What causes this????

[Brantel]

I have learned one thing....EGT temps are highly subjective with tons of variables!!!

Seems every source I look at for written documentation have different ideas on what normal is for EGT's. They are more consistent on the fact that the important thing is the spread between them and the amount of change that you get from full rich to all the methods of leaning etc.

Thanks!!!

Originally Posted by Radomir

I wouldn't expect EGTs in the 1400s at full rich... 1200s would be numbers I'd expect to see.. (maybe even high 1100s, depending on probe location etc).. All this is just a general thought.... you could nit-pick a bit.. so at best this is just a guideline..

[Brantel]

I have ordered new plugs and new distributor block, distributor gear, rotor gear and points for the mags. Capacitors check good. The harnesses are brand new.

Will install the new plugs, change the parts in the mags and retime them to the engine and try this again...

Thanks!

Originally Posted by schristo@mac.com

sounds like it might be a bad plug in that cylinder? fires good under low pressure / power but not under high so the egt goes way down. did you check the plugs in a tester?

Originally Posted by OceanBob

Lower temps happen for three reasons. A plug that doesn't fire a cylinder that has low compression and a rich mixture. I've had a PRIMER leak that sucked extra fuel into the manifold causing a rich mixture on one cylinder. like I said LONG SHOT. It was a bad O ring on the primer plunger. Just tossing ideas around.



Bob

Originally Posted by OceanBob

I agree with Stephen. The first thing to check is the plug.



1. Replace both plugs in #2 (don't waste time moving plugs around)



2. If the problem persists then borrow an ignition harness tester and check for a short in the plug wires. This is a portable box that shoves 100,000 volts into the unhooked plug wire and if you have a breakdown in the insulation the spark will jump thru that weak spot. It will also jump the gap if you have a hairline crack in the plastic rotor cap. You can hear it crackle.



3. Chrome cylinders have problems (cast iron rings) breaking in sometimes and you might check the cylinder walls in #2 to see if you have any scratches (broken ring?) that are fouling the plug with oil and preventing ignition. (this is a long shot however)





4. A real long shot would be a valve that 'sticks' and lags preventing full compression in that cylinder. Or it sticks because you have a broken valve spring which allows the engine to run at low rpm but simply can't dance fast enough at higher power. It's hard to see this problem but if you run out of options above you will have to remove the rocker cover, both plugs, move the cylinder to bottom dead center, shove a nylon rope into the cylinder, slowly bring the cylinder up until the rope is crushed against the valves; using a lycoming valve spring depressor compress the cap around the valve, remove the keeper keys and lift off the spring assembly to check for a broken spring (or weak spring). *as a side bar, in the old days if we had a valve that leaked air during a compression check on a lower time engine, we would chuck the loose valve up in a drill motor, lift it off the seat, spin it up and pull it back to clean the garbage off the valve and seat....usually this worked..hahaha....Political correctness prevents me from naming this sort of Valve Job. :-)



Good luck..! Let us know what you find.



Bob

[jabarr]

What model carb do you have?? I agree with Radomir that your EGTs seem a bit high ( I know the numbers are a bit subjective, but I would be concerned with numbers that high)--A test would be to lean to peak and see how much rise you obtain. I would like to see a good 150 degrees rich of peak when running full rich. I think Don at Airflow says more like 175-225 degrees.
see this for a little more info-- http://www.vansairforce.com/communit...ight=4164+carb

[Brantel]

It is a 10-3878 that was modified by Bart at Aerosport to be richer for use on an RV.

Originally Posted by jabarr

What model carb do you have?? I agree with Radomir that your EGTs seem a bit high ( I know the numbers are a bit subjective, but I would be concerned with numbers that high)--A test would be to lean to peak and see how much rise you obtain. I would like to see a good 150 degrees rich of peak when running full rich. I think Don at Airflow says more like 175-225 degrees.
see this for a little more info-- http://www.vansairforce.com/communit...ight=4164+carb

[170 driver]

I recently had this problem on another type of plane,,,changed the harness and had the mags overhauled,,still no good,,,tested the brand new unison spark plugs in a bomb tester and 2 were not firing under pressure, the plugs had less than 5 hours on them. I tested some other brand new. never run unison plugs I had and 1 of those was also bad. I switched all to champion and the problem is fixed. If you have unison plugs, I'd consider switching them.

[Bill Phillips]

first noticed after takeoff and during climb out. Scared me for a couple flights until I mentioned it to a Bud who advised me to tap the brakes after take off as the wheels were still spining. Sure enough that was the fix. You might try that before taking your plane apart. Best of luck, Bill RV-8a wiring

[airguy]

Originally Posted by Bill Phillips

first noticed after takeoff and during climb out. Scared me for a couple flights until I mentioned it to a Bud who advised me to tap the brakes after take off as the wheels were still spining. Sure enough that was the fix. You might try that before taking your plane apart. Best of luck, Bill RV-8a wiring

Vibration, sure - but I don't think that would cause an EGT drop!

[Mike S]

Originally Posted by airguy

Vibration, sure - but I don't think that would cause an EGT drop!

Unless the EGT probe was a bit faulty.........

[IowaRV9Dreamer]

I get both a vibration and EGT drop at full power too.

So I added an engine monitor to my tired old O-235. At cruise or low power, I get EGT spreads in the 40-60 degree range.... but at full throttle my back two cylinders (3 & 4) run about 200 degrees cooler EGT. There is a noticable roughness also. I've been told this is just a byproduct of poor fuel/air distribution inherent to the carb installation. The engine seems to be making rated power (RPM) and more or less "performs" as expected for a high time little engine. I have recently rebuilt mags, replaced plugs & wires, and set valve tappet clearance. All with no real change.

[truflite]

Check your air induction tube for that cylinder. Make sure it is not leaking. It could be going well lean of peak which could lower the temp drastically. We had a similar problem on a C182. After a mountain of money, tightening a hose clamp fixed the problem. Won't cost you anything to try it.

Dave Nellis