Hi Richard,
please see my replies below;
regards, Terry
Richard Holder wrote:
>
>
>
>>Hi Richard,
>>
>>We have an XS with a 912S and the Skydrive carb heat (hot water). We
>>have had the engine run rough both on the ground and in the air, both
>>cured within less than a minute after carb heat is turned on (usually
>>after 10-15 seconds). We don't leave the carb heat on all the time
>>because it raises the hot start temperatures, although it doesn't hurt
>>performance when on (we have carefully checked this). In the air we
>>have had carb ice at normal cruise and when throttling back. The carb
>>temp gauge (supplied with the carb heat kit) indicates a few degrees
>>from 0 deg C whenever we have seen this carb ice condition.
>>
>>We have also had the engine run rough due to what we believe is vapor
>>lock. This has only happened in the winter time (when there are more
>>volatiles in the auto gas), after a hot start, and typically after
>>taking off from a higher altitude field. We have since added more
>>insulation in the carb and fuel line areas, protecting them from the
>>exhaust 'hot spots'.
>>
>>regards,
>>Terry Seaver
>>A135 / N135TD
>>
>>
>
>Thanks for this, How does the carb temp gauge work, is it in contact with the
carb body or does it fit inside somehow. I have (currently) my external temp
probe
blue-tacked to the side of one of the carbs with the metal touching (I hope)
the carb body. I will look at the temp very carefully from now on.
>
The carb temp gauge that came with our Skydrive kit is a small, self
powered, digital gauge. The temp sensor fits into a hole in the carb
heat block, which is press fit to the carb, between the carb and the
intake manifold.
>
>I am more sceptical about vapour lock. You get it in winter but not in the
>summer
? I know that there are more volatiles in winter fuel but In summer there
is much more heat around (esp in the land of the free !) Do you have the return
fuel line back to the fuel tank ?
>
>
We have the factory supplied fuel return, and tested the return flow
immediately after the last 'vapor lock' incident (return flow was about
2.2 gph, which is the same as it was 300+ hours ago at initial engine
startup). The most compelling evidence of vapor lock was as follows;
I shut the engine down in flight at about 8,000 feet altitude, to test
the glide ration with the prop fully feathered (it was about 13:1, as I
recall). On restart, the engine ran rough, worse at higher throttle
settings. I finally turned on the boost pump, which killed one back of
the engine momentarily, with the fuel flow going from about 5 gph to 9
gph, momentarily. After pulling the throttle back (and turning the
boost pump off) the engine ran better, and by the time I got into the
pattern at the airfield I had been testing over, the roughness was gone.
It is my assumption that there was a vapor lock partially blocking the
fuel flow. At higher throttle settings the blockage causes rough
running (perhaps only on carb is affected). When I turned on the boost
pump, I believe it forced a 'bubble' of vapor through to the carb(s),
killing that side, and caused the momentary surge in fuel flow.
>I have another question :
>
>In the cold today it did NOT want to start. I have noticed that during the
>summer
it started on the button. Now it takes several (in the case of today 12 minutes
or so - not continuous !) churns on the starter. If it doesn't start the
prop tends to "bounce" almost hitting a (?) compression stroke and reversing
its direction (maybe 50 degrees)
>
Our cold starting problems have been because our carbs are not sync'd
well, or because the plugs are fouled with lead. We normally run
unleaded auto fuel, but on trips have to use 100LL, which fouls the
plugs in less than 25 hours.
>
>Might this be lack of battery zoom so that the battery cannot overcome the
>compression,
or is it a sign that a mag is firing early ?
>
>I got it to start today on only one mag. Maybe this means that I have a
>timing fault.
>If so how can I check ?
>
The ignition timing is fixed by an induction coupling at the flywheel,
one per ignition 'side', no adjustment necessary or possible.
>
>I flew today but between uncovering it and being ready to leave (see above !)
the blue sky disappeared and a 1300 overcast appeared. I ended up doing one
circuit
(pattern). Air temp was 4 celcius (40 F) no misfire but I hardly used a
steady throttle ! I have fixed my throttle lever box problem and the throttle
cables are now pulling evenly. I haven't got the balance right as it shook a bit
at 5000 but was OK at 4800. However as in the past the misfire problem seemed
to "accumulate" - appearing on a second flight in a minor way and on a third
flight worse I need to fly more ! In each case in the past the misfire went
away after less than 60 seconds and a change of throttle setting.
>
Could very possibly have been carb ice.
>
>I live in hope that I have fixed it - probability .6 or 60% chance. (Sorry I am
a mathematician !)
>
>Thanks for your help !
>
>Richard
>Richard F.W. Holder 01279 842804 (POTS)
>Bell House, Bell Lane, 01279 842942 (fax)
>Widford, Ware, Herts, 07860 367423 (mobile)
>SG12 8SH email : rholder@avnet.co.uk
>Europa Classic Tri-gear : G-OWWW, EGSG (Stapleford)
>PA-28-181 : Piper Archer : G-JANA, EGSG (Stapleford)
>
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