O- 200 external carb heat

[zenairdave]

So it's winter and oil temps are nil, and the dreaded run rough at full throttle on climb out has stricken me. In the process of reading all the wonderful possibilities I got to wondering.

First off, my application is Experiamental CWC, J3 open cylinder with O-200A

I have turned this kind of into a two part problem.
First , what changed?
Outside temps means engine is running much cooler. I have used Mogas all summer. (no extra additives)
My thought is I have a valve not seating completely when the block is very cold. (after chasing down fuel flow to carb, carb heat, spark plugs).

Then I started looking for ways to "warm" up the engine while operating in the cold.
This is the theory part of the problem next.

When safely on the ground and back at the hanger and trying to figure out what is going on, I noticed that when carb heat is not being used, it gets dumped out the belly of the engine cowl. Same goes for cabin heat.
Is there a really good reason that the cabin heat or carb heat when not being used (applied) could not be plumbed to the front of the carburator and let it warm the exterior of the carb so to speak?
Then I looked for ways to keep the cold out of the enqine cowl
The engine cowl has room around the air intake which allows cold air to blow in and hit the carb, I have since sealed this area up but have not flown since.
This plane struggles to show 140 degrees oil temp in the heat of summer so blowing the unused heat around the carb and then over the oil sump seems to me would be a win win deal.
Carb ice would be less likely if the carb is not frozen from fresh air and fuel vaporization which would make the application of carb heat needed far less.

Am I off base here?

 

[wronghand]

It may work. As long as you plumb it so the heat can still be dumped overboard in the warmer months. Rough engine on climb out in the winter is frustrating. But a dead egine due to a vapor locked carb in the warmer months could be a really bad day...

 

[DENNY]

Do you have the same symptoms on climb out once the motor is warm? If so it is most likely not a valve. Do you have problem with rough running at startup (morning sickness) that sounds more like a stuck valve. You could be running lean because you are getting a much more 02 in with the cold air. Just more stuff to muddy the waters.;-)
DENNY

 

[N86250]

My thought would be lean mixture too. If you pull on some carb heat does the roughness get better? If it does it's surely a mixture problem. You did check all the intake rubbers and gaskets for leaks? On the small engines, A65s, There is a baffle for the air filter for winter ops. Only it supposedly is to help prevent engine stoppage on final with the throttle closed. It's for the opposite of your problem but it's simple to fabricate and maybe it would somehow help. I have seen J3 carburetors covered in frost seconds after landing and shutdown on a steamy july day. No operation problems and no carburetor icing on that day. Does that mean something? Does a carburetor always operate at or below a freezing temperature? Is carburetor icing more dependent on humidity or ambient air temperature?
Has your mixture cable slipped and you no longer are going full rich? jrh

 

[zenairdave]

Do you have the same symptoms on climb out once the motor is warm? If so it is most likely not a valve. Do you have problem with rough running at startup (morning sickness) that sounds more like a stuck valve. You could be running lean because you are getting a much more 02 in with the cold air. Just more stuff to muddy the waters.;-)
DENNY

Problem started in the summer on Av Gas but only ran rough at about 400 ft on a second climb out.
Started and ran great unless I did a touch and go. carb heat would reduce rpm further and drop EGT but feeling would remain rough. Reduced power/ drop to cruise RPM would smooth things back out.
Tried flying around at full throttle for 10 minutes and smooth as glass.
Figured had to be reduced fuel flow under full load. took screen out of carb and put it back on fuel line and dumped the whole tank in a bucket. Runs like a garden hose in 3 pt attitude which pretty well eliminated gascolater screen, fuel filter and screen in carb. Plugs all looked very clean but big gaps. Installed 8 new plugs. Checked intakes for cracked rubbers or looseness. Seemed good. Switched to mogas 91 shell which has no ethenal but did use 5 gallons of regular,(likely with some ethonal) to get home one day.
Avoid go rounds and touch and goes and you would never know anything was amiss,
Then the cold weather came and problem became easier to repeat.
During takeoff it seemed like egt's wanted to climb over 1400 which I had not seen before and experimented with full carb heat to keep the egt at 1200 to 1300. flew 20 minutes that way.
Reducing power seems to be the only way to smooth it out.
Still starts very easy in cold and runs good until about full power time. cough cough and then seems to clear out for take off but the last time at about the time you clear the end of the runway, it goes into auto rough. loses some power when it happens. Full carb heat drops the rpm but rough stays. EGT drops from 1400 to 1200 while carb heat on.
Tired of risking my neck so I tied it to a truck and played around with cowl off and various rpms. I had a valve that was leaking when pulling over the prop but after a good workout on the ground with some carbon clean and 100 to 1 two stroke premix(storage idea until warmer weather) the valve sealed back up. So, is the cooled engine causing the tight valve to be worse or do I have something else going on? I came from the two stroke world so I do really good pre flight warm ups. At this point I am waiting for some warmer weather to see if that changes how it runs before I start taking things apart.
On a side note, if I shut it down with ignition, I get some raw gas leaking out the throttle shaft on both sides. Shut down with idle cut off and it stays dry.
Carb has 300 hrs/15yrs on rebuild and it looks to have got replacement throttle shaft bushings when it was done.

 

[zenairdave]

My thought would be lean mixture too. If you pull on some carb heat does the roughness get better? If it does it's surely a mixture problem. You did check all the intake rubbers and gaskets for leaks? On the small engines, A65s, There is a baffle for the air filter for winter ops. Only it supposedly is to help prevent engine stoppage on final with the throttle closed. It's for the opposite of your problem but it's simple to fabricate and maybe it would somehow help. I have seen J3 carburetors covered in frost seconds after landing and shutdown on a steamy july day. No operation problems and no carburetor icing on that day. Does that mean something? Does a carburetor always operate at or below a freezing temperature? Is carburetor icing more dependent on humidity or ambient air temperature?
Has your mixture cable slipped and you no longer are going full rich? jrh

I only recently discovered there was a cover for the filter which should richen the mixture by causing some vacuum and decreasing the ram air effect, but I am not really getting high EGT reading which makes me question the possiblility of being lean.

 

[zenairdave]

I will have to check that one but the cable feels normal and shuts off where it always did.

 

[zenairdave]

I hate threads that drop you when they find the problem but don't tell you what happened. Like most of the tough cases, I think I had several things stacking up which hid the obvious solution.
1st off, i had the climb out stumble a couple times in the warm but it got worse in the fall and in the winter(below freezing) the stumble got worse with just trying the add power for take off.
After a lot of research, possible causes that take a bit of work to determine if they are the cause include but certainly not the only things....
Shifted/restricted exhaust baffle, collapsed or over inflated hydraulic lifer, stuck/sticky/lazy valve, excessive dry valve lash clearance. worn camshaft/lifters. I decided to tackle all those items at once by pulling all four cylinders. (one at a time) As I pulled them, I removed and decarboned the valves, checked the rings for blow by and general inspection. Re installed and went to the next. Before removing, I already knew I had decent compression on all cylinders.
First cylinder had all the ring end gaps lined up but otherwise good, Second cylinder also was fine. Third cylinder appeared fine at first but as I was re installing the Intake valve, noticed it did not drop all the way in to the seat. It took a bit of a push. All the stems and guides had been good to this point, including this one so why did it not drop in? Carbon build up in the combustion chamber about 1/2" by 3/4" and 1/16" thick was just enough to scrub the lip of the valve at about 1/4" open. I can only assume as things got hot, side pressure on the valve when open made the valve hang open longer than it should have. I did find black residue in the whole intake track so likely the cylinder was igniting the fuel before the valve got 100% closed. Thus the power and rpm drop. Simply chipping away the carbon in the non machined area of the head solved this. For good measure , the 4th cylinder was removed and oddly enough had the only valve that was not still in new condition. The face was badly erroded but looked like it was still sealing reasonable well. Rather than update to a new guide and valve, I located and installed a NOS valve that matched the guide. Luckily lapping it in proved the seat was still in good shape. I kind of wonder if the valve was soft because the stem was marked with the incorrect part number for the stem diamater. I would guess that might have made it a chinese knock off part that had been installed without a log book entry at some point. Anyways, I was suspicious of the dry valve clearance and removed, cleaned and tested against a new Superior lifter and found my used ones were still tighter tolerance than the new one. I re installed them dry per the manual so I could measure the valve lifer clearance. Visually the camshaft lobs looked as they should and lifter faces did not seem hollowed out in any way.
It was harder to see some lobs more than others but zero corrosion and no metal in the screen which had not been removed for several oil changes produced no metal.
Anyways, when I measured the dry clearance, 4 were wider than max. .030 to .110. Not by a lot but it took four .030" over length pushrods to solve the clearance and one also took a different tappet to tighten things up to within the .110 range. The exhaust baffle was still attached and it has the bail welded into the discharge pipe to prevent full blockage if it does come loose. So with valve clearances corrected, hanging valve freed up and good mechancial condition otherwise, I was expecting great things. It started first tick over after a prime. The climb out rpm drop is gone. No hesitation when thottle is applied. Those symptoms are gone. You will think I would be set but there is still a rough running at 75% power or greater that I can not find. I have done zero investigation into the 400 hr mags so those are next on the hit list. At least I am flying again and at 55% power it purrs along at 95 mph in the heat but applying full power does not reward the PIC with much of a thrill. I wont rule out prop balance but I think timing is the next to confirm. The other symptom that is driving me nuts is with the ma 3spa carb, I cannot get any rpm rise prior to cut off despite screwing with the idle mixture between 3/4 and 2 turns. It idles fine with the odd skip at 650. Warm the idle is a bit higher at 700 ish. I know that should mean lean at idle but I cannot get a change. (I have had carb open and blew through the idle circuits but thats no guarentee that they are clear).
Frustrated but flying......

 

[skywagon8a]

.......This plane struggles to show 140 degrees oil temp in the heat of summer so blowing the unused heat around the carb and then over the oil sump seems to me would be a win win deal......
Am I off base here?

This idea has some merit. I know of one engine failure accident in the winter due to oil pressure loss with a kidney oil tank Continental engine powered airplane. It seems that the oil had become congealed leaving just a small amount of oil near the pickup tube to circulate within the engine giving normal temperature readings. As the oil warmed a glob of congealed oil shifted and blocked the oil pickup tube. While I have never seen what you suggest being done, it would be a reasonable idea to ensure that all of the oil was up to temperature.

..... You will think I would be set but there is still a rough running at 75% power or greater that I can not find. I have done zero investigation into the 400 hr mags so those are next on the hit list...... but I think timing is the next to confirm. ...

While flying in the greater than 75% rough range, do a mag check. Switch between the mags and notice if or what the difference is.

The other symptom that is driving me nuts is with the ma 3spa carb, I cannot get any rpm rise prior to cut off despite screwing with the idle mixture between 3/4 and 2 turns. It idles fine with the odd skip at 650. Warm the idle is a bit higher at 700 ish. I know that should mean lean at idle but I cannot get a change. (I have had carb open and blew through the idle circuits but that's no guarantee that they are clear).
Frustrated but flying......

This sounds as though the float level is too low. Which could be causing the rough running at higher rpm by having a mixture which is just a little too lean for that power setting. You do want the rpm rise at idle cutoff. The no rise at cutoff could be a symptom of low float level.

 

[zenairdave]

Good points so I went flying last night in the haze. Before starting I turned the idle mixture screw out another 1/2 turn which is enough it almost touches the fuel cut off arm.
Still no rpm rise on shut down.
Up I went. At 75% , things are still working pretty good. push the throttle the rest of the way and if anything you get a slight 50 rpm drop. Mag check revealed that both are set at the same time and all plugs are firing. About 100 drop on either at full or 75% throttle setting. I still need to confirm if they are both set at 24 degrees but at least they are both the same at any rpm. Next test was climb over the front seat and release the primer. I didn't get enough nerve to pull it out much, 1/16" maybe. it started running rough and while juggling the stick, got it locked back in again. Full throttle gives about 1200 egts and part throttle just under 1300 without leaning.
So that brings me back to float level which I can believe could be the culprit but when I checked it earlier, the Viton needle was in good shape and the float was parallel to the gasket surface, everything was clean so after blowing through whatever passages I could find, put it back together without adjustment. (that was before I realized I had a hanging intake valve)
Has anyone first hand experience that a float level being 1/16" low would give a poor power syndrome?

 

[cubdriver2]

You can pull the carb drain and install a barbed fitting and a foot of clear tubing after you level plane front to back. Clear tubing held against outside of carb will show fuel level inside carb with fuel on.

Glenn

 

[zenairdave]

You can pull the carb drain and install a barbed fitting and a foot of clear tubing after you level plane front to back. Clear tubing held against outside of carb will show fuel level inside carb with fuel on.

Glenn

I just bought the fitting. Any suggestion on fuel height. I think the book says 7/32 but that seems to be from the gasket to the top of the float???

 

[cubdriver2]

I just bought the fitting. Any suggestion on fuel height. I think the book says 7/32 but that seems to be from the gasket to the top of the float???

Bottom seam of carb to top of fuel. I lift tail till carb seam is level.

Glenn

 

[zenairdave]

Bottom seam of carb to top of fuel. I lift tail till carb seam is level.

Glenn

That seems higher than the manual suggests. Fuel would never reach the top of the float which means it should never reach the bowl gasket height??????

 

[skywagon8a]

....Up I went. At 75% , things are still working pretty good. push the throttle the rest of the way and if anything you get a slight 50 rpm drop. Mag check revealed that both are set at the same time and all plugs are firing. About 100 drop on either at full or 75% throttle setting. I still need to confirm if they are both set at 24 degrees but at least they are both the same at any rpm.

Your ignition is operating properly. You may be able to reduce the rpm drop by adjusting the timing.

Next test was climb over the front seat and release the primer. I didn't get enough nerve to pull it out much, 1/16" maybe. it started running rough and while juggling the stick, got it locked back in again. Full throttle gives about 1200 egts and part throttle just under 1300 without leaning.
So that brings me back to float level which I can believe could be the culprit but when I checked it earlier, the Viton needle was in good shape and the float was parallel to the gasket surface, everything was clean so after blowing through whatever passages I could find, put it back together without adjustment. (that was before I realized I had a hanging intake valve)
Has anyone first hand experience that a float level being 1/16" low would give a poor power syndrome?

You can ignore the EGT numbers. They are only there to give you a relationship. The numbers mean nothing.

I would need to look in a manual for the correct float level for your carb. Are you certain that it is 1/16" low? If so, that is your trouble. Glenn's method will work without having to dismantle the carb. Remember that the measurement is taken with the gasket installed. Bottom of the gasket to the top of the float. Glenn's method tells you the fuel level, not necessarily the same as the measurement given in the manual which is the distance between the gasket and the float.

 

[zenairdave]

Your ignition is operating properly. You may be able to reduce the rpm drop by adjusting the timing.

You can ignore the EGT numbers. They are only there to give you a relationship. The numbers mean nothing.

I would need to look in a manual for the correct float level for your carb. Are you certain that it is 1/16" low? If so, that is your trouble. Glenn's method will work without having to dismantle the carb. Remember that the measurement is taken with the gasket installed. Bottom of the gasket to the top of the float. Glenn's method tells you the fuel level, not necessarily the same as the measurement given in the manual which is the distance between the gasket and the float.

I have no idea about the current fuel level other then when I had the carb apart and held it upside down the floats sat about parallel to the gasket material. I never measured it because I was looking for an intermitant problem, not a poor running issue. The 1/16" was more a statement that if the level is off, it sure is not by much and was wondering how tempermental the float level is if only a slight amount from correct. I know the original owner lowered the level 15 years and 400 hours ago and it was not touched since.

 

[skywagon8a]

..... I know the original owner lowered the level 15 years and 400 hours ago and it was not touched since.

This is enough for me to take the carb apart, measure the float level and to make sure that it complies with the carburetor manual. A level lower than called for will create symptoms which you describe. I don't have a manual in front of me but the number 17/32" rings a bell. Perhaps someone else here can confirm the measurement.

You do want ALL of the required fuel to be flowing at full power. There is extra fuel flowing at full power for cooling and to prevent detonation. If the level was previously adjusted lower, then it is not happening.

 

[zenairdave]

I have not had time to test carb fuel level yet but I can add that with hot weather it is slightly better. Also up to 3/4 throttle , it makes great power. Advancing past that just makes it run poor with zero or negative rpm change. Will report what happens when I get time to pull the carb again

 

[N86250]

I have not had time to test carb fuel level yet but I can add that with hot weather it is slightly better. Also up to 3/4 throttle , it makes great power. Advancing past that just makes it run poor with zero or negative rpm change. Will report what happens when I get time to pull the carb again

Did you have the rear case off? We had an A65 that had been assembled by a homebuilder. It started easy. Idled fine. Ran good up to 1500 RPM. Then the more throttle added, the worse it got. When it was wide open it was belching black smoke, missing, and ready to quit. After a carb overhaul and a carb exchange and different mags made no difference, it was found that the cam gear timing was three teeth off. You could just see the marks with the mags removed. Reset the cam gear timing and it ran great. Could your cam timing be a tooth off? jrh

 

[zenairdave]

Good to know what symptoms show for cam timing being off but I don't believe the back has been off for either 450 hrs at overhaul or 150 since an electric start was added when there had been none. So many of the rough run or low power problems come from different issues.
On a different engine that I have been watching too many people having there fingers on the same engine, I noticed they used C90 pistons with a C85 crank. Currently they have low power and low rpm at full throttle. (kind of my complaint but a totally different reason.) I am thinking they detuned a C85 with that move that would be about a 5 to 1 compression ratio. They assumed since the cylinder was the same, that so were the pistons.

 

[zenairdave]

More follow up. I pulled the carb and put the elbow and clear hose in place of the drain plug and turned the fuel on. with carb level the fuel height was about 11/16" from the gasket surface. (I used a wrench to guesstimate the height) Sadly, I could not find what the fuel level height should be but at least I would be able to see how much it changed.
I removed the bowl and inverted it. The floats were very close to level to the gasket surface but would have to move more to be called level. I tweaked the tab that touches the needle just a bit and the floats sat more level. I grabbed the specified drill bit (7/32") and slid it under the float. It lifted the float about the thickness of the gasket which I did not remove so I had to be close. Unless you were looking for a difference, most guys would not notice how little it changed. Put it back together and with the fuel connected, refilled the bowl. At first I did not think the level changed but as I wiggle the carb around trying to duplicate the position I had it in for the first test, I think the level raised about 1/8" give or take. I really wasn't expecting any change because from what I know, as long as the level is high enough, it works. The book says otherwise and after putting it all together and test flying it, eureka. It will now take full throttle and leaning actually raises the EGT. Although the plane flew for almost 15 years with the lower fuel level, something changed when I adjusted the overly loose dry valve clearance. The float is now set per the manual and although I have to lean it a fair bit in 80 degrees, I think it will like the cold weather better now too.
Full discloser: When I was smelling like fuel, I also removed two good primer Orings and installed two new ones. Also the last time I had the carb off I thought the accelerator nozzle was supposed to sit outside the small 1/2" venturi and assembled it that way. When I had it apart this time, I put it back inside the venturi. That change seemed to make no difference when assembled either way.

 

[skywagon8a]

The measurement is supposed to be taken with the gasket in place. Don't mess with it, richer is better than leaner.

 

[zenairdave]

The measurement is supposed to be taken with the gasket in place. Don't mess with it, richer is better than leaner.

Somehow this has morphed into a carb tuning thread. LOL
I set the float level assuming the gasket was not supposed to be part of the measurement. :x
This weekend with a passenger, 90 degree OAT, I used most of a very long paved runway at 700 ASL. It runs okay at full throttle but some experimentation with leaning revealed that leaning brought back a lot of missing power and changed the C65 power back to O200 power. I will be taking it apart a third time and setting the float in the middle of the two positions I have had it. There must be a happy medium someplace.

 

[zenairdave]

Pulled the carb for the third time and set the float with the gasket in place at 7/32. While I was at it, I replaced the lock o seal washers on the spider for good measure and noticed the last guy used regular washers but at least they were only finger "tight" . The engine still is on the rich side at 800 ASL and 90 degrees F but I can live with that until I see how close it is when the temps get below freezing.

 

[zenairdave]

OAT is closer to freezing now and leaning is no longer needed. I guess I have the happy medium for carb float setting. Engine is running smoothly and I am actually starting to trust it again.

 

[Pacerfgoe]

The fuel level should be 7/16" down from the top of the gasket. If you had 11/16" thats likely your problem.
I just rebuilt....Ahem....repaired my ma3-spa carb, and set mine to 7/16" and have no issues at all, runs great with a slight increase in RPM at cutoff.

 

[zenairdave]

The fuel level should be 7/16" down from the top of the gasket. If you had 11/16" thats likely your problem.
I just rebuilt....Ahem....repaired my ma3-spa carb, and set mine to 7/16" and have no issues at all, runs great with a slight increase in RPM at cutoff.

You have to be careful we are talking apples to apples here. float level and fuel level are two different measurements one might use. The book I have says 7/32" from the gasket to the top of the float. I ultimately lowered my fuel level by using the 7/32" setting but did not recheck what fuel level that gave me because I could not find any book that indicated fuel level measurement. I started lean and raised the fuel level too much(rich) and then had to come back to a happy medium.

 

[Pacerfgoe]

You have to be careful we are talking apples to apples here. float level and fuel level are two different measurements one might use. The book I have says 7/32" from the gasket to the top of the float. I ultimately lowered my fuel level by using the 7/32" setting but did not recheck what fuel level that gave me because I could not find any book that indicated fuel level measurement. I started lean and raised the fuel level too much(rich) and then had to come back to a happy medium.

I understand the difference.
7/32" is the correct float level measured with the gasket in place
With the float set correctly, the fuel will fill to within 7/32" of the top of the float
7/32" + 7/32" = 14/32.... = 7/16"

 

[zenairdave]

I will be marking that dimension down in my Carb manual because I could not find it when I wanted the info.

 

[zenairdave]

Quick follow up to this old problem. My Accelerator pump check valve which is factory set and not to be adjusted, was leaking. Changed out the assembly and it leaned out the full throttle excess rich problem to a more normal slightly rich at full throttle. Explains why local carb repair attempts failed because it was not possible, or obvious to me, to test the 'accelerator relieve valve. I assume the during a carb repair, this part is not generally replaced unless it gets to a facility that can fully test flow the carb and they find the problem.