fuel burn?

[KG]

Hi all.

My airplane (B model with an O300-D) is burning more fuel than it used to. This is after the pistons and rings were recently replaced. The cylinders were lightly honed during the piston replacement.

I had been flying down low at high power settings to break in the new rings so I thought maybe that was the culprit. Today, with 12 hours on it since the pistons and rings replacement, I took it up to 5500 ft. I set 2500 rpm and leaned it and came up with about 10 gph. (oat was 48 degrees F).

Previously I consistently used 8 gph... maybe 8.2 at the most.

My ECI engine analyzer has a digital tach that indicated within 40 rpm of my mechanical tach so I think the rpm is pretty accurate. I leaned two times.. once the old fashioned way of just leaning until it ran rough and then enriching it a little and again using my ECI fuel flow analyzer. Both had the same result... 9.8 to 10.2 gph.

During the 12 hours of operation since they piston replacement, I have confirmed that my fuel flow instrument is fairly accurate by comparing fuel purchased to the instrument reading.

What could cause my fuel burn to go up?

I suppose a leak downstream of my fuel flow transmitter... but I don't see any signs of a leak (no blue stains that I can see).

The only other wild guess that I can think of is perhaps an intake leak somewhere causing it to run lean, thereby causing me to have to run richer to compensate. Anybody else think that might be it?????

I'm taking it in for an oil change the first of next week. Would like some ideas to discuss with the mechanic. Any suggestions?

Thanks.
Keith

[blueldr]

Try setting your RPM with the tach that gives you the desired RPM with the lowest throttle setting. You mentioned that there was a difference of 40 RPM. If you were right at the edge, 40 RPM might make a difference. I don't suppose that you have a manifold pressure gauge with an O-300D engine.
Past experience has shown a considerable difference in fuel burn with a 100 RPM difference.

[minton]

Have you done a compression check/blow by since the top? Fuel flows don't change through a fixed jet carb so the power being generated is leaking to somewhere and not out to the prop. .

[KG]

Try setting your RPM with the tach that gives you the desired RPM with the lowest throttle setting. You mentioned that there was a difference of 40 RPM. If you were right at the edge, 40 RPM might make a difference. I don't suppose that you have a manifold pressure gauge with an O-300D engine.
Past experience has shown a considerable difference in fuel burn with a 100 RPM difference.

That's what a buddy of mine told me. He suggested that I might be at a higher power setting than I think I am.

I do have a manifold pressure gauge. I don't usually pay much attention to it. Maybe I should.

I haven't had a compression check done but I will next week when they change the oil.

[jrenwick]

Where are you located? Is the density altitude similar to what it was before the piston replacement, or is it lower now due to colder temperatures? What's your TAS? Is it what you would expect for the fuel consumption you used to have?

[KG]

Where are you located? Is the density altitude similar to what it was before the piston replacement, or is it lower now due to colder temperatures? What's your TAS? Is it what you would expect for the fuel consumption you used to have?

I knew someone would ask about airspeed. Today, the day I decided to go up to a higher altitude and try to firm up the fuel burn numbers, is the day my little bug guard / flapper / pitot tube cover decided to stick closed so I was flying around with no airspeed indication.

This could just be a combination of several little things... tach a little off, fuel flow meter a little off, slope where I park while fueling causing me to not be completely topped off sometime, lower density altitude... Guess I'll keep a watch on it and see what happens.

[FredMa]

Are you under the impression that your engine with only 12 hrs since repair should already be broken in? It surely isn't. What type of cylinders were used steel or chrome? What type of oil are you using? what have you noticed with oil consumption? A compression check is most likely a waste of time and money at this point. You could very likely be chasing a problem that doesn't exist. I would personally not concern myself with fuel burn on an engine that isn't broken in. Only after I was sure the engine is properly broken in would I worry about fuel burn.

[blueldr]

One of my flying friends used to brag that he had one of the fastest Grumman Tigers going. A few weeks after I bought a digital tach checker, he asked to borrow it to see how his tach was doing in flight. When he came down, his his face was longer than his arm. His tach was 125 rpm slow! The Tiger has a FP prop on a 180 HP Lycoming O-360, so he was pulling considerably more power than he thought he was. His airplane wasn't as fast as he thought it was. He had failed to consider his fuel burn versus his HP being developed.

[Jr.CubBuilder]

As silly as it sounds, make sure you dont have any leaks from your drain sumps. A very slow leak can leave almost no trace except a sump that is always damp, but if you are flying once a week it can throw off your fuel burn buy a gallon.

[GAHorn]

If you are running slightly more power than previously...you may be running the carb in the power-enrichment phase without knowing it.
An accurate tach is important when making these comparisons.

Also, if your carb float is sticking it might blow fuel overboard without much evidence.

[canav8]

Are you under the impression that your engine with only 12 hrs since repair should already be broken in? It surely isn't. What type of cylinders were used steel or chrome? What type of oil are you using? what have you noticed with oil consumption? A compression check is most likely a waste of time and money at this point. You could very likely be chasing a problem that doesn't exist. I would personally not concern myself with fuel burn on an engine that isn't broken in. Only after I was sure the engine is properly broken in would I worry about fuel burn.

The other thing I would like to add Keith is, Have you seen the CHT temps break yet? If you havent, its not broken in. D

[KG]

I wasn't under the impression that it was fully broken in. I was wondering if there were anything associated with changing pistons and rings that would cause an increase in fuel burn. I think I have narrowed it down to a combination of: high rpm, tach off a little, my digital fuel flow gauge off a little, and --- possibly the main thing --- fueling on an uneven surface that was leaving me a couple of gallons short.... making me take more when I topped off at another field. I think my fuel burn is ok.

Having said that, I did see lower CHT's last time I flew (with about 20 hours on the new pistons). It was a cold day so I don't know if that was why they were lower or not. The EGT's did settle down and stabilize... right after the piston change they were all over the place with #4 being the hottest. They are now back to what I normally saw with #2 the hottest. So I'm thinking it is probably broken in or close to it.

Good news is that I just changed the oil and had a sample taken for analysis which showed great improvement in the metals in the oil, now in the normal range after the pistons were changed. Never did figure out what was wrong with the cylinders and pistons the first time but something was wrong and it looks like the pistons and rings fixed it.