Topic: Knocking/Tapping Noise: False Neutral'd into Red Line

[YoungBlood]

Hi everyone,

I could use some help figuring out what may be the cause of this abnormal noise. Thank you in advance, and my apologies for being absent!

Background: Went to shift 2nd to 3rd, bike went into false neutral (nothing new, happens intermittently), revs up to 10k (that doesn't happen often, if ever), loud knocking/tapping noise coming from 4th cylinder (seems to come from top-end, valve area), tapping was synchronized with rpms (higher rpm, greater frequency of taps/knocks), tapping continues at idle.
Immediately stopped running the bike, got it on a trailer, it is sitting.
What I've checked: I pulled off the valve adjustment covers for cylinder 4 and 3, looked around (it was dark) for any noticeable damage, missing nuts, broken valve adjustment screws -- I did not find anything. Specs of the intake valve for cylinder 4 were .002 ish, around .0025 and .002. I could not check the exhaust valve because I can't find my .003 feeler gauge. It was also dark out, had little time to spare, and did not check all valves for clearance specs.

Questions: What should I do first? I am planning on rebuilding the engine this winter, but I would like to keep riding into November.
Do valve adjustment, cam chain adjustment to see if that solves the problem?

Side info that may be in relation to this problem: I lately have been pushing this bike hard now that I solved my hesitation problems. Friends of mine say I will kill the bike pushing it to red line as often as I do. I will push the bike to its maximum speed at times too (110 with lack of compression).
Cylinder 1 has 90psi, cylinder 4 has 110 (last I checked  roughly 1 month ago)
There is a continuing back fire at idle, and on deacceleration (possible a leaking valve seal? possible I have blown the seal finally?)

[BenelliSEI]

Do a compression test. My guess: you’ve bent a valve and lost a cylinder. Don’t ignore a “knocking”. It will not fix itself and only get worse.

[seanbarney41]

ok.  I shift at redline nearly every shift.  I have missed plenty of shifts and over-revved plenty.  The worst that ever happened was valve adjuster locknut came loose, slow 30 mile ride back to some tools, reset valve clearance.  Done.  This is not to say you did not somehow damage your engine.  But I dunno.  Overall, these are some extremely tough old engines and will survive amazing amounts of abuse...until they don't anymore.

[PeWe]

Same has happened to me.
Missed a gear, lots of rpm.
Tapping sound as adj nut came loose.
Opened the tap cover and found the nut in a small cavity beside covered in oil.
Real luck, no need of pulling engine. Easy when it was cyl 1.

A magneto on a flexible arm is useful if not visible beside the adj screw. Something like this or even slimmer.

[ekpent]

Loose tappet adjusting nut hitting the valve cover was my first thought as well. Never happened to me but did a friend of mine while we were out riding. As for compression numbers are you using a quality tool or a Harbor Freight ?

[Jerry Rxman Griffin aka MuthaF'er]

Yes, do a compression test. While your compression is definitely low you may find that #4 has changed due to a bent valve.

Get a bore scope and look for signs of a hit on the piston, etc.

[YoungBlood]

Thanks everyone! It's nice to see a common line of thinking on what may be the cause.

Ek: the compression tool I last used was not mine, but it was the version they sell at AutoZone. Definitely not HF
Benelli: all four exhaust pipes were hot when bike was idling, but I will check the compression first, compare the new readings with the old, and then do an adjustment
PeWe: The noise could be from a lost adj. screw on the #3 intake, or #2 intake, I was unable to pull those caps off in the dark to check. I'll be sure to pull all them, look around, and then do a valve adjustment.
Sean: The bike is about 25 miles from where I live (the city), I can't let it sit at my Ma's place for too long. I might just do a complete valve adjustment and ride it back slow.

Questions: Last time I did a compression check, I did not use my electric start because of the problems I've had with the button blowing to pieces. Is it okay to use the kick start and kick it until it reaches its highest value on the gauge? Last time I measured compression, this is what I did (approx. 10 kicks).

[PeWe]

When nut fall off, the adjuster screw can still be there with a few mm lash. Only nut came loose on my CB750. No other damages.

[ekpent]

Or if that is the problem the nut can still be on the treads and hitting the inspection cap which may leave a little tell tale sign. Won't be hard to see if one is loose when you get in there.

[pekingduck]

I doubt the problem is related to the valve adjuster nut.  It was fine until over-revved, and the adjuster nut didn't get knocked loose.
Likely bent valve

[Don R]

 I tagged a valve once, it was a brand new intake valve activated by an F2 cam on stock K springs and tight clearances. I shimmed the cylinder a bit when I repaired it.  This 836 had an unusually high piston/deck height.

[YoungBlood]

Hi everyone,

Thank you to all your suggestions and insight. The culprit was a missing valve adjustment nut on the number 3 intake valve. I was unable to find it with just the valve adjustment ports taken off. What I ended up doing was removing the tank, loosening all the valve cover's screws, and lifting the cover as much as I could.

Needless to say, I found the nut, redid the valve adjustment. And everything is back to normal.

THANK YOU!

[PeWe]

A happy ending! 
Good idea to verify that all nuts are tightened enough.

Check the lash on all valves at the same time.
I use 0.10mm on all instead of the really tight stock 0.05/0.08mm. Not much needed to get no lash at all.
That on my stock K2.
(More on hotter cams)

The tappet screws/nuts are steel with M6 fine threads so they can be tightened rather hard. Harder than the fragile M6 threads in the alu holding the engine covers.