Rookie engine timing questions...

[Andrew K]

Recently I had the timing checked and adjusted on my Colt's O-235, fixed pitch prop. Everything was running fine before the changes but now there are a few issues that I wanted to ask about. I was so used to flying the plane the way it was that the new setting feels wrong and I would like your opinion to see if my engine was set the right way before the change or if it is running right now and I just need to get used to it.

When the timing was checked it was set to about 29 degrees BTDC. The mech said it should be set closer to 25 degrees BTDC and made the adjustments. Firing up for the first time after the adjustments the first thing I noticed was a lower (idle in the 600 RPM area). I thought it was maybe because the engine was cold but it used to idle closer to 800 when cold prior to the timing change. Everything else is smooth, run up is smooth and mag drops are within limits so I decided to fly around the pattern. It was a much cooler day since I had last flown yet the plane felt more sluggish and seemed to take it's time picking up speed. I noticed the max RPM I was able to achieve during climb out was 2350 @ 80 Mph and 700 fpm. Even during summer I could have sworn with the same loading (solo/light/low fuel) I could have seen higher climb rates but can't remember my RPM for the life of me. In the pattern I seem to have to open the throttle more to achieve the same feel as before.
After a few touch and gos I fly out of the pattern to see how she does in cruise. In level flight at full throttle it will slowly reach right below redline @ 2600RPM and seems to be nice and smooth.
Once back on the ground and ready to shut it down, RPM 1000, pull the mixture and it stops firing but then immediately diesels for 5 to 10 seconds more and when it finally appears to stop it fires backwards and the prop spins the other way before totally stopping and I know this can't be good. Prop ends up stopping in the vertical position if that means anything.

So my questions are:
1. Is 25 degrees BTDC right for my engine (O-235)?
2. What is a good idle RPM?
3. What kind of RPMs do you see during climb out? Is 2350 out of 2600 normal? (I wish I could remember what it used to indicate)
4. Are my dieseling and backfiring (?) problems related to the timing change or a possible fuel issue where the carb needs to be adjusted to match the new timing?

I know with the cars I tinker with if you advance the timing you can get more power so I wonder if that is why this timing setting feels sluggish.
5. Is there any reason I wouldn't want to go back to the 29 degree (even if 25 is recommended)? More heat or wear on the engine? If not I think I would prefer to go back to the way it was.
6. If I should stay with 25, and everything else is alright, what would be the first thing to do to stop the dieseling and backfiring issues?

Sorry that this is such a lengthy post but I want to get some background information regarding my issues before taking it back to the shop so that I am more familiar with the concepts.

Andrew

[Bultaco Jim]

1. Is 25 degrees BTDC right for my engine (O-235)?
2. What is a good idle RPM?
3. What kind of RPMs do you see during climb out? Is 2350 out of 2600 normal? (I wish I could remember what it used to indicate)
4. Are my dieseling and backfiring (?) problems related to the timing change or a possible fuel issue where the carb needs to be adjusted to match the new timing?

1. Yes, 25 is what's called out by Lycoming, - and your A&P HAS to set it there. Having said that, your engine is a low-compression mill, meant to run on 80 octane fuel. Higher octane fuel (100LL) burns slower, so, all other things being equal, it will run better at 28 or 29 degrees - as you have found out.
2. I would say 700 to 900 rpm would be good. (Lycoming says not to idle less than 1000)
3. 2350 is exactly what my Colt climbs out at. Most planes are pitched so as not to over rev when trimmed out at cruise. I believe at gross weight. So it sounds like yours is fine.
4. It shouldn't diesel or backfire. Did this start right after the timing change? The carb shouldn't need adjusting to match the timing, but you should check to see if the mixture cable is adjusted correctly, - it may not be cutting all the fuel off when pulled all the way out, which would give the engine just enough fuel to try to run.

[Andrew K]

Thank you for the information, it is a relief knowing that my engine isn't way off the page except for the dieseling part.
It would diesel before the timing adjustment for just a split second every once in a while so maybe the new timing is just making it easier for what little fuel is making it in to fire but it never kicked back the other way before.
I will look at the mixture cable for sure. Thanks again!

[Steve Pierce]

The compression ratio on the O-235-C1 is 6.75 to 1 and according to the type certificate data sheet the timing should be set at 25 degrees. The high compression O-235 engines are 20 degrees. I have never heard of Lycoming recommending idle set at 1000 rpm. I usually set it at 650 rpm.

[andya]

the jacobs (round engine on some airplanes) has a mag and a distributor like a car. the mag is set for 30 degrees advance. the distributor(been told its a modified late 40's Hudson) has an advance curve that has it at zero advance for low RPM and advances to 30 degrees to match the mag at some RPM above 1000-1200 rpm. when idling about 600-700 rpm and if you switch the ignition sw to distributor (advance of spark is much less than 30, maybe close to zero) the rpm will drop a little over 100 rpm and it runs one heck of a lot smoother, yes Virgina, timing will affect idle rpm

[Bultaco Jim]

The idle shouldn't be "set" at 1000rpm, it should be "operated " at 1000rpm. Lycoming recommends not idling below that in service letters 185b and 192b. It has to do with plug fouling, and the effects of using 100ll in older engines designed for 80 octane. 100ll has a lot more lead, but also has a lead scavenging agent which only works when the plugs are 800 degrees, which is attained at 1000 to 1200 rpm. Since your "in the air" idle speed is 2-300 rpm higher than static, setting it at 1000 would be too high. Sorry if I caused confusion. I would also like to add my own reason for not idling real low, -" pounding " of the bearings.

[Steve Pierce]

Sorry, I read too fast. I agree about idling fast. I also run up to about 1200 and pull the idle cut-off to stop the engine and lean all the time except in climb. Before I started doing that I cleaned plugs every 25 hours on my O-235-C1.

[Bultaco Jim]

I do the same thing. Back to the problem, I sure hope it's as simple as a poorly adjusted mix cable. The dieseling is a worry.

[zalto]

As noted, the correct timing is 25d BTDC. This is not to say that the engine will not perform better with the timing at a more advanced setting. There is a mag company out there that sells adjustable mags. You replace one of the conventional mags on the engine and tune the new mag for optimum efficiency. It may only be legal for the experimental crowd though. The big problem I see here is with the engine wanting to continue running after the mixture is pulled. This may be a problem with the primer leaking or the carb itself (float level, etc.) as well as an improperly adjusted mixture cable. I think the major reason that these engines develop the "deiseling" problem is that they develop a lot of carbon deposits in the combustion chambers. Another good reason to use good leaning practices. I have also noted that when I shut my colt down, it shuts down normally but the prop always stops in the vertical position. Does anybody know the cause of this?

Thanks,
Dusty

[Steve Pierce]

I would check the idle mixture setting. When you pull the mixture to idle cut-off at 1000-1200 rpm it should increase 20-50 rpm and die.

[andya]

As I have also noted that when I shut my colt down, it shuts down normally but the prop always stops in the vertical position. Does anybody know the cause of this?

Thanks,
Dusty

mine does this also an it has to do with the prop being mounted horizontal when the #1 cylinder is at top dead center (actually the prop will wind up being horizontal with any cylinder top dead center in this case).
I suspect the prop either bounces back from getting to top dead center or just makes it over top dead center when slowing down and winds up in a somewhat neutral position compression wise with it vertical.
THis is an ideal set up in the event you have to hand prop the engine as you have good/best leverage on the prop to pull it thru top dead center when propping by hand.

[zalto]

Thanks Andya,

It makes sense now. Actually a good thing to take note of. It always bugged me why my drivers side windshield wiper (in a truck) always wore out first. I finally figured that one out on my own. (somehow)

Dusty