Author Topic: Compression and Plug Diagnosis  (Read 1077 times)

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Offline bunghole

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Compression and Plug Diagnosis
« on: May 25, 2008, 11:28:31 AM »
I finally got around to running a compression test on a new to me '77 CB750K.  I'm planning on rebuilding the engine, but I figured I'd collect some information first.  Here is what I found

Cyl 1 - 120 psi, plug is black and sooty
Cyl 2 - 120 psi, plug is black and sooty
Cyl 3 - 120 psi, plug is black and sooty
Cyl 4 - 125 psi, plug is normal

Also, there is a pretty good oil leak coming from the head gasket near cyl 1.

So I'm assuming I have a bad head gasket which is resulting in the low compression numbers and the had gasket is probably blown between 1, 2, and 3, resulting in the black, sooty plugs and lower compression.

Would you experts concur?

Thanks!
'05 HD FLSTCI (Heritage)
'06 Suzuki DRZ400S
'08 Honda VFR800
'77 Honda CB750K

Offline cafe750

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Re: Compression and Plug Diagnosis
« Reply #1 on: May 25, 2008, 12:44:23 PM »
Before you did your compression test, did you adjust the valves? Also, did you hold your throttle wide open? You can wind up with completely different numbers by changing those two factors...
"It's an old motorcycle, the wind is supposed to blow your head around, it's supposed to leak oil, the brakes should suck, and every now and then, it should scare you so bad you piss your pants."



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Offline TwoTired

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Re: Compression and Plug Diagnosis
« Reply #2 on: May 25, 2008, 05:41:15 PM »
The compression is within 10% for all cylinders.  So, this looks good.
Since most auto store compression gauges are for larger displacement engines, they add volume to the squish chamber, and this lowers peak readings from Honda values, as they use a special low volume compression tester.

I would access your cylinders and head gasket to be good.

If your oil leak is between the head and cylinder block, this is common, as there is an oring seal at each end of the cylinder block and with heat and age it becomes brittle, cracks, and leaks.  To replace those, you do have to replace the head gasket, as well as the cylinder base gasket once it is disturbed. So, you are looking to remove the top end of the engine for those leaks.  Is the leak that bad?

Since you don't say if your compression test had the throttles open, you said nothing about any tune up history, or if you have any air filter or exhaust mods from stock, I'm going to have to assume you haven't done a carb vacuum sync either, and you also don't know if you have any vacuum leaks between the carb and cylinder head.

You also didn't say if the air filter(s) is clean, what Number the spark plugs have on them, or how old they are.

If you have the stock air box are all the couplers in good, non-leaking condition?

And, are all the idle screws set to the same setting?  How certain are you that the idle circuits in the carbs clear and operating correctly?

But, I'm no expert.
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline bunghole

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Re: Compression and Plug Diagnosis
« Reply #3 on: May 25, 2008, 07:33:38 PM »
Thanks guys!

The throttle was wide open and the choke was full on as well.  This is a new bike to me that I haven't even begun to tinker with.  It starts and runs and did when I got it, but I need to do a full tune up.  I don't know much about its history.  As far as I know, there have been no mods to the box and filter.  However, the exhaust is a four into one.  I'm not sure which manufacturer.  Plugs are D8EA.

One more thing, the bike will run and once warm with the choke off, the bike will idle at around 1800 but will cut out after about 20 seconds.

By the way, this is with out a doubt the best and most helpful forum I've ever been a member of.  Thanks to all of you!
'05 HD FLSTCI (Heritage)
'06 Suzuki DRZ400S
'08 Honda VFR800
'77 Honda CB750K

Offline TwoTired

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Re: Compression and Plug Diagnosis
« Reply #4 on: May 25, 2008, 11:59:41 PM »
Compression tests are done with the throttle and choke open so that the engine gets unrestricted air to compress.  Applying choke negates opening the throttle.

If it won't idle below 1800, I suspect one or more slow jets in the carbs are blocked, perhaps #4.

An old or dirty air filter will behave as partial choke, and soot up the spark plugs.  Something to check anyway.
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline bunghole

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Re: Compression and Plug Diagnosis
« Reply #5 on: May 26, 2008, 12:21:21 AM »
The Honda Service Manual instructs to open the throttle AND choke.  Is this incorrect?
'05 HD FLSTCI (Heritage)
'06 Suzuki DRZ400S
'08 Honda VFR800
'77 Honda CB750K

Offline TwoTired

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Re: Compression and Plug Diagnosis
« Reply #6 on: May 26, 2008, 01:23:35 AM »
The Honda Service Manual instructs to open the throttle AND choke.  Is this incorrect?

No, it is quite correct.  An open choke is the off position.  You stated you did compression test with choke full on, which is a closed choke, not open.
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline bunghole

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Re: Compression and Plug Diagnosis
« Reply #7 on: May 26, 2008, 02:26:18 AM »
UUUUhhhhhhh!

I pulled out the choke thingy.  Like I was starting it cold.  Did I do it wrong?

Stupid Beer!
'05 HD FLSTCI (Heritage)
'06 Suzuki DRZ400S
'08 Honda VFR800
'77 Honda CB750K

Offline bryanj

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Re: Compression and Plug Diagnosis
« Reply #8 on: May 26, 2008, 06:01:08 AM »
Yup you did it wrong
Choke OPEN or OFF is when in running position
Choke CLOSED or ON is in starting position
Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

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