J115PLSOR cylinder bank temp difference

[Harkonnen]

Ran into an interesting case, 2005 J115PLSOR, 60 degree V-4 2-stroke looper, aftermarket cylinder head gauge showing temps separately for port and starboard.

At WOT, port bank runs about 30F cooler (120 vs. 150F). Has anyone seen this?

What I know so far:
1. Probably real - heat cam confirms, gauge calibrates correctly.
2. Probably not one cylinder dead - starts and fires with any three cylinders disconnected, heat cam shows uniformly cooler head.
3. Probably not ignition - new coils and powerpack: replaced as a result of an earlier incident, spark looks good.
4. Probably not thermostats - swapped between banks, same picture.
5. Power loss - maybe. Fails to get to full RPM, but that may be an unresolved trim issue.

Less than 10 hours after break-in on a new powerhead, compression good across the board, carbs redone at that time, new Boyesen reeds.

Another observation: if I throttle up to 2000 rpm and just let it sit there, both banks warm up to 140F, and then port temperature drops.

What am I missing here, and do I panic yet?

Thanks!

 

[dingbat]

Unlike a pressurized automotive cooling systems which share a common heat sink/source, the temps on open loop cooling system are not homogeneous.

The intake side of the loop is always going to be cooler than the exhaust. The colder the intake the higher the temp differential between intake and exhaust

 

[Harkonnen]

The colder the intake the higher the temp differential between intake and exhaust

Except is is not coolant, and it is not intake/exhaust. It is port cylinder head vs starboard cylinder head, temperature of actual metal.

 

[Harkonnen]

Read the manual finally ("if all else fails..."). Thermostats open at 143F+/-, idle head temp is 143F+/-, WOT 160F max. Thermostat bypass valves open around 1800 rpm.

What bothers me is that port temperature drops just as thermostats are supposed to open, except that it should not matter, because at 2000 rpm flow should be controlled by thermostat bypass. And there is no corresponding drop on starboard side.

There must be a piece I'm missing here.

 

[racerone]

Replace springs on bypass valves ??

 

[Harkonnen]

Replace springs on bypass valves ??

I like the idea, but I suspect it may not be that simple.

The springs are fairly new - they came with the replacement powerhead - and even with the thermostats removed entirely, the port bank still runs cooler. That makes me think less about the control side and more about a downstream restriction on the starboard side causing flow to favor the port bank.

The reason I am leaning that way is this: if I feed water through the flush port and let it flow through the block, the amount of water reaching both thermostat openings appears roughly equal. However, if there is a restriction downstream of the starboard head, I would expect greater water retention on that side, which in turn would divert more flow through the port side and make it run cooler.

Interestingly, the starboard side of the midsection also appears warmer, although I cannot tell whether that is simply hotter exhaust or evidence of reduced water flow.

Is there an easy way to measure cooling water pressure without investing in expensive tools (or mechanics)? That is, before I pull SB head and start poking around with a wire.