Misfire or ??

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Dana

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Just when I thought I had everything sorted out... I've started to have a new issue. I'm hearing a sound, hard to describe but something between a rattling and popping sound, only at full throttle. I can hear it in the plane and people on the ground can hear it, too, when I take off. No noticeable effect on power or rpm, unlike the magneto coil issue I had last year which caused a brief rpm drop. Engine is a Mosler CB40, with an almost new (30 hour) Slick 4316 magneto with two poles shorted out, .016 gapped BPR5ES plugs with Great Plains adapters for the shielded Slick harness. Mag timing is correct at 28° BTDC. I put in new plugs and set the valves with the oil change less than 5 hours ago. I haven't yet, but will, try new plugs even though the ones in there are almost new, recheck the valve clearance, and if all else fails try swapping with an identical new magneto that a friend has for his Minimax but hasn't installed yet.

Dana
 

Dana

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That's worth checking. Compression feels good, though.

Dana
 

StarJar

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When you said rattling, I immediately thought about a head leak. You could, a few decades ago often hear VWs on the street making that sound, with intermitent pops.
Don't know its direct effect on compression. That is a bit odd, I would think also.
Edit: It might open up only under the force of combustion?
 
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Topaz

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Any chance you're running rich enough under full-throttle conditions to get some ignition of unburned fuel in the exhaust system? Similar to a backfire, but under power. It wouldn't affect power output at all, but would produce the "popping" sound you're hearing.
 

Dana

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When you said rattling, I immediately thought about a head leak. You could, a few decades ago often hear VWs on the street making that sound, with intermitent pops.
Don't know its direct effect on compression. That is a bit odd, I would think also.
Edit: It might open up only under the force of combustion?
Could be. I have no VW experience other than this engine. Certainly can't hurt to retorque the heads, some people say you should anyway after some running time. I had them off for a valve job last fall. Only problem is that one nut is under the rocker box so you can't get to it with anything but an open end wrench, and I really don't want to pull the boxes as it's a real PITA to put them back on again.

Any chance you're running rich enough under full-throttle conditions to get some ignition of unburned fuel in the exhaust system? Similar to a backfire, but under power. It wouldn't affect power output at all, but would produce the "popping" sound you're hearing.
I don't think so, I played with the mixture yesterday without affecting anything. But a misfire with proper mixture might well combust in the exhaust (afterfire).

Somebody on another forum suggested an intake leak. Unfortunately it's going to be too windy for the next few days to take the plane out of the hangar and run it.

Dana
 

StarJar

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Could be. I have no VW experience other than this engine. Certainly can't hurt to retorque the heads, some people say you should anyway after some running time. I had them off for a valve job last fall. Only problem is that one nut is under the rocker box so you can't get to it with anything but an open end wrench, and I really don't want to pull the boxes as it's a real PITA to put them back on again.
You should first look for exhaust soot around the cylinder/head seal area. If it's a leak or loose head you should see some dark residue there.
 
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don january

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keep an eye on your crank could be a fracture just starting to come out of hidding.
 

StarJar

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Exhaust residue is often white.
I spoze it could be white. My main point is that he can find out rather quickly, without running the engine and all that.
I've usually seen where it sprayed out in one area, top, side, or bottom, but with a flashlight it should be visible, on the base of the head, in the crack between cylinder and head.
You A&P's don't see that much with all those sealed jugs. Lol.
 

Dan Thomas

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I spoze it could be white. My main point is that he can find out rather quickly, without running the engine and all that.
I've usually seen where it sprayed out in one area, top, side, or bottom, but with a flashlight it should be visible, on the base of the head, in the crack between cylinder and head.
You A&P's don't see that much with all those sealed jugs. Lol.
We see exhaust leaks all the time, at the stack flanges, at all the stack and muffler connections, and so on. These things run red hot and when they get old, they warp and erode and crack and we have to check them carefully for leaks. Cracks are distressingly common, especially on the Lycoming installations in 172s. Various exhaust parts makers have tried beefing up stacks and mufflers, and some of their mods seem to make it worse. When you start laminating stainless sheet in the form of doublers, you end up with a part whose internal sheet gets hotter than the outside sheet, the inside expands more than the outside, and the outside gets torn.

Only a few times have I encountered cracked heads, but they're not unknown. They used to be more common.
 

Hot Wings

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If I were a betting type I'd have to second StarJar. If it's only noticeable under load that indicates a head leak rather than an exhaust manifold. If it's fresh there may be no visible sign - yet. In really bad cases the pushrod tubes even get black soot on them. A leak down type compression test is the quickest way to diagnose this.

If it is a head leak simply re-torquing the heads won't help. The only proper way to fix it is to pull the heads/cylinders and lap or re-machine the seating surface as needed. A VW is just like any other air-cooled engine with removable cylinders. The mating surfaces have to be true, clean and void of any material that can compress or flow. That means no paper gaskets, paint, or excess sealer under the cylinders. I know VW used paper gaskets under the cylinders but it was a mistake that caused plenty of problems.
 

ekimneirbo

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Dana, do you have access to a bore scope to check the piston top and cylinder internals with? You could have a valve sticking intermittently.
 

StarJar

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Dana, is it like a 'shuddering' sound (along with popping)? If so I put all my money on a loose or leaking head.
If turns out to be, you might as well remove the head to examine if the shoulder is smooth enough to simply re tighten, and no warping, IMO.
Edit: Ah, hadn't seen Hot Wings post. I'm glad we agree.
 

StarJar

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I hope we are both wrong...
Yeah, but it's really not that bad. With only a single cyl. head, he should be able to lap it (if that's what's needed) without even removing the cylinder. (As long as he cleans everything good before reassembly, IMO.)
Edit: Hmm maybe scratch that. I forgot about cyl. studs in the way of lapping. But on the other hand its the alum. only that might need lapping, you could do it with another cylinder if you had one. It's recent, so it might just need to be cleaned. Depends what caused it IMO.
 
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delta

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Normally the head nuts would have to be loose for quite a while to burn grooves deep enough to require lapping. Go ahead and check the ones under the rocker with an open end. You shouldn't be able to budge them at all either way.
There still could be something going on in your mag even though it's new. Dirt, point gap, cam wobble, loose condenser... Wouldn't hurt to check.
You should be able to tell by looking at your plugs if mixture or timing is off.
 

BBerson

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Check the mag drive coupler when you pull the mag off.
The Carb can be the cause also. Pete Plumb had trouble with the Zenith carb on his O-100. Dual carbs might be better on a twin
 

ekimneirbo

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Lots of good suggestions here Dana, but it kinda gets back to "checking everything" which doesn't help pinpoint your problem. A look inside the combustion chamber should tell you if combustion is normal and give you some clues if one cylinder is different in appearance from the other. A sticking valve still gets my vote as a likely source since you mention the popping noise, even though you mention a recent valve job. Did they do any guide work or just grind them?
 
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