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11 Answers

Engine ran rough on startup

Asked by: 810 views , , ,
Aircraft Systems, General Aviation, Student Pilot

During engine startup today on a C152, I did my normal procedures. The aircraft was previously flown a couple of hours ago, so I used about 2 shots of primer, I then proceeded to crack the throttle, master on mixture rich and turn the key. The airplane has a 10 second limit on the starter, so i turned the key and let it turn over for a few seconds and almost had it started. After this my instructor tried to start it and then added about 3 more shots of primer, and once it started it ran rough, almost as though it wasn't firing on all cylinders, it did that for maybe a minute and then it ran normal. My question is, what would have caused it to run rough for a minute or so, it has never done that before and just wondered if the extra shots of primer made it do this.

11 Answers



  1. Derek Schwalenberg on Oct 04, 2013

    Primer may not have been locked.

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  2. Ryan Jones on Oct 04, 2013

    Why would that have affected it?

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  3. Derek Schwalenberg on Oct 04, 2013

    If it isn’t locked it can make the fuel pressure unstable.

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  4. Derek Schwalenberg on Oct 04, 2013

    A partially open primer allows raw fuel to get into the engine intake without atomizing as required for proper combustion.

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  5. Ryan Jones on Oct 04, 2013

    Oh ok I see, thanks ill check that next time!

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  6. Joe Platt on Oct 05, 2013

    A stuck valve perhaps, discuss this with your mechanic.

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  7. Matt on Oct 05, 2013

    Please get this checked by a qualified mechanic before you fly again. This is the classic symptom of a sticking valve in one of the engine’s cylinders. It’s easy to check for and can prevent a catastrophic engine failure. It can even be repaired without removing the cylinder. If the primer was left unlocked, that only causes roughness at very low power, below 1500 rpm.

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  8. Ryan Jones on Oct 05, 2013

    Matt, what about a spark issue causing this? It only ran like this for a minute or so and then fired off like normal, is it possible that spark plugs could cause this on startup?

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  9. Ryan Jones on Oct 05, 2013

    Also, if it was a valve issue, would it repeatedly do this on every start up, or be very seldom?

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  10. Brian on Oct 05, 2013

    Might have been the primer, or the stuck valve, or a little water in the fuel, or some lead on a one of the spark plugs, or a mag on it’s way out, or probably any number of other things that I’m not qualified to discuss. It’s nice to come to a forum to get some ideas on what it might be and I will never criticize a person for trying to understand or learn.

    That being said, it’s still important to let qualified eyes take a look at the aircraft if something unusual occurs. Even the worlds best mechanic can only speculate probable cause without actually seeing the aircraft. Get it looked at.

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  11. Matt on Oct 06, 2013

    Ryan, what happens is that the valve sticks while the engine is cold, causing the roughness. Then when it warms up enough (it doesn’t take long), the valve starts working normally. But what causes the sticking can eventually cause the valve to stick while the engine is running in the air. When this happens, if the valve sticks open, it can hit the piston, which can break the piston and trash that cylinder. The worst-case happened to an airplane that I owned, a Citabria with basically the same engine as a 152. The valve stuck, the piston broke up, the pieces caused the engine to seize, instant off-airport landing. I don’t wish to denigrate anyone involved in this situation, but you may find people poo-pooing this and trying to convince you that it’s perfectly normal for a 152. A sticking valve is dangerous and needs to be repaired (if that is what’s causing the problem). Investigating it can be done with a simple compression check, which costs very little.

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