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1978 KZ750 TWIN - Blown Head Gasket? 17 Jan 2019 13:32 #796957

  • Jessmansimon
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I noticed over the past month that the driver left pipe was turning blue. I did a bunch of carb work to try to remedy the problem over the month. When I finally had enough, and STILL the spark plug on that side was black, I noticed some oil coming from the cylinder head housing the exhaust pipe. I looked closer, and could see all along the head gasket, there is a bit of oil seepage and a lot of oil seepage on the left side! So, I found the problem.

My question is, what caused this? I am going to replace the head gasket, but what can I do to prevent this in the future. I read that I should be re-torquing the head bolts every few months. Should I be worried about piston seizure? The bike runs fine, but the one cylinder is definitely getting air from the gasket hole/failure. This is also my first time replacing a head gasket, I shouldn't have to mess with the valves, correct? How can I visually check if they need replacing while I'm in there?

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1978 KZ750 TWIN - Blown Head Gasket? 17 Jan 2019 13:36 #796958

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Jessmansimon wrote: ...................... I read that I should be re-torquing the head bolts every few months.......................


Where did you read that? :blink: Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)

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Last edit: by 650ed.

1978 KZ750 TWIN - Blown Head Gasket? 17 Jan 2019 13:56 #796961

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I read it on some forum. And it makes sense. All the vibration and heat and cooling? could loosen a bolt for sure. If its not seated right with enough pressure, makes sense it would pop a leak. Maybe not every few months, 6 months it could have said?

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1978 KZ750 TWIN - Blown Head Gasket? 17 Jan 2019 15:10 #796969

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It doesn't make sense to me at all. I've owned my 1977 KZ650-C1 for almost 42 years and have never re-torqued the head, and it has not any head gasket leak in the more than 61,000 miles I've put on it. Also, the manual does not call for that to be done.

I would strongly suggest that you buy a Kawasaki Service Manual that covers your bike's exact year and model and follow the maintenance schedule detailed in it rather than pay too much attention to folks on online forums who may or may not know what they are talking about. Until then you may want to look at the KZ750 Twin online manual at the link below. It includes the 1978 models. Ed

www.kzrider.com/modules/ServiceManuals/K...l%20KZ750%20Twin.pdf
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)

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1978 KZ750 TWIN - Blown Head Gasket? 17 Jan 2019 17:05 #796975

  • Nessism
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The factory manual for the 750/4 calls for retorquing nuts, bolts, fasteners every 10,000km but they don't specifically call out the head nuts, which frankly surprises me. I always retorque the head nuts every other oil change. And remember, you have to back off the nuts enough to get them moving before making them up and retorquing.

In terms of what needs to be done, personally, if pulling off the head I always take the entire top end off. Replace base gasket and all the seals, and even the rings. Lap the valves and replace the valve stem seals too. OEM gaskets and seals only, unless you can't get them. It costs a little more to do things this way but it's your best chance of long term success.

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Last edit: by Nessism.

1978 KZ750 TWIN - Blown Head Gasket? 18 Jan 2019 07:33 #797016

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If not already done, would assure correct ignition timing, and that the advancer is properly functioning.

Head torquing after head gasket replacement is done on a stone-cold engine. After torquing on the stone-cold engine, run/ride until engine attains normal operating temperature. Allow engine to get stone-cold (sitting overnight is best), and then -- without loosening anything -- torque again.

Just a general re-torque in an effort to reduce a leaking head gasket should be done on a stone-cold engine. I first try to torque each fastener in the factory prescribed tightening sequence without first backing anything off, which may result in zero movement as if already sufficiently tight. If so, then, as Nessism says, back off the nuts enough to get them moving before tightening them to specs.

I'm uncertain whether it's best to first slightly back off all the fasteners before the re-torque, or just back off and re-torque one at the time, or if it really matters. I think the most important thing is to have the engine stone-cold, and to tighten in the correct sequence.

Good Fortune! :)
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KZ900 LTD

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1978 KZ750 TWIN - Blown Head Gasket? 18 Jan 2019 09:51 #797017

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You may be surprised when you pull the head to see that your gasket was fine, but once the head is off you will have to change it. Another possibility is that you have a leaking valve seal. That will require a little more work than just changing the head gasket.
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.

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1978 KZ750 TWIN - Blown Head Gasket? 18 Jan 2019 10:36 #797019

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well, the plugs are not oily. Like, not wet. Just black sooty. So I don't think oil is getting into combustion. I will find out soon enough though. Thanks for all the advice. I ordered the head gasket, and will replace it. I'll make sure to examine the valves for lapping purposes. I want to check out the piston rings, but I work on the bike in the parking lot. Is there a way I can tell from above? Without taking the block off the base? Probably not. The block, base and crankcase look brand new. No oil and shiny.

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1978 KZ750 TWIN - Blown Head Gasket? 18 Jan 2019 12:02 #797028

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I don't know of any way to tell if rings are a problem unless you pull the cylinders off, other than a compression test.

Check your compression before you take the head off. Check it first with a warm engine. Then drop a teaspoon of oil into each cylinder and check again. If compression is higher the second time round you probably have ring issues. If it's the same more or less then your rings are probably OK but you may have a valve issue.
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.

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1978 KZ750 TWIN - Blown Head Gasket? 18 Jan 2019 14:20 #797030

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I would perform a leak down test and compression test rather than adding oil to check compression. Even a very small amount of oil added to a cylinder can make a big difference in the compression readings. I have seen more than one person advocate adding "about a teaspoon" of oil to each cylinder when doing a "wet" compression test. That may not sound like much oil to many folks, but a teaspoon is actually right around 5cc, and that is a lot. As an example of how little oil it takes to change the compression ratio I offer the following diagram. It uses a hypothetical cylinder with a total volume (cylinder & head) of 100cc and a dry compression ratio of 10:1. A little oil added changes things quite a bit, so if different amounts of oil were added to different cylinders one can see that the compression ratios might vary a lot. This example is not precise and is not any specific engine and it doesn't take into account oil that may seep past the rings, etc. It only is meant to demonstrate in very general terms how adding oil can change compression ratios even in a perfect engine. You can see how quickly the compression ratio jumps as oil is added to the cylinder. Ed





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1978 KZ750 TWIN - Blown Head Gasket? 18 Jan 2019 16:58 #797042

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Yes, that's true mathematically, and if too much oil is used then the results can be skewed, hence not very useful. I've done the math for this engine below.

In this case this engine design compression ratio is 8.5/1. Bore and stroke are 78 mm. Doing the math gives a total piston volume displacement of 373 cc in one cylinder. The head volume at TDC (at 8.5/1) would be about 44 cc. (This assumes no carbon buildup) Assuming all of the oil stays on top of the piston the new compression ratio would be 378/39, or 9.7

It's also worthwhile to point out that what's shown in the diagrams is only a single piston cycle where the oil remains on top of the piston, displacing head volume. During an actual compression test the cylinder will go through 10 to 15 or more complete cycles. (The engine is cranked over until the pressure gauge no longer shows an increase.) During this time the oil will spread to the cylinder walls and coat them, causing an increase in compression if the rings are in poor shape.

Compression for 4 mm on top of the piston would be 377/40 = 9.4. For 3 mm would be 376/41 = 9.2. For 2 mm would be 375/42 = 8.9. For one mm would be 374/43 = 8.7

It's important to keep in mind that the test isn't done to determine the actual engine compression. It's to compare two readings to see if there is a significant difference between the two tests that would indicate whether low compression is due to rings or to valves.
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.

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1978 KZ750 TWIN - Blown Head Gasket? 18 Jan 2019 17:01 #797043

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With 372.7cc f cylinder displacement 5cc or even 10cc is not going to cause any damage on a cylinder that is low on compression.
I only add oil to a cylinder with low compression. This is to determine whether the rings or valves are the cause, as stated by hardrockminer.


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