Light Aircraft Association

Dear Friends,

Sadly, I have been here before, with tails of engine woes, which I thought were behind me now, but find myself head-scratching and thankful that today I was operating from a reasonably long runway !

Allow me to explain and I would invite any help...

After I had obtained my Cessna 120 in May, I had the old Eissman mags replaced with new Slick units. After this we flew it back to my base without incident and did some circuits, before experiencing a power loss on take off. Landed, cleaned out the gascolator - thinking this to be the cause and tried again. Same thing once again, so grounded it for further, more in-depth inspection.

I then carried out a fuel-flow check to the carb and found this to be very satisfactory. The very old and creaky (original 1946) fuel selector switch was removed and replaced with a new Andair unit. My new inspector then checked everything thoroughly, to try and ascertain the RPM drop, from my episode above, and carried out the following :-

1. stripped, cleaned and over-hauled carburetor (jet was loose and dirt in bowl)
2. re-set idle mixture setting (was too rich)
3. removed and cleaned all 8 plugs (they were very black with heavy carbon deposits) and re-gapped two of them that were out.
4. re-timed magnetos (the left mag was 10 degrees out)

This was 2 weeks ago and after all of this, with all other functions etc. of engine inspected and checked, we put it all together again and started. It ran very well, smooth and keeping good RPM at full power. We did a high speed taxi, with tail up and, as RPM maintained correct level, we flew without incident for 20 minutes. After this the 120 was put away as I was off to Oshkosh and vacation.

Today, I hoped to finally fly again. I put a quart of oil in, topped off with 20 litres of Avgas and off we went. Engine checks were good, 2 good mag drops, carb heat showed suitable drop and idle was good and smooth at 600rpm. At full power (2,300 RPM) on the roll, after 10 seconds and just as the wheels came off the ground, it lost power (100 - 150 RPM). We returned to the hold, did power checks again and, now on the other tank, tried again. Sure enough, after a short run just as the wheels came off it dropped power again, almost as if the throttle had been retarded like in a practice engine failure !

So, back to the hold, full power engine run holding on the brakes and, after 10 - 15 seconds, the power slowly dropped. We changed tanks again and, same again, after 10 - 15 seconds it slowly lost power (100-150 RPM).

I called my inspector, who is a great engine guy and also owns and flies an aircraft with a C-85 and he's at a loss to know what it is.

Could it (would it) be anything to do with suction in the tanks from poor tank venting ? I have what I thought were the correct caps - screw on (almost like a car radiator type) with two holes in each, right through the cap, so you can see directly inside, not vented through a second skin, as some are. Also, my caps do not have the small tube angled into the airflow, as some do. Surely, the 'weight' of fuel in the line from the tank would be enough to give pressure to the carb (tanks were both 3/4 full) but would the air past the caps, as we got to speed (60 mph) create a low pressure and create a vacuum ? Though, power was dropping after a short time when revving at full throttle but static on the ground, so this cannot be the case (?).


Help !

Many thanks
Graham

You said replaced with new mags, are they brand new or just new to you?
Could be a coil/ condenser problem.
Have you still got the old ones?
Have you changed any of the fuel lines lately ? ( the old "flap" gotcha).
What prop have you got and what static rpm?
Was it Ok before you changed the mags?

Welshman, you got me there ! I blame fatigue and stress...mine

As for the mags, they were brand new from Champion. I changed the mags because the old 'original' Eissman were making it impossible to start the thing with the pull handle and I am not wanting to hand prop it.

No fuel lines changed, only new Andair switch. The prop is a McAuley 1B90CM 71 x 46.

Graham

Have you checked the carb heat for correct operation , holding closed, or maybe the flap is moving on the shaft.

Are your carbs floats set correctly? Also, you said you put a new fuel selector switch in...did you re-check the fuel flows after doing this? And re-check the carb bowl? It's possible that work on the fuel line system might have introduced/dislodged some crud...

From the info given , it would seem that you can rule out the mags which would normally mean fuel.
Two suggestions so far are valid and would produce the symptoms, Low fuel level in the carb would reduce flow at full chat and inadvertant carb heat would defineatly do it.It is possible for the flap to wear and close off at full chat.
Unlikely but can the mixture be vibrating closed? Check all linkages for wear/tension.
I had the carb heat problem on a 150 so I guess its a similar system?
Do you have a cht guage? that may show some clues.

thanks for all help,

to answer a few points - fuel flow check done after replacing fuel selector switch and this gave flow of 1 ltr in 23 seconds - exact same for both tanks = 41USG/hr.

Carb was removed, cleaned and over-hauled subsequent to the fuel flow check. So far, 'crud' in the system seems a distinct possibility.

New carb heat cable fitted this week, though problem was prevelant before this - and subsequent too, of course. Reason for change, cable snapped during carb removal because of outer sleeve not giving enough free movement of the cable. I have ruled out the carb heat cable as a cause, all connections secure and fitted to a new air box, so nothing worn out there. Don't have cht gauge.

Going forward, I will check filters at fuel tank to lines and also primer line to ensure nothing loose there.

as ever, all help much appreciated.

thanks
Graham

As per above, did you check float level?

When you carried out your fuel flow checks did you have the filler caps on tight?

I ask because one possible cause could be a blocked or partially blocked tank vent. As fuel is used it causes a depression in the tank and reduces flow. Worth a check if all else fails.

I'll watch to see how your problem eventually is solved and hope to learn another lesson. We never stop learning.

Good luck.

Harry Hopkins

If all was well before the it is something that can build up and cause the increasing problem.
If you want to check the fuel flow you mus do it through the carb and not the pipe leading into it.
If you have a Marvel there is a drain plug at the base of the float bowl and you must check through that.. there are pencil filters in the carb inlet that can and do gradually block up and the fuel flow will slow or stop. pull it out and check and clean it. The stromberg also has inlet filter so check that.Keep us updated.
Graham.

Which carb is fitted? Stromberg or Marvel-Schebler?

Hi Graham,

If I can bring my contribution, a few remarks:
-luckily, the symptom apprears on ground test. You can enquire very accurately from there.
-in diagnostic, you need method, otherwise, you may miss the elephant in the corridor. "i'm sure it is OK", "I've already checked that" often lead to hours of turning around and missing one clue.
-a way to make diagnostic is to start from big parts to finish on details. If I were facing your trouble, I would check:
1/ Fuel supply: as explained by Graham D., in the carb itself, you can have trouble. To check through the drain hole or to check with the bowl off, acting directly on the float, is the best way.
2/ compressions: do they read OK hot, after power loss?
3/ timing: can the timing move with time/temp? Is the timing OK when hot with a lamp test?
4/ mags: do it behave the same with only the L or only the R mag on? Can you make the test, 10 min on one side then 10 min on the other side?
5/carburation: if you can get a cheap thermometer and use it as a EGT, can you read a substential increase in EGT prior or at power loss?


By acting this way, you can definitely sort fings out and sneak cause more rapidely. Do not hesitate to document, as many other contributors, I know mecanics can be very ungratful and sometime irksome, you learn every day and at every new case.



Bertrand

Dear All,

I am very grateful for all your input, suggestions and thoughts...trust me all will be considered and employed to help diagnosis.

I have a Stromberg carb fitted.

I am out of the country until September, so whilst appreciative of all the helpful guidance, am unable to begin to use any of it until then. However, i will endeavor to update you when I have started remedial actions !

thanks again
Graham