Help! Uneven Compression on New Motor

[///M5]

I built a 62t/61x motor, and after running it in for 6 hours I tested my compression. The readings I am getting are 175 on the MAG, and 200 on the PTO.

The engine is running great, and the power is awesome, but the readings are freaking me out, so I pulled the head to inspect the cylinders and domes. There are some slight vertical marks on the PTO Cyl, but nothin abnormal except for the different burn pattern, and there are no scuffs, or signs of seizures.

I would think that at 200psi cranking comp I would for sure be detonating in that cylinder, but the plugs are rich and the ski is not overheating.

Any ideas or help will be appreciated.

Thanks,

Motor type/setup = 82mm Bored and Ported 61x cylinders, Pro-X Pistons, Ported 62T cases, ADA Head w/ 36cc domes, B Pipe, MSD ignition, RAD charging flywheel,

 

[yamabro]

Are the ADA o rings new?
Did you re torque the head after warm up?
Have you checked your squish?

 

[///M5]

Are the ADA o rings new?
Did you re torque the head after warm up?
Have you checked your squish?

Yes
No
No

 

[yamabro]

I would definitely check that squish clearance.

 

[///M5]

Yeah I will do that, and I'll post the readings.

The head was not leaking and all orings were sealed when the head was removed

 

[///M5]

I would definitely check that squish clearance.

Checked the squish. Both holes are reading right around 1mm

 

[OCD Solutions]

Is it an illusion from the camera flash or is there major pitting in the cylinder wall in the third pic? Like whomever bored it did not take enough to clean it up right.

 

[Nuklear6]

Is it an illusion from the camera flash or is there major pitting in the cylinder wall in the third pic? Like whomever bored it did not take enough to clean it up right.

I thought the same thing about the pit marks but when you zoom in I think it's the flash reflecting off the texture of the cast piston top.
OP If you have the means cc the domes and make sure they are not miss marked. Also use a friends compression guage as a second opinion. The odd part of all this is a 1mm over 701 with stockish deck height and 36cc Ada domes should have a cranking compression of near the lower cylinder (aprox 175psi). We know your deck height is close to stock as your squish numbers are a hair low but very close to the target Ada shoots for. The squish clearance can be +/- a few thousandths with varying base gaskets.

 

[Nuklear6]

Just looking at the pics again and I don't see a build date on the bottom of that head. It's either a quite old Ada or a fake if it doesn't have a build date. Flip it over and post a pic normally can tell by the engraved font.

 

[badvinny]

If you find nothing wrong with the head, I would pull the cylinder, inspect the Ring tips and re-measure the ring gap. Then I would Re-hone the cylinders again, the hone marks appear coarse and minimal for only 6 hours, it should still look freshly honed. While you have that caliper out measure from the cylinder top and down to the exhaust port and ensure both cylinders exhaust ports are identical.

 

[Freakrider]

Just looking at the pics again and I don't see a build date on the bottom of that head. It's either a quite old Ada or a fake if it doesn't have a build date. Flip it over and post a pic normally can tell by the engraved font.

I noticed that too...Also notice the rear cylinder is running more rich which helps give it slightly higher #'s due to more oil seal.

 

[yamanube]

If you don't have the means to measure the volume of the domes, just swap them and re-check compression.

 

[///M5]

Is it an illusion from the camera flash or is there major pitting in the cylinder wall in the third pic? Like whomever bored it did not take enough to clean it up right.

It's the reflection from the WD40 on the piston crowns on the super shiny cylinder walls

 

[///M5]

Just looking at the pics again and I don't see a build date on the bottom of that head. It's either a quite old Ada or a fake if it doesn't have a build date. Flip it over and post a pic normally can tell by the engraved font.

It's the old style head, I'll get close up pics later it's at a friends place now

 

[///M5]

If you find nothing wrong with the head, I would pull the cylinder, inspect the Ring tips and re-measure the ring gap. Then I would Re-hone the cylinders again, the hone marks appear coarse and minimal for only 6 hours, it should still look freshly honed. While you have that caliper out measure from the cylinder top and down to the exhaust port and ensure both cylinders exhaust ports are identical.

I did the ring gap before building. I can measure to the top of the port now without pulling the Cyl, so I'll check that before pulling it.

You think the crosshatch looks worn?

 

[///M5]

If you don't have the means to measure the volume of the domes, just swap them and re-check compression.

Good idea, I can't believe I didn't do that yet.

 

[///M5]

I thought the same thing about the pit marks but when you zoom in I think it's the flash reflecting off the texture of the cast piston top.
OP If you have the means cc the domes and make sure they are not miss marked. Also use a friends compression guage as a second opinion. The odd part of all this is a 1mm over 701 with stockish deck height and 36cc Ada domes should have a cranking compression of near the lower cylinder (aprox 175psi). We know your deck height is close to stock as your squish numbers are a hair low but very close to the target Ada shoots for. The squish clearance can be +/- a few thousandths with varying base gaskets.

I can make something to cc the domes, and I have access to 35cc and 37cc domes to help diagnose. I will install different sets, torque them down and see if it's still 25 psi off.

Yeah, It's really weird that my problem is high comp. low comp would be so much easier to figure out.

 

[Captain PegLeg]

If you don't have the means to measure the volume of the domes, just swap them and re-check compression.

Quick way to rule out head/dome issues here.

 

[badvinny]

I did the ring gap before building. I can measure to the top of the port now without pulling the Cyl, so I'll check that before pulling it.

You think the crosshatch looks worn?

Yes, the crosshatch doesn't look right from the photo, I have bored cylinders for over 30 years, maybe it's just the photo? Putting a hone back in to scuff it back up a little more won't open the clearance too much and will only re-seat the rings. If the ring gap was too tight it will 'butt', although it shouldn't be a 25 psi problem. Swap the head domes first.

 

[///M5]

Yes, the crosshatch doesn't look right from the photo, I have bored cylinders for over 30 years, maybe it's just the photo? If the ring gap was too tight it will 'butt', although it shouldn't be a 25 psi problem. Swap the head domes first.

I adjusted the ring gaps, but they were pretty much spot on to spec from the box. I think that the glare from the WD on the walls is throwing the photos off. The final hone was to 320.