Oscillating power

[jackcarrozzo]

Well, now I'm really lost.

The motor starts easily. For the first few seconds all 4 cylinders fire and everything is happy. Then, cylinders 3 and 4 stop firing and the motor hobbles along on cylinders 1 and 2.

If I increase the throttle a ways, all four cylinders will fire again and make tons of power (about 3200rpm) for maybe 20 seconds. Then, cylinders 3 and 4 will stop firing and it will come down to 2000rpm. The interesting thing is that after about 30 seconds, all 4 cylinders start firing again and the cycle repeats.

Here's a video of the tach while this is happening: (don't mind the CHT, it's not connected)

http://vimeo.com/21246585

I pulled the plugs, and interestingly enough the plugs on cylinders 3 and 4 were quite fouled, as if those two cylinders were running very rich. I swapped those plugs to 1 and 2, and the same thing happened (just to check).

The only two things I can think that would cause this are:

(1) Somehow the plugs on 3 and 4 foul quickly, then while they are fouled and not firing they get cleaned off, work for a bit, foul, stop working, repeat.

(2) A lose wire or something to that effect in the ignition system.

I checked the valves etc on both sides, they are fine. Compression is the same on both sides. Motor runs beautifully when all 4 fire, but alas, that's only half the time.

Cylinders 1 and 2 aren't fouling. I think my mixture is correct, since when I pull the mixture knob out while running the RPM decreases.

Any ideas?

Thanks,

-Jack

 

[maddogbrown]

Jack,

How close to fouling are the 1&2 spark plugs, by visual inspection?
You should lean very slowly and see an increase in RPM if the 3&4 are flooding/fouling.
Is compression, and or leak down good on all cylinders?
Is there a choke on the carb?
What RPM's are getting when running well?
(see the RPM's upon reread, looks close to full power)

Mike

 

[jackcarrozzo]

Plugs on 1&2 look fine to me, here are the plugs:

As soon as I start to lean, the RPM drops.

Compression is good, what's a leak down?

There is no choke on the carb, it's a standard slide carb.

When all the cylinders are running, full throttle gives 3200-3250 RPM.

Thanks,

-Jack

 

[maddogbrown]

Jack,

A leak down test is a system that applies a known amount of air pressure to the cylinder through the spark plug hole and then can measure the amount that is left inside of the cylinder.

While there is pressure applied to the cylinder one can some times hear the pressure leak, either through the exhaust, or the crank case (rings, listen to the oil cap) or in rare cases through the carb (intake valve). Please be careful when running this test, when pressurizing the the cylinder hold the propeller, otherwise it can windmill and strike you.

Mike

 

[jackcarrozzo]

Ahh, interesting, I will read up on that. Are you thinking the 3&4 side has some sort of compression leak that comes and goes?

Actually now that I think of it, if the leak is small, thermal expansion could open it up, it leaks, rpm drops, it cools and closes, the motor runs up again... etc. Interesting indeed.

Come to think of it, the motor started doing this after I had the heads off. It wouldn't surprise me if perhaps that head isn't seated well. Very interesting indeed.

Thanks for the tip, I will head to the field in the morning and investigate.

-Jack

 

[thomefran]

What type of ignition system do you have? Sounds like floating points, but I'm not a mechanic

 

[jackcarrozzo]

Single mag, a Slick 4016. I am not positive the internal timing is correct, however after fooling with it for a day this is the only configuration that gets the motor to run at all.

-Jack

 

[maddogbrown]

Jack,

Just by looking at the pic I would say that the 1&2 plugs are leaning out and starving for fuel, causing the pulsing running that you are seeing. The gaps on 1&2 are not even... can't see 3&4, but they don't look that bad to me, is there some foreign objects that are closing the gaps to the electrode that is showing that it is fouling (are they ever wet with fuel)?

Not seeing an increase in RPM when leaning is showing to me that it is already too lean. Lean Slowly.

Maybe pull the plug wires as it is running to find the culprit. Dangerous.

Mike

 

[jackcarrozzo]

Well, I know it's 1&2 that are running while 3&4 aren't since I can put my hand on the exhaust tubes from the left side, while the right side is very hot.

I will check the gaps, that is a good point. There is no mixture control on the carb for "more rich", so nothing really I can do there other than increase the fuel pressure (by... filling the tank higher, I guess)

-J

 

[maddogbrown]

Jack,

Good that you are checking the egt's, even by hand...

Just thinking here, how are you numbering your cylinders?, (doesn't matter as long as you do it the same way that the mag is doing it) and left side is the pilot side?

And no, if the fuel level makes a diff you have different problems, should run the same or close till out of gas. At some time you should do a worst case seneraio where the tail is down to max climb out angle, and low on fuel, but that will come later...

Right now you are saying that one side of the motor is dropping out at full power and at temp.

Are you hand propping? Hard to get accurate reading with compression test with hand propping.

Just shooting in the dark here,,, check the fire of each plug in a lawn mower or something.

Mike

 

[jackcarrozzo]

I am number the cylinders, from the pilot perspective as front right, near right, near left, far left = 1 2 3 4. I think that's the standard method.

The fuel pressure has a small effect on mixture; since most of us don't use a fuel pump, more fuel pressure means a richer mixture (with a slide carb). I have heard a couple guys mention they knock the mixture back a tiny bit once they are moving since the air into the vent tube increases the tank pressure, and thus enriches the mixture.

Yes, I'm hand propping. Honestly the best way I can tell compression is the feeling when I turn the prop; all the cylinders feel quite stiff, and they are the same resistance ie roughly the same compression.

-J

 

[mike1]

check your intake to head joint and tubes. sounds like you may have an air leak.

 

[thomefran]

That would be my next item to check. Warm the engine, then do a compression check on that side. Keep us posted Jack C.

 

[dayflyer47]

it literally could be something as simple as these plugs are not made secure on the inside structure and are breaking down. I read your post but I didnt see where you had simply replaced those plugs or better switch them with one and two and see if the problem moves.

Dave

 

[jackcarrozzo]

So turns out I was right: the mating surface between the heads and jugs was tight when cold, but very loose when warm. I ran the motor ("ran" might be too strong a word here) for a few minutes, then loosened one head, jiggled it a bit to seat it, and torqued it back to spec. Heated the motor again and did the other side.

I am quite pleased to report that all 4 cylinders now run happily.

However, my timing is still screwed up. I tried a ton of different internal timing settings for the mag today, and found a couple that "work". Some of them run great at high RPM but won't idle, and the others idle beautifully but die at higher speeds.

I would really like to borrow a known-working and properly-timed mag from someone to put on the motor just to verify that the trouble I'm having is indeed ignition-related, and not anything else.

Anyone in New England have a spare mag around? I'll buy it from you for $700 or whatever the going rate is, and hopefully you'd give me the money back when I return the mag ;-) I'd only need to borrow it for a weekend.

Failing that (since unfortunately I believe all you guys are farther west and quite out of driving distance anyhow), is there a place to find manuals for this sort of thing? I've been googling quite a bit, but apparently this particular mag is older and no one much used it.

Thanks a ton for the input everyone,

-Jack

 

[Outlandar]

Hello Jack
"is there a place to find manuals for this sort of thing?"
Here are the links to find the information you need. First go to Harry Fenton's. Harry worked for Slick Electro/ Unison out of Rockford. Harry was the Slick rep. at the shows. I am not sure if Harry is still with Slick. I hear there is now Joe Lugie as the rep. for Champion Airmotive doing the shows.

Harry Fenton Slick Electro/ Unison Rockford Il.
http://www.bowersflybaby.com/tech/fenton.htm
You will find posts on:
Slick Mag Gear Alignment
Timing the Slick 4001 and 4003 Mags
Aligning Gears in Slick Mag

Joe Lugie Champion Airmotive. I do not have any contact information for Joe Lugie

You can also look at:
Tim's updated Slick Timing Document
Updated for better readablity and more completeness
http://www.myrv10.com/tips/maintenance/Mag_Timing/tims_slick_timing_info.html

Yours, Brian Dempsey
Where are you located? I may have a mag. Contact me offline with your phone number and we can discuss this.

 

[jackcarrozzo]

Brilliant, thanks. I'm reading through the links now, and I'll send you my contact info shortly.

-J

 

[Gaston]

Hi Jack

I had nearly the same problem maybe 15 years ago...

don't blame it on the mag too fast...

I would check for air leaks in the intake system,as you recently pulled the heads...
then get rid of these bad and expensive plugs and install some cheap and efficient NGK B6HS or equivalent...

and give it a try...

Gaston

 

[jackcarrozzo]

Hey Gaston,

I'm fairly certain the plugs are ok, since I can either get the motor to run beautifully at high RPM and badly at low, or the opposite by changing the gearing in the magneto. Since I just guessed at the internal timing of the mag, it's very likely it is wrong.

I'm waiting to hear back from an A&P near here who thinks he has the shop manual for this particular mag, that should clear things up.

Have you had your plane out yet this year?

-J

 

[Gaston]

Jack

My insurance coverage will be in force on April 1st...
It's coming fast...the plane is almost ready, only the wings to put back in place

Jack drop these plugs into the garbage can ,and install Good ones...
Automotive type are best...

defective plugs may fire at low rpm and missfire at high rpm as well as they may behave the opposite way...that's common

It won't cost you a lot to give it a try...

Gaston