Topic: Motor locked up today. What's next?

[Yamanube]

Finally got my '75 cb750 on the road today, PO said it ran great last year and it ran fine today, for about 15 minutes. Pulled into my parents driveway and it sputtered and stalled. Tried to kick it over and it wouldn't turn. I don't really have the time or money to do a rebuild. It had oil but I think it may have overheated?

[Gordon]

Let's get the obvious out of the way first:


You had the transmission in neutral or the clutch lever was pulled in when attempting to kick-start it? 

Will it shift through the gears?

Will the rear wheel spin with the transmission in neutral or clutch lever pulled in?

[Yamanube]

Yes, yes and yes.

[HondaMan]

Fuel. Pull off the float bowls and see if the level is low or empty. If the bike's been sitting much, the filter inside the petcock may be clogged with gunk.

Fuel lines: they are often replaced and misrouted on that bike. They should come out of the petcock, then run horizontally, through the bracket holes on the carb mount, then down into the carb tees. I see lots of them that are routed in big loops down and back up to the carbs: they will run OK until the tank is about half empty, then have troubles.

[Yamanube]

The motor is locked up, won't turn over.

[wrenchmuch]

Did it squeal before it stalled ? Did it stop abruptly ?

[Yamanube]

I don't think it made any wierd noises, just sputtered a couple times then stopped. I had the clutch in when it stopped.

[ekpent]

Have you done any engine or carb work to it prior to riding it yet this year? Now that it has sat for awhile and totally cooled down any change?.I think I would pull the plugs,try some lube in the cylinders and snick it into maybe 4th gear or so and rock it or push it to see if I could get the motor to turn again for starters.On the ride before it died was it running quite rough,no gas or extreme lean would cause an overheat but would be evident in its performance.Did you ever peek inside the oil tank when it was running to make sure the oil pump was working or notice that your red oil light was on?  Sorry for your problem,good luck on it.

[fire113]

...that sounds real bad, maybe You are right with overheating or You have a bearing damage at the crankshaft.
It seems that something block the engine. But without opening the engine it is very difficult to analysis.

[Yamanube]

It seemed like it was losing power a bit right before it stopped. I haven't gone out and looked at it now that it has cooled off, pretty upset about it, don't really want to touch it right now. It seemed to be running well prior to. Didn't check to see if the oil pump was working but when I pulled the cap the oil was bubbly on the surface.

[Yamanube]

Update: I went out just a couple minutes ago and put
it in 4th and rocked it back and forth a couple times and it loosened up. Kicked it a couple times and it fired up but made some noise in the valvetrain or topend.

[scottly]

You might want to pull the tappet covers and see what the rockers and adjusters look like...

[Yamanube]

Is there something in particular I should be looking for?

[bistromath]

Usually when a motor seizes it gets harder and harder to run for a little while before it finally locks. Like it takes some throttle to keep it lit, and then finally loses power and dies.

These motors make a lot of valvetrain noise to begin with, so if you aren't used to it, it might seem abnormal.

There are two main causes of seized motors (other than a rusted piston): a seized piston, from overheating or lack of oil; or a seized camshaft, from lack of oil to the top end. Don't know that I've ever seen a crank bearing actually seize.

It's a good sign that the motor freed up for you. First step is to verify that the top end has oil. Take the tappet covers off and look for evidence of fresh oil up there. You can kick the motor over, or even run it for a short time with the tappets off, and look for oil up there.

[fire113]

...try to see if there is someting very different to the other rockers and adjusters.

What kind of noise does the engine make ? all 4 cylindes are working ?

[HondaMan]

The motor is locked up, won't turn over.

Oh, sorry. Read it too fast, missed the "Stuck" part. 

Remove the plugs and spray some WD40 into the cylindrs and let it soak for an hour. Then try to kicj it over. If it frees up, the pistons may be stuck for some reason. If it doesn't, it's probably going to need a teardown to find out what's up.   

[Yamanube]

...try to see if there is someting very different to the other rockers and adjusters.

What kind of noise does the engine make ? all 4 cylindes are working ?

Its a pretty decent clatter, not a hard knock but definitely noisier than normal.

[Yamanube]

Usually when a motor seizes it gets harder and harder to run for a little while before it finally locks. Like it takes some throttle to keep it lit, and then finally loses power and dies.

These motors make a lot of valvetrain noise to begin with, so if you aren't used to it, it might seem abnormal.

There are two main causes of seized motors (other than a rusted piston): a seized piston, from overheating or lack of oil; or a seized camshaft, from lack of oil to the top end. Don't know that I've ever seen a crank bearing actually seize.

It's a good sign that the motor freed up for you. First step is to verify that the top end has oil. Take the tappet covers off and look for evidence of fresh oil up there. You can kick the motor over, or even run it for a short time with the tappets off, and look for oil up there.

That sounds pretty much exactly like what was happening before it stuck. I will try and inspect the valvetrain a bit tomorrow and see if it is getting oil. Is bubbly oil normal? What would be the reason for no oil in the topend? Bad oil pump? Plugged passage or line?

[scottly]

...try to see if there is someting very different to the other rockers and adjusters.

What kind of noise does the engine make ? all 4 cylindes are working ?

Its a pretty decent clatter, not a hard knock but definitely noisier than normal.

Pull the tappet covers! I had a valve adjuster screw break off before. Buggered the inside of the cover before it got spit out. And I would have described the sound as a "decent clatter".

[wrenchmuch]

Unfortunately plugged head passages are a common problem along with lifting cam towers . When the towers lift off the head pressure can be lost to the cam towers . There are cover mods to try and address this .

[ekpent]

As to the bubbly oil in the oil tank that would be normal and means it is moving and hopefully feels hot after fully warmed. Did you do any kind of engine tear down at all where you may have gone a little overboard on silicon sealer or such. Bits are bad for oil passages,can plug them up. Also good gas flow is crucial on keeping an air cooled motor running cool-too lean means to hot.Fingers are crossed that your cam etc are still good.

[Yamanube]

As to the bubbly oil in the oil tank that would be normal and means it is moving and hopefully feels hot after fully warmed. Did you do any kind of engine tear down at all where you may have gone a little overboard on silicon sealer or such. Bits are bad for oil passages,can plug them up. Also good gas flow is crucial on keeping an air cooled motor running cool-too lean means to hot.Fingers are crossed that your cam etc are still good.

The oil tank was warm, I have only owned the bike for a few weeks and this is the first time I have had it on the road (besides the rest drive)

[scottly]

Nothing you have mentioned would make me think of a lubrication problem. Again, remove the rocker covers, and have a look. It IS the easiest thing you can do, at this point.

[Yamanube]

Pulled the motor out and the valve cover off and this is what I found. The valve train on the two right cylinders was bathed in oil and looked nice the two left cylinder had some residual oil pooled in the bottom but the camshaft was bone dry. On the very end the bolt in the camshaft had come about halfway out and was just sitting there and the nut for the bridge that holds the camshaft down had come off and bounced around in there for a while, came to rest right next to the cam (where they are in the pics). (excuse my noob terms I am new to 4 strokes, smokers are my forte). The cam lobes are pretty worn on that side and the journal is pretty hashed along with the surface it rides on in the head.

SO, what does this mean? Does the lack of oil on the cam mean that I didn't have oil going to the valvetrain on that side (even though there is oil puddled around the valve springs? What kind of damage might have been done to the piston(s)/bore? What all do I need to fix this problem, all the "framework" that holds the valve train on those two cylinders, new camshaft and valves?

These pics are pretty much exactly how it looked when I opened it up (besides the one where I pulled the cam holder cap thingy off

[bistromath]

Jeeeeeeeeesus. Well, I suppose in some way it's better than the usual camshaft-splitting-in-half thing.

Good news, and bad news. The good news is, on the 750 I believe you can replace the cam towers and save your head. The bad news is you'll want to pull the head to find out exactly why the right side oil passage wasn't flowing.

The engine is definitely salvageable. If you pull the head, you'll want to pull the cylinders to replace the base gasket as well, since it will likely start leaking if you don't.