Engine stutter when cold

Jesse · #1 05-20-2014, 04:21 PM

We just had a new development with our RV-10 engine. It has 1,350 hrs SMOH and just had a thorough condition inspection. 3 plugs were replaced and all intake and oil drain tube hoses were replaced.

When the engine is cold, it starts fine, but when we advance the throttle it quits unless we get the throttle past a certain point and then it runs fine. In other words, it idles just fine and it runs fine above, say, 1,500-1,700 rpm, but in between it won't stay running. Once it warms up a little, like a couple of minutes, then it runs fine and keeps running fine.

It doesn't seem to make any difference if it has the boost pump on or not, so I don't think it is a mechanical pump failure.

Any thoughts?

krw5927 · #2 05-20-2014, 04:31 PM

Sounds somewhat like classic Lycoming "morning sickness", a.k.a sticking exhaust valve. You'll find lots of threads in the archives on diagnosing and repairing sticking valves, written by very knowledgeable folks.

David Paule · #3 05-20-2014, 04:50 PM

It might also be simply insufficient priming or something like that, if the effect doesn't last longer than a few seconds.

I thought I had a sticking valve on one cylinder. That cylinder worked fine after it warmed up for a few minutes. Turned out to be bad Champion spark plugs. I replaced the plugs and all was fine. They had about 600 hours on them and the pints were excellent. When I measured the resistances of the dozen plugs, they were all over the place.

Caveat, my engine is an O-470 and it's not on an RV.

Dave

Jrskygod · #4 05-20-2014, 05:29 PM

If it started right after the intake and plug replacement that is exactly where I would start looking. Could very well be an intake leak. Check those clamps.

MarkW · #5 05-20-2014, 05:47 PM

Since an engine needs to be a little rich when cold, I would suggest it is running too lean at idle. Have you done the idle mixture check by looking for a increase at cut off?

rv7charlie · #6 05-20-2014, 05:49 PM

Carb or injection? If carb, accelerator pump? (Already lean because it's cold, leaner when throttle is opened slightly?)

Charlie

RV10inOz · #7 05-20-2014, 05:58 PM

List of things that must be in good order.

Plugs gapped 0.016-0.018" or fine wires which normally need no gapping.
Plugs all 5000 ohms or less.
No intake leaks - very common problem.
FCU been off for maintenance ? The aneroid in there gets full of fuel dye gunk and becomes not so useful. Some require every 500 hours or so.

Older Lyc's suffered from valve sticking, newer ones not so much.

E. D. Eliot · #8 05-20-2014, 09:03 PM

Don't know much about aircraft engines - only a little about auto engines but I would first remove the three 'new' plugs that replaced the three 'old' plugs and run the engine with the three 'old' plugs reinstalled. It's not that uncommon for new plugs to be bad and this test won't cost you anything but time. Also, run the resistance test on all of the plugs. Good luck.

PCHunt · #9 05-20-2014, 09:42 PM

Quote:

Originally Posted by RV10inOz

.............. Older Lyc's suffered from valve sticking, newer ones not so much.

Interesting! Didn't know that, and would like to ask what was changed in "newer" engines to alleviate the problem? Can a "newer" engine be identified in any way?

Thanks.

RV10inOz · #10 05-20-2014, 11:52 PM

I suggest finding a good engine builder (and they all think they are ;-) ) and ask but Lycoming have made changes over the years to the guides.

If you check out the SB's relating to wobble tests you might get some ideas. Possibly Walt or some of the A7P's will give you a better answer.