Husqvarna 435 cylinder/ piston fault or something else?

[adw]

It's a entry level saw 120 is enough for it, as for the exhaust gasket missing, not an issue.

[DaveyCrockett]

55 minutes ago, adw said:

Remove the flywheel, on the back will be the part number of the flywheel, you then match the unit to the parts list that shows that flywheel.

This is a Husq 435 II  I have used the parts lists from L+S engineers website for numbers.

 

The flywheel has the number 580 27 98 cast in the back,  The parts list for series II quotes 580 27 98 01.

The ignition module fitted is marked 579 63 88-02.  The parts list for series II quotes 579 63 88-03.  Is this okay? I think the last two digits are just a revision number.

 

I have checked lists for 435 and 435E.  They use a different parts for flywheel and ignition module.

 

I think I have the right parts fitted.

 

 

 

[adw]

The latest part number seems to be 04, however I know from experience when they just fire but do not run it is an ignition unit issue.

[spudulike]

I recently did an MS200t that would spark across a 7mm gap with a good spark, it had a slight break in the HT line and under compression wouldn't fire at all. A new HT lead sorted it.

120psi is poor, very poor. I would expect at least 150psi with 170psi being good. This measurement is a bit subjective as different gauge types give different readings. The first pull can tell you a lot about the real state of the engine. If it doesn't make 70psi + after an energetic first pull then something is amiss!

ADW has a reasonable😉 experience of Husqvarnas so it is worth taking note of what he said about the match of flywheel to coils and coils giving issues!

[DaveyCrockett]

15 minutes ago, adw said:

The latest part number seems to be 04, however I know from experience when they just fire but do not run it is an ignition unit issue.

The only tests I can really do for the module is to jump the normal gap 0.5 mm  which gives a fat blue spark and it jumps a modified plug with a 2mm gap but I know the stresses on the module are higher when it makes the spark jumps under compression.  It sounds like I should take a punt on a new module.  I can always send it back if it doesn't work.

 

I have taken the taken the saw apart again to check ring wear and to provide some pictures.  I've had it apart so many times now it didn't take long!  The ring gap is 0.3 mm with the ring in the bore, so negligible wear.  I think the scratches on the front of the piston are nothing to worry about.  The blow past, wouldn't usually worry me if the saw was running.  The scratches on the back of the cylinder are I think caused by burrs on the ends of the piston ring.  It's not nice but I don't think it would stop the saw running.

 

 

 

 

[adw]

Are you sure the piston wasn't in backwards? the marks on the inlet seem to match that on the exhaust side of the piston.

[DaveyCrockett]

17 minutes ago, adw said:

Are you sure the piston wasn't in backwards? the marks on the inlet seem to match that on the exhaust side of the piston.

It was in right.  I first saw both sets of scratches through the exhaust port, and I did check the arrow on the crown was pointing front.

[DaveyCrockett]

1 hour ago, spudulike said:

I recently did an MS200t that would spark across a 7mm gap with a good spark, it had a slight break in the HT line and under compression wouldn't fire at all. A new HT lead sorted it.

Well I can measure a couple of resistances as well as the spark test.  HT to ground = 1.6 kOhm,  cut out contact to ground = 60 kOhm.  Doesn't prove much but HT lead good in this case.

[spudulike]

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?

[adw]

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