Misfire or ??

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bmcj

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Is there any way you can duplicate it on the ground safely with the cowling off? Maybe you can see or hear (stethoscope) something. If you can replace the prop with a deadweight flywheel, then you could work closer to the engine and not have the prop blast blowing away sounds or exhaust leaks.
 

Dana

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The valves were ground; the guy doing the work looked at the guides and said they were OK. Plugs look good, no evidence of improper mixture or anything.

At this point (next time I get back to the airport) I'll check for leaks at the heads and at the intake manifolds, check the cylinder stud nut torque, try new plugs, and maybe pressurize the cylinders (I don't have a leakdown tester but I can pressurize the spark plug hole and listen for air escaping, which is how I found the bad valve last time).

Dana
 

StarJar

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Kind of an obscure place to mention this, but IF it turns out to be a loose head; on VW's you have to realize that with the bolt pattern outside the circle of the cylinder, it's important to be very careful to make sure the head is going down flat on the cylinder and then turn the nuts just a little at a time, otherwise one nut can teter-totter the head on the cylinder and cause an uneven seal.
 

Dan Thomas

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If you can replace the prop with a deadweight flywheel, then you could work closer to the engine and not have the prop blast blowing away sounds or exhaust leaks.
That won't let the engine build combustion pressures. No load on it. I once engineered the installation of an RAF Subaru conversion into a Glastar; it had the flywheel and I could run it in the shop without a prop. Opening the throttle even a small bit let the engine run way up toward redline.
 

Dan Thomas

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Kind of an obscure place to mention this, but IF it turns out to be a loose head; on VW's you have to realize that with the bolt pattern outside the circle of the cylinder it's important to be very careful to make sure the head is going down flat on the cylinder and then turn the nuts just a little at time, otherwise on but can teter-totter the head on the cylinder and cause an uneven seal.
It's been an awful long time since I worked on my VW (1970/71) but I remember that it had eight studs per head, or four per cylinder. I could see the head itself being distorted by someone overtightening things, trying to stop leaks, and the gasket surface would then be wavy. The head would need light milling to get that surface in plane again. I used to run into this sort of distortion when remanufacturing compressors, and found that the specified head bolt torques really were critical. Besides distorting the head, it would distort the cylinder bores because the bolts went into threads in the block and the cylinder walls would be pulled inward at those points. The VW doesn't have that problem.
 

StarJar

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It's been an awful long time since I worked on my VW (1970/71) but I remember that it had eight studs per head, or four per cylinder. I could see the head itself being distorted by someone overtightening things, trying to stop leaks, and the gasket surface would then be wavy. The head would need light milling to get that surface in plane again. I used to run into this sort of distortion when remanufacturing compressors, and found that the specified head bolt torques really were critical. Besides distorting the head, it would distort the cylinder bores because the bolts went into threads in the block and the cylinder walls would be pulled inward at those points. The VW doesn't have that problem.
Well that reminds me, with full VW heads (2 cyl. and 8 studs) the whole head will come down straight (on the fore/aft axis) as you turn one lower nut, and compress the pushrod tubes. But not so with a single cyl. head, because there aren't two lips that it's resting on on the upper side; only one.
Also as you compress the pushrod tubes, by turning the nuts it can cause confusion about how the head is squaring up on the cylinder.
These are minor things, but are good to be aware of on V Dubs.
 
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BBerson

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The valve guides need to have carbon cleaned out when a valve job is done. Looks good is not enough info.
If the carbon isn't cleaned then sticking will soon happen.
When the carbon is removed, then check the valve guide bore diameter (clean metal).
See if it meets specs or too loose.
If too big you need new guides.
The valve stem must contact metal, not carbon to cool.
I did suggest cleaning your guides when you had the heads off.
 

N8053H

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If it was a head to cylinder leak you will hear it as you pull the prop. I say go one step colder on the plug. You are getting pre ignition from to hot of a plug. This will show up at a high MP setting when running. Lower the rpm's and if it is a hot plug it should get better as the rpms are reduced. Try a one step colder plug. You may need to go two steps colder.
 

Hot Wings

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on VW's you have to realize that with the bolt pattern outside the circle of the cylinder, .
It would be an interesting engine if the head bolts were within the cylinder circle! :gig:

But I know what you mean. Torque the lower bolts a bit first and the P tubes tend to line up the sealing bits.
 

StarJar

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It would be an interesting engine if the head bolts were within the cylinder circle! :gig:

But I know what you mean. Torque the lower bolts a bit first and the P tubes tend to line up the sealing bits.
Yeah, I guess bolts will always be outside the circle:gig: I probably should have said the mating surface.
But when you think about it, the single cylinder head is more prone to getting an uneven start on an additional axis.
I think the bottom line in this case is; start with the bottom two nuts, to get it very flat ei. as soon as there is an increase in torque, stop and back them up a bit until you can get them all happy at like .5 ft/lb, (while closely watching the parting crack on all sides) then slowly proceed in a crossing pattern.
 
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Dana

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The valves were ground by a reputable shop. When I say he said they "looked good" I'm paraphrasing; I assume he cleaned them and checked them to specs when he had the valves out.

It looks like it's likely a loose head. I didn't take the cowl off today, no time, but I put a wrench on the nuts I could reach. I didn't have the proper size metric wrench with me, just an oversize english open end wrench, but one of the nuts turned a little bit even with relatively light torque. Also though it was hard to see, it looked like there may have been a bit of gray (as opposed to the light rust everywhere else on the outside of the cylinders) in the space between the top fin and the head. I'll retorque them properly next time I go to the airport and see what happens. Probably won't be until next week, though, the weather doesn't look that great.

Dana
 

Hot Wings

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But when you think about it, the single cylinder head is more prone to getting an uneven start on an additional axis.
Could be true for the twin. Maybe we need to hook up the vacuum infusion pump to the spark plug hole and suck the head down before starting the torque sequence? :lick:
 

N8053H

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When my head to cylinder started leaking, yes I have had this issue, you could hear it when pulling the prop through. If it is indeed a leaking cylinder you should be able to hear it when the engine is not running and you pull the prop. I could on mine when this happened.
 

N8053H

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Keep us posted as to what you find here. I made the comment about heat range plugs because I had this issue with a car once. At around 180 mph it would start to miss and backfire. This was around 4500 rpm's. The engine was built to turn 7000 rpm's. under load and 12,000 free reving rpm's. I was told to go one step colder on my plugs. She then ran to 198 mph. at 6500 rpm's just what I was looking for. Yes this was a street car. Before changing plugs the plugs were really white and the exhaust pipes were white also. When you mentioned this the first thing that came to mind was my hotrod I built and the problem I had with that. I could be wrong but something to think about.
 

don january

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Dana I see alot about the upper end of your engine but just to past note, if your crank is gettnig weak and she flexe's at high RPM maybe that's whats throwing your timing off, Note not all the time will it snap. Be carefull and keep an open field in sight
 

Dana

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As I said above it looks like at least one cylinder head nut is loose, so it seems likely that's the problem. But winter has returned to New England, at least for awhile, so I haven't been able to do anything further. I will post a followup when I have something to report.

Dana
 

StarJar

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I know you probably intend to pull the head off although you didn't specifically say so. I would say 50% for educational purposes if nothing else, as you will be able to see the path of the leak, and also be able to clean that area and run your finger over it.
If it was car, I would just tighten it (but first back all the nuts off to 7 ft lbs.) because its not worth the hassle of untangling the head from everything to get it off.
Like I was saying before the single head is no automatic thing to get straight because any single nut can cause an uneven start, especially with the pushrod tubes pushing on one side.
On a full head, starting softly with one of the two bottom-center nuts, the head is pulled flat onto the cylinder.
 

Hot Wings

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Just a thought - There are 2 sizes of VW head studs and each require different torque values. Torque the 10mm head studs to the 8mm value and the head will leak. Torque the 8mm to the 10mm value isn't a problem. The studs will fail in yield before reaching the specified value.

All new blocks from VW have the 8mm and that is the most common torque value listed. The Global probably has the 10mm studs?

An old school test to verify the integrity of the 10mm head stud threads in the block was to torque them to double the value. The logic being that simulated the actual tension load when the engine was hot. If they held all was OK. If the studs pulled the threads out and you had a junk engine to repair.
 

TFF

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How much was it flown between re installing the heads and now? Sounds like they need a 5-10 hr torque check to keep it leak free.
 
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