

[Guest Gimlet]

2 hours ago, adw said:

I would not expect a miss fire due to a fuelling  issue, more likely  the earth cut out wire shorting out, or the ht lead shorting against the cylinder.

You may be onto something there. I have had an issue with the kill switch stopping working. Had to use the choke to stop it. It was a dodgy connection. Fixed it but even now it sometimes takes a very firm push on the switch before it cuts the engine. 

 

I'll check the HT lead tomorrow. 

I may sound like a mechanical moron in this thread. I'm not, but I'm currently in a rented house with nowhere to work on my tools. I'm in the process of buying a house which has a forty foot workshop in the garden. Wish I was in there now. I have had it fettled in a trice. 

[seanshaugh]

Possibly could be ignition module loose either ,rocking a bit when  you move the saw a certain way had a 365 do something similar 

[Guest Gimlet]

Well the HT lead is OK. But thinking about the kill switch,when you put pressure on the tip on the bar the body does flex on it's damper spring right where the switch and all the wires are. 

 

I fired it up today and let it sit on the ground ticking over, then squeezed the two halves of the body together right above the kill switch. And sure enough, it coughed and picked up again when I let go. 

 

It's definitely something electrical around the kill switch area where the chassis moves on the damper spring. 

Is the ignition module located in the same place?