engine roughness

[337G]

I have a 1976 337G and I am experienceing rough running on the front engine especialy after a cold start. If I do not give it a shot of prime it will stall or the other thing I do is keep it up at around 1500 rpm. It smoothes out after the engine is warm and will then idle at 1000 rpm but still with a bit of roughness. After flying and I shut down for a short while and go and re-start the engine it again runs very rough and then smooths out some what after getting up to normal temps. I have been told that this is normal but I dont agree. Any one else have this same experience?

Craig B.

[KyleTownsend]

Ours runs somewhat rough in the first few seconds after starting while the RPM's are below 1000. However, it smooths out pretty quickly after I lean it back for ground operations and let the RPMs stabilize.

[SkyKing]

Craig,

You don't indicate 'where' in Canada you're located, but my first thought is that your plugs frosted over, although there are a myriad of things that could cause the roughness... like a bad lead. But my first guess would be fouled plugs, especially the lower ones if yours is a high-time engine, which you also don't specify. On startup, me thinks you should be warming-up the engine above 1000 RPM so that the oil pump doesn't cavitate. Try 1100 RPM and I'll bet your results will improve.

SkyKing

[WebMaster]

I remember GMAS telling us that we should warm up slightly above 1200 RPM. I don't remember why, except it had to do with the Mags having a spring, which would have retracted above 1200.

[SkyKing]

Well, not exactly Larry... it's actually a little lower than that, around 800 to 1000 RPM with the cowl flaps open and you stay in this range until the oil temp starts to come up. In this RPM range the mag impulsers stop clicking, reducing the wear on the little tangs or ears... and it also brings the timing up to 20 deg from the retarded condition, which also helps bring on more heat. After the temp comes off the cold end of the gage, then you can bring the engine up to say 1100 or so for warming on an especially cold day.

SkyKing

[337G]

Cold is not an issue up here any more Skyking so frosted plugs would not be an issue however perhaps a bad lead or two may be worth investigating. I thought fouled plugs too but after trying to work that out it does not seem to make a difference.

I can't pull the power back below 1200 RPM or it will quit. I run it at 1300 and it is very rough and wants to stall. At the point when I feel that it will stall I give it a shot of the "low" fuel boost pump and it recovers but still runs rough. This may continue for 1 or 2 more times and then it smoothes out enough to run at around 1200 RPM...roughly but it does not stall out on me. After I have been flying and then land it seems to "work" better and the engine allows me to bring it below 1000RPM however it still has a roughness about it.

FYI...the rear engine runs fine.

In any case I have the engineer on it and he knows the 337 very well.

Thanks in advance for every ones input!

Craig B.

[Walter Atkinson]

Craig:

I will bet that this is ignition-related. When the engine is cold, it requires a bit more charge to jump the electrodes. Try an in-flight, LOP mag check, and I'll bet the problem will identify itself. It likely will either be a weak plug or two or a weak harness.

Good luck.

[kevin]

For another viewpoint, several mechanics have told me to avoid mag checks during climb or cruise, which is what I interpret an "in flight" mag check to mean. Depending on what your problem is, you could put a dangerous or damaging level of stress on the engine, according to them.

Please consider this before making your own decision on what to do.

Kevin

[Walter Atkinson]

Kevin:

The only reason I post the following is to point out that my recommendation is based on a lot of real-world AND research experience. Just for the record, my qualifications are:
ATP SEL&S, MEL&S, CFII, MEI, A&P, piston engine researcher (Carl Goulet Memorial Engine Test Facility).

A lot of mechanics worry that an in-flight mag check is dangerous but have no data to support their concern. I continue to ask "What stress is it they are concerned about?" So far, none of them has ever had a suggestion as to just what could be a problem. When asked for specifics, they don't have any or are usually grossly misinformed about the effects of single mag operation. (Maybe it's just a fear of the unknown.)

We have been doing in-flight mag checks on EVERY flight for many years.

One should realize that during single mag operation, the stress on the engine goes DOWN as the thetaPP is later, the pressures are lower and the CHTs run cooler. (The EGT's go up as a result.)

PLEASE, let's not offer conjecture about some unknown and unidentified boogie man as a reason not to do something very helpful.

One of the best ways to stop detonation is to simply switch to a single mag. I've demonstrated that countless times on the engine test stand.

Please offer any reason that anything bad could happen during an in-flight mag check. If there is such a ligitmate concern, I'd like to either dispell it, or if I have never examined the particular issue, I'd like to do so.

Specifically, just what is so dangerous about doing an in-flight mag check? Maybe I can set you mind at ease.

[big al 08]

if your reply wasn't so caustic, i would consider it very funny, except it would require someone to have a texas hat size. i subscribe to the big head theroy; but not that big. sorry big al with no alaphbet soup after my john handcock.

[Walter Atkinson]

While I got a chuckle out of your wisecrack, it was non-contributory to furthering the group's understanding of the issue being discussed.

Why not offer some data on the subject--if you have any. Or even ask a question for clarification about something you don't understand.

I would be most interested in seeing any data that refutes what I said above.

Folks, there was already a great thread on this topic a couple of years ago titled:

"Mag Check in Flight"

It discusses power reduction, the reasons for doing so, altitude, etc. Do a search on this site and you'll find it.

I personally don't know jack about technical issues, but that thread seemed to make sense and seemed to have some sound reasoning for everyone's position.

and in the old days . . . we then held hands around a camp fire and sang Cum Bya M'Lord.



IMHUO

Keven
________
E60

[big al 08]

i do know "jack" it's a sad story, he's now homeless and and a drunk (maybe he's my missing twin). hey walt as soon as i return from dallas (they have the worlds largest cowboy hat streacher) i'll be able to contribute w/o humor. it's a joke! i value/respect all opions. just not necessarly buying aluminium siding today

[kevin]

Here is a link to the thread Keven mentioned. Thanks Keven, I had forgotten we discussed this before...

Mag Check In Flight

Kevin

[Walter Atkinson]

Kevin:

Thank you for that link. There is some misinformation in it I'd like to address. To wit:

1) Someone said that during an in-flight mag check that EGTs and CHTs would rise. During any mag check and especially LOP, the CHTs will fall when operating on one mag. Why? Because the thetaPP is occuring later and the intracylinder pressures are lower, therefore less heat is transferred into the head. This is not arguable. It can be observed by any of us. Simply WATCH the CHTs fall while on one mag. This means there is less stress on the engine during single mag operation. Try it an WATCH for yourself. CHTs will go down while on one mag. If you leave it there long enough, they will go down significantly.

2) The concern over de-tuning an engine if there is a dead mag is a red herring. It's not an operational issue. If it were, all of the twins used for training and all of the jump planes in the world would have detuned engines. This is one of those remarks made by an old wise mechanic that sounds so ominous that we dare not argue, but it's just not an operational issue. I asked several premier engine builders how often they have had to rebuild an engine because the crank counterweights were detuned and they looked at me like I was crazy. It sounds scary, but it's poppycock.

3) I have a turbocharged engine. I have no concern about doing in-flight mag checks on a TC'd engine. I do one right before the let-down on EVERY flight. I do them at between 85 and 90% power, LOP. That is most diagnostic. I know of about 700 pilots who do these routinely. One would think that if there were a bad problem being caused by this, SOMETHING would have shown up in at least one of these engines? So far, ZERO reports of any difficulty.

If anyone has any DATA to the contrary I would be very interested in looking at it.