WHAT TYPE OF CARBURETOR?

Post by **GAHorn** » Wed Feb 09, 2005 11:22 pm

Dear Confused,
Have no idea who "Gabby" is.

But I can tell you that the main discharge nozzle and the throttle shafts are different between the MA3-SPA for the C-125 and the C145/O300 engines. My guess is that the engine isn't gettin' all it should be.

FredM · Post by **FredM** » Thu Feb 10, 2005 6:39 am

I have never seen the correct rpm. The prop was pitched at 55 when I bought the plane. I had it repitched to 51 and the rpm is still low. Static rpm on the ground is at 2300. The most I see in the air is 2500 - 2550.

N170BP · Post by **N170BP** » Thu Feb 10, 2005 7:00 am

I apologize for suggesting this if you have already
done so, but have you verified the accuracy of the
tach with a hand-held strobe?

If not, I'd do that right away, and with any luck,
the tach itself may be off (reading too low).

Post by **GAHorn** » Thu Feb 10, 2005 4:22 pm

FredM wrote:I have never seen the correct rpm. The prop was pitched at 55 when I bought the plane. I had it repitched to 51 and the rpm is still low. Static rpm on the ground is at 2300. The most I see in the air is 2500 - 2550.

Fred, static RPM is only supposed to be about 2300. The type certificate specifies 2230-2330 rpm static.

blueldr · Post by **blueldr** » Fri Feb 11, 2005 4:42 am

Fred,
If you can get 2300 rpm static, your engine is putting out properly. If your max rpm in flight is no more than 2550, your tach. is way off. Electronic tach checkerare no more than $50 these days and are well worth having for peace of mind.

Post by **GAHorn** » Fri Feb 11, 2005 3:49 pm

Blue ELDER ....as Ole Gar (sittin here beside me) calls 'im...

That may not be correct in all cases. Depending upon altitude and prop pitch....not all aircraft will get 2550 inflight except maybe in a dive!
Just because the engine is rated at 145 hp at 2700 rpm....doesn't guarantee that all engine/airframe/prop combinations will actually achieve that. The guide-rule to use is......: You may determine how much hp you're developing based upon how much rpm you are getting. (But just because you're not getting 2700 rpm does not necessarily indicate a defect.)

zero.one.victor · Post by **zero.one.victor** » Fri Feb 11, 2005 6:35 pm

I think maybe what BL is saying is that if you can get 2300 static but only 2550 in level flight, the tach might be haywire. If you can get 2300 static, you should be able to get 2700 in level flight no problem. I can get 2700+ in level flight with mine, but don't quite get 2300 static as I recall.

Eric

blueldr · Post by **blueldr** » Sat Feb 12, 2005 4:49 am

George,
I don't believe I've ever seen an engine that either so sick or tired, or has such a fat prop, that it wont make 2550 turns in flight
that will make 2300 static. From zero to full speed and it wont pick up 250 rpm? Not very likely. The one I'm flying right now is pitched at 50" and turns just under 2300 static . I cruise it at 2600 and 2700 is easy and that's on an electronic tach checker.

FredM · Post by **FredM** » Sat Feb 12, 2005 5:10 am

I have checked my tach with a hand held electronic tach but you have given me an idea. I should probably check it in the air instead of only on the ground. Are the specs in the TCDS for standard prop pitch or does the 2300 - 2330 figure include everything from a climb prop to a cruise prop?

zero.one.victor · Post by **zero.one.victor** » Sat Feb 12, 2005 7:29 am

The TCDS does not specify prop pitch, or labels such as "cruise prop", or "climb prop"-- it specifies the allowable static rpm with the particular prop & use. For exampe, the static rpm for the 1A170 prop most of us run (the 1A170DM76XX) is 1) 2230-2330 for a landplane & 2) 2300-2525 for a seaplane. Whatever twist produces that is OK, the TCDS don't care if it's a 50"pitch or a 15".

Eric

Post by **Bruce Fenstermacher** » Sat Feb 12, 2005 3:50 pm

Fred I wouldn't be surprised if your tach is off by at least 100 at the top end ans mine is.

My tack reads about right at 1800 then reads about 50 high at 2300 and then 100 high when it's reading 2700

I'm running a 1A170DM7654 which would be a semi cruise prop for a 170 according to the numbers. I have a freshly overhauled engine which can turn this prop no higher that 2600-2650rpm at 3000ft. There can be so many little differences between props and engines that are supposed to be the same that it is difficult to say what a given prop / engine should do exactly by what another prop/engine of the same combination will do. Of course they should be in the sam ball park.

Another ballpark way of comparing your engine /prop combination is to compare your speed for a given rpm in formation with another 170 of a like configuration. You may find that your engine /prop is pulling your plane faster or acting like it has more pitch than the plane with a like configuration and that would explain a your lower RPM.

In the end I'll bet you find a little tack error and a little prop "error".

zero.one.victor · Post by **zero.one.victor** » Sat Feb 12, 2005 5:12 pm

I can't find it in a quick look thru my stuff just now, but I have a formula written down somewhere for the prop pitch/rpm /airspeed relationship. You can use any two components to find the third. It seemed pretty accurate when I plugged in my airplane's numbers. I think it's probably on this site somewhere if you did enough of a search to find it. It was *something* like pitch times rpm divided by 2356 equals mph.

Eric

johneeb · Post by **johneeb** » Sat Feb 12, 2005 5:57 pm

Eric,

I used my "Bomar Brain" to work on the formula you are refering to. I came up with:

((Pitch x RPM) devided by 1056), If we use Bruce's prop numbers and RPM we get ((54 x 2650) devided by 1056) = 135.51 MPH. The font size & color should warn you of the value of this information.

While this formula gives us the amount of air the prop is processing, it is not the same as airspeed of airplane, at least not mine and from what I read here certainly not George's.

Johneb

Sun Feb 13, 2005 12:08 am

Well I only wish my 170 went 135 mph

That interesting thing about that formula is that if you adjust the variable (1056) so that it gives you the airspeed your airplane is indicating then you can play with the other numbers to see how they might effect the final indicated airspeed, ie what if I change my prop pitch to 52 or cruise at a lower or higher RPM.

zero.one.victor · Post by **zero.one.victor** » Sun Feb 13, 2005 6:02 am

Yeah, John, that 1056 is the number I was trying to remember. This formula seems to work for my airplane:
51 X 2400 / 1056 = 115.9 mph
51 X 2550 / 1056 = 123.1 mph
51 X 2700 / 1056 = 130.4
I've been flying around with my 2 buddy's in their Grumman Yankees lately. They used to both fly Luscombe's so I had the hotrod of the bunch, now I'm the slowpoke so I've been setting the power at around 2550 so as not to fall too far behind. It seems to produce this 120-125 cruise speed, and is real smooth. I know a guy who had a 145-powered Swift, all his other Swifter bud's had big engines so he just ran his wide open all the time to keep up. It never seemed to bother the engine. I don't like running mine wide open, but 2550 or 2600 feels pretty good.

Eric