Husky 55 with 028 piston

[Nitris223]

Over the last couple days I have been playing with a 55 and wanted to try a 028 stihl pop up piston.
I first set up the cylinder and cut the base so it would be .040 thousands with a base gasket and stock piston.
Then I started fitting the 028 piston. I cut .030 of the main part of the piston and went 10mm wide. Then I took .060 off the dome top. It cleared 1 spark plug but not others , so I milled a spot for the ground strap to fit with a ball end mill.

Got the saw running for about a minute and then it died.
I could get it going with the choke and then high idle it would run. As soon as I take it off high idle it starts to act up. Tried leaning it out to 3/4 and no good. Turned it up a 1/4 at a time and it always seems like it would bog out with the throttle. Went all the way to 2 turns out. Still couldn't get it to run long enough to try some adjustments.
The port timing is
Intake 77
Exhaust 106
Transfers 127
BD 21
Squish is .024

New fuel filter, new impulse rubber, intake boot.

Not sure if it's the carb, fuel vent or something in the timing.
Saw ran fine before the experiment.
I never did a pressure and vac test . I guess that's next sometime this week.

Any ideas would be great.

 

[thompsoncustom]

not sure how close everything ended up in the spark plug area your piston didn't come up and kiss the ground strap did it?

The timing numbers you have won't cause any issues and are pretty good. If I was porting the saw I wouldn't change them much. 80 on the intake and 121 on the transfers is what I would aim for. You can also take your squish down to .015 without issue.

 

[NSEric]

You can put an extra spark plug washer under the spark plug to gain some clearance if it hit the plug.
I think you may have too much compression, there's a point where it starts to hurt power, somewhere around 250psi.

 

[Nitris223]

Compression is sitting at 175 psi.
I pulled the carb apart and it was spotless. I threw in new gaskets and diaphragms anyways.
Fuel line split when I pulled it off the carb so I replaced that as well again.

Pressure and vac test were also good.

Put it back together and set the carb to factory settings and it would fire up and die shortly after taking it off high idle.
Always have to restart with choke so it's not getting enough fuel.
I'm going to pull the jug tomorrow and see if the impulse hole is slightly plugged and also check the fuel vent.

If that don't work then I'll put the stock piston back in and see if it runs like that.

 

[pioneerguy600]

Run a piece of wire down into the cylinder base through the impulse nipple,that will clear any blockage if it is blocked. No need to remove the cylinder just to open up the impulse corridor.

 

[steele_109]

Did you pressure test the carb? I'm working on a 353 that has the same symptoms and the carb was leaking down.

 

[Nitris223]

Not yet. I'll do that tonight maybe.

 

[Nitris223]

I pulled the jug again and everything looked OK until I started comparing the location of the impulse hole. This 46mm new cylinder was slightly off compared to the 45mm that came off. Also the notch in the actual case was very little. I took a small dental bit and elongated the impulse hole closer to the edge of the bore.
Back together and runs perfect. Nice throttle response compared to stock piston.
Now I just need to test it in some wood to do the final tuning.

 

[Nitris223]

This is what I did to the top of the piston for spark plug clearance. Went a little to deep but I got a spare piston if things don't look OK once a piston wash gets developed.