C-85 mixture

1quick46 · Post by **1quick46** » Sun Dec 28, 2008 18:42

Hi all. I have a C-85 powered chief that is driving me nuts, it is usally to rich or to lean depending on the OAT, we turn the big screw and it is perfect on a warm day, then on a cold day it hesitates horribly, set it for a cold day and i lose warm day performance etc etc... is there a way to put a mixture control on this carb...

Post by **joea** » Sun Dec 28, 2008 19:12

Many of the carbs on this engine have a mixture control, but its been wired "full rich" and left that way.

My Chief with a C-85 had the mixture control on the carb operational and it worked fine.

Have your A&P look at the carb to see if it has one, and if it does then you can put a cable control on it to make it functional. Now, if it does not have the mixture arm and control on the carb, but there is a cover over the mixture section on the carb, you may be able to buy the bits to make the mixture functional.

One last thing, this type of mixture is not ever going to work until you get up at altitude unless you run the engine RPM's up. It will not kill the engine on the ground, you still have to use the mag switch to do this.

Joe A

Roger Anderson · Post by **Roger Anderson** » Mon Dec 29, 2008 09:14

1quick46 wrote:Hi all. I have a C-85 powered chief that is driving me nuts, it is usally to rich or to lean depending on the OAT, we turn the big screw and it is perfect on a warm day, then on a cold day it hesitates horribly, set it for a cold day and i lose warm day performance etc etc... is there a way to put a mixture control on this carb...

A while back, I had an Ercoupe (C85, Stromberg) with the same issue. Unfortunately, and no matter what we tried, it continued until I sold it. After starting, it would barely run off idle and up to about 1500 rpm, coughing and missing badly. We rebuilt the carb, checked for induction leaks, messed with mixture, float level, on and on. However, all the symtoms would leave when carb heat was applied, indicating a lean mixture at low rpms. I checked with the Ercoupe group and found a few others with a similar situation. Finally, I just started using card heat for start, taxi, and initial takeoff roll, and left it on on during a go around until full power was developed. Oh, after the engine was fully warm, the issue did not occur. Realizing this contains no answers, it has happened to others. Reference the mixture control, I tired it, didn't really care for it, and like many others, wired it rich.

Post by **joea** » Mon Dec 29, 2008 10:04

Roger,

Not sure how much you know about carbs, and I am far from being an expert on them either. A carb has two circuits, idle and running. The two circuits overlap and the idle circuit provides power up to and including around 1800 rpm or so. (the "or so" comment is because I am not sure where it stops) The main running circuit starts coming in around 1500 rpm and takes over when the throttle is pushed in and RPM's go over 1800.

I have seen several situations where the carb had issues with the idle circuit but ran fine on the "running" circuit. What the problem is would be a big guess but in the above situation would try to see if a problem could be found in the idle circuit.

Paul Agaliotis · Post by **Paul Agaliotis** » Mon Dec 29, 2008 10:39

The thumb screw is for idle mixture. If it needs adjustment in relation to OAT then you have a carburettor problem. The float levels are generally out of whack.
Paul

1quick46 · Post by **1quick46** » Mon Dec 29, 2008 11:49

On my aircraft the HORRIBLE stumble is approx 1200-1400 rpm on takeoff roll, also it seems to backfire on back throttle indicating to me a lean condition. the question is why does the Idle mixture screw correct the issue when turned. but then when the OAT changes all my progress is lost....Maryland weather has been down in the teens up to 65 lately..also the float has been checked twice and been found correct...

Paul Agaliotis · Post by **Paul Agaliotis** » Mon Dec 29, 2008 12:13

The stumble would be from the lean mixture. Enriching the mixture via the thumb screw would decrease the problem but has not solved why the mixture was lean to begin with.
The backfiring when in glide with the throttle closed is generally from a leak in the exhaust gasket or broken pipe.
If the engine is backfiring when the RPM is held at 1200 to 1400 you have a hernia size intake leak.
Paul

Roger Anderson · Post by **Roger Anderson** » Mon Dec 29, 2008 13:44

joea wrote:Roger,

Not sure how much you know about carbs, and I am far from being an expert on them either. A carb has two circuits, idle and running. The two circuits overlap and the idle circuit provides power up to and including around 1800 rpm or so. (the "or so" comment is because I am not sure where it stops) The main running circuit starts coming in around 1500 rpm and takes over when the throttle is pushed in and RPM's go over 1800.

I have seen several situations where the carb had issues with the idle circuit but ran fine on the "running" circuit. What the problem is would be a big guess but in the above situation would try to see if a problem could be found in the idle circuit.

Joe,
I'm with you and far from being an expert, although my dad pretty much was. We focused also on the idle circuit, and on about the third disassembly it was probably the cleanest and most adjusted carb around. Our A&P also took his turn with it checking head pressure, float level, etc, but to no avail. It was about the only time I've ever seen persons just give up on ideas. I clearly reported the issue to the purchaser of the Scarecoupe and he flew off with the carb heat out, a happy aviator. roger

P.S. An after thought. He should have been a happy aviator! Two weeks later, the LSA and SP thing was finalized making an Ercoupe 415C an LSA...and the price immediately went up on them about 10 to 15 grand. I like to buy high and sell low.

MikeB · Post by **MikeB** » Mon Dec 29, 2008 14:19

Assuming all the other things are OK ( no intake leaks, base gasket tight to manifold, float level, etc.) I would suspect the carb passages are somewhat crudded up. That would be my next step after all the obvious have been checked. Having said that, these little Continentals with an updraft carb are pretty 'cold blooded' and it's not unusual to have to use some carb heat at below freezing temperatures. Especially at start up. At least the 65's on our airport need a lot when the temperature is down. Once they're warmed up it's a different story.

I remember the old (gas engine) trucks I used to work on had a cable and flap to take either cold air from the outside or warm from under the hood and they had a choke and accelerator pump, too to help things along......I know, I'm dating myself

.

Mike Berg

Post by **joea** » Mon Dec 29, 2008 14:31

1quick (what is your name? we are a friendly group here)

The rpm you are having the problems at are directly in the "transition area" where it goes from the idle circuit to the running circuit.

As others have mentioned, I would yank the carb off and overhaul it, and Paul is spot on about the backfiring, so while the carb is off check for vacuum leaks and such while you are there.

My guess is that you have several problems here and not just one in the carb. As Mike says, it sure sounds like the idle circuit is partially plugged up and only way to cure that is to clean the carb out.

Just my 2c worth and the above advise is worth what you paid for it!

Joe

MikeB · Post by **MikeB** » Mon Dec 29, 2008 15:47

One other thing; if the throttle shaft is loose in the housing (worn) it could cause a lean condition. Anything that upsets the air/fuel ratio toward the lean side. I'm not an A & P so I don't mess with aircraft carbs but the fundamentals are the same for all.

Mike

1quick46 · Post by **1quick46** » Thu Jan 01, 2009 18:17

Sorry, My name is Tony, I used to own a speed shop in Maryland and race a 740 Hp 4.6 mustang, name just kinda stuck. flew the Chief today OAT 28 degrees. flew with the carb heat on. although it was not making max rpm all the other issues were gone. on another note I am not making any oil temp, the scott guage was stuck at or below 100, I also verified the guage is correct after landing..tomorrow I am going to tape up the eyes, and lower opening and see what happens.. I am wondering if this motor is just not getting any heat in it.

Roger Anderson · Post by **Roger Anderson** » Thu Jan 01, 2009 20:20

1quick46 wrote:Sorry, My name is Tony, I used to own a speed shop in Maryland and race a 740 Hp 4.6 mustang, name just kinda stuck. flew the Chief today OAT 28 degrees. flew with the carb heat on. although it was not making max rpm all the other issues were gone. on another note I am not making any oil temp, the scott guage was stuck at or below 100, I also verified the guage is correct after landing..tomorrow I am going to tape up the eyes, and lower opening and see what happens.. I am wondering if this motor is just not getting any heat in it.

Hi Tony. A winter cover for the lower opening is a pretty standard attempt to increase oil temp. On my A65 it seems to help some. However, and others probably know better, but I think covering the eyes could have adverse effect on your cylinder head temps. Let others chime in here.

Post by **joea** » Thu Jan 01, 2009 20:25

Tony,

Thanks for the name. You will get more replies to your posts if we know who we are talking with!

Regarding the oil temps, pls look here: http://joea.com/phpBB2/viewtopic.php?t=3001

Four pages of discussion about oil temps on Aeronca's. I personally would not put anything on the "eyes" on the cowling, its just too sensative with regards to CHT and your cylinders.

Joe A

National Aeronca Association Bulletin Board

C-85 mixture

C-85 mixture

Re: C-85 mixture