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on 6/5/03 1:19 PM, John Stevens at [EMAIL PROTECTED] wrote:
> I have been having an issue with my coupe at idle - she runs fine at 900
> rpm or better but seems to want to stall below this mark unless I put
the
> carb heat on. I have a C75-12F with a stromberg carb in mine - the
engine
> has just been "0" timed and have only put about 6 hours on her thusfar.
>
> What I have found is that the RPM decreases with carb heat during run-up
> (1500+ rpm) which is what I expect, the mags have 0 differential between
> them during run-up...
>
> Any suggestions would be appreciated.
>
> John Stevens
-----------------------------------------------------
Yo, John-
The Continental Operator's Manual, Form X30012 (1980) states:
"If no drop in RPM is observed when operating on either magneto alone,
the switch circuit should be inspected.
WARNING. . .Absence of RPM drop when checking magnetos may
indicate
a malfunction in the ignition circuit. Should the propeller be
moved by hand (as during preflight) the engine may start and cause
injury...
CAUTION . . . Do not underestimate the importance of a pre-takeoff
magneto check. When operating on single ignition, some RPM drop
should
be noted. Normal indications are 25-75 RPM drop and slight enghine
roughness as each magneto is switched off. Absence of a magneto drop
may be indicative of an open switch circuit or improperly timed
magneto.
An excessive RPM drop usually indicates a faulty magneto or fouled
spark
plugs."
Because you have the Stromberg carb (without idle cut-off), you kill the
engine by turning off the ignition switch. If your ignition circuit had
the
problem described, turning off the switch wouldn't stop the engine!
(Coupes
with Stromberg carbs pass a built-in P-lead check when turned off!).
Get your magneto timing reset (28º BTC for the right mag and 30º for the
left). You didn't indicate whether your accessories were "0 timed" when
your engine was. Presuming everything is sufficiently "to spec" to be
airworthy, accurate timing should result using proper procedures. It
isn't
luck, it isn't "art", and it isn't rocket science.
I suggest the disks and holes under the cover of your carb's mixture
control
be inspected and cleaned, by a properly certificated person, of course
:<).
Greg's suggestion to check the idle circuit passage and jet also sounds
"on
target".
Induction system air leaks can give you fits. Automotive starting fluid
judiciously applied can help locate them on a running engine (increased
RPM
when the vapors are sucked in), but the procedure involves so many special
considerations to be effective and safe (no, you don't do it in the air!)
great care in proper assembly, airtight from the start, is much
preferable.
hope this helps,
William R. Bayne
<____|(o)|____>
(Copyright 2002)
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