PA30 fuel flows and exhaust popping during landing

[jbravo65]

I have a 69 PA30C (non-turbo). I have a Shadin Fuel flow meter and I consistantly show between 1-1.5 GPH ff more on the left, during take off, cruise and decent from cruise altitude. This is even after I lean both engines to the same rich of peak temp at cruise. I havent checked actual fuel used yet but will do that next time I fill up. This is my first twin and didnt really used to pay close attention to the ff difference on the twins that I rented. I just know the ff's were pretty close on the stock analogue guages on the Seminole and Senecas that I flew previously.

I guess my first question is: If it turns out that I am actually burning 1 to 1.5 GPH more on one engine than the other, would it be worth spending the money trying to trouble shoot the cause? If I find out that the guage is off, what would be the steps to check that. The Shadin guage was installed about 10 years ago. By the way, I have just about all the Knots2U mods, including the nose cowl.

Second thing I'm curious about is when I am pulling back the power just before landing, there is quite a bit of exhaust popping. I was told that this is because of the short exhaust stacks that this aircraft has. Is this true? My stacks are cut about 1 inch beyond the cowl opening and do not have extentions.

Third and last concern is that both oil temps are about 220 to 230 during climb out with cowl flaps wide open and about 200 to 210 with cowls open about 25%. CHT's are all 350 and below. These temps are shown on my digital engine analyzers.

[steen]

Welcome to Twinkie country. You will love the bird.
You don't say which engine is high on ff but you should know that the Shadin will show a higher ff on the right when the heater is running, but only about .5gph. Other than that, I have no clue.....my Shadin is about the same age and leaned to the same egt they read within .1. And they are dead accurate.
NT Twinkies are not known for being hard to cool and your high oil temps are worrisome. Even in extremly hot temps my oil temps, and cylinder temps, stay cool during climb with the cowl flaps closed except when it's above 90-100f and still hot at 8000', then I might have to open cowl flaps a little. Suggest you make sure your engine baffling is really tight and well sealed and then consider when your oil coolers were last overhauled.
You might check the vernitherms, but for both to have failed would be a big coincidence. By the way, book says climb at 125 but I use 150 to about 6000', then 140 to 9000 and 130 on up to my usual 12,500' cruise.
The rate is about the same, you cover more ground and cooling is good.
Excessive popping (afterfiring) on throttle close for landing is an indicator that idle mixtures are too rich, easily corrected and easily checked by careful notice of rpm rise on shutdown....you should have about 50-75rpm rise as the mixtures are closed. More than that you are rich.
I suggest you turn the idle mixture knobs 1/2 turn toward lean and see what you get.
Let us all know what you find about the high oil temps and fuel flows.
Steen

[jbravo65]

mine is showing the left engine burning more than the right. I'm going to fill it up and check the fuel in both tanks so see where they're at. I'm hoping its just a matter of adjusting the K-factor in the Shadin meter. Other than that, maybe the fuel servos if in fact they are not buring the same amount.

Its been about 90+ degrees here in Fresno, Ca and the oil temps can reach 230 while sitting on the ground too long. The previous owner said his oil temps have run between 200 and 220 on both engines since he's had the plane for the last ten years. I usually climb out at about 130 mph all the way to cruise and I'm getting 210 to 220 on climb out and about 200 in cruise. cht's are all 350 or below.

What are your typical oil temps during climb and cruise? Do you have the Knots2U or LoPresti nose bowl mods?

[Kristin Winter]

I have seen some of these things in my 69 twin. You are going to have to approach it scientifically.

As for the fuel flow, set up in cruise and switch both engines to their respective aux tanks and run for a period of time. The longer the more accurate. I would recommend at least an hour. Fly the rest of the flight on other tanks. When you fill the test tanks (usually aux) you can see if now is burning more and get an idea of the accuracy of the Shadin.

One thing to check, were your fuel servos last overhauled at the same time? If not, they may not be set up exactly to match each other.

As for the popping, I am guessing too rich an idle mixture.

As for oil temp, check baffling. I used a digital camera up on top of the engine, with the cowlings on, to inspect the baffle seals. Next, check the actual tempature. If you get the oil hot, try physically taking the temperature of the oil, either through the dipstick or by draining some.

That should be enough to start with.

[N3322G]

Welcome to Twin Comanche ownership.

Have had several of the issues you mention and Kristen's advice right on target. Also, you might check what have been termed the sniffle valves at the bottom of the engines. After maintenance, I have found that mine were frequently disconnected and recently replaced them and their hoses/tubing.

[Stuart Baxter]

[charles Melot]

[Zach Grant L1011jock]

[Terry Tibbitts]

I realize this topic has been well discussed, and I have read everything I can find in the archives, but here goes: I have just restored a 1969 PA-30 that has been setting for a while and we are getting the popping on landing. I have heard some say "yeah the twin Comanches do that because of the short stacks" and I am thinking that might be true. The idles are set perfectly, the idle mixtures are set perfectly, injectors are clean, fuel burns look right on both engines, temps are all in the green, everything is straight and true EXCEPT, we get that slight popping on landing (only) when you pull back power for the flair. Should I search deeper for the cause, or is this just normal for some of our engines? Other data: Engines are at TBO but good compressions and appear to be in good shape. Mags are timed. Plugs are clean and gaps are set. No obvious leaks in the exhaust or intakes. One of the RSA units is over 20 years old, but seems to be doing its thing correctly, but other than that, I can't find a reason for the backfiring on landing.

Other ideas? Just keep flying?

Thanks!
Terry

[N3322G]

Hi Terry,

So glad you have restored the Twin.

44 years of Twin in the family. When all is correct, no popping on landing - that being said - we frequently just fly with it. IIRC Zach Grant, Comanche guru says it is a idle richness thing and he adjusts his seasonally. I'm not a mechanic so we don't.

IMHO it has nothing to do with length of exhaust stacks - we flew with short ones and for a decade now, with the extensions - we can have popping in either case.

Things that have helped are: fuel pumps off during taxi, lean during taxi, fine wire plugs.

Hope this helps.

[Charles Schefer]

Terry,

I agree with Pat and other responders comments here. Zach is on target in his post regarding the idle adjustment. The popping you hear is simply too much fuel making it thru the cylinder unburned, going in to the exhaust pipes and then detonating in the exhaust. In a car we'd call this a "backfire" When you land you bring your engines to idle and the RSA's idle mixture should then come into play. If it's set too rich you will end up with the popping.

Here's some more food for thought... when did my muti-engine training it was in a Twin Comanche. I was taught to always bring the mixtures back 1/2 way during taxi or the plugs would foul by the time you got to the run-up block. I also found that during landing there was a lot of the poping you describe. By way of comparison my Twin Comanche does NOT do any of that. The idle mixtures are adjusted properly. I do NOT lean at all during taxi and I get NO popping whatsoever on landing or during idle. The length of the exhaust stacks has nothing to do with it.

Here's something else to check... how dirty / sooty are your gear doors? I clean mine, and the gear after every flight. A year or so back I was finding that the left door was always sootier than the right. Also my IA watching me do a run-up from outside commented that there was visible soot in the exhaust on the left side during a full rich - runup. To further this my fuel flow on the right engine at idle (1K or lower RPM) was around 1.5 - 2.0 gph whereas the left engine was exactly twice that. Furthermore at idle if I slowly leaned the mixture to cut-off the right engine would show no more than 50rpm (if that) rise before the engine died. This is correct. My left engine on the other-hand showed an almost 500rpm increase before the engine cut-off - not correct.

I thought we'd need to overhaul the left engine RSA unit but they were both overhauled with the engines not to far back. Anyway my IA and I were able to adjust the left engine RSA unit to get the idle mixture correct. No more soot, no more rpm rise (no more than 50) when leaning to cut-off at idle and the fuel flows at idle on the two engines are within 0.1gph of each other and more typically they are identical.

Do the test others have described. With your engine(s) idling, slowly pull the mixture back to cut off and note the max RPM rise. You should not see more than 50 or so rpm increase before the engine dies. More than that and you will need to adjust your idle mixtures. If there isn't enough adjustment then the units need to be overhauled. Call Heritage Aero and they know a good local shop that does this very well.

One more comment on adjustments... the way we did it was my IA and I used wireless 2-way radios with headsets. With me in the cockpit and him on the ramp under / behind the engine we slowly made adjustments with my IA making the adjustment while I called out readings on fuel flow and idle RPM during full rich and pull to cut-off on the mixture. It took us about an hour to get it perfect but we did get it quite perfect. Download the service manual from the ICS home page tech pubs and look up the procedure. The procedure for adjustment is important. The idle speed and mixture settings must be adjusted in the correct order to get the right result. It's not hard but do follow the procedure or you will not get what you're looking for.

Good Luck,

- Charles

[Terry Tibbitts]

Thanks Charles and Pat. I have done all of these things you suggested prior to my post with the same results, but I will do the idle mixture control procedure again and see if I can get any change in performance.

Terry

[Zach Grant L1011jock]

Terry,
Just a reminder, the idle mixtures are not density compensated in any way. A 20 degree F difference or a 2000' pressure altitude difference could be enough to cause this to happen even if the engines were set up properly at the then current conditions. Most people complain of this if they set the engines up in the winter and fly into summer. Going the other way is usually not noticed very often as the engines run, but they are lean on the ground at the lower density altitudes. I suggest taking the engines to extremely lean mixture (engine almost unable to run) and working gradually richer from there. As was mentioned before in the thread, its a two man job and you should be able to advance the thumb wheel one flat at a time and check without having to restart everytime.

-Zach

[Neil Fogle]

I don't claim to be the ace of the base even though I have an A&P with IA. I am basically a parts changer and would rather pick someone else's brain than re-invent the wheel. I have owned, flown, and maintained a twinky for the last ten years so I have dealt with most of the issues.

Oil temperature: I had a high oil temp reading on my left engine that would react somewhat to cowl flap position and speed. After replacing all the hoses, getting the cooler rebuilt, changing the vernitherm, checking the baffling and who knows what else, I determined that it was faulty indication. They no longer make the resistance unit for the accessory case so I ordered a different unit from Air-parts of Lock Haven that requires a resistor to match the indicator. They supply all the necessary parts. Problem solved. I found out later that Webco has some used units in stock that will do the job with out the paperwork.

Okay, lets talk about the exhaust popping. When I first started flying my twinky it would not only pop while landing but on taxi out as well. After making sure the idle speed and mixtures were set properly it would no longer pop on taxi out but would still pop somewhat when I pulled the throttles back during flare. I finally solved the problem by sending the Fuel Controls to Ken at Aircraft Accessories of Oklahoma (918-835-9924). What I would suggest is that you look at the data plate on the fuel control and note the dash number at the end of the serial number. Call Ken with that information and see what he recommends. I can't remember but mine were a dash 5 or 6 and the current fuel control dash numbers are up in the teens. There had been several factory modifications since mine were overhauled.

Fuel flow gauges: The so called fuel flow gauges in the twinky are fuel flow gauges in name only. What they do is measure the fuel pressure at the spider and assume the higher the pressure the higher the fuel flow. This is reasonable accurate when everything is working normally. If you have a clogged injector ( as number three will do every so often, but that's another story) your actual fuel flow will decrease some what but the Piper gauge will show a higher fuel flow due to the higher pressure at the spider. If you have a JPI EDM 760 or the equivalent with fuel flow capability that will give you CHT and EGT foe each cylinder it is easy to tell what is going on and where. A partially clogged injector will indicate low CHT and High EGT for that cylinder and low fuel flow while the Piper gauge will indicate high fuel flow.

Oh, BTW, Burro Zach.