Wing tips #8 compression and leakdown

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mikenixon
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Wing tips #8 compression and leakdown

#1

Post by mikenixon »

Cylinder compression basics
One of the many things I hear from customers is "Well, sure, compression is low, but at least they're close to the same in all the cylinders." As if that makes up for it. While it's true that being close to the same is beneficial, it is not so important that it will ever trump what the actual number is. Let me say it again. I will trade compression evenness across the board any day for a nice high average number. No contest. What the average compression is is many times more important, and this is ever more critical as the age of our bikes lead into the antique era. Cylinder compression is the foundation. I ask my customers to take compression readings before they send me their carburetors. If nothing else, it gets the idea of compression on their radar, and keeps it there. I get a surprising amount of pushback on this.

Cylinder compression's importance
Cylinder compression is so basic a lot of us overlook it. Here are a few wrong ways of looking at this important test. One, it isn't a test relegated to when your bike has over 80,000 miles on it. It's important at any time, regardless of mileage. Rather than thinking of a compression test as a yardstick of old age, instead consider it a routine checkup able to catch any number of possible issues before they take you by surprise. Two, forget all that Internet nonsense about warming up the engine first. I don't know where that came from. With massive amounts of factory training, decades in the votech field, official oversight of the curriculum of the nation's largest mechanics training school and later responsible for all of the training of a Big Four manufacturer's 1200 dealers, not to mention over 45 years experience, I only just heard of this a few years ago. I can assure you no career tech warms up an engine to test cylinder compression. Third, forget the teaspoon of oil. That's not really part of a compression test. It isn't even a good test, certainly not as good as the leakdown test it is trying to replace. Fourth, best practice is always to adjust the valve clearances before performing a compression test. Put them a little on the loose side. And use a good quality tester. Stay away from those rubber tipped jobs and testers with different sized screw-on adapters.

Compression and leakdown tests as a pair
One of the things career mechanics learn is how to use a compression test and a leakdown test in engine troubleshooting, that is, use them together. People who are not vocational users of these two tools may not understand this, and might think one is simply a substitute for the other. But this is not so. The best use is to compare one against the other. Kind of like how exhaust gas analysis works, wherein two gases compared reveal a third picture not possible with either. Here's an example of how that works with compression and leakdown tests. Let's say the engine is running poorly. A compression test indicates good compression, over 150 psi. But, the subsequent leakdown test is not so promising, showing a 20-30 percent loss. Wait. How can this be? How can the engine have high (good) compression but also high (bad) leakdown? Easy. Receded valves. Even badly grooved valve faces often manage to seal adequately and average some sealing when repeatedly banged open and closed during a compression test. However, the completely static nature of a leakdown test will expose the bad valves. Okay. Now another example. How about low (good) leakdown combined with low (bad) compression? Huh? Is this possible? You bet. But what does it mean? This is the classic symptom of retarded valve timing, typically from worn or mistimed cam chains or belts, which results in low cylinder compression, despite the good valve and cylinder seal. So you see, the two tests -- compression and leakdown -- work best not individually but in tandem and that is the best way to use them to get a complete picture diagnostically.
Last edited by mikenixon on Sat Jan 12, 2019 5:43 am, edited 1 time in total.
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Rednaxs60
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Re: Wing tips #8

#2

Post by Rednaxs60 »

Mike - been reading your Wing Tips threads. This one is timely as I have been investigating various issues and do believe a valve job is in the not to distant future for my '85 LTD. It had 125 across the board last fall, but this has fallen to around 90 PSI of late. The engine does not use oil so I expect the rings to be good. The gas we use nowadays is not the best and the valves do get deposits.

Know about the leak down test but have not purchased the test equipment yet. This is on the tools list as I will soon have two of these bikes on the road, need one just in case one goes down for maintenance/work.

Thanks for putting the time into these posts.

Cheers
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Re: Wing tips #8

#3

Post by mikenixon »

Thanks Rednaxs60.
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gltriker
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Re: Wing tips #8 compression and leakdown

#4

Post by gltriker »

action1 "Fourth, best practice is always to adjust the valve clearances before performing a compression test. Put them a little on the loose side. And use a good quality tester. Stay away from those rubber tipped jobs and testers with different sized screw-on adapters."

Respectfully inquiring. Why not? Are those different sized screw-on adapters you caution to stay away from, those inexpensive ones not equipped with a pressure holding Schrader valve? If yes, please delve a little deeper into your reason why.

If the different sized screw-on adapter-14mm to 12mm- without a pressure holding Schrader valve provision, seals leakfree onto the cylinder head's spark plug gasket surface, and the compression tester's fitting with its pressure holding Schrader valve installed, seals, leakfree, into the different sized screw-on adapter, what is your objection to utilizing a different sized screw-on adapter?

note. Remember....adapter seals leakfree is in this query.

( I'm not concerned about different tester hose diameters and lengths, etc. . Read it all, already. )

What I want to know from you, utilizing your applied Professional knowledge, is the distance of the compression pressure holding Schrader valve's proximity to the cylinder head combustion chamber, itself, important?
I think Yes is the correct answer.

If I am correct, an engine's effective working cylinder bore/ cylinder head combustion chamber's displacement ratio is not calculated with the additional volume a completely removed spark plug, of correct length, would add to the equation. I am correct.

Same testing error as utilizing a different sized screw-on adapter not equipped with a pressure holding Schrader valve will present. Yes? No?
The reason why most fellows can't report compression test readings from their GL1000 engine higher than the typical 140psi range. Including me. We'll never achieve the FSM test reading of 171psi, cold engine, without proper equipment.

I don't have a proper 12mm test hose assembly, with an integral pressure holding Schrader valve in its screw-in fitting, and have to use one of those 14mm to 12mm screw-on adapters with my Snap-On gasoline engine compression tester. :oops:

Show us your professional grade gasoline engine cylinder compression tester and, most important, its functionally correct 12mm test adapter, please. tumb2

Done. ;)
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Re: Wing tips #8 compression and leakdown

#5

Post by mikenixon »

Hi. Not sure what is behind this question, but what I meant is easy to explain. The screw-on different diameter adapters are loathed by pro techs because inevitably they come unscrewed when removing the hose from the engine after a test. When this happens in a modern (1990 and later) engine whose spark plug holes are 3 to 4 inches or more below the valve cover, you have an interesting day ahead of you trying to get that piece out. Not inspiring. Even in older engines not having deep plug wells, it's no fun.

I didn't mean anything about length of hose and all that controversy. Nonsense, that. One more example of folks on the Internet majoring in the minors.
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Re: Wing tips #8 compression and leakdown

#6

Post by mikenixon »

The best is of course Snap-On. I don't say "of course" because it is Snap-On but because this particular tool is matchless, the best design for this type of tool. The hoses are one-piece, robustly made, with swaged-on integral threads. Nothing to separate when unscrewing the hose from the head, and the hoses are so thick and sturdy that over 40 years of using them hasn't diminished their ability to be unscrewed without twisting or flexing. Motion Pro is having some duplicates of the Snap-On style made, lower quality but also much lower price. The hoses are not as sturdy, but they work well enough. Check it out. I supplied this tool to all of Kawasaki's training centers. Not as good as Snap-On, but for intermittent use, not bad.

One of my regular customers just found a good used Snap-On compression tester with hoses on eBay.
Last edited by mikenixon on Sun Oct 13, 2019 8:07 am, edited 1 time in total.
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mikenixon
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Re: Wing tips #8 compression and leakdown

#7

Post by mikenixon »

"The reason why most fellows can't report compression test readings from their GL1000 engine higher than the typical 140psi range. Including me. We'll never achieve the FSM test reading of 171psi, cold engine, without proper equipment."

No. Not so. The reason is the engine needs work, in most cases more valve work than cylinder, but work in either case.
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