Topic: Blew up my 750 today

[American Locomotive]

It had always made me nervous that my 750 had no rev limiter, especially since my transmission could be a little finnicky if you didn't move the shifter with authority. Well it bit me today merging onto a state highway. I lazily shifted into 2nd, got on the throttle and about a second later it popped out of gear and the tach went screaming past 11,000 RPM. When I got it back in the gear, the engine had a loud tapping that varies with engine speed.

The good news is the engine still runs on all 4 cylinders, and cranks evenly with no obvious signs of a cylinder being limp. The tapping seems to coming from under the valvecover. I was thinking it might be a bent valve, but I don't think it'd idle smoothly and evenly with a bent valve. Maybe a damaged rocker or cam?

Any ideas or suggestions before I start tearing into it?

[seanbarney41]

bet all that happened is a loose valve adjuster, so re-adjust your valve clearances and ride on

I have missed shifts like that literally hundreds of times.  Only once has an adjuster come loose...re-set valve clearances and all was well.

[robvangulik]

Just hope the adjuster nut is still in its place, or you'll be fishing with a magnet in the dark....

[madmtnmotors]

I've had an adjuster loosen at high rpm as well. Makes a hellish racket until you get the adjuster cap off and get the adjuster set back where it belongs.

[scottly]

When it happened to me, the adjuster screw hit the underside of the aftermarket tappet cover, and snapped off above the rocker. I had to remove the cam cover to find it. It was sitting in a drain hole, with the nut still on it, and it took a while to find it since it looked at first glance like it belonged there.

[American Locomotive]

Alright guys, you got me feeling a little better. Hoping for the best when I tear into it this weekend.

[Bailgang]

When I took my 77 F2 to the drag strip for the very first time I blew a shift so bad it floated the valves and making all sorts of tapping sounds once it calmed down. I had 2 adjuster come loose and a adjuster nut came off as well. Fortunately the nut hadn't dropped down into the engine so it was easy to retrieve and having a frame kit from our very own "754" made it possible for me to remove the valve cover and sort everything out (reset valve lash etc..) without having to pull the engine so I was back in action in no time. Yeah I got real lucky being the F2's have such a notorious reputation for dropping valves.

[my name is nobody]

Had an adjuster back off once when I was a kid and owned a k3. Like everybody else, it was from a missed gear during a street
race. Took it to the dealer, and the tech was able to eliminate the noise with a valve adjustment. Hope yours is as simple

[PeWe]

Happened me plenty of years ago.
Nut came loose from adjuster screw. I found it inside when open the tappet cover, it was just beside in a cavity covered with oil. Could fish it out of it without lifting cover nor the entire engine.

[TwoTired]

With all these testimonials, I feel I must add mine...

I never had a Honda tappet adjuster come loose on a tappet I adjusted.  (I crank 'em down really tight.)

Not even with an 'unintentional' torture test.  You see, my 74 CB550 came to me with smashed gauges from a wreck.  The bone yard sold me 76 Cb750 Gauges for replacement, without telling me the tach ratio was wrong for it.  Before figuring that out, I was quite annoyed that I could never achieve book red line from the engine.  One day, while being particularly annoyed (and rather stupid) I held the throttle open, with engine unloaded, until it would rev no higher and held it there for 10 seconds-ish.  The tach indication never neared redline, of course.
I don't exactly know what "floated" to prevent destruction, the valves or the points.  But, the engine behaved the same both before and after the ordeal.

I still have that small dent in my forehead from smacking it when I later understood my error, though. 
Tough motors, these.

Cheers,

[PeWe]

I bought a good looking tacho for my CB750. From a late CB500 said to be same except for red line at 9500 which should fit my tuned engine perfectly. I understood not when it idled a lot according the tacho.

[American Locomotive]

Welp you guys called it. Nut backed off and lash adjuster came out. Unfortunately it's pretty mangled. Does anyome know what years work with my 76 750F? I see there eems to be a difference from k0-k6 adjusters and K7 adjusters. What about the F motors?

[754]

Ness go me there is a difference .  Shoukd be easy to find one..
 Where are you ?

[American Locomotive]

I'm in Rhode Island USA.

[seanbarney41]

did you find the nut though?

[American Locomotive]

Yes, probed around blindly with my magnet and found it.

[goldarrow]

Did you find the other half of that tip of adjuster?

[seanbarney41]

Did you find the other half of that tip of adjuster?

looks like the tip is just screwed up into the rocker, hopefully rocker did not get bent or threads damaged

[seanbarney41]

also, guys following this thread...I don't know what the torque spec for the locknut is, but don't be timid about snugging these up guten teit (german engineering torque spec lol).  They are very well built and you will not have problems then, even with missed shifts and over revs.

[American Locomotive]

Did you find the other half of that tip of adjuster?

The tip that contacts the valve is all is there, but one side of the "flathead" portion you put your screwdriver in was broken off since I've had the bike. I assume the previous owner had broken it off.

[HondaMan]

I'd stick another one in, and not miss a day of this fine summer to ride! Especially if it runs fine, nothing serious enough to worry immediately happened. I often see these engines with valve 'smiles' tapped into the pistons, most often when the 2nd gear dogs have been worn, which causes them to half-shift unless you're real deliberate. Then they can pop back out, like yours...someday when it's all apart, send the C2-C5 gears to Fast By Gast in Buffalo, and he will fix them up better than new!

I once tore down a K2 that had this issue (and 70k miles on it), and the owner told me he had not used 2nd gear in almost 15 years because it would not stay there. He just shifted from 1 to 3, to 1, whenever he stopped, didn't bother him a bit! He did ask me to fix it while I was in there, though...

[American Locomotive]

That's the first time it's ever happened. I've gotten false neutrals between other gears, but that's the first time it's ever popped out. Hopefully it was just a fluke. I'll probably change the oil and see if that improves the shifting situation. Certainly can't hurt.

[PeWe]

4:th gear can also enter wrong, not completely in. You'll feel it when happen, gear to 5:th and gear down just for sure. Easy that the foot will kick out the gear during heavy acceleration too.
Undercut gears (2-4) an improvement, though.

[HondaMan]

That's the first time it's ever happened. I've gotten false neutrals between other gears, but that's the first time it's ever popped out. Hopefully it was just a fluke. I'll probably change the oil and see if that improves the shifting situation. Certainly can't hurt.

Here's a hint, an a clue (no, I don't want to start an oil thread...) :
if the gears don't shift well when the engine is hot, there is not enough zinc in the oil. These engines were made when we had almost 400% more zinc in our oils, now outlawed because it harms catalytic convertors in cars. Our bikes don't have them...so, you need oil that contains more zinc. This is almost universal, unless you are buying oil in off-road equipment shops.

The better oils today for our bikes are the Golden Spectro synthetics, the Bel-Ray EXL Mineral type, and (yes, it's true) AMSOIL in the 20w50 non-Harley 'motorcycle' oil they offer. All of these should be in 20w50 weight for the CB750, unless you ride in cold weather (then 20w40 is OK). Don't use 10w40 oil in this engine, as it will cause rapid wear and stiff shifting, every time.

You can also get ZDDP additive (zinc) in little bottles of 2 to 4 ounces. If you have ordinary oil in it (like Valvoline or Shell Rotella), and it is at least 20w40 weight, you can add this stuff to "save" this oil change. I use about 1/3 bottle with the BelRay EXL Mineral, and it makes it shift like the old Castrol XLR 20w50 oil, which was made specifically for this engine, by Castrol, beginning in 1969. Alas, it disappeared in 2002, though...

[American Locomotive]

I have not changed the oil since I've had the bike (which is only been ~700 miles or so), but I did add some JASO-MA rated 20w-50 I picked up from the parts store. I usually ran Rotella 15-40 in my CRF450, and liked the way it shifted and how the clutch felt, but that's a liquid cooled motor, not a hot running air cooled one like the 750. So I elected to use the Valvoline JASO-MA 20w-50.

I'll see about getting some ZDDP additive.

Also, just out of curiosity: Is it normal for the shifter to still move when you each the end of the shift range? By that I mean, when I'm in 5th gear, I can still move the shifter upwards, but it has no resistance and doesn't do anything. It will only do something if I push it down to downshift. In 1st gear, I can still push it down, but it has no resistance and doesn't do anything. It's different behavior than my CRF or old XR200, where the shifter stops moving once you hit the end of travel. When I hit 5th gear on my CRF, the shifter will not move up - only down.