Magneto Timing Question

Dale,
Thanks for the additional thoughts. Bottom line: I'll recheck my technique
and the points before "bumping" the mags.

Thanks again, Dick




"Dale Scroggins" wrote in message
news

"Dick" wrote in message
news:2c2qk.130$p72.75@trnddc05...
Dale,
If OP means original poster, that is me. For the record, my timing is 2
deg more advanced but still BTDC by 32 deg. After an A&P rebuilt and
installed my mags about 100 hours ago, I'm not sure if they were
installed per the manual spec 30 deg or the 32 deg I found by test
recently. A different A&P
doesn't have a problem with it either way.

I any case, I am just curious about any range established per "rule of
thumb", experience or paracticality..

Thanks for taking the time to give your opinion.


snip

I can't give you a rule of thumb. I can pass along an informed opinion
developed over the last thirty years as aircraft mechanic, with several
thousand hours of that spent working on small Continentals.

1. You shouldn't run your timing more advanced than specified. Two
degrees is too much excess advance for your engine; 30 degrees advance is
a lot of advance for an engine that turns about 2,000 rpm most of the
time. Continental specified 30 degrees to get good performance at a cost
in durability. When I set mag timing, I'm happy only when it's within 1/2
a degree. But I would worry if it were 1/2 a degree too much advance on a
small Continental.

If the mechanic who put the mag on set it right to begin with, then your
points are wearing faster than the cam rubbing block, and the timing will
continue to advance as you fly. And the e-gap, or internal mag timing, is
drifting too. If the timing shifted 2 degrees in 100 hours, you have
something wearing too fast.

How, if you could not see the prop flange markings, did you establish TDC?
Are you fully confident that the advance is 30 and 32 degrees? By the
way, if you can see the flange markings from the top, but not the bottom,
you could time the mags using a cylinder other than #1. The second A&P
may have doubts about your methods. How did the first A&P set the timing;
could he see the prop flange markings, or did he use a timing wheel? The
wheels can be inaccurate if improperly set up or if the pointer friction
is excessive.

2. How old are your cylinder heads? Not since overhaul, not since
cylinder barrel replacement, but since the heads were new? Have they ever
been welded? How old is your case? How many hours total time? Who did
the overhaul, and what was the condition of the mating surfaces and
bearing journals? Continental hasn't made a C85 in quite a number of
years, and advancing the timing a couple of degrees, especially from a
fairly advanced setting to begin with, will result in significantly higher
cylinder pressures. Are all your engine parts up to that?

The questions I asked above would be questions I would have in making the
decision to reset the timing. But, in the end, I would probably use the
time-rite or timing wheel and recheck the mag timing. If it had any
excess advance I would reset the timing. If it had two degrees excess
advance, I would pull off the point covers and look at the points.

Dick wrote:
The specs for my C-85 are upper plugs mag @ 28* BTDC and lower @ 30*BTDC.

My results are upper @ 30* BTDC and lower @ 32* BTDC.

I couldn't find a "flower pot" with attached degree wheel to borrow and
attach to the prop hub so used a little "mickey mouse" circular protractor
with a free swinging needle/pointer.

Two separate readings confirmed my 30-32. TDC was identified and impulses
unloaded before taking the readings.

My question is whether my results are acceptable for a low speed ,
experimental or need to be readjusted??

If need to be redone, I'd appreciate a little on why, please.

Thanks, Dick

PS: If listening G, I think "highflyer"(?) gave me some mag advice
during an "annual" a few years back which was appreciated.


Teledyne Continental has issued MSB94-8C-1. With rare exceptions (mostly
for turbo'd engines), the magneto to engine timing tolerance is +/- 1
degree.

Your results, per the manufacturer, are one degree out of specification.

Rip
A&P/IA

Actually found my original technique was poor and was able to use Dales
advice of timing a different piston to the upper case split. Right on @ 30
and 28deg per spec. Thanks to all who advised.
"Rip" wrote in message
...
Dick wrote:
The specs for my C-85 are upper plugs mag @ 28* BTDC and lower @ 30*BTDC.

My results are upper @ 30* BTDC and lower @ 32* BTDC.

I couldn't find a "flower pot" with attached degree wheel to borrow and
attach to the prop hub so used a little "mickey mouse" circular
protractor with a free swinging needle/pointer.

Two separate readings confirmed my 30-32. TDC was identified and impulses
unloaded before taking the readings.

My question is whether my results are acceptable for a low speed ,
experimental or need to be readjusted??

If need to be redone, I'd appreciate a little on why, please.

Thanks, Dick

PS: If listening G, I think "highflyer"(?) gave me some mag advice
during an "annual" a few years back which was appreciated.
Teledyne Continental has issued MSB94-8C-1. With rare exceptions (mostly
for turbo'd engines), the magneto to engine timing tolerance is +/- 1
degree.

Your results, per the manufacturer, are one degree out of specification.

Rip
A&P/IA

The TCM shop manual for my IO-550 engine says to set the magneto timing plus
or minus 1° from spec.

Dennis

Dale Scroggins wrote:

How did the first A&P
set the timing; could he see the prop flange markings, or did he use a
timing wheel? The wheels can be inaccurate if improperly set up or if
the pointer friction is excessive.


I used to have a nifty device that installed into the plug hole on the
number one cylinder. It had a probe and a set of cards(each had marks
for several engines) that installed on it that would directly read the
cylinder position. The device was entirely mechanical and had a very
well made wooden box for a case. I inherited it and sold it years ago
since I wasn't likely to ever work on an R-2800 (most of the cards were
for big radials, but lycoming and continental were well represented as
well) I expected that it was probably standard kit for A&Ps

Charles

On Aug 22, 7:58 am, Charles Vincent wrote:
Dale Scroggins wrote:
How did the first A&P
set the timing; could he see the prop flange markings, or did he use a
timing wheel? The wheels can be inaccurate if improperly set up or if
the pointer friction is excessive.

I used to have a nifty device that installed into the plug hole on the
number one cylinder. It had a probe and a set of cards(each had marks
for several engines) that installed on it that would directly read the
cylinder position. The device was entirely mechanical and had a very
well made wooden box for a case. I inherited it and sold it years ago
since I wasn't likely to ever work on an R-2800 (most of the cards were
for big radials, but lycoming and continental were well represented as
well) I expected that it was probably standard kit for A&Ps

Charles

They're called Timerite and are still available but quite pricey.