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Mystery cold start issue


FlyingMonkey

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Hey all...

After completing my Rotax rubber change, on first starting the engine I had some problems.  The engine would not turn over, and we could not figure out why.  We confirmed fuel flow and spark.  My battery was on the weak side, so we jumped it from another source.  It still took several attempts, but it finally started.  I chalked it up to a weak battery and/or air in the fuel lines.  I have since replaced my battery, and have had no issues since. 

Until today.  The airplane sat for a week and a half due to poor weather, and I went to start it this morning and it just would not start.  The engine turned with vigor on the new battery, but just wouldn't catch. I tried 7-8 times, then pulled the upper cowl and checked everything.  I confirmed plug boots were seated, the chokes on both carbs were getting full travel, etc.

I tried it again, no luck.  Then I advanced the throttle halfway and started it.  It caught immediately.   I let it warm up, then shut down and re-cowled.  On restart it caught on the first blade.  I flew it to a local fly-in, where it sat for several hours.  On leaving once again the engine caught on the first blade.

I don't even know if this is really a problem, but to recap, this only seems to happen when the engine is cold and has sat a week or more without running.  It has had zero issues in any other circumstance.  I don't think the ignition modules have issues, because hot starts are always instant.  I'm wondering if there might be a problem with one of the carb chokes inside the carb, or with an idle circuit.

 Any ideas?

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I wouldn't rule out the ignition modules, especially if they are the older non soft start type. The symptoms described were like what my engine was doing before I replaced the modules with the newer type. Next time it won't start, try the Ice bag trick. If the engine starts after chilling the modules, then they could be the problem

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19 minutes ago, acresflyer said:

I wouldn't rule out the ignition modules, especially if they are the older non soft start type. The symptoms described were like what my engine was doing before I replaced the modules with the newer type. Next time it won't start, try the Ice bag trick. If the engine starts after chilling the modules, then they could be the problem

My understanding is that the problem with ignition modules occurs when they are hot.  My airplane has never failed to start instantly when hot.  Does that problem really occur with (very) cold starts?

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On my plane, the starting problem was worse during the warmer months after sitting for a few days. During the cooler months it would start OK. Also once I got it started, it too would start easily when hot only to revert to hard starting after sitting a few days.

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Andy, I had an idle higher than I wanted so I adjusted my idle without going thru the whole procedure of setting choke idle and rebalancing the carbs.  This changed the way my choke operates.  I need to crack my throttle about 1/8" from the stop now to keep the engine from running rough during cold starts.  My choke also has always seemed to provide too much enrichment on those "less than extreme cold" days so I have a reduced setting position for the choke on these days.  I need to get into my choke settings at my next condition inspection and straighten things out.  I'm wondering if this might be your situation too?

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Back when I got my first CT in 2007 I sent out the carbs for the 200 hour inspection. When they came back I installed them on the airplane, and it wouldn't start. I was told to blow into the vent tube on the carb to prime the engine and see if it would start, and it did. My problem was that the starting carbs had been mismatched to the carbs. I swapped them out and everything went back to normal.

I doubt this is your problem, but if you had them off it might be worth a look.

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Roger, battery is brand new and cranks the engine very fast (obvious change over old battery).  Carbs also just synced.

Update from today:  Engine started beautifully whether cold or hot.  I think I'm just not going to worry about this unless it becomes a regular occurrence, since it's not easily reproducible. 

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33 minutes ago, Ed Cesnalis said:

If  you ign mods are original you will have to replace them. There is no free lunch.

Our old CTs like to start on the first blade except when they don't.  Still a bit of 'art' mixed with this 'science' especially the warm starts.

Well, maybe.  Roger says the number that have needed replacement is a small percentage of the fleet.

I agree there is some art to starting, and cold starts are harder than hot starts for me, but before this latest event I never needed to try more than 2-3 times to get things going under any conditions.

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So this is interesting.  

One of my co-workers is a very accomplished drag racer and engine builder.  I described the starting issues I had, and the circumstances that it only seems to happen when the engine was not started for a week or more, and it seemed to start when I went to half throttle.  He started nodding immediately, and said he had similar problems this summer with his carbureted race engines.  

His diagnosis:  excess atmospheric moisture.  We've had an immensely wet summer this year in Georgia, with lots of rain and high humidity.  He believes that moisture and condensation collect in the carbs and intakes, and the longer the airplane or car sits without starting, the more moisture can collect.  This explains why it happens after a week or two of sitting on a cold start, but not just sitting overnight.

I'm not saying this *is* the problem I've had, but I like that it fits very well the behavior I have seen.  And the fact it won't cost me any money to fix is a bonus as well... :D     

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21 minutes ago, Anticept said:

I've opened the ignition system on mine after it had sat for a while. Yes, they do collect dew in there, and not any small amount.

Good to know Corey, that makes me feel a bit better about this theory.  Well, except for the "dew collecting in my ignition" part.  :fainting-1344:

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Another possibility... If I leave the fuel valve open I will have difficulty starting or rough starts days later, but not hours later or even the next day. Apparently the float valves leak just a tiny bit and flood the engine... or maybe evaporating fuel gums up the carbs. I have a system now to ensure I remember to shut the valve as soon as I turn the key off.

Mike Koerner

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12 hours ago, Mike Koerner said:

Another possibility... If I leave the fuel valve open I will have difficulty starting or rough starts days later, but not hours later or even the next day. Apparently the float valves leak just a tiny bit and flood the engine... or maybe evaporating fuel gums up the carbs. I have a system now to ensure I remember to shut the valve as soon as I turn the key off.

Mike Koerner

That's a good thought, I know some folks leave the valve open all the time.  I always shut off the fuel as soon as I stop the engine.

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16 hours ago, procharger said:

If you have to go to half throttle to start sounds like its getting flooded??

I don't know how it could be flooded with the choke on and the throttle at idle, with the fuel valve only being opened a few moments before starting.  If it is moisture in the carb/intake tract, the extra fuel might "wash" the water out to enable starting...? 

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