I have a ’48 C-170 w/ stock C145 engine. It has 2420 hrs TT, and 670 SMOH. Mags are Slicks. Left one was rebuilt and got a new coil in Oct ’01. Timing is last set to 30/30. Harness is shielded, not a cross-over type and lower plugs are fine wire electrode. Oil consumption is negligible (1 qt / 5 hrs). Compression has been steady each year and even from all jugs in the high 60’s to mid 70 psi. Prop is McCauley 7653. I used to see 2300+ static RPM. Now only get 2100. Mag drop on left is 200 with roughness, right drop is 25 and smooth. Fuel burn has increased with this loss of power. Was 8.7 now over 10 GPH. I typically run at 2600 to 2700 RPM in cruise at 3000 to 6000 MSL. Leaning is done just rich of roughness in cruise which shows 1425 degrees on EGT. Burning 100LL w/ MMO 2 oz to 10 gal.
Q1: How would you diag the RPM drop on the left mag?
Q2: Why is fuel consumption up and power down?
Q3: What adjustments would be made to maximize power.
Reduction in Power - why?
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
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zero.one.victor
- Posts: 2271
- Joined: Tue Apr 23, 2002 12:11 am
The mag timing is spec'd out at 28 left and 26 right. Seems like 30/30 should work OK if the fuel octane is high enough. If it doesn't ping at high manifold pressure with 100LL,I guess it's OK. Still not per spec's though.
How much time on the plugs? I can't recall but it seems like I remember the LH mag fires the bottom plugs? I don't have any experience with fine-wire plugs,but if they're high time it cold be that the resisitors are going south. That happened on some regular style Champions on mine--the plugs looked great but ran crappy. Replacing the plugs cleared up the problem.
Maybe try switching plugs top/bottom & see if the excessive mag drop with the LH mag is still there. Don't know what to tell you on the fuel consumption--if you have a two-piece venturi,maybe make sure it's OK.
Eric
How much time on the plugs? I can't recall but it seems like I remember the LH mag fires the bottom plugs? I don't have any experience with fine-wire plugs,but if they're high time it cold be that the resisitors are going south. That happened on some regular style Champions on mine--the plugs looked great but ran crappy. Replacing the plugs cleared up the problem.
Maybe try switching plugs top/bottom & see if the excessive mag drop with the LH mag is still there. Don't know what to tell you on the fuel consumption--if you have a two-piece venturi,maybe make sure it's OK.
Eric
Actually, Eric posted it backwards. It should be 26 degrees BTC on the Right, and 28 on the left which should be firing the bottom plugs.
If you are running at high power settings (2600 rpm plus) and also leaning just barely rich of roughness, combined with the advanced timing you've been running, you might be running too lean. (A burned exhaust valve is possible here. Advanced timing can also lead to cracked cyl. heads.)
Firstly, you should check out the mags completely as to points, condenser, dist. block, and cap, then time them correctly.
Clean/inspect and test (if possible) the plugs.
But there is a chance you've developed a burnt valve and may find a current compression test will reveal that.
Pg 12-16 of the Service Manual will also give some guidance on how to check the mags for moisture.
Don't forget that ign. harness can also break down, and a high-tension test may be a good idea. It's a simple check if you have the box. (Check with your local mechanic.)
If you are running at high power settings (2600 rpm plus) and also leaning just barely rich of roughness, combined with the advanced timing you've been running, you might be running too lean. (A burned exhaust valve is possible here. Advanced timing can also lead to cracked cyl. heads.)
Firstly, you should check out the mags completely as to points, condenser, dist. block, and cap, then time them correctly.
Clean/inspect and test (if possible) the plugs.
But there is a chance you've developed a burnt valve and may find a current compression test will reveal that.
Pg 12-16 of the Service Manual will also give some guidance on how to check the mags for moisture.
Don't forget that ign. harness can also break down, and a high-tension test may be a good idea. It's a simple check if you have the box. (Check with your local mechanic.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
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zero.one.victor
- Posts: 2271
- Joined: Tue Apr 23, 2002 12:11 am
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zero.one.victor
- Posts: 2271
- Joined: Tue Apr 23, 2002 12:11 am
Actually George,YOU have it backwards! Mag timing should be listed as 28 LH 26 RH,as the LH mag fires first--that's 28 degrees before top dead center. Ha!gahorn wrote:Actually, Eric posted it backwards. It should be 26 degrees BTC on the Right, and 28 on the left which should be firing the bottom plugs.
Eric
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funseventy
- Posts: 230
- Joined: Fri Oct 25, 2002 11:46 pm
There are so many places that you are defficient that you must first fix those before we diagnose your problem. You may not have a problem or you may have a bigger problem than you think based upon the poor set up of the engine.
If the engine isn't running efficiently it will not produce power. Then it will take more manifold pressure to hold the same RPM and that is why the fuel burn has gone up. You have flown the airplane long enough to figure the fuel burn but you should have noticed that a 200 RPM drop is excessive long before that. If it is a rough drop you could have one or a group of dirty plugs. Also remember if you have had the plugs out and you dropped one, throw it away. It may look good, and it may even test good, but IMHO it will not last and you will be troubleshooting it later.
Start the engine and let it run for 3-5 minutes on the bad mag and then shut it down. Grab your craftsman screwdriver and drag the handle accross each exhaust stack. It will slide easily on a hot stack and it will grab like sandpaper on a cold one. There you will find a bad plug or wire.
Let's get the engine somewhere close to spec and then revisit your problem. I agree with George in the fact that you may have run lean, you may have burned a valve, or you may have cracked a cylinder head. But let's try to be optimistic about getting it cleaned up and timed right.
If the engine isn't running efficiently it will not produce power. Then it will take more manifold pressure to hold the same RPM and that is why the fuel burn has gone up. You have flown the airplane long enough to figure the fuel burn but you should have noticed that a 200 RPM drop is excessive long before that. If it is a rough drop you could have one or a group of dirty plugs. Also remember if you have had the plugs out and you dropped one, throw it away. It may look good, and it may even test good, but IMHO it will not last and you will be troubleshooting it later.
Start the engine and let it run for 3-5 minutes on the bad mag and then shut it down. Grab your craftsman screwdriver and drag the handle accross each exhaust stack. It will slide easily on a hot stack and it will grab like sandpaper on a cold one. There you will find a bad plug or wire.
Let's get the engine somewhere close to spec and then revisit your problem. I agree with George in the fact that you may have run lean, you may have burned a valve, or you may have cracked a cylinder head. But let's try to be optimistic about getting it cleaned up and timed right.
Eric, I got my info from the TCDS for the engine. They listed it in that order! 
(Truth being, that I originally misread your post, and got completely upside/down/inside/out and backwards.) 
(Truth being, that I originally misread your post, and got completely upside/down/inside/out and backwards.) '53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
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BloomerJohn
- Posts: 55
- Joined: Mon Apr 22, 2002 3:28 pm
Pulled the cowl, covered front w/ cardboard, started on left mag and ran at low rpm for 1 min. Shut down, measure each cyl in same location (exhaust valve guide area) using Snap-On instant IR digital thermomiter. Found left forward jug colder by ALOT. Changed plug, reran test, all cyls now within 5 degrees of each other. I will now set timing and check it out once the frost is out of the ground and I can get to the strip.
RE Timing: Several 'old guerues' in this area still stand on the recommendation of timing set at 30/30 for o-300's using 100LL. Say power is increased, but may start just a bit harder.
Any comments from this group??
John
RE Timing: Several 'old guerues' in this area still stand on the recommendation of timing set at 30/30 for o-300's using 100LL. Say power is increased, but may start just a bit harder.
Any comments from this group??
John
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zero.one.victor
- Posts: 2271
- Joined: Tue Apr 23, 2002 12:11 am
Fuel octane level has nothing to do with allowable timing advances with any aircraft engine I'm aware of. By the logic of the "old guerues', if you can find some 130/145 Avgas, you could probably advance it to 35/35 or even more! Right? Boy! Then you wouldn't need that engine conversion!
Fuel Octane is not the limiting factor here. This engine only needs 80 octane and any excess octane is simply blown out the exhaust. It is not "available" for engine modifications. If you fill up an air tank that's designed for 120 p.s.i., ...can we fill it up with helium up to 160 psi? After all, helium has molecules that are smaller so they don't push as hard, right?
Ask the "old guerues" what engineering data they used to come up with that recommendation, especially what the cylinder pressures are when that 30/30 is used, and how does that compare to the design limits of the cylinder.
Without spending a lot of time doing research giving dates/part numbers and mods. (which would just support doing things like the factory says anyway) you should know that the background of these cylinders involves numerous design changes, one of which involves cylinder pressures vs mod status. Superior's millenium series was approved for 28/28 timing in the O200, but none of them were allowed that advanced (and increased pressures) in the O300. There's a reason for that. It damages the cylinder and piston.
Ask the "guerues' which one of them will sign your logbooks that they made that unapproved adjustment to your engine, which one of them (knowing the timing is wrong) will be willing to sign the logs approving it for return to service?
(And think about the $750 per cyl plus labor it'll cost you when the cylinder develops a head leak at the juncture with the barrel.)
Ok. I'll climb back down now.
Fuel Octane is not the limiting factor here. This engine only needs 80 octane and any excess octane is simply blown out the exhaust. It is not "available" for engine modifications. If you fill up an air tank that's designed for 120 p.s.i., ...can we fill it up with helium up to 160 psi? After all, helium has molecules that are smaller so they don't push as hard, right?
Ask the "old guerues" what engineering data they used to come up with that recommendation, especially what the cylinder pressures are when that 30/30 is used, and how does that compare to the design limits of the cylinder.
Without spending a lot of time doing research giving dates/part numbers and mods. (which would just support doing things like the factory says anyway) you should know that the background of these cylinders involves numerous design changes, one of which involves cylinder pressures vs mod status. Superior's millenium series was approved for 28/28 timing in the O200, but none of them were allowed that advanced (and increased pressures) in the O300. There's a reason for that. It damages the cylinder and piston.
Ask the "guerues' which one of them will sign your logbooks that they made that unapproved adjustment to your engine, which one of them (knowing the timing is wrong) will be willing to sign the logs approving it for return to service?
(And think about the $750 per cyl plus labor it'll cost you when the cylinder develops a head leak at the juncture with the barrel.)
Ok. I'll climb back down now.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
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BloomerJohn
- Posts: 55
- Joined: Mon Apr 22, 2002 3:28 pm
Problem is BACK. Run-up was great. Equal drop...50 RPM. Strong Static...2350 RPM. 30 min into flight during cruise, I checked mags by switching on one at a time. Ran smooth on right with slight rpm drop. Left ran like s--t! Sounded and felt like on cyl was only firing intermittently. Top plugs (left Mag) are massive electrode and quite old. I am going to order a set of new ones today. Wires were replaced about 10 yrs ago and look in good condition. Left mag was sent out for rebuild two yrs ago. They replaced a bad coil and tested..said it was fine. Would a plug wire problem show up intermittently when warm?
John
John
I had an intermittent plug firing problem awhile back.
After cleaning, all plugs would test fine on one of those
combination cleaner/tester rigs (where you pressurize
a small chamber with air from an air compressor and
you can test fire the plug).
I think the problem with these testing rigs is they don't
replicate the pressure *and* heat environment the plugs
endure during flight at cruise speeds.
Bought a new set of plugs and it's been running like a
Swiss watch ever since.
Bela P. Havasreti
'54 C-170B N170BP
After cleaning, all plugs would test fine on one of those
combination cleaner/tester rigs (where you pressurize
a small chamber with air from an air compressor and
you can test fire the plug).
I think the problem with these testing rigs is they don't
replicate the pressure *and* heat environment the plugs
endure during flight at cruise speeds.
Bought a new set of plugs and it's been running like a
Swiss watch ever since.
Bela P. Havasreti
'54 C-170B N170BP
I have seen many times where a magneto coil go open as it heats up. It would work fine at first then over the course of the flight degrade to the point of intermittant firing (a high rpm dropoff). Not really that uncommon to do so. But if you think it is only one cylinder (A steady rpm drop) see if you can find someone with a lead tester, this way you can eliminate the ignition harness as suspect.
Vic
Vic
Vic
N2609V
48 Ragwing
A Lanber 2097 12 gauge O/U Sporting
A happy go lucky Ruger Red label 20 ga
12N Aeroflex
Andover NJ
http://www.sandhillaviation.com
" Air is free untill you have to move it" BB.
N2609V
48 Ragwing
A Lanber 2097 12 gauge O/U Sporting
A happy go lucky Ruger Red label 20 ga
12N Aeroflex
Andover NJ
http://www.sandhillaviation.com
" Air is free untill you have to move it" BB.
Had a similar problem and did not have access to a lead tester. Borrowed a used harness and left my harness in place, just sort of layed the borrowed harness on top and to the side, hooked it up to the mag and plugs - the engine ran great. Because of that, I have a harness you can borrow if you can't gain access to a lead tester. Sure am enjoying the new harness before I fly much and spread oil all over those pretty new wires.
Joe
51 C170A
Grand Prairie, TX
51 C170A
Grand Prairie, TX