Topic: oily plugs again, could it be valve seals?

[markreimer]

Hi guys, I've got an irritating problem with my cb750 k4 that I haven't been able to solve for some time now.

If I ride my bike everyday, it runs and starts perfectly. No smoke at start-up, good power.

If I don't ride it for a few days, it is pretty hard to get it started. Once it does start, it'll smoke a bit, then clear up and run smooth.

If I leave it for a week, it will barely start. When it does start running, it is often not on all 4 cylinders. When I start to ride it a block or two, the cylinders that aren't firing suddenly start firing and it's back to normal.

If I leave it longer than that, it will not start, unless I replace/clean the plugs.

When I remove the plugs, they are wet and black - not wet with gas, it's definitely oil.

I tried cleaning the carbs several times first thinking it was a carb issue, but nope. All clean with new jets/gaskets/etc. I pulled the plugs to check for spark when all cylinders weren't firing, and I'd have some wet plugs with no spark. When I replace the plug, it's back to a strong spark.

Also worth noting, when some cylinders don't fire at start-up, it varies which cylinders fire and which don't. IE: Some days cylinder 4 is dead. Other days its 2 and 3, and so on.

Other details on the bike: It has a Dyna-S e-ignition, clean carbs, the battery is 1-year old and I keep it charged up. This problem only started in the fall. Before that, I rode it around 10K km this season without issue.


It seems clear to me that oil is slowly dripping onto the plugs as it sits. It's not enough to wet the plugs badly over a day or two, but the longer it sits, the more oil drips onto the plugs. I've never opened up an engine before, but I understand the exhaust valves are directly overhead the plugs, yes? Could they be leaking oil onto the plugs?

And second question, would it then be the valve seals, the valves themselves, or the valve guides causing the issue? Potentially all? I'd really rather not have to pull the head off and I've read you can replace the seals by only removing the cam cover by using compressed air to keep the valves closed, correct? Any tips or advice would be great. It's winter here now, so I've got about two months of shop time available to fix this before spring hits...

[matt mattison]

When you went through the carbs, did you check the needle jet (emulsion tubes) and needles for wear. If they're worn, you will foul plugs in the mid range. When pulling them for inspection, you will have a black plug. The only way to really check the needle jet, as far as I know , is compare it to a known good jet and needle. You would compare how they "fit" together. If your certain its an oil fouling condition, I would perform a leak down test. The leak down test won't tell you specifically what the problem is as much as what it isn't, if your looking for bad stem seals or guides. If you pass the leak down test, your rings are ok. Now you know that your oil control issue is in the top end. If you fail, you have bad rings, and your looking at a total rebuild anyway. If you can get to all the valves in the 750 , while in the frame, I would recommend getting a valvemaster sping compressor tool. Many others have good homemade methods to removing valve springs  whilst the motor is in the frame, I'm just a little less ambitious. Worn valve seals at this point wouldn't really be a surprise, while your in there you can check for guide wear as well. A typical bad valve stem guide and or seal will make you smoke at start up, after sitting a while. Rings will constantly make a smoke cloud. Have you tried using 20/50 weight oil? The heavier viscosity oil will sometimes slow down an oil control problem. Mind you, this is not a fix, but another way to help isolate your problem.

[markreimer]

When you went through the carbs, did you check the needle jet (emulsion tubes) and needles for wear. If they're worn, you will foul plugs in the mid range. When pulling them for inspection, you will have a black plug. The only way to really check the needle jet, as far as I know , is compare it to a known good jet and needle. You would compare how they "fit" together. If your certain its an oil fouling condition, I would perform a leak down test. The leak down test won't tell you specifically what the problem is as much as what it isn't, if your looking for bad stem seals or guides. If you pass the leak down test, your rings are ok. Now you know that your oil control issue is in the top end. If you fail, you have bad rings, and your looking at a total rebuild anyway. If you can get to all the valves in the 750 , while in the frame, I would recommend getting a valvemaster sping compressor tool. Many others have good homemade methods to removing valve springs  whilst the motor is in the frame, I'm just a little less ambitious. Worn valve seals at this point wouldn't really be a surprise, while your in there you can check for guide wear as well. A typical bad valve stem guide and or seal will make you smoke at start up, after sitting a while. Rings will constantly make a smoke cloud. Have you tried using 20/50 weight oil? The heavier viscosity oil will sometimes slow down an oil control problem. Mind you, this is not a fix, but another way to help isolate your problem.

Hi Matt,

The rings were replaced three years ago by the previous owner. I did a compression test around 6 months ago and all four cylinders had great and even compression. I very much doubt that the rings are bad, thankfully.

I was under the impression that I can not get to the valves on a cb750 with the engine in frame. The cam cover can't be removed with the engine in the frame at all, I believe.

Does a good compression test with fairly new rings make it pretty likely it's a top end leak?

[matt mattison]

The leak down test will tell you if your leaking air past your valves. And by listening closely, you can usually tell if the air is escaping thru the exhaust , intake or past the rings ( if rings, you will hear it thru the crankcase somewhere.

[MRieck]

The guides, seals and valves are 39 years old. Change them.

[ShermanPain]

I changed my 650's this year, not particularly hard if you dont mind taking the engine apart. good to clean everythign else while its apart too. I was getting smoke at lower rpm and i would notice it at a traffic light. if i gave it some revs it would go away and i was told thats a symptom of valve seals. went away after i changed them. I used a cheapo valve spring compressor from princess auto with a nut on the end for more reach.

[lucky]

The valve guide seals are probably hard (and cracked?) as a rock unless they were replaced.

[markreimer]

You may have thought this thread was dead, but oh no... I was just working up the courage to actually pull out the engine and find out what was what.

Long story short, I found a few interesting things:

- Pulling an engine is a lot easier with four sets of hands than two

- valve seals seem to be in good shape. They seem pretty new actually, so I'm thinking the PO replaced them when he did the rings approx. 3-4 years ago.

- The top-end of the engine (specifically the cam shaft and rockers) all look to be in great shape. It was nice and clean with a thin coating of oil over everything. Well actually to be honest, I haven't the faintest clue what a good looking top end looks like, so maybe I should leave that assessment to the professionals here. Looks ok, no?

- There is a thick layer of carbon buildup on top of the pistons and at the top of the combustion chamber/valves. I don't have any experience with engines, so I have no clue if this is normal or not. Help? My plugs have always been pretty black, but all my tinkering with the carbs to lean out the fuel/air mixture apparently hasn't done much. So far as I'm concerned the bike runs great. The insides don't seem to support that though. How much carbon buildup is typical?

- Based on what you can see here, is it safe to assume I should take the head to a machine shop and have the seats re-cut? Or is the only way to know that by measuring the play in the valves? I wanted to pull all the valves out and measure how much play there was, but my DIY valve compressor wasn't making me happy so I'm going to wait till later this week once I have the real tool.



I'm hoping the responses here can serve as a guide for others doing valve jobs for the very first time. I've searched these forums and found a few good tips, but nothing that definitive. I bought valve lapping paste and a lapping stick/suction cup, as well as new valve seals. Based on what I see, my gut tells me it's time for new guides and seats though?


Summary of the bike:
1974 cb750K, roughly 30K miles. I've owned it for 2.5 years. PO replaced rings right before I bought it. I've ridden about 15-20K KM since owning it. No issues what-so-ever until this past autumn when the plugs starting getting oily while it sat, causing some cylinders to not fire when I started it. Ignition is a Dyna-S, which I installed right when I bought it.

[markreimer]

a few more photos

[markreimer]

And a few more

[markreimer]

Last ones... gotta save smaller files next time.

[markreimer]

Also forgot to attach this. I believe this is cylinder #3. It looks like scratches on the cylinder walls. I did a compression test at the beginning of last season and had great even numbers across all four. As I mentioned before, the PO put new rings in just before I bought the bike. Any ideas what would cause these marks? Is it something to worry about even?

[70CB750]

Carbon deposit is quite normal on running engines.  Did you do leak test on valves?  The head looks good to me, I would lap the valves, test them for leaking and put it back together.

Leak test demonstrated here by our own Rigid:

Honda CB750 cafe SOHC4 valve leak check.AVI

[ekpent]

Don't think we ever heard what your air filter,pipe and jetting combo was on your 750. Looks like it was rich. Did you notice blue oil smoke when running or excessive oil consumption from the tank ? Not that its your problem because it would smoke badly but the orientation of the oil scraper ring is very important when they are replaced.

[Vinhead1957]

Those "scratches" are crosshatch marks and those are pretty when they are there.  Minimum wear after rebuild.  You do have excess carbon though!  Especially on cylinder 1.   I think your carbs are OK but the valve seals are not doing their job.  Did you have oil loss? I had this on a bike and it was using a quart of oil per 1500 miles. The seals may be the problem but guides and stem to guide clearance needs to be checked also. 

[markreimer]

Don't think we ever heard what your air filter,pipe and jetting combo was on your 750. Looks like it was rich. Did you notice blue oil smoke when running or excessive oil consumption from the tank ? Not that its your problem because it would smoke badly but the orientation of the oil scraper ring is very important when they are replaced.

Ah right. I have the stock air box with a K&N air filter in it. The pipes are unbranded so far as I can tell. They are 4-2. They are in rough shape, with some exterior rust and some small pin holes. My intention is to replace them once I have the valves cleaned up, most likely with a cycle-x 4-1.

I'll need to double check my jets, but I'm pretty sure that I have 110 mains, 40 slow, and the needle clip is right in the center.

Those "scratches" are crosshatch marks and those are pretty when they are there.  Minimum wear after rebuild.  You do have excess carbon though!  Especially on cylinder 1.   I think your carbs are OK but the valve seals are not doing their job.  Did you have oil loss? I had this on a bike and it was using a quart of oil per 1500 miles. The seals may be the problem but guides and stem to guide clearance needs to be checked also. 

I was wondering about the seals. I've only inspected one since I've only removed one valve thus far. It didn't seem to be super hard or old, but I'll look again. Either way I have replacements that will be going in.

I had some oil loss, which was the root cause of my problems here. Oil would drip down into the cylinders when I was parked. For general around-town riding I didn't have significant oil loss. My head gasket and cam cover gaskets were both in poor shape and would weep oil if I was riding a bit harder though, say on the highway with a passenger.

I'll be picking up a valve spring compressor tonight and checking the stem-to-guide clearance. As for the guides themselves, anything I should be looking for?

Oh and last question, am I safe to use a brass brush/wheel to clean off all the carbon? I don't want to polish off anything that shouldn't be..

[Vinhead1957]

Stay away from the gasket surfaces and it will clean up pretty easy. Hondaman said in an earlier post to raise the valve about 6 mm and check side to side on the stem. What is the condition of the guide inside the intake/exhaust.  Mine were very wet and carbony.  Bright side they cleaned up easily.

[markreimer]

Bad news.

I picked up a valve spring compressor today and was able to remove all 8 valves without an issue. Then I started to test how much play the valves had in the guides. I could feel quite a bit of noticeable wiggle room just by hand. The first test of an exhaust valve confirmed - I had .007 of play. Factory spec'd max play is .004, nearly half. All the other valves were similar. The intake valves weren't as bad, but still out of spec. I tested them with by pushing the valves out roughly 5-7mm and used a dial gauge as you can see in the photos. My manual didn't indicate how far to push them out to measure play. Either way, I guess it's time to order some new guides and find a machine shop in town. Anyone know of a good shop here in Winnipeg......?

As for the valve guides themselves, does anyone have recommendations of what guides to buy, or what to avoid? Also I read in the manual that I should order guides that are one size oversized and have the inside honed to the size of the valves, but it doesn't say what one size over actually is... Forgive me, I'm new to this. Would these be suitable?

http://www.davidsilverspares.co.uk/CB750K4-FOUR-1974-USA/part_105795/
http://www.davidsilverspares.co.uk/CB750K4-FOUR-1974-USA/part_105794/

Or the kit from cycle-x?


I also noticed a bit of scarring on one of the camshaft supports. It's only on the one side. It seemed to be covered in oil when I pulled the shaft out, so I'm not sure what to make of it. Any ideas?

[markreimer]

Latest update:

I dropped off the head at a local engine shop here, Competition Engine. It was recommended to me by a local custom bike and hotrod builder, so I feel confident about it. They've measured the valves and they are in good shape. I'll be re-using them. They're also checking the head for flatness and cleaning it all up.

I bought new APE guides and some seals from http://4into1.com/ last night, and Competition will be installing them as soon as they arrive.

So I guess that puts me in a holding pattern until the guides arrive and I get the head back. In the mean time I'll turn my attention to cleaning up the tops of my pistons and the block surface. One question though, it looks like there are two o-rings on the intake-side of the engine, near cylinder 2 and 3. Then it also looks like there are longer rubber seals that go around the studs, but I can't quite figure out what they are. Best guess is there are 8 of these between the block and head: http://4into1.com/ne-brand-cylinder-stud-gasket-honda-cb750/

Which I think are #3 in this diagram: http://www.motosport.com/cruiser/oem-parts/HONDA/1976/CB750/CYLINDER

Is anyone able to weigh in here? I have a new head gasket and ordered 8 of those rubber seals assuming they are for around the studs.

Also, now that the head is off, are there any other things I should address while the engine is out? I mean things that are just a good idea, not in response to a specific problem? I have new 'pucks' for under the camshaft tower, new head and cam cover gaskets, new valve seals and guides, and a new tach seal. Any other things to look out for would be good to know sooner than later! Thanks!

[iron_worker]

There should be 4 o-rings between the cam towers and the head I believe. The oil jets sit in the inside 2 holes (above cylinder 2 and 3) and the o-ring goes around the jets and seals the oil in and forcing it into the cam tower bearing journals. I believe the o-rings on the outside 2 holes just seal the oil into the cam tower and it doesn't go anywhere. I believe the cam towers were made this way so they could be identical parts left and and right.  (Just a thought ... if your outside 2 o-rings were missing then this could be the reason why there is some scarring on your one bearing journal. If the pressure was not held in by this o-ring then there may not have been sufficient pressure by the time the oil reached the final cam bearing.)

The 8 longer cylindrical rubbers are called stud packing. From what I understand they were put in place on the later 750s (K2 and later?) to attempt to stop oil leaking out at the head gasket. However, from what Ken at Cycle X has told me this was also a failed attempt and the cause of the problem was in poor head gasket design. Ken told me that the stud packing is not even required if you run a MLS (multi layered steel) head gasket... so this is what I'm going to try. I also have a k4 btw!

You'll definitely want new cam tower pucks and from what I can gather it's a good idea to use a little bit of sealant around the ege of these as they are installed.

Your head looks to me like your valve seals were not doing their job ... even if they are soft and new the probably wouldn't work effectively if your guides have excess play in them. Getting your guides replaced and the valve play back in spec should help a lot although for the cost of them I would definitely replace the valve seals anyway.

Good luck with it!

IW

[markreimer]

There should be 4 o-rings between the cam towers and the head I believe. The oil jets sit in the inside 2 holes (above cylinder 2 and 3) and the o-ring goes around the jets and seals the oil in and forcing it into the cam tower bearing journals. I believe the o-rings on the outside 2 holes just seal the oil into the cam tower and it doesn't go anywhere. I believe the cam towers were made this way so they could be identical parts left and and right.  (Just a thought ... if your outside 2 o-rings were missing then this could be the reason why there is some scarring on your one bearing journal. If the pressure was not held in by this o-ring then there may not have been sufficient pressure by the time the oil reached the final cam bearing.)

The 8 longer cylindrical rubbers are called stud packing. From what I understand they were put in place on the later 750s (K2 and later?) to attempt to stop oil leaking out at the head gasket. However, from what Ken at Cycle X has told me this was also a failed attempt and the cause of the problem was in poor head gasket design. Ken told me that the stud packing is not even required if you run a MLS (multi layered steel) head gasket... so this is what I'm going to try. I also have a k4 btw!

You'll definitely want new cam tower pucks and from what I can gather it's a good idea to use a little bit of sealant around the ege of these as they are installed.

Your head looks to me like your valve seals were not doing their job ... even if they are soft and new the probably wouldn't work effectively if your guides have excess play in them. Getting your guides replaced and the valve play back in spec should help a lot although for the cost of them I would definitely replace the valve seals anyway.

Good luck with it!

IW

Are you sure there are 4 o-rings under the towers? I only had two, above cylinder 2 and 3 as you described. On the outside two holes, there were not o-rings. When I look at the parts diagram here:

http://www.cmsnl.com/cb750-four-k4-usa_model482/partslist/E++04.html#row91318ME5003

I only see o-rings on the inner cylinders. I could add o-rings to the outer one easily enough if that would help..

[Irukandji]

you work fast mark, you're way ahead of me in being a mech. But seeing as you have the engine down so far ,the cam chain and tensioners are on your list to check also? Though not related to your primary concern.

[lucky]

The guides, seals and valves are 39 years old. Change them.

I agree. Why mess around.

[markreimer]

Hi guys,

Here's the latest update on the head.

I just received my valve guides from 4into1.com

I bought this set of 8 valves:  http://4into1.com/ape-high-performance-bronze-valve-guides-honda-cb750/

I realized while looking at them that they seem to be the same 8 valves. Aren't the exhaust and intake valves different sizes? Did I just waste a bunch of money buying double the valves I need?? All the listings for this product on all the sites I've visited say that this package is good to redo all the valves in the head. I'm confused!

[jneuf]

The diameter of the valve heads will be different, intake is 32 mm I think, and exhaust is 28 mm (stock sizes). That being said, the diameter of the valve stems is exactly the same for the intake and exhaust valves.

I've also heard nothing but good things about Competition Engine!