Topic: Loud tapping sound that stays with RPM

[tonymasone]

Just this last Sunday I was hauling pretty fast on my 74' cb750 accelerating hard and doing high rpms and all of a sudden this loud tapping sound started that was synced up with RPM both in and out of gear with or without the clutch depressed. The bike didn't have any change in power at all or act funny in any way other than the noise. I immediately went home and parked the bike. The sound has not gone away but one thing I noticed is my choke is now 2x easier to pull (a lot less friction than usual) and doesn't seem like its coming out the whole way. It seems to still work considering the bike will not start cold without it being pulled. I don't know if the two are a complete coincidence but it definitely happened at the same time.

Thanks ahead to anyone who can help!

[jahmic]

FWIW, I'd say that it's a coincidence between your choke pull and the tapping sound since your choke cable just connects to an actuating arm on the carbs to close the butterflies...and it shouldn't be related to any tapping noises that synch with the engine speed.

I suppose if something came loose in the carb throat due to a bent arm or damaged butterfly, then it could produce some noise and cause some issues with the choke pull as you've noticed...but it seems very unlikely that the noise would synch up with the engine speed so well.

My suggestion would be to remove your airbox, pull the choke, and watch to make sure all of the butterflies move in unison, and nothing is binding the arm or the cable.  If that checks out, time to look elsewhere.

Where is the sound coming from?  Pinpointing the location will be a HUGE help in diagnosing the problem

[Grabcon]

Have you pulled the tappet covers and inspected the tappet screws. In my youth which was years ago I had seen several times where a tappet screw broke for some reason and then of course loosen up which created a loud tapping noise. This always happened when the rider was cranking hard on the bike. The power really didn't seem affected till the tappet screw had back out most or all of the way. When the power started to drop the motor the exhaust sound on the cylinder where the screw broke would get I guess what I would call a hollow sound. This was a sure sign that the screw broke. Hard to tell on 4 to 1 and 4 to 2 exhaust.

Now with that said I had only seen this happen when custom tappet covers were put on the motor. I seems like there was slightly less clearance and the tappet would hit the inside of the cover and break off.

Don't know if this will help but just a thought.

If you did break a tappet screw that in itself will present a whole other set of issues finding the pieces. They fit very nicely down the oil drain holes in the head.

Best of luck, I hope this is not the issue.

[tlbranth]

I'm with Grabcon. Probably a tappet.

[smpls70]

Yup , my 750 made the same noise , took several try's to get the adjument right, but finally went away. I'd check and see if those valves are in spec .

[Grabcon]

As I pondered a little more two other things came to mind. Check for loose spark plugs and exhaust flanges.

Also don't forget the big screw drive to the ear and probe around the motor listening for the loud tap.

[cameron]

Tappets. Thats how they got that name

[tonymasone]

Thank you everyone for the responses, I will begin inspecting tonight. Which side should I be inspecting? Intake side or exhaust side? (or both) What sort of adjustments can I immediately make?

Thanks,

[lrutt]

both sides, .002 intake and .003 exhaust. Just give them a run through. Can't hurt. should be done in half an hour tops.

[tonymasone]

Hey all,

As usual I can rely on this forum as a great source of information, the very first tappet cover I pulled off (shifter side 1st cylinder intake side) and the tappet nut is completely gone... Judging from the 0 change in performance it was probably backing its way out for a long time and has me a little excited in a way because I haven't been able to get the power out of the bike that it should have and this valve adjustment might be the final step.

Question is now where did that nut go??

Last question, where can I get a replacement nut? Is there anywhere I might have a chance to get one local?

Thanks again,

[Grabcon]

Glad to hear that you found the issue. Where did the nut go? Under your valve cover. If you can pull the cover without pulling the motor great, maybe you can find it. Personally I would make the effort to find if even if it means I need to pull the motor.

Where do you find parts. Any local cycle salvage yard would be a good starting point.

Best of luck.

[CB750F2]

That nut will be sitting under the valve cover somewhere - more than likely not far from No1 valve inspection covers. Try and locate it with a torch and a small inspection mirror. If you fail to do so tilt the bike and try again. You could also try removing the valve cover bolts and lifting the valve cover. You will not be able to remove the cover but you may be able to lift the cover enough to help locate and remove the nut. You must find the nut. Pat

[cameron]

Find that nut.
Pat said it. You need to find it..

And, gently, valve adjustment should have been your FIRST step in your quest for more power.. not your final step

[Duanob]

Try and locate it with a torch and a small inspection mirror.

In America we call that a flashlight. A torch could end up kinda tragic of the fuel tap is left on.

[jacquesleclochard]

Been there. Fiddled around with a magnet for hours. Finally, I had to pull the engine.
Pull the engine. Or lift the cilinder head up so slight that you can put that magnetic tool in between.
You're looking for this little flexible magnet, good luck:

[Bankerdanny]

Try and locate it with a torch and a small inspection mirror.

In America we call that a flashlight. A torch could end up kinda tragic of the fuel tap is left on.

I was going to say the same thing. LOL