0-300A Rebuild running problem

[cfiatzph]

I have a 0-300A on a C172 (yes I know not a 170) anyhow. Went through and a got another rebuild on the carb, installed, it is not running rich anymore, shuts off with the mixture control etc, good mag checks, carb heat checks. However, the manifold pressure only goes up to 28 inches and the tach come up about 75 rpm short of what it should according to the book anybody have any ideas? This is full power and the stop on the throttle is hitting full throttle as is the mixture full rich, it does this regardless of wheather the carb heat tube is off or on. Anybody? Fresh rebuild, 50 rpm drop on the mags, new cylinders (ECI) etc.

Thanks

[cfiatzph]

Also I get anywhere from 45 psi to 55 psi depending on RPM, according ot the POH max is 50 psi. Any ideas here?

[1SeventyZ]

What elevation are you at? If you're above, say 3000 ft, you'll need to ground lean at full throttle to see your max rpm. I think the AFM or POH only states 2230-2330 as a reference range for static rpm. Sounds like you got your carburetion problems fixed, might be time to double check your ignition timing.

I assume you are talking about oil pressure in your 2nd post. Many others have the same question about high oil pressure, or at least higher than the original spec and upper limit from the POH. George gives a good answer in that thread.

[ak2711c]

28 inchs of manifold pressure at full power sounds right to me. In fact you must be close to sea level to get that much manifold pressure. As far as the rpm goes, test your tach with a digital tach checker. Mechanical tachs are famous for being off, even new ones. With a fixed pitch prop the TCDS will give you a range of rpm for a full power static run up. Make sure your prop is the same length, pitch, and model as what is listed in the TCDS. That rpm is probably around 2250-2350 I would guess, but don't take my word for it. What rpm are you getting? Are you doing it static or on takeoff? The rpm check must be done while stationary on the ground. Note that this rpm is not the engines rated horsepower rpm of 2700. Hope this helps.
Shawn

[cfiatzph]

I get just short of like 2200 like 2180 or so I think I should get like 2230 according to the book. This was done at sea level, and the manifold pressure before start is the same as the altimeter setting on the altimeter, I was under the impression that at full power I would get the same amount.

Thanks

[N2865C]

Also I get anywhere from 45 psi to 55 psi depending on RPM, according ot the POH max is 50 psi. Any ideas here?

I had the same issue. It was discussed here. http://cessna170.org/phpBB2/viewtopic.p ... l+pressure

[GAHorn]

A slight loss of manifold pressure from static barometric pressure is normal due to induction system losses. The static rpm specified in the TCDS (2230-2330) is for the original McCauley prop (DM/MDM series) in standard pitch ranges (51-55) at standard atmosphere (Sea Level, 29.92 in. Hg, 59-degrees C. Any change of density altitude will affect that static rpm observed. (Colder air that is more dense will result in a lower rpm due to prop loading. Less dense air due to higher density altitude will result in lower rpm due to hp loss.)

[lowNslow]

I talked to TCM field rep about this several years ago. ( I was concerned about a TCM Ovhl Manual warning about valve lifters with excessive oil pressure.)
TCM field rep said not to worry. Oil pressure up to 60 is now considered acceptable by TCM. Earlier engines went through several modifications with regard to lifter bodies that provoked differenct comments regarding oil pressure, but since lifter design was finalized TCM has found no problems with oil pressures up to 60.

I just wish TCM would put this in writing, would avoid a lot of discussion with IAs at annual time.

[cessna170bdriver]

I get just short of like 2200 like 2180 or so I think I should get like 2230 according to the book. This was done at sea level, and the manifold pressure before start is the same as the altimeter setting on the altimeter, I was under the impression that at full power I would get the same amount.

Thanks

If you're getting 28" MP at full throttle at sea level, that's about 2" of induction loss, which seems a bit on the high side (assuming your induction filter is in good shape). One remote possibility is the intake flow divider in the sump tee just above the carburetor. The older sumps had a sheet metal divider that could work loose and partially block the intake, which would cause a MP loss. The newer sumps had the divider cast in as part of the sump.

Miles

[Bruce Fenstermacher]

My experience tells me, wow 28" of MP, that's great. I live and fly at nearly sea level and I have a MP gauge which isn't very common on aircraft with a fixed pitch prop.

When I bother to look at a gauge to see what my power setting is it's not my tach but my MP. I then glance at my tach to be sure I'm not exceeding red line and that is all I use my tach for.

Since on takeoff I don't care what my MP is, I'm concerned with how fast the prop is spinning and if it's spinning fast enough to get me off the ground, I've never looked at my MP. So I can't say what it gets to.

I do know that from about 1000 ft asl in a full throttle, full rich climb I'd expect to see no more than about 25.5" of MP. Can't say what RPM because I never look at it because it's going to do what ever the engine can do and it's not anywhere near red line. No need to waste time looking at the tach.

I'm running a DM model full length McCauley with the pitch set at 54".

Wouldn't the pitch of the prop and RPM of the engine can with it change what MP is achieved? And wouldn't changing the pitch of the prop produce a different MP for a given RPM?

[GAHorn]

I talked to TCM field rep about this several years ago. ( I was concerned about a TCM Ovhl Manual warning about valve lifters with excessive oil pressure.)
TCM field rep said not to worry. Oil pressure up to 60 is now considered acceptable by TCM. Earlier engines went through several modifications with regard to lifter bodies that provoked differenct comments regarding oil pressure, but since lifter design was finalized TCM has found no problems with oil pressures up to 60.

I just wish TCM would put this in writing, would avoid a lot of discussion with IAs at annual time.

They did, a long time ago. Show him page ONE of the TCM O-300 Operator's Manual Form X-30015 (FAA Approved) under Specifications: Oil Pressure, cruising 30-60 psi.

[cfiatzph]

Well I'm glad I don't have a oil pressure problem. It was just wierd because the other 0-300D I have with fairly fresh overhaul comes real close to min idle pressure and never gets above 39 PSI or so. And the old overhaul on the -A was the same exact way, just was'nt sure what was changed (it could be a million things during the rebuild process).

Thanks

[cfiatzph]

A slight loss of manifold pressure from static barometric pressure is normal due to induction system losses. The static rpm specified in the TCDS (2230-2330) is for the original McCauley prop (DM/MDM series) in standard pitch ranges (51-55) at standard atmosphere (Sea Level, 29.92 in. Hg, 59-degrees C. Any change of density altitude will affect that static rpm observed. (Colder air that is more dense will result in a lower rpm due to prop loading. Less dense air due to higher density altitude will result in lower rpm due to hp loss.)

So what your saying is 2175 or so at 75 degrees at sea level is probably a good power check.

Thanks

[blueldr]

Where on the engine are you tapping off the manifold pressure?

[lowNslow]

I don't have a copy of the operators manual to check but I believe, for those of us with C-145 engines, it still shows 45psi as max.