Engine quit during takeoff when full throttle applied

[Magman]

Dan post #14.

I agree that air with a 27 degree rise ( 23 ambient to 50 degrees)

is too cool.

To me it sounds like a duct is off and he is taking air from lower cowl.

Had the same experience re need to lean.

 

[Dan Thomas]

Downwind is about 18", not 23" like cruise. Carb temp was showing 25 C in cruise at 23"/2400 rpm.

That's not much rise. 25C is only 77°F, a 54° rise over your 23°F ambient. Carb heat definitely needs looking at. Or your carb temp gauge is way off.

 

[brcase]

My vote is Carb Ice. Unfortunately I don’t have a carb temp gauge into 172 o-300 engine I was flying this morning. But I was able to duplicate this issue.
Two landings in a row carb heat on power off from downwind to the runway OAT 18 degree F. On power up no response from the engine. Pull the throttle back and pump the thottle a couple times and it came right back. Third landing left the Carb heat off (un-natural feeling for me) touched down and engine responded right away.

I bet if the OP and watched his carb temp during the descent he would have seen he was right in freezing zone. Even though on Downwind with power he was showing 50 degree F. Wish I had, had a carb temp gauge this morning to verify what I suspect.

I did try clearing the engine on short final on the 2nd landing and in flight it responded ok. I think it was only an issue when the throttle was fully closed and at near ground idle speed.

Brian
CFIIG/ASEL

 

[Dan Thomas]

If this still seems low I'll have it looked at, but the ducting seemed fine when I did the oil change last month.

The rigging of the carb heat control at the airbox needs checking too. It might only be operating part way. And the heat muff on the exhaust might have big leaks in it.

 

[brcase]

Thanks. Do you feel that your carb heat is in any way deficient or in these types of atmospheric conditions is it just the way things are? I agree that it would be somewhat uncomfortable turning carb heat off, especially in my case if the carb heat at idle was still keeping the temp needle in the green (I can't recall if it was, and the carb temp gauge in the airplane is not in the best place to be read while in the pattern, being on the far right side of the panel).

Dan, thanks for your help! The airbox was rebuilt about 9 months ago by an IA due to a crack and was working well upon installation. I will have an A&P look at the exhaust and check the airbox again to be sure.

The carb heat as it is now definitely reduces EGT by about 300 degrees F with mixture full rich at max continuous power (23"/2450 RPM), in addition the the increase in carb temp it's producing.

I think I'm going to modify my procedures as follows during high probability carb ice conditions:

1. Apply carb heat prior to the 500 ft power reduction to 23"/2450 during pattern work.

2. If performing T&Go, Leave carb heat on until after takeoff power is applied unless:

3. Carb temp isn't staying well above the yellow arc (5 C) when reducing power for descent "abeam the numbers" in the pattern. In this case, turn carb heat off.

Unfortunately this introduces a lot of extra mixture/carb heat control manipulation at low altitude, not ideal. It's just more motivation to save money for an airplane with a more modern powerplant, I guess.

After thinking about it a bit, I am not 100% sure carb ice is the entire answer. The A-65's and a lot of the C-85 engines I have flown will die after a power off descent to landing and applying the throttle a bit to quickly. Interesting I don't usually notice them doing this on the initial take off, perhaps that is because we have a bit of power in to taxi onto the runway. I have always thought these engines didn't have an accelerator pump and that might be the cause, but I really don't know if they do or not. The fix usually is to crack the throttle open and let the engine pick up about 100 rpm and advance the throttle normally. It only seems to happen when on the ground at low speed and full idle. So maybe it is just the opposite that at that low of RPM and maybe especially at low temps where the fuel does not vaporize as quickly the accelerator pump is actually flooding the engine, (to Rich). Just a little more RPM may be all it needs to pull that rich mixture or more air into the engine and start firing again. I never seem to have issues with go arounds.

I am just speculating based on my observations.

Brian

 

[Dan Thomas]

After thinking about it a bit, I am not 100% sure carb ice is the entire answer. The A-65's and a lot of the C-85 engines I have flown will die after a power off descent to landing and applying the throttle a bit to quickly. Interesting I don't usually notice them doing this on the initial take off, perhaps that is because we have a bit of power in to taxi onto the runway. I have always thought these engines didn't have an accelerator pump and that might be the cause, but I really don't know if they do or not. The fix usually is to crack the throttle open and let the engine pick up about 100 rpm and advance the throttle normally. It only seems to happen when on the ground at low speed and full idle. So maybe it is just the opposite that at that low of RPM and maybe especially at low temps where the fuel does not vaporize as quickly the accelerator pump is actually flooding the engine, (to Rich). Just a little more RPM may be all it needs to pull that rich mixture or more air into the engine and start firing again. I never seem to have issues with go arounds.

I am just speculating based on my observations.

Brian

The A65 and many C85s had the Stromberg carb, which had no accelerator pump. It had an accelerator well that stored a bit of fuel ready for sudden delivery on acceleration, but opening the throttle too quickly from idle would still cause a stumble. The OP's carb will be a Marvel Schebler or one of its successors and has an accelerator pump.

 

[hindsight2020]

Unfortunately this introduces a lot of extra mixture/carb heat control manipulation at low altitude, not ideal. It's just more motivation to save money for an airplane with a more modern powerplant, I guess.

Congratulations on completing step #1 of problem resolution: acknowledging the problem as one.

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