Husqvarna 435 cylinder/ piston fault or something else?

[DaveyCrockett]

23 minutes ago, spudulike said:

Those marks on the piston and bore are very strange. Is the ring location pin still in place on the inlet side? 

I don't think they will effect compression as they look to be below the exhaust port roof but are strange, not something I have seen before.

 

The fact the plug cap to ground is measuring continuity is probably enough to show the HT lead is OK.

 

ADW mentioned the match between coil and flywheel, on some saws, there are a number of flywheels and coils available. If you don't use the correct combination, the plug can spark at the wrong time and the saw will never start. He will know the correct combination and if your part numbers are OK.

 

It looks like the top end is OK, ring end gap is fine, this leaves fuel and ignition. Issues tend to be fuel related but some saws can have inherent weak coils.

 

Was the plug getting wet with fuel when you were pulling it over? Did you try a bit of WD40/carb cleaner/GT85 down the carb to see if it would fire. 

 

When the carb was stripped, was there a decent amount of fuel in the pump AND metering side of the carb?

The ring location pin seems solid.  I've tried to move it with tweezers and a strong magnet.

 

When I strip the saw everything is very wet.  I think it is unburnt fuel from  missed ignitions rather than running rich, but could be a carb issue.  The exhaust when it fires seems on the smoky side, but the saw is so far off running properly I'm reluctant to judge it like a running saw and blame richness. The fuel mix is fairly recent - a few weeks old.

 

It does fire on carb cleaner for 1/2 second but seems to make no power.  It doesn't scream away for a couple of seconds like some things do, that gives you confidence compression and ignition is right.

 

I believe it had fuel in the metering chamber, but brain was not thinking leanness at that point, because of wet engine internals.

[spudulike]

OK, sounds like is getting fuel then........ possibly the needle valve in the carb is stuck open.

Does the saw still fire, has it fired with this flywheel and carb setup?

These sorts of issues are difficult to sort and often, you just eliminate parts being faulty and the only part left is the route cause.

[DaveyCrockett]

1 minute ago, spudulike said:

OK, sounds like is getting fuel then........ possibly the needle valve in the carb is stuck open.

Does the saw still fire, has it fired with this flywheel and carb setup?

These sorts of issues are difficult to sort and often, you just eliminate parts being faulty and the only part left is the route cause.

Carb was pressure tested before and after disassembly.  Needle valve holding 15 psi and reseating and holding pressure when pressing purge bulb.

 

I just have the original flywheel and ignition module.  I believe they are correct matched parts.  See earlier post for part numbers.  Behaviour always the same, on cleaner or carb. Fires but runs down in 1/2 second, no power at all.

 

Crankcase  passes vacuum and pressure test too.

[bmp01]

If the piston is at bottom of its travel are those marks above the piston crown ?

Have you got a picture of the inlet side of piston please ?

I can only imagine someone has had something like a scriber poking in through exhaust port while moving the piston up and down. Any damage to the top of the exhaust port ?

[DaveyCrockett]

13 minutes ago, spudulike said:

These sorts of issues are difficult to sort and often, you just eliminate parts being faulty and the only part left is the route cause.

My trouble is that every part I  look at, I think, well that should probably work.  But ignition and engine together (forget the carb) - nothing ever looked less like a working saw 😩

[spudulike]

Sounds like the carb is OK then. Was the gauze strainer checked? 

Both flywheels and coils can't be faulty - very strange one, difficult to pin it down without inspection.

[DaveyCrockett]

48 minutes ago, spudulike said:

Sounds like the carb is OK then. Was the gauze strainer checked? 

Both flywheels and coils can't be faulty - very strange one, difficult to pin it down without inspection.

There was a little debris in the strainer,  nothing horrendous. 

Did you mean -  both flywheels and coils CAN be faulty?

Edited December 30, 2020 by DaveyCrockett
thought a bit more

[DaveyCrockett]

18 minutes ago, bmp01 said:

If the piston is at bottom of its travel are those marks above the piston crown ?

Have you got a picture of the inlet side of piston please ?

Any damage to the top of the exhaust port ?

Yes they are visible through the exhaust port with the cylinder assembled.

 

Here is the inlet side of the piston.  The scratch above the ring groove I did removing the ring😳.

 

Top of exhaust port looks perfect.

 

[bmp01]

Nothing to see there,  looks peachy. Marks in the bore are not causing any wear.

I have nothing useful to add, certainly interested to hear the outcome though....

 

 

 

[DaveyCrockett]

1 hour ago, adw said:

I am sticking with the coil, seen too many like this.

I'm leaning in the same direction.

 

Although the piston and cylinder are far from a thing of beauty, and in a formula 1 car engine, internals like this would hit the bin, this is a consumer grade chainsaw and I don't believe what I have seen would stop the saw running completely.

 

The ignition module however ... I can imagine things inside which I can't see, that could easily cause the problem.