"slightly retarded timing"

R

RobertGerace

Guest
In my quest to run LOP my first task has been to get my temps correct.

First some background:

I have a new JPI, which was using the OLD ALCOR leads to NEW JPI TIT probes. The old probes used to be in the WYE, and when I got new exhaust the NEW JPI probes were placed in the inboard riser, aft of the aft-most cylinder. The JPI is 'piggy-backed' on to the ALCOR.

My old temps were 1450 at 16.5gph with CHT in the low-to-mid 300's (using the ALCOR and factory CHT gauge, as well as a Hoskins for FF.)

After the JPI and exhaust, the JPI continued to show about the same numbers, but the ALCOR went off scale high. My A&P calibrated it, and was convinced that it was correct. I could only lean to 20gph with a temp of 1550 on the ALCOR, but the JPI was still showing 1450. At the urging of my A&P, I adjusted the JPI to match the ALCOR.

Attempting to run LOP, I had to lean to 12.1gph in order to be at 1625dF, TIT. Any leaner and the engine quits. Any richer and the JPI alarm goes off as I'm exceeding redline of 1650.

This LOP test convinced me that my ALCOR was lying to me, and my A&P is finishing up an installation of a NEW ALCOR and NEW leads as I type this.

However, along the way, someone on the CPA board posted the fact that he noticed my CHT's throughout all of this were in the low 300's and should have been in the mid-to-high 300's compared to my EGT values.

He said if you have a high TIT and a low CHT it is usually "slightly retarded timing." I asked my A&P to check it out, and it was, in fact, 2 degrees retarded.

What, exactly, does this mean?

What other bad things happen with slightly retarded timing?

What causes it to be retarded in the first place? (I got new mags in September.)

What changes in smoothness, power, airspeed, fuel consumption, etc. should I look for with the timing set properly?

Thanks!
 
You get more uncombusted mixture buring in the exhaust manifold e.g. higher TIT, and less power produced, e.g, lower CHT. 2 degrees won't make you run rough, though usually. Amazing what we find out when we look... :-0
 
Bob,

Bruce got it on the retarded timing. Advanced timing can be bad, too, in a different way. Once you get everything set with the JPI, you'll want to log the readings and check them after maintenance. Thanks to that process, I readily discovered that the shop mis-set the timing one time.

One question.... why did you piggy-back the JPI on the Alcor? My preference is to use separate probes and leads, providing me a ready cross-check in the event of failure of one unit or the other. Yes, I've experienced failure of the TIT probe (and a couple of JPI EGT probes) and was able to diagnose & readily resolve the issue by having redundant instrumentation. Certainly reduced the pucker factor when a gauge reads funny with a turbocharged engine....

just a thought.

bill
 
Bill,

My first (and probably best) thought was to provide redundant probes as well. I let my A&P talk me out of it because he assured me that the risk/reward of having a single probe or lead fail had less severe consequences than the risk/reward of drilling another hole in the exhaust system.

Since I had just replaced all the exhaust with the idea to prevent leaks, fires, and damage to structure, I was of a mind to 'drill as few holes as possible' and was ripe for him to make that suggestion.

In hindsight, I wish I would have insisted that he drill separate holes.

I had another chance to get it right this past weekend, but didn't think to insist that they are separate. I am now coming to the realization that I may end up having to take it all apart a third time to do it right...sigh...
 
It never stops, does it?

Mine is in for horizon gyro replacement this week.....
 
I think when it comes time to move up to a twin, I will just go buy Robert's 310. By that time everything that could go wrong will have been fixed at least twice. :)
 
Mark S said:
I think when it comes time to move up to a twin, I will just go buy Robert's 310. By that time everything that could go wrong will have been fixed at least twice. :)
Click to expand...

Man,

You're not kidding. It is going to be SWEET. It is going to be PERFECT. Then I'm gonna do something stupid and sell it to start all over again on a 340A...no...421C...no...Conquest...no Cheyenne....no Citation....BAWAHAHAHAHA Heaven help me! :D
 
RobertGerace said:
Bill,

My first (and probably best) thought was to provide redundant probes as well. I let my A&P talk me out of it because he assured me that the risk/reward of having a single probe or lead fail had less severe consequences than the risk/reward of drilling another hole in the exhaust system.

Since I had just replaced all the exhaust with the idea to prevent leaks, fires, and damage to structure, I was of a mind to 'drill as few holes as possible' and was ripe for him to make that suggestion.

In hindsight, I wish I would have insisted that he drill separate holes.

I had another chance to get it right this past weekend, but didn't think to insist that they are separate. I am now coming to the realization that I may end up having to take it all apart a third time to do it right...sigh...
Click to expand...

Robert, if you have connected two indicators to a single thermocouple probe, you are asking for trouble IMO. I'd check with the tech's at JPI before deciding to leave things this way. Personally I'd either go with two probes or a single readout. IME the probe is by far the most likely part to fail here, so two displays on one probe don't seem to add much redundancy. If you're really worried about probe failure just carry a spare, they don't take up much room or useful load. An inflight failure of TIT in a twin can be easily dealt with if you know how the engines normally compare and have an accurate fuel flow display.
 
This may sound crazy, but part 43 requires a check of engine timing at every 100-hour or annual inspection. The reason is in that the timing can be off 2 degrees. The vibrations and heat changes the timing over time and will mess up you performance.

Stache
 
RobertGerace said:
He said if you have a high TIT and a low CHT it is usually "slightly retarded timing." I asked my A&P to check it out, and it was, in fact, 2 degrees retarded.

What, exactly, does this mean?

It means that your engine is not operating as efficiently as it could.
What other bad things happen with slightly retarded timing?

Nothing usually

What causes it to be retarded in the first place? (I got new mags in September.)

The mag fires when the points open, so when you timing was set at installation, it was adjusted correctly, but now the cam follower foot has worn, closing the point gap, this causes the points to open late and close early.
What changes in smoothness, power, airspeed, fuel consumption, etc. should I look for with the timing set properly?

I doubt you will see any thing except the readings on the EGT

Thanks!
Click to expand...

This thing tells me my message was too short.. :) funny before I typed this there was nothing out of Quotes
 
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