So I've been trying to track down this issue for some time now. As you can see in the video my engine (go-300) makes and holds power smoothly at any power setting until I give it full throttle. At full throttle it will make 2700rpm for a few seconds then "stumbles" for lack of a better term. It will bounce around between 2500-2600rpm. To isolate fuel/ignition I ensured the mags were timed properly and did a run up with each mag independently and the issue persists on both so I'm pretty much down to fuel.


Right after it started doing this I lost a good bit of compression in once cylinder so I started pulling jugs, I found 4 cracked Pistons and another had broken rings. I cleaned/lapped valves and honed out all the cylinders, re-assembled with new rings/gaskets and also pulled apart/cleaned the carb and used a gasket kit from Kelly aerospace to re-assemble that. I'm getting 35ish GPH flow from the fuel line to the carburetor so I know fuel flow to the carb isn't an issue. Has anyone seen this before or maybe point me in the right direction? I think my next step is to pull the trigger on an overhauled carb but I'm not going to be happy to drop another $800-$1,000 on a part that doesn't end up fixing the problem. My carb appeared to be in fine shape when I disasembled it, it cleaned up nicely. I did need to adjust the float a little since it was set a bit too low.


http://youtu.be/mW9DET-Qfbg

When you say you timed the mags, did you verify the internal timing of the mags?

MTV

Check the throttle cable clamps. Sometimes adel clamps find there way in there and they are usually not correct. I forget the right part number but it can be different for different airframes and engines. The outer sheath will slip in the clamp then return to the correct position when you pull the throttle back to idle.

Does the engine make 3200 for takeoff?

mtv wrote:When you say you timed the mags, did you verify the internal timing of the mags?

MTV

I did not althoug the mags were overhauled less than 40 hours ago. I also did a few full throttle runs on each magneto independently and it did the same thing, just at 100(ish) rpm less. Since it did it on both mags independently I kind of ruled out the possibility that this is ignition related. Maybe I should still check the internal timing though, thanks for the response.

Mister701 wrote:Check the throttle cable clamps. Sometimes adel clamps find there way in there and they are usually not correct. I forget the right part number but it can be different for different airframes and engines. The outer sheath will slip in the clamp then return to the correct position when you pull the throttle back to idle.

Does the engine make 3200 for takeoff?

That's a good I deal that I hadn't thought of, I don't know if a slipped cable would cause the power to jump around like that though....I'll check that out tomorrow, thanks for the idea.

TCDS says with my prop I should be seeing between 2650-2750rpm static. It seems to hold 2700 for just a few seconds before beginning to drop down and bounce around.

I have been pushing the mixture to full rich when running it to full throttle and if I pull the mixture slowly when at full throttle it does nothing to help, just starts loosing more rpm when I get it pulled somewhere between 1/8-1/4 of the way out.

I have been scouring the Internet for clues or other people with similar problems, one similar(ish) case I found was a guy with an 0-300, that ended up being weak valve springs. I don't know if there is a tool to check to make sure they have enough tension or not. It does run up to 2700 and run smooth for a few seconds though so I'm not sold on it being a mechanical thing.

This is driving me crazy, I have been on the hunt for a Avcon O-360 mount for my 175, I really hope to find one soon so I can just throw this GO-300 in the trash.

Thanks for the responses, I know I'm not a frequent poster but I am a long time lurker. I'm just getting this airplane in shape to fly and will hopefully be able to come up with some content to post of the strips on the islands in Lake Michigan this summer.

I would bet ya a beer both magneto condensers are bad.

PAMR MX wrote:I would bet ya a beer both magneto condensers are bad.

Do you know what would cause condensers to fail? And is there a way to check them or are they easy enough to replace? I am an A&P/IA but to be honest I have little to no experience on prop jobbers, I'm learning this stuff now since I bought this steaming pile of an airplane.

I'm looking at the magneto overhaul paperwork, they were overhauled in January of 2015, probably less than 20 hours have been put on them since then. The invoice shows the capacitors were changed but no mention of condenser.

Thanks for the reply!

Check your ignition switch. I recently heard a similar story about the owner of a Cessna 180 (1955) spending all kinds of money chasing a similar sounding issue. Turned out the ignition switch was essentially killing one of the mags. Not sure if this AD applies to your ignition switch but I would look into it. There are two switches and two AD's. One for ACS and one for Bendix.

The owner dropped thousands chasing this demon and it turned out to be a simple switch. Worth a try.


http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/AOCADSearch/B8ABD56539B4684886256A3E00759DBF?OpenDocument

http://www.faa.gov/aircraft/safety/alerts/aviation_maintenance/media/2010/2010_11_alert.pdf

http://www.ercoupe.net/ads/76_07_12.html

Nuther thought or two:

Almost sounds similar to a "flutter" ?!? could you be getting air in through some place in the induction system above the carb.?!?
Don't remember of the 0-300 or geared version GO-300 have induction system with rubber hose connections with cable clamps.

At full throttle there should be a power enrichment function that adds more/excess fuel to keep temps down and prevent detonation. Maybe you are getting detonation?!?

Maybe set the RPM around 2,000 and lean it as if you were going burn a plug clean.
When it starts to miss at 2,000 see if it is a similar (miss - shake - tach. needle motion)
This may simulate a missing mixture enrichment mechanism.

Best I can do for now.

Wannabe sleeping on it.



Capacitor and condenser are/should be synonymous.

Right after it started doing this I lost a good bit of compression in once cylinder so I started pulling jugs, I found 4 cracked Pistons and another had broken rings. I cleaned/lapped valves and honed out all the cylinders, re-assembled with new rings/gaskets and also pulled apart/cleaned the carb and used a gasket kit from Kelly aerospace to re-assemble that.

Wow, that sounds like detonation! And it's across multiple cylinders. I don't suppose you have more engine data or history information?
Is there any chance the method of engine timing is wrong with the geared engine, degrees on the prop do not equal degrees on the crank?

It does read as if the timing advance for the geared engines is determined differently from direct drive engines. And it's possible the effects of incorrect advance wouldn't present until you get into those higher RPM, then power loss as the previous poster suggested.

http://www.tcmlink.com/pdf2/msb94-8d.pdf

Yeah capacitor condenser same.
I know what you mean about being new to Pistons.
I work on mostly turbo props now witch is boring as all hell. Wish Pratt Whitney put magnetos on a pt 6 it would make it interesting.

The condenser is super easy to change if that's it. It likely wouldn't test bad since is kinda works. I had this bad batch go through my airplanes one year. The engines would mag check fine with zero hint of an issue but the engine would loose power at take off power. Just like yours. Some did this out of the box.
A mag check at cruise power rpm of 2200 or take off power it was obviously a mag problem. Run that engine up. Do your mag check like normal then run it up to full power do a mag check and I bet it points to some ignition problem.

Often time the standard 1700 rpm magneto check is inadequate to check the health of your magneto system. After all who flys around at 1700 rpm? There is one problem with a high rpm or cruise flight mag check. If you accidentally switch the sucker off its going to go BOOM and scare ya and maybe brake some shit.
Keep us posted when ya find out. I love arm chair mechanicing [emoji848]


Sent from my iPhone using Tapatalk

Check the finger screen in the carb. Not long after buying my C170B, I had a similar situation with my O-300 and had no indication of fuel starvation problems at the sumps, gascolator, or with the fuel line disconnected from the carb. However, we found the fine screen in the carb to be packed full of very fine fibers that had apparently passed thru the screen in the gascolator. The fiber packed filter would pass enough fuel for the engine to run smoothly at 1500-1700 rpm but it would stumble at full throttle/high rpm. We never did pinpoint a source of the fibers and believe them to be form a rag or tape that a prior owner left in the tank.

Zzz wrote:It does read as if the timing advance for the geared engines is determined differently from direct drive engines. And it's possible the effects of incorrect advance wouldn't present until you get into those higher RPM, then power loss as the previous poster suggested.

http://www.tcmlink.com/pdf2/msb94-8d.pdf

I found that service bulletin and used it when I did the magneto timing, I hear you though, likely ignition problems are exaggerated at higher rpm's where the load is higher.

BlackWater wrote:Wow, that sounds like detonation! And it's across multiple cylinders. I don't suppose you have more engine data or history information?
Is there any chance the method of engine timing is wrong with the geared engine, degrees on the prop do not equal degrees on the crank?

Yeah it does seem like possible detonation, that's the biggest reason I want to get this situated before I try and fly it.

I purchased the plane last fall, flew it home (don't remember any of these issues) and refurbished the interior/panel so it hadn't run between September 15 and about February of this year. The engine is original to this airplane (1959) and has 1425hrs on it (never been overhauled). It has sat for 20+ years in a barn between the late 80's and about 2011-12. When I pulled the jugs off I was happy to see no corrosion on the lifters/cam. There is a tiny bit of play in the gearbox but I'm assuming that's normal. I'm currently hunting for a Avcon or similar Mount to do the Lycoming O-360 swap, as its really not possible to overhaul this GO-300 with the current parts situation. I'm hoping to get a few hundred hours out of it, enough to buy me some time to find a 175/O-360 mount.

Thanks for all the suggestions, I have a list written down and I'm heading back to the airport now. I was speaking to another local mechanic yesterday and he suggested looking at the idle mixture screw. When I pulled the carb apart I do remember it being very close to full in (if not fully in). Although I didn't touch it and put it back as it came off figuring I'd adjust it when I got this thing running correctly. The local guy says it affects mixture throughout the rpm range and even at full throttle so I'm going to start there today. If that doesn't help I'm going to rig up a fuel line to a gas can and go directly to the carb inlet so I can rule out the entire fuel system up to the carb. If that doesn't work I'll go back after the mags or carb. I'll let you know what I come up with today....

BeeMan wrote:Check the finger screen in the carb...... The fiber packed filter would pass enough fuel for the engine to run smoothly at 1500-1700 rpm but it would stumble at full throttle/high rpm. We never did pinpoint a source of the fibers and believe them to be form a rag or tape that a prior owner left in the tank.

I had this issue with my old C170. Due to the goofy fuel line plumbing on the ragwing, the carb strainer is a PITA to check. Annualled it with three different mechanics over the years & none of them bothered to check it, or told me to. It crapped out on me about 30 miles from the nearest airport-- I limped home with the engine cutting in and out, started at 1700' AGL and ended up at about 700. Made her though! Strainer was packed with red fibers, looked like someone had left a red shop rag in the tank & it slowly dissolved.

Another thought... check out Aircraft Magneto Service's troubleshooting page.
http://www.aircraftmagnetoservice.net/# ... uide/c15r3
It mentions "Most "bad mag checks" are spark plug related."
Check your plugs, even if they're new. Take a ohmmeter and check resistance through them, from the center electrode to down inside where the cigarette on the lead goes. New spec is 800-1200 ohms, replace the plug if it's more than 5000 ohms.

I have had a bad plug that only showed up above 2100 RPM Do a mag check at 2500 RPM see if it shows up.
DENNY

I recently encountered a situation in an O-200 powered aircraft where a bad plug was smooth at about 2200 rpm but cut out under load at 2500ish. It manifested on a takeoff and I swore I was losing the engine. I pulled carb heat, then experimented with different throttle positions, and it would only start to stutter at full power. I never would have guessed bad plug.

This sure is fun, playing diagnostician from afar. Everyone ends up learning.

Upon reflection, I think the rpm range of these engines is so narrow that timing is unlikely to be the culprit that would cause a problem only in a narrow rpm range. I'm going with a partially blocked screen in the carb/gascolator that passes enough fuel to run smoothly up until a threshold demand.

Or the bad plug.

Today's findings so far.....

So the idle mixture screw was only about 1/2 turn from the full in (lean) stop. I removed it, cleaned it up and ran it to 3 turns out. After that, I did the EAA method of setting the mixture and idle screws and it ended up at about 2 turns out from the stop. Now when I push the throttle in and run the rpm up to about 2650ish it seems to hold much smoother. I can still see the tach bounce about 50ish rpm, this leaves the throttle about 1/2in from the full stop, if I push it in the rest of the way it does the same vibration/100-150rpm loss thing.

This is a vid I took of just setting the power to peak rpm (still 1/4-1/2" off the full stop)

http://youtu.be/8u7-BtGJqIk

The fuel screens have all been removed and cleaned, from the tank fitting screens to the gascolator screen to the carb finger screen, all spotless.

I'm going to run back out there and do the resistance check on the plugs. When I inspected them before I installed them last week they all appeared to be relatively new as the gaps were good and the center electrodes were round, although I didn't check resistance. I'll report back with my findings.

Thanks again for taking the time to help, I desperately want to get this thing in the air!

So I pulled the plugs, here is a pic of them along with their resistance values right below them. I see the #2 and to a lesser degree #5/#6 are sooted up. The rest appear to look like good burning cylinders. While I have them out I'm going to clean them out and might as well do a leak down on the cylinders before I put them back in....



EDIT: I did a leak down, all cylinders were between. 68-74/80. The engine had cooled off already and there is only maybe ~20 mins of runtime on these rings/hone job so far. The only leaking I could detect was through the rings on cylinder 6, which is the only chrome cylinder I've got on it and that was the cylinder that had 68/80.

I'm at the pint where I don't know if this rpm fluctuation is even a problem..... It seems to have gotten better since the idle mixture screw adjustment. I've got some new cowl seals due to be in on Tuesday, I think I'm going to put the cowl seals on, cowl it up and try flying it, maybe that stumble will go away once some wind is going though the prop?

Newvizor

Don't waste your time looking for an AVCON motor mount: At least not from the alleged AVCON Co. Lots of other postings about them. Some almost humorous. May re-write some of it on request .

Best bet (last I checked) is still DELAIR in Porterville Ca. Young fellow name of Chris took over the business after Harry Delicker died. Chris had worked for Harry for years and has done some of the best "show" quality work I have ever seen. They also have larger fuel tank kits. I installed the larger of two options in my 170-B. I also put in their "forward" mounted muffler system.

They (Delair) have a full STC with kits and materials for 0-360 conversions.

Old farm boy trick was to start an engine in the dark so see if the wiring is winking at ya in little sparks.
Don't remember ever trying it on an airplane.

Wannabe a different Chris C

Are those all Champion massive electrode plugs?

Do they all have the same P/N? Looks like maybe some may not have internal resistors, while others do, which could be messing with your ignition. Otherwise, you have some bad plugs there (cracked internal insulation, maybe, could be shorting out at high power settings).

I am just guessing at the cause, this is internet advice. It sounds like either an ignition or an air/fuel ratio problem.