Seized Engine

[Kahoy]

Hello everyone! I'm a new member on this site and this is my first post and plead for help.

My project is a Honda 79CB750K that was in storage since mid-80's. Very low mileage bike and I'm also the original owner. Due to improper storage and harsh storage condition the engine on the bike seized. To free the seized engine, I took all plugs out and pour penetrating oil in the cylinders and let it do its job for a couple of days. I also stored it in a heated garage. Then I put the bike on 5th gear and start rocking the bike back and forth until I can see all the pistons move up and down. That took couple of hours. Almost time for celebration, but noticed a problem I could not figure out. When the piston start moving easy, I took out cover for the generator to access the crankshaft. So with a 17mm box wrench I continue to turn the engine. To my surprise, I could only turn the crankshaft approximately 1-1/2 revolution and it would stop. I could turn it the other direction with the same outcome. I'm don't want to apply too much pressure since I'm not sure what causing the problem. When this bike was put on storage it was running and nobody touched the bike since then.

From what I saw it appears that pistons for cylinder 2 and 3 may not be reaching the TDC. I need to measure and perform accurate measurement. My statement was based on using a screw driver sitting on top of the piston. i just though I share that clue.

I'm looking to see if anybody had similar experience and perhaps could point or suggest to me the things to look for to further investigate the cause for the blockage (for the lack of better word) and solve my problem. I'm tempted to open the valve cover and cylinder head but not sure what to look for. This is my first venture with this engine. I currently ride a bike with a boxer engine.

I would appreciate any thoughts you guys/gals may have. Thank you in advance - Nino

 

[amc49]

Engine must turn at least two full turns at crankshaft to be considered loose and free. Obvious something is not there. What was the 'harsh storage condition', that is not a heated garage. If you can't get it loose it has to come apart to find out why. I personally never try to run a stuck engine anyway, stuck enough to not turn it means automatically it comes apart. Too much stuff tears up from the ragged stuck condition being forced to loosen by itself and running only. Stuck = destroyed where I come from and waste of time working on it. Most of the time anyway.

Turn clockwise ONLY at alt on right side, turning backwards and cam eating slack in chain rather than positively moving can bend valves as the timing changes. Why one generally never turns engine backward except for a specific reason. Could be bike stored when it badly needed a chain adjusted.

If engine in any way got water inside it then pretty much kiss it off.

 

[Kahoy]

AMC49 - Thanks for your thoughts and quick reply. The harsh storage condition was a backyard shed exposed to weather extremes of the NE. I don't believe water got inside the bike, but not entirely sure.

Today I have raised piston 2 and 3 to the highest point it can go and poured more penetrating oil in both cylinders. I plan to continue rocking the bike in a couple of days in hope to unstuck the engine further if I don't hear additional thoughts and suggestion from other members. This is my last ditch effort before removing valve cover to investigate what is causing the problem. I also need to find a shop manual, in addition to my Clymer manual, before I start tearing down the engine. Wish me luck and thanks again!

 

[amc49]

http://www.cb750c.com/modules.php?name=Your_Account&redirect=viewtopic&t=301

Go there, sign up for free and the OEM service manuals are in the technical documents section for the taking.

 

[Dave]

One of the valves is stuck. The crankshaft completes 2 revolutions while the camshaft completes 1.

STOP ROCKING it! You may have bent a valve already. Pop the valve cover and look for a valve with excessive clearance. If you find one try to move it. If it moves but won't come all the way up, it's probably bent and will have to be replaced. If it's frozen, free it (you can tap on the top of the valve with a brass punch and small hammer).

WARNING On top of each typical valve is a cylindrical valve cap. Inside the top of each cap is a shim. These shims are all different thicknesses and are how the valve clearances are adjusted. DO NOT mix them up. Prep a numbered shim holder before removing any of them. An empty card board egg carton works good.

You can download manuals here http://www.cb750.com/threads/2505-69-78-and-79-83-Service-Manuals?p=12718&viewfull=1#post12718

 

[Kahoy]

Thank you AMC49 and Dave for the link and suggestions. The manual will surely help me. I did found paper copy of the shop manual on eBay, but its quite pricey. I'll save my bucks for parts that I will surely need.

Dave I also suspect that I may have a frozen valve(s), but mine was a guess since I have no experience on this type of engine. I'm keeping my fingers crossed that I have not bent any valves at this point.

Question: Once the engine is turning and the valve cover is off, should I crank the engine using the starter to verify that lubrication is reaching the top end? I plan to measure the oil pressure before I start the engine.

 

[Dave]

Question: Once the engine is turning and the valve cover is off, should I crank the engine using the starter to verify that lubrication is reaching the top end? I plan to measure the oil pressure before I start the engine.

I think you would need to use the starter to get the oil moving enough to see it.

I've never actually thought of doing this as oil passages very rarely clog.

 

[amc49]

DON'T crank that engine until it is 100% freed up, bent valve and you'll find clearance like said or not bent but merely stuck in place and that tight says rusted solid, a cylinder has gotten water in it. Engine may be worthless, seen it plenty. The bad storage conditions thing says it all.

Luck..............

 

[Kahoy]

My valve tool (Part#07964-4220001) finally arrived and work will resume after the W-UConn basketball game this morning. Here's what I'm planning to do to loosen the stuck valve. Once I located the problem valve, I'll remove the valve shim to exposed the top (or tail near the keeper end) of the valve. My Clymer manual (page 95) does not clearly shows if the bucket is a cylindrical hollow part that is open on both ends. I hope that is case so I could safely tap directly on the valve and not on the bucket. Thanks guys!

 

[Kahoy]

I did confirmed the intake valves on cylinder #4 are frozen. When the crankshaft is turned, it is visible that the shim and bucket do not move up and follow the cam's movement. In fact, it is visible to see a large gap between the cam lobes and shim. (I took picture but don't know how to upload it.) The shims and bucket can be spun/move but I'm not able to move it up or down. Tried tapping it over the bucket area but no movement. I soaked the two valves with penetrating oil to loosen, but if this approach does not work, which is likely, I would need to remove the intake cam to reach down and have access to the valves.

Question:This may be dumb question! Instead of removing the exhaust cam, would I be able to have access to the valves from the carburetor intake to reach the top of the exhaust valves? I'm thinking that if this is possible, and not a dumb idea, I can spray penetrating oil in the area and push/turn the valve up or down to hopefully loosen them. What do you guys say - really dumb idea? Thanks.

 

[amc49]

Highly likely that once valves stuck open to drag behind actual crankshaft turning action you have bent the valves to have them never close to run correctly again.

Going through carb intake does nothing to help, you are then below the stick point if engine still in bike, it needs flipping upside down to do what you propose.

Push the valves down and you bend them more against piston top or the exhaust pair (they too collide if both open at the wrong time), can then bend the exhausts too. All in knowing exactly where that crankshaft is at all times meaning you have to have engine experience to know that. You can't just keep working engine back and forth once the valves have stuck on their own, more damage after that.

In my garage head coming off now, I actually would have sooner. Engine with that much sticking in it is already seriously corrosion damaged in some way, never saw one that wasn't. They don't just stick tight from lack of movement.

 

[Kahoy]

amc49 - Thanks for the feedback. I have noted your points and concerns and I truly appreciate your continued support sharing your knowledge.

After flooding the #4 intake valves for a day and saw some of penetrating oil go down I tap the two valve lifters. At first it felt very solid. After few minutes of tapping and pouring oil over the valve both valves started to move down a bit. At that point I stop and poured more penetrating oil over the intake valves and again will let it soak for a while.

Regarding the concern on piston position, I did verify that the piston is not on TDC with a screw driver inserted on spark plug hole before I started tapping.

Another thought came to mind to provide lubrication to the intake valves is to pour oil on cylinder #4 to lubricate the valve stem. Would that help loosen the valve from the other side, or is that a bad idea? Thanks again.

 

[750Nooby]

My son owns a 1973 CB750 mostly all original, a few minor addons. He bought the MC from a friend of mine 4 years ago knowing their were issues. I now have the bike and would like to finish the project for him. I am recently retired and have a small workshop to do the work. I joined the forum today to learn what I can about the Honda. I am a Yamaha guy so it is another new adventure.
The engine seized for no apparent reason known to us. I am willing to try to get this engine running. This is not my only project so time is no issue.
Beyond what I read here are their other things to look for.