272 new build low compression

[rfarr94]

I put a Hyway 272 top end kit on a husqvarna 61. It came with a pop up piston and caber rings. I plugged the decomp hole. When I first built it I did a compression test and only came up with like 116 psi. Now I’ve got 1 tank of fuel through it and if I get the high screw where it seems right it will run ok but as soon as you lean on the saw at all the saw has no power and the chain just stops. Also it doesn’t want to start quite right. Even when warm it needs to have the choke on first. Did another compression test after one tank and still only 120 psi. Even after the piston breaks in I feel like I’m not going to get the compression. I would’ve thought compression should be 155 or better. What am I missing here?

 

[huskyboy]

What’s the squish? (piston to head clearance)

 

[Nutball]

Use thin solder (.025") twisted over itself and insert it through the spark plug hole and let the piston smash it, then use calipers to check the thickness. Can you see any deep scoring in the cylinder wall?

 

[rfarr94]

Sorry forgot to mention in original post that squish was .018 with no gasket and I did a base gasket delete. And cylinder and piston are about perfect as far as I can tell.

 

[Nutball]

What gauge are you using, has it measured over 150psi on another saw before? And, (forgive the stupid question) you did remember to put 2 stroke oil in the gas right? Could the decompression plug or even the spark plug be leaking due to defective threads?

 

[huskyboy]

Somethings leaking or you have a huge combustion chamber… if your compression tester is accurate.

 

[Nutball]

I wonder if the port timing numbers could be off far enough to make it low on torque too: high exhaust causing low compression and low intake? I have a 395BB Hyway cylinder I need to time on a 395. On a different crank case it looked like it could have only 13deg blowdown.

 

[rfarr94]

Got the gauge from advanced auto. I have got 155 psi from another saw using it. No stupid question anything can be overlooked but yes the fuel was mixed 40:1. I tore the saw back down today and piston and cylinder still look good. I took the decompression plug out today and used motoseal on the threads to make sure it was sealed not sure if that’s the right stuff to seal the decomp or not. Would calipers be accurate enough to measure if the bore is too big? I timed the saw I’ll have to check my notebook tomorrow but I’m pretty sure exhaust opened right at 100 and blowdown was about 16 degrees I believe. I’ll get the exact numbers tomorrow. An air leak shouldn’t affect compression at all should it?

 

[davidwyby]

Throw a glug of oil in the bore and see if sealing the rings helps.

 

[Nutball]

Check the ring end gap somewhere above the exhaust port

 

[Maintenance Chief]

Why was the p/c replaced in the first place ,performance? Or was the old one trashed?

 

[Czed]

I have 4 different gauges
They all read different.
And i have a couple 372 hyway popup cylinder kits they both landed in the 180s
Without a base gasket
I haven't tried the 272 kits.

 

[rfarr94]

Never checked ring end gap before, do I just put the ring in the cylinder above the exhaust port and check with feeler gauges?

 

[rfarr94]

I wanted to upgrade the 61 to a 272 and I checked 2
More saws this morning with the gauge both saws above 150 and 155 so I don’t have any reason to think the gauge is off.

 

[huskihl]

Why was the p/c replaced in the first place ,performance? Or was the old one trashed?

Most likely because everyone thinks that an Asian big bore with a popup will give them more power. When in most cases, going 2mm bigger in bore size is required to get a similar amount of power as OEM

 

[Stump Shot]

If the cylinder is a little more out of round than usual when, it could take much longer to break in and explain the low comp.
Not a big fan of the pop up piston for performance as a whole, true they can add some grunt at times, they also can loose a lot of zip.

 

[Mastermind]

Not a big fan of the pop up piston for performance

We don't need no stinkin popups !!!!!

 

[Stump Shot]

We don't need no stinkin popups !!!!!

 

[rfarr94]

Would you be able to maeasure if the cylinder is out of round with calipers or would you need something more accurate than that?

 

[Mastermind]

Would you be able to maeasure if the cylinder is out of round with calipers or would you need something more accurate than that?

Use the ring. Push it up with the piston and check the gap in several different places. Holding a light behind it may show you if it is out of round enough the cause low compression. I doubt it is that far out though.......can't imagine it.