OK bright and experienced minds. I'm stumped. My skywagon has a single channel EGT and no CHT. During cold weather the other day, on take off at full power, the EGT was about normal. At 500', when I pulled the power back to 24.5" the EGT went UP significantly. I played with the aircraft on the ground and again in the air and this is a constant and repeatable response to pulling the power back. I have to pull the power to about 20" to get the EGT to come back down to a normal high power indication.
At my A&P's suggestion I did check for intake leaks and found one loose clamp but tightening it produced no change whatsoever. Also idle RPM is normal. I am running 92 octane MoGas and have been for the last year.
Any ideas would be appreciated.
FZ

Make sure you don't have a fuel leak between where the line comes off the spider to the injector. That can cause your EGT to wizz up there.

Gump

GumpAir wrote:Make sure you don't have a fuel leak between where the line comes off the spider to the injector. That can cause your EGT to wizz up there.

Gump

Or a manifold leak. Pull power back, vacuum goes up and sucks air through a crack or compromised seal, leaning the mixture and sending EGT up.

Flying zebra, I know you checked for intake leak, but how?

Just did a visual for loose clamps. I didn't think to ask my A&P for another way to check. What is a good way?

Have you had any backfires recently during start?? This could blow one of the intake hoses out, maybe take a good look around the crossover tube hoses in front. If you have help, somebody can spray some ether around the intake hoses while idling on the ground, you should find an intake leak this way....concentrate on the side that your EGT probe is located.

If I'm not mistaken, the carb has a richer setting during full throttle operation which could mask this leak during a climb out using EGT, and show up only when the power is pulled back such as you describe.

This situation is a good reason to have a 6 point EGT/CHT instrument installed IMO

I'm embarrassed to admit that my nice new 14 channel EGT/CHT oil temp, carb temp analyzer is sitting in its nice new box, on my shelf, where its been for about seven months. I'm planning on doing a thorough refurbish this spring and that analyzer is part of the collection of STC'd stuff sitting there waiting for this damn house to be finished. So, for now, I'm making do with the single Alcor gauge.
No backfires but I will pull the cowling and really look and maybe draft my buddy/A&P to spray ether and see if we can pin it down.
All thoughts and feedback appreciated.
I'll go with the manifold leak unless anybody else has any ideas?
FZ

Check the primer lines for cracks and areas they may have been rubbing on the cooling fins. Small holes there can do exactly what you describe...but pretty much just at the cylinder the primer line is attached to from my experience (actually, when mine finally cracked in flight, it made the cylinder very unhappy, and went so far lean it would not come back into control even at full rich).

You can check for vacuum leaks in the manifold with the cowl off and a person at the controls to run the engine and hold the brakes. With the engine at idle (nicelow pressure in the manifold), you can use a propane hose to probe for leaks. I used a tube heater with the hose/regulator removed, and turned it way down low...it doesn't require much. You bring the tip of the nozzle around the area of each rubber intake manifold coupling and check for an increase in RPM, and also near the rear carb coupling.

This helped me find one really bad leak and another tiny one in the couplings. In the bad one, the RPM went from 800 to 1300...it was that obvious. The small one was barely perceptible.

GumpAir wrote:Make sure you don't have a fuel leak between where the line comes off the spider to the injector. That can cause your EGT to wizz up there.

Gump

Oooops, focused on the Skywagon part, not the 180 part.

Carry on.....

Gump

I removed the cowling and found that one hose had slid to the point that it looked likely that it was the culprit. Moved it and tightened it and that helped but didn't cure the issue completely. the EGT still rises slightly when reducing the power.
All of the hoses and clamps are okay but not exactly pristine, so I'm thinking about replacing them all. Can that be done without removing the individual manifold pieces? The hoses just downstream of the carb don't look like anything to write home about either. Same question there, can I replace them without removing the carb?
I will get my A&P buddy involved but want to get all educated and prepared before I drag him onto the scene.
As always, thanks for the feedback.
FZ