High CHT's still, fresh overhaul 78 hours 0-200

[6597]

Well, I am still struggling with high CHT's on take-off and cruise, below is a link from my old post on the subject-

viewtopic.php?f=7&t=1787&p=12157#p12157

Have been conversing with some folks on the subject and was curious what folks are getting for CHT's on their climbs and cruises. I have a ring gauge on the #2. By the time I'm done with run-up I'm getting close to 250F just on the ground with ambient 65F outside.

Now I've gone over the baffles, intakes, primer lines, etc dozens of times, even fashioned a cooling lip for the '46 lower cowl, all to no effect. Idle mixture gives a nice 50rpm rise on cut-off, I think I have it as rich as I can make it.

Grasping at straws, am thinking the nozzle isn't correct in the MA3SPA 4894, was overhauled by D&G and they did a superb job. Looking at their parts replaced list shows they still used the old venturi and nozzle. I had unusually good gph values before and after carb and engine overhaul.

New Millenium cylinders, the old Continentals had the opposite problem in that I couldn't keep things warm enough, go figure...

So, bottom line, anyone have high CHT issues and what did you do to fix the problem? Enquiring minds want to know (before summer gets here)...

[6643]

Have you verified the accuracy of the gauge?

How's your static RPM? If the performance is normal I don't think you can blame the carb. What behavior do you get when you lean it in cruise?

[6597]

John,

Static is 2400, if you remember I have that 71/42 prop on the 0-200 with the 337 from a previous owner. I had to back the throttle linkage down a little to make high static on the new engine. I remember reading somewherre that full throttle movement forward may induce a little more 'extra' fuel for cooling but for the life of me can't remember where I read that.

As far as leaning during cruise, I don't with the new engine/OH carb, the CHT simply takes forever to come down on the half hour flights I take.

Yes, gauge verified wth boiling water.

Check your email, I'll send you a correspondance I've been having with Victor and Randy-

[6643]

I remember reading somewherre that full throttle movement forward may induce a little more 'extra' fuel for cooling but for the life of me can't remember where I read that.

Yeah, most carbs do. I'm not certain about the ones on the O-200 but I'd be surprised if it didn't. So, if you adjusted the throttle so it doesn't open all the way, you're not getting that.

As far as leaning during cruise, I don't with the new engine/OH carb

I was suggesting that as a diagnostic tool. If you don't get any rise when leaning slowly then you're at or leaner than best power.

The idle setting does contribute to the mix at other throttle settings, although not a whole lot. I doubt that is the root of your problem.

If it were mine I'd have the prop repitched so the static was right at the maximum and do away with the throttle position hack.

[6597]

If it were mine I'd have the prop repitched so the static was right at the maximum and do away with the throttle position hack.

Just got off the line with Western Aircraft Propeller, $400 for a re-pitch, on it, 71/44 should work, I'm just a tad over 2400rpm on the static at full throttle adjustment on the carb.

I was suggesting that as a diagnostic tool. If you don't get any rise when leaning slowly then you're at or leaner than best power.

Next flight I'll do this. I should have thought of this, just like on idle mixture testing.

The idle setting does contribute to the mix at other throttle settings, although not a whole lot. I doubt that is the root of your problem.

Seeing as I'm getting close to 250F just after run-up we will start from scratch again on the idle mixture to make sure we are as 'rich' as possible for now. Going into launch with an already hot engine isn't working out for me unless I make the most shallow climbs.

Thanks John! I sent you an email reply a bit ago.

[8342]

Did you check the timing?
Tha will affect the temps.

Rick

[6597]

Did you check the timing?
Tha will affect the temps.

Yup, even dropped it back to 24btdc to see if there was a difference, no change. (0-200's with correct cylinders are timed 28btdc)

[6597]

John,

So I went flying this morning to test the rpm rise on lean at cruise and found it was perceptible, maybe 25+ or more depending on how I read the tach, so I can definitely say there is a rise in rpm, just not very much at all. I repeated the test 3 times, even locked the throttle so there was no creep (vernier) and was in steady stabilized cruise. Surprisingly the CHT did not rise much but it did rise, maybe 25F.

Fortunately it is a very cool morning over here in Vancouver WA.

I have not double checked the idle mixture adjustment as my IA is out of town. It does give me a 50 or so rise in rpm on shutoff at it's current setting. Even though it may prove little effect on the matter I still do have rather high CHT's (250-ish) just on getting to the run-up and mag check prior to flight. I will suggest to my guy that we see what the actual turns on the idle mixture screw are and re-set it for rich on the ground. Pretty much all we have to work with at the moment.

I did re-adjust the throttle control with another A&P and my static max is 2400 so I have full throttle movement. The adjustment was minimal at most.

I still can't figure the wide difference between the Continental cylinders and the Milleniums, but others have noticed the Milleniums do run hotter, at least on the internet forums FWIW. Thanks John!

[6643]

So, a 50 RPM rise at idle indicates the idle mixture is correct, and a 25 RPM rise in cruise indicates it is slightly rich of best power.

Based in this, my advice is to put tape over the CHT gauge and go flying.

[6597]

So my A&P had a spare MA3SPA carb laying about and we studied the idle mixture screw and it's total movement, being careful not to screw up the needle/seat. What we found is that it had about 4 outward turns before the needle felt slightly 'loose' in the threads. 5 turns out and it was definitely loose and therefore unsafe. We measured my current needle turns, and after much deliberation carefully turn it in to see what it's actual current measurement was and then backed it out an additional half turn.

Viola', we found the idle mixture screw can in fact contribute significantly to richening the open circuit mixture. It seems rather sensitive to adjustments also. My climbs are now in the 370-380 range repeatable 3 times on 75 OAT days. Cruise came down to 300-325-ish and I can finally lean at cruise with confidence. I still get a small rpm increase at cruise over-lean. That's good, I'm still a little rich to begin with. The bother is that at idle/cut-off I get a 100rpm climb before a clean shut off at 750rpm. I'll take that as not being optimum but it sure beats a hotter running engine. Oh, and my taxi to the run-up area doesn't get above say 210 or so.

I'll check my plugs at oil change in a few hours but so far so good.

Thanks John for putting a bug in my ear on the idle mixture, I think I found the culprit and she's running significantly cooler now!