750 TSCC Rebuild or Replace?

[Rich82GS750TZ]

I figured I’d start a new thread as this has nothing to do with the clutch issue that I successfully resolved, or so I thought.

I have spoken too soon. Was riding for about 1-1/2 hours yesterday. Twice I noticed my throttle seemed to hang. So I pulled out from a stop sign to head home. Went to shift into second, didn’t get it all the way past neutral, I think and the RPMs shot up way past redline, heard an awful rattle from the engine, I hit the run switch to off and pulled into a parking lot. Started the bike, awful rattle ensued. Limped for about 1/2 mile the the bike wouldn’t stay running.

Called a buddy with a trailer. Good thing he wasn’t working, I guess. Got it home and immediately pull the clutch apart. Everything seemed fine there. I thought for sure I screwed up something there. Nope.

So I thought I’d check the throttle. Sure enough, the nifty little piece I made from a wire hanger, that my bike has lived without for 15 years, was pushing the throttle cable too far between carbs 1&2 so that it caused the throttle linkage to hang up.

I am so afraid that I’m going to have to pull the head and inspect. I think I just killed my engine.

I have no idea what to do next. Looking for educated guidance and suggestions.

Before I pulled the clutch cover, when I tried to start it, engine cranked but sounded gutless.

To say my confidence in my mechanic skills is shaken would be an understatement. Should have thought to check proper throttle cable operation before heading out.

FML

On a somewhat positive note, I’ve been informed that I’m going back to work on Monday.

I think the first thing I do today will be to remove exhaust and oil pan and see what falls out. Will post pics.

I feel like someone shot my dog. And worse, that I gave that person the gun and the bullet.

[dorkburger]

Ugh, sorry to hear.
Good news is that it didn't cause an accident, and that you were able to make this post.
Sounds pretty bad, but as you are planning to do- assesss the situation and go from there. I think yours is the plain bearing (non roller) engine and isn't known to be as robust as the roller engines. If the engine turns out to be junk, I think the 750 oil pump gears have some resale value (if they are ok) for people building hi performance engines.

As far as your questioning your mechanical skills - we've all been there. Learn- sometimes the hard way- and carry on.

[storm 64]

I have a complete 82 750 engine if you need it. Or just the head if that is all you need.. Don't worry to much, we'll get you fixed up. 👍

[Big T]

Before you pull the head, inspect the cam timing,
the chain may have jumped a tooth or two
Also, check compression and see if it's still good (after you check the cam timing)

[Carter Turk]

Not likely the reason for the rattly sound, but maybe on the over rev, blew out an exhaust gasket or two. Or the exhaust manifold bolt/studs were loose.

[Rich82GS750TZ]

Thanks Big T, I’ll do that.
I’ve checked compression w/finger over the plug holes and all 4 suck and blow when I manually advance the crank. Don’t have a gauge but can go buy one if necessary.

Likewise, 4 straws resting on piston tops move up and down when I manually advance the crank. 1 with 4, 2 with 3. Seems normal.

I’ve pulled the oil pan and with the exception of a bit of crud, a strange plastic washer, bits of old gasket, and the tiniest sliver of some kind of metal, I don’t see anything near as horrifying as I was expecting. Oil pickup screen looks clean. Those screws holding the screen in place are well stuck and I’m not gonna take try any harder to take them out. I can’t see anything obviously damaged from this view.

Does removing the big nut in the middle do anything for me?

Next step, remove valve cover and check timing?

noob question: could over-revving the engine cause the timing to jump, with no other damage? I couldn’t be that lucky. Too soon to tell, I guess.

[GSXR7ED]

Throttle Issues

Glad you are safe and no accident happened...as Glen said.

I had a similar situation happen to me when I was out in NM. I shifted gears and when I would disengage the clutch, the engine revs went through the roof! Somehow, keeping the bike in gear would shoot the RPMs through the roof.

I didn't get a "rattle" sound you you did but I was able to turn the ignition off and roll to the shoulder in neutral.

Once I got the bike back to my apartment, I took apart the throttle side grip to investigate. I'm looking for the thread with my notes but I can't seem to find it.

In any case, can you be more descriptive on the sound your bike made? Did it sound like something internally broke?

Meanwhile, I'll have a look for my post about that situation.

Ed

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[Rich82GS750TZ]

It did sound sort of like a chain rattle, maybe. Honestly, I had just turned on music to my helmet. (Yes-Union, not that it matters). But the fact that I did have it running, poorly, for about half a mile, leads me to believe that all may not be lost. I’ll know more when I get the valve cover off. Got me thinking that the little bits of black plastic found in the pan are pieces of cam chain guard. Be back shortly.

[Rich82GS750TZ]

Update
Warning: the following pictures are not for the faint of heart; extreme violence. (I’m trying to have a sense of humor about this)

Removed valve cover and found that the chain had indeed chewed up the guide inside the cover




Found my rattle. Intake side valve adjusters, all 4, were completely sheered off where they connect to their shaft (red). One of the adjuster nuts on #3 exhaust was gone from the adjuster screw (yellow). It was there on Monday when I checked clearances, I swear.


With timing mark at TDC......


.......this is the position of the cam shafts. Something incredibly violent apparently happened here.


Years ago I bought a cheap, rough looking spare head on eBay because it had all exhaust bolt holes intact. I’ve since cleaned all mine up so never used it. It has all adjusters intact but I’m sure could use new seals, springs, and valves lapped.


At this point I’m not sure if I want to keep going or take Norm up on his spare engine offer.

[GSXR7ED]

Ugh!

You wasted no time investigating...that’s wild!

Calling Mr Norm...using reverse polarity! (Rush song reference)

Ed

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[Redman]

Wow.

You did get in there quick.

I can lend no analysis of that valve adjust followers (if that what they are called) catastrophic failure. All four intake . . . at once.
Maybe some pieces-part-shards getting thrashed around can account for the cam chain guide getting chewed up.

AM not too familiar with that valve adjust mechanism..... did it/they come unbolted... or did it break...?
Either way, little parts-shards went through engine...?

Ah... never mind. You can see I am of little help.
You dont need to explain to me.

[Rich82GS750TZ]

Dave
There are 8 Rocker arms #9 on the fiche, 4I, 4E, that ride on 4 rocker arm shafts. They appear to made of cast? Steel?


Yes, all 4 intake rocker arms broken. This shows that the cast metal piece broke. I propped it up on the head wall.


This is what one looks like when not broken and mounted on its rocker arm shaft. Red line shows where mine broke.

[zuluwiz]

Wow! I've never heard of those parts failing before. And all at once on the intake side only! Very strange. Not a mechanic, but this engine is a strong candidate for a total tear down. I'd keep disassembling until I was sure I'd found all the parts, and all the damage. I'd be looking for a replacement engine. After installing that, that would be the time to rebuild this one. (Just my opinion, FWIW.)

And just as an addendum, that looks like fatigued steel, not cast iron. No way would I trust cast iron for an job like that.

[Redman]

.
.
.
Yes, all 4 intake rocker arms broken. This shows that the cast metal piece broke. I propped it up on the head wall.

........

Thanks Rich,

Have you ever noticed that SOME people require to have things explained to them more than others. haa haaa

After a while I did notice that fracture in your PicOfWeek photo, that follower-RockerArm broken.

I am not a mechanical engineer, not a metallurgist, but I worked with one for several years who was investigating mechanical failures. Notice how a portion of that fracture surface along the one edge is smooth. I would suggest that portion was cracked first, and things were still in operation, and the remainder of the piece was flexing and then those mating surfaces in the area of the crack were wearing smooth against each other, and then later the remaining area let loose all at once.
THat probably doesnt help you much, but just something I noticed.

....
..................Went to shift into second, didn’t get it all the way past neutral, I think and the RPMs shot up way past redline, heard an awful rattle from the engine, .. ...

,

Might that be the awfull "cam slapping" , "valve float" ?
Where rpm is so high that the valves are not following the cams, cam pushes valve down so fast that the valve doesnt return and the next time the cam comes around the valve is still down some, so in case of this cam rocker system, the rocker is floating around, and gets slapped hard on next time around . . . . .
I dont know, I am asking just for this conversation

I have experienced that only a couple times, on shim-bucket engine. Maybe is worse on this follower-rocker system used in these 4 valve per cyclinder engines.

.
Monday when I checked clearances, . .
.

.....

[TeamDar]

Thanks Rich,

Have you ever noticed that SOME people require to have things explained to them more than others. haa haaa

After a while I did notice that fracture in your PicOfWeek photo, that follower-RockerArm broken.

I am not a mechanical engineer, not a metallurgist, but I worked with one for several years who was investigating mechanical failures. Notice how a portion of that fracture surface along the one edge is smooth. I would suggest that portion was cracked first, and things were still in operation, and the remainder of the piece was flexing and then those mating surfaces in the area of the crack were wearing smooth against each other, and then later the remaining area let loose all at once.
THat probably doesnt help you much, but just something I noticed.

That is what case hardened steel looks like when fractured. The inner part looks more grainy that the outer .060” hardened surface.