Cylinder compression and related (cam musings, pt 5)

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mikenixon
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Cylinder compression and related (cam musings, pt 5)

#1

Post by mikenixon »

1970s Japanese valves
One of the scandals of powersports is that Honda and Kawasaki from the mid 70s through the mid 80s employed soft, very fast-wearing valves in their engines. On those engines having correspondingly hard seats, the valves last less than 15,000 miles, and cylinder compression quickly becomes a fleeting hope. The GL1000 and GL1100 at least have softer seats so valve recession is slower in them. But it still happens, just as with every engine. And since the OEM valve is plated (actually plasma-coated), refacing the valve as was commonly done on old cars is out of the question. Saving the odd NOS appearance on eBay or the expensive of sourcing from CMS -- when they have them -- factory valves for the four-cylinder Wings are all gone. Two Japanese companies have stepped up and are providing aftermarket replacements, and this keeps head work on these engines possible as time marches on. However, the quality of the replacement valves is not consistent. Out of four I recently ordered, half were out-of-round and had to be sent back. Of the two sources for these valves, the ones coming in green boxes appear to be better quality than the ones shipped in brown paper.

Valve seats
A cylinder head's valve seats are a mystery to many, with the usual hype and unfortunate misinformation obscuring the plain facts. No one for example needs five angles on their valve seats. In fact, without even trying, a good three-angle valve job becomes five-angle in practice. So it's just hype. Tooling for valve seat work takes a dizzying array of forms. There are too many choices for the initiate to make sense of it all. However, very good work can be done with tools that don't cost as much as your house, if you are careful, knowledgable and proficient. And even the valve seat machines that are as big as pickup trucks have their drawbacks. The real proof of whether a valve seat is sealing in partnership with its valve is the vacuum test, and this is the test pro engine rebuilders use. It's eye-opening. See my video.

https://youtu.be/uUwl_yBrORU[/youtube]

The importance of cylinder compression
You can restore low cylinder compression by doing a proper valve job, and in most cases expect to see in your 70s Honda-- including the Wing-- 150-160 psi at sea level. I do this many times each year. This is short of the book's 170 because you still have 40-year-old rings and cylinders, not to mention worn cams. On the Wing, cylinder boring is a bigger project than in say a 70s inline four. But it's do-able with the right equipment, and given no more than average cam wear, doing the two together, both valves and a bore, will easily net you the manual's 170 psi. Just the 150 or so attained from a valve job will work very well for you. And sometimes you can "cheat" and get really close to it from just loosening valve clearances. I tell all my customers: shoot for 150. Anything over 150 psi is a bonus. By the same token however, compression significantly *under* 150 is a compromise. The engine just will not be as sharp. It won't be "all there". Carburetion will not shine, acceleration will be saggy, ignition adjustment will not be very fruitful, the idle will not be rock steady. 1

Where carburetors come into it
Which leads to this. Informed tuning steps make a huge difference on the GL1000 and GL1100. Did you know the GL1000 carburetors contain over 32 removable jets? These automotive-design carbs demand a lot from the knowledgeable, professional carburetor rebuilder, let alone the do-it-yourselfer. They are unlike 90 percent of other Japanese bike carbs, and there are many things to know, things not very intuitive and not easily discovered. And finesse isn't just handy in this case, it's imperative.

Where ignition comes into it
But as demanding as the GL1000 carburetors are, ignition is the big bugaboo on this bike. Even among pro mechanics that I have known, few could handle setting up the GL1000's point ignition. The power to work well is there in the point system, but uncovering it is a lost art. Half of the challenge is that the Wing's ignition system is driven by a wiggly rubber belt. And substituting a Dyna or other electronic system does not change this. Nor does it remove very much of the other challenges of ignition tuning on this bike. Electronic ignition isn't the silver bullet; in fact, it *adds* complications. Dyna ignitions are not "plug and play". The ignition's two sensors have to be adjusted relative to each other, timing indicators are less intuitive and more confusing, and if you want factory timing specifications, a modification must be made to overcome a built-in defect Dyna manufactured into their system. So Dyna installation is no less challenging than with the points system, and in some respects more so.

Compression and ignition should precede carb work, that should be clear. But good tuning is possible and oh so rewarding. Here is a video illustrating that fact. This bike received only carburetor rebuilding and engine tuning, no engine work. Note how responsive to the throttle the engine is.

I hope you have enjoyed this journey into the world of camshafts. With these five articles we first explored the role cams play in engine tuning, and along the way discovered why over-camming is bad. We then dissected the cam, putting under a microscope the cam lobe's geometry and in the process dealt with the little-understood connection between valve clearance and cam design, and reviewed how cams are best measured. The importance of preparing the cylinder head for a non-original cam was next, with several specific, painstaking areas of the necessary steps pointed out. We then covered cam wear both normal and severe-- neither lubrication-related by the way-- while touching on the vagaries of cam chains and cam chain tensioners. Finally, the ethic of engine tuning as it relates to cams, valves, valve seats and compression was celebrated and explained, and at the same time the importance of taking care of these and ignition *before* giving attention to carburetion was stressed. I trust this series has expanded your understanding and appreciation of your bike, and encouraged you in its maintenance.

1 Naturally, compression numbers are always relative to sea level. Above sea level, oxygen is depleted and readings decrease. Mechanics correct for altitude mathematically.

https://youtu.be/CDnzwDWhN24[/video]
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sgwilly
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Re: Cylinder compression and related (cam musings, pt 5)

#2

Post by sgwilly »

Thanks Mike!
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Re: Cylinder compression and related (cam musings, pt 5)

#4

Post by desertrefugee »

Extraordinary Mike. Extraordinary. Once again, thank you for sharing. Your contributions here are invaluable and should be broadcast far and wide. I can't think of any other author who's offered up (or is capable of doing so) such a broad swath of insight into the inner workings of these vintage machines. I know, personally, I am compiling it offline. All of it. Thanks again.
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Re: Cylinder compression and related (cam musings, pt 5)

#5

Post by Track T 2411 »

Another wonderful series, Mike. Thanks. I'm curious, are the 'middle if the road' aftermarket valves offered coated like the originals, or of better (longer lasting) materials? I'm just pondering the cost/ benefits of actually doing a 'proper' head rebuild, such as getting the cams and rockers hardened, valves, seats, guides, etc. I know it would probably cost more than what I've invested in the bike already, lol!
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Re: Cylinder compression and related (cam musings, pt 5)

#6

Post by mikenixon »

I have assumed for some time that coming from Japan they like Honda's original suppliers are coating them. But they might not be, and some think they are not. But they know no more than I do from what I can tell. So I say, hmm, treat 'em like stock. At their inexpensive price, you can afford to replace them when they recede. Should have 15,000-20,000 miles of good use though.
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