The International Cessna^® 170 Association

I want to install a carb temp probe in my 0300D engine. I am not at the hangar but I assume I have the MA-3SPA carb. My mechanic cannot find a port for the sensor. Can anyone advise where it is? I hope the answer is not that there isn't one

It probably should be mounted on a threaded boss on the carb box. I’ll try to find one that I have and post a pic.

If your carb does not provide a threaded boss for the CAT probe….(not all do)….you might consider using the one already existing in the intake manifolds. (One likely already is used for a primer on the left manifold….but the right manifold might allow it if you don’t already have a MAP gauge there.

Personally, I think CAT indicators are tits on a boar. YMMV

Check out Figure 79 of the 170B IPC.

cessnut wrote: ↑Sat Jan 25, 2025 8:48 pm Check out Figure 79 of the 170B IPC.

Beat me to it…

Here’s a carb box I have with the provision for adding carb temp. Thanks to Gary and George for addition info.

Thanks for all the responses. I think I found the port and it already has a sensor in it with the wires cut off! There is cross brace clamped onto the exhaust stacks with a central bolt through a bracket securing it to the carburetor. Partially hidden behind the bracket is the sensor. I assume this bracket is common. I need to figure out how to file the side of the bracket or move it partially or both to give me the clearance necessary.

IPC shows the brace applicable to s/n 20267 and on. Its prone to cracking along the tab at the carb attach points.

Alan, thanks for pointing out this interference between the probe and the brace. As discussed here, the brace (p/n 0550159) has several revisions. Not sure how they differ from each other or if they all interfere with this probe mounting. Here’s a couple of photos showing the interference you refer to. Judicious use of a die grinder would provide the necessary clearance. The probe, p/n 0550209-1, is a Richter B-5. Spruce wants $445 for it. I’m looking for cheaper alternatives. UMA has several that fit a 1/4-28 hole. Unfortunately, none of them match the necessary 89.68-91.08 ohm resistance @ 0°C.

GAHorn wrote: ↑Sat Jan 25, 2025 8:11 pm… Personally, I think CAT indicators are tits on a boar. YMMV

George, I’d appreciate your reasoning on this opinion. I think you may be right, but mainly because I would ignore it as it has no idiot light or horn accompanying the needle’s being in the yellow arc. But then, if it had such an annunciator, without a relative humidity sensor as part of the circuit, such a warning light/horn would be going off so often it would be totally ignored. The IPC shows four possible options for these 2” instrument holes — CHT, manifold pressure, suction and CAT. If an electronic engine monitor wasn’t in the panel, what would you fill an empty 2” hole in the panel with?

The Aviation Consumer and the FAA have reports on the two types of ice detection instruments; the optical sensor and the temperature sensor. The FAA report, while dated, has some interesting findings. The best place to mount a temperature sensor, they say, would be on the throttle plate. It’s the coldest place in the carburetor. In over 150 hours of testing they only had ice accumulate on the venturi twice - and then it was only light frost which didn’t affect engine operation. Ice accumulates on the throttle plate. The most accurate indicator of carburetor ice is revealed by the idle jet pressure. ”Large pressure fluctuations were typical during carburetor icing and were detected prior to engine performance degradation.” Check out these two graphs: They write, “establishment of accurate, sensitive, more direct instrumentation, … will assist in solving the problem.” It seems to me that the FAA’s NORSEE program is designed for a problem like carburetor ice. Surely, someone can come up with a borescope type camera mounted in the carburetor air box fitting that Jim pictured above aimed up at the throttle valve. Such a camera could remotely transmit the picture to an iPad/iPhone. The challenge would be keeping the lense clean.

Got the cross brace bracket ground out and everything is looking good.
And yes, I have since safety wired the oil temp sensor.

Thanks all!

But its also interesting where you have put the oil temp sensor. Normally that plug is not used and as its somewhat difficult to get to. In my opinion its not a bad idea to remove that plug occasionally to rid any sludge in the area in front of the carb to prevent internal corrosion of the sump.

n2582d wrote: ↑Wed Jan 29, 2025 6:02 am

GAHorn wrote: ↑Sat Jan 25, 2025 8:11 pm… Personally, I think CAT indicators are tits on a boar. YMMV

George, I’d appreciate your reasoning on this opinion. I think you may be right, but mainly because I would ignore it as it has no idiot light or horn accompanying the needle’s being in the yellow arc. …

My thinking on this is that the tachometer is a pretty good indicator that a loss of power occurs … and a Manifold Pressure Gauge is as well. Setting engine power and taking note of RPM and/or MP …. a loss of either is time to apply FULL carb-heat to check for it.

A CAT-indicator is not of itself any better at detecting carb ice… because 1) it has no knowledge of the other ingredient necessary for carb ice: …. humidity, … and 2) if the CAT-Indicator is in the range conducive to carb-ice …the recommended corrective-action is to apply “partial” carb-heat to bring it out of that range…. which will give (IMO) a false-sense of security while simultaneously reducing potentially developed-power, …removing air-filtration, …and violating the contrary and widely-held advisory to avoid operation with partial carb heat (encouraging poor-habit-disregard of that advice which can prove hazardous when operating other aircraft not so equipped.)
Finally, a CAT-Indicator can fail and/or fall out of calibration….adding yet another unnecessary variable.

This opinion is primarily applicable to simple, normally aspirated engines like our original carb’d engines. (If you are operating a Martin 202 or similar… disregard this advice and pay attention to your factory-equipped CAT and use the AFM-recommended Induction Heating Provisions.)

GAHorn wrote: ↑Mon Feb 03, 2025 8:43 pm
…
This opinion is primarily applicable to simple, normally aspirated engines like our original carb’d engines. (If you are operating a Martin 202 or similar… disregard this advice and pay attention to your factory-equipped CAT and use the AFM-recommended Induction Heating Provisions.)

A little off topic, but Dad flew Martin 404’s for Piedmont Airlines in the mid 1960’s and said that if it was snowing you wanted to actuate carb heat periodically because its intake was right out in front of the engine, and if allowed to fill with snow, even the R-2800 took some seconds to swallow the several GALLONS of water that resulted when carb heat was applied.