importance of WOT during compression test?

[seestheday]

I re-read the compression test guide after doing one on my bike and realized that I'll probably have to do it again because I didn't do it at WOT.  Can anyone tell me what this does?  Just curious, and kind of hoping I don't have to do it again because I've already pulled the carbs and would have to put them back on to do the warm up right.

For reference, here were my compression numbers:

Cold
115
110
128
134

Warm
120
110
114
138

#4 plug oil fouled

Wet
125
122
130
155

Spec is 170 +/- 14 psi

UPDATE: Answer - WOT is important to ensure that the cylinders are able to take in as much air as possible.  If you do it without WOT you might not get as high a reading as you would with WOT.

[seestheday]

More info, I plan on removong the head anyways as I know that the gasket is bad.  I have oil leaking there and have seen very small bubbles coming out while the engine is running.

I'm curious as to what info I need before pulling it all apart.

[robs750]

seestheday wrote

  Just curious, and kind of hoping I don't have to do it again because I've already pulled the carbs and would have to put them back on to do the warm up right.

With the carbs removed,wouldnt that be the same as WOT?
Also looks like you did the test with the engine warm.

Before you pull the head,do a leakdown test to check the valves.

[seestheday]

No way to warm up the bike with the carbs off, other than an external heat source - and I don't think that is the same thing.

Does it matter if I just do it cold?

I hate to buy a tool I'm only going to use once or twice, so I'm going to look into borrowing what I need for a leakdown test.

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "robs750 [via Honda CB750'S]" <[hidden email]>

Date: Mon, 10 Dec 2012 05:04:12 -0800 (PST)

To: seestheday<[hidden email]>

Subject: Re: importance of WOT during compression test?

seestheday wrote

  Just curious, and kind of hoping I don't have to do it again because I've already pulled the carbs and would have to put them back on to do the warm up right.

With the carbs removed,wouldnt that be the same as WOT?
Also looks like you did the test with the engine warm.

Before you pull the head,do a leakdown test to check the valves.

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[cdaiscool]

WOT allows the air to be pulled in. If you don't have the throttle to open it, it can't suck in the air to pressurize to give you readings. It'll pull in some, sure, but not NEARLY as much as it (theoretically) could.

As for warm vs cold, the rings will be slightly contracted, causing leaks around it. When warm, this is a better way to be sure your rings are sealing properly. However, it won't cause a huge drop, maybe 20 psi, for cold vs warm. And you can put just a little oil in 'em and retest to see if it changes at all. No change means the rings are sealing beautifully.

Careful how much oil you put in though - too much, and you're changing the size of the chamber and it will go up due to less space; oil doesn't compress.

[seestheday]

Thanks, I might just warm the engine up via an external heat source and then do it with the carbs off.

Maybe I have better compression than I thought.  I hope that is the case because those numbers don't look so good given that factory spec is 170.

[shinyribs]

Checking compression with the carbs off is just as effective as checking at WOT. When doing a leakdown test is doesnt matter if you have carbs on,or WOT,since your are just checking the sealing of the valves.Not how much air is being ingested/compressed. Your introducing your own compressed air source when doing a leakdown,in other words.

[TOOLS1]

I made my leakdown tester out of an old spark plug, and an air hose fitting. Just break the porcelain out of the plug, then tap it with a 1/4 pipe tap. Screw in a male air hose fitting, and you have the leakdown tester.
TOOLS

[Lucky 1]

I air cannot come into the engine what will it compress?

[Lucky 1]

Does not matter if the engine is cold.
What does matter is that the engine has run recently and oil is on the cylinder walls.
Cold air is more dense so compreession numbers would be slightly higher.

[Lucky 1]

Engine does not need to be warm, It just has to have oil that has
circulated in the cylinder walls to seal the piston rings.

[Lucky 1]

To rebuild the engine you need lots of tools.

Besides sockets and wrenches you need:

Dial indicator for checking valve wobble in the valve guides.
Dial indicator with gauges to check the cylinder bore.
Micrometer to check the piston if the ring lands are not trashed.
Drill press to press in new valve guides.
Flat steel table or flat formica table,
Thickness gauges. For measuring ring end gaps.
Gasket scraper.
Vernier calipers 6 inch ,0 - .100 in one revolution.
All engine cases and cylinder to be cleaned spotless. Hot tank or ultrasonic tank.
Good workshop manual.
Allen wrenches for pulling the plugs in the crankshaft oil galleys, plugged up with years of gunk.
Machine shop nearby to machine parts.
A new set of valves and valve springs.
Neway valve cutter set.
Watch this video AFTER you have replaced the valve guides.
http://www.youtube.com/watch?v=QIVj_L1akl4

[seestheday]

Are you saying that I absolutely need all of those tools before I should consider removing the head?

I know that at the very least I have a bad head gasket that is leaking oil, not sure what else I will find once I open it up.  I was hoping not to have to do a complete rebuild.  If I open it up and things are very bad, then I'll have to reconsider my options at that time.

I have all of the tools you listed below, with a few exceptions, notably:
dial indicators
drill press
new valves & valve springs
new valve cutter set
flat steel table, is it just a very flat surface that is needed?  Would glass do, or does it need to be sturdy?

I would buy all of these if needed.

[seestheday]

Can you define "run recently"?  Within an hour?  day? week?

[Lucky 1]

Within one day.

[robs750]

Lucky 1 wrote

Besides sockets and wrenches you need:

Machine shop nearby to machine parts.

Neway valve cutter set.

Those seat cutters are pricey.
Wont need one if the machine shop can do the head.

Might be able to lap the valves also.

[Lucky 1]

You would not want to buy the valve cutting set just to do ONE valve job. If the local machinist cannot cut all three angles or does not know what the angles are, then you need to give it to a machinist that does know or buy the tools and then sell them to the machine shop so they can make more money with them or you save them to make some money.


And NEVER put a valve seat cutting tool on a electric drill!!!! Just 1.5 turns is all it takes.
Unless you want to buy a new cylinder head. I do not know where in Japan you could find new valve seats.

Remember a car machine shop is not really the right place.
It needs to be a machinist that understands motorcycles.
You may find a shop that does both but it is rare.
Car people are car people know what I mean?

BTW I forgot you need the special socket to remove the clutch nut. =$25.00

Remember ....Once you start taking the head off you are committed to finish the job.
If you take it apart and give it to someone else they may not know how it goes together and where the O rings and locating pins belong.

It is better to not even take it apart.
I hate getting engines that beginners took apart. No offense to beginners.
Generally they strip bolts and break things.

[Lucky 1]

A glass table top would work.
A formica table top would work if it is not warped.

[Lucky 1]

What about the cleaning tank or containers???

[Lucky 1]

Are these numbers AFTER the valve adjustment?