Number 4 cylinder running hot!

Try angling the  carb  do lean that carb about 19 degrees with the fuel bowl closer to the cylinderhead  or top facing in  as the intake manafolds fo not draw even air that allows more fuel to go to the 3 or 4 cylinder . 

Okay so you are talking CHT.& EGT

1. In the standard Rotax 9 configuration you only have two CHT's. Usually on #2 & #3. Do you have additional CHT on #1 & #4?

2. Unless your Rotax is a very early model, the CHT is cylinder coolant temperature.

3. Coolant temperatures of 186F(85.5C) is well within the normal operating range of up to redline/danger of 120C. 

4. Oil temperature of 158F (70C) is a bit on the low side for cruise.

5. It is not unusual to see small differences in EGT probe readings caused by differences in cooling air flow, slight air:fuel mixture differences, etc.

6. I have no experience with pusher configurations however in tractor, we usually only monitor the rear two (# 3 & #4) cylinder EGT's because these are most likely to be the hotter, due to operating in the prewarmed air from #1 & #2. Cylinder/exhaust manifolds #1 & # 2 are likely to have cooler temps, particularly#1 as it is slightly further forward than # 2. The confused airflow in a pusher/gyro set up may further confuse EGT readings.

6. "White" plugs is a sigh of too lean a mixture/engine running hot - you will need to increase fuel : air ratio.

7. Airflow can effect mixture. Constricted airflow (eg blocked air filter(s) will cause the engine to run rich (dark/black plugs). A hole in a inlet manifold to head gasket may cause that cylinder to receive additional air and run lean. Leaning the mixture will cause the engine (cylinder) to run hot (light/white plugs).

Speculation: If only one cylinder is running significantly hotter than the rest, this may be something to do with a leaking inlet gasket/manifold at that cylinder.

Thanks for your help, everyone.

I do only have the two standard coolant temp sensors, but I do have 4 EGT sensors.

I have the next size large in main jets hopefully showing up this Friday to add more fuel across the board.

The air filters are unobstructed as far as I can tell.  And your statement about an air leak may be something.  Installing new manifold gaskets should not be too difficult.

I have never heard about angling a carb before.  Wouldn't that mess with how the float system is setup?

Thanks!

If we are indeed talking exhaust temp I would agree with a possible air leak causing that cylinder to be somewhat lean. 

Has anyone in the engine’s history changed its stock main jets? We’re the needle clips all in the #3 position? I rarely ever see an engine running in the high 1400F area and none I the 1500F area unless someone has messed with the carbs. Have you checked you jet sizes to see if they are stock jets? Has any Maint. Been done prior to this new high temp development?

This could be easily a bad temp probe.
I wouldn’t put larger jets in. That isn’t your problem and installing a larger jet is masking the real issue. All the other Rotax engines like yours don’t need larger jets and run just fine on stock jets. Find what’s causing your single cylinder issue.

Could be an air leak, someone tampered with the stock carb setup, poor air flow, etc…

p.s.

 Carbs are supposed to be mounted straight up for proper function. It’s in the literature.