clunking sound under acceleration

I have just purchased a '75 MK III, having lots of fun replacing, adusting etc as it had only done 80 miles since a restoration 20 years ago.
Anyway lost compression, diagnosed as the head not re torqued, I replaced the gasket and very carefully put the head back on as I had read about damage to the con rods, she started first kick but now has a very annoying "clunk" tractor like sound that you can only hear when you twist the throttle under way, very hard to hear at idle and revving whilst in neutral. Only appears under engine load

I have had mechanics listen with screw drivers etc to pin point the source to no avail, the alternator checks out OK, the valve clearences have been rechecked and are spot on, cam chain looks like it is tensioned, primary tensioner is good, the chain is not strected, the bike runs nicely at cruise but as soon as you open the throttle she sounds like a tractor.
I also had prior to the loss of compression the rear brake jamming up , which meant a couple of times I put the engine under strain before I could get of the road and release hydraulic pressure, I have since put a kit through the master cylinder and re set correctly. Any ideas? has putting the engine under strain caused this annoying noise?

Did it clunk before replacing the head gasket ? If not then it is related to that little operation. Clunking before replacing the headgasket is a different thing. Being harvest season now you could put it to work in a field and make it pull it's weight until a winter teardown - investigation ?

Transmission layshaft bearing OK? Does the kick starter drop on hard acceleration in first?

Glen

You need to isolate the noise to the major assembly - crank case, cylinders, primary drive, or gearbox.

maylar said:

You need to isolate the noise to the major assembly - crank case, cylinders, primary drive, or gearbox.

Click to expand...

+1

As stated earlier, using intuition and logic I would say if the noise was not there before you pulled and replaced the head then it is very likely something done during reinstallation of the head that now what gives you the clunking.

Is there something fretting under the tank? I recall the fuel tank can reach a harmonic and bang against something and make a clunking sound right in rythym with the motor.
Headsteady set up correctly?
Now that you are firing on two cylinders, this indicates new found torque and spectaral frequency - are the iso's in good shape?
Exhaust pipe hitting somewhere?
Were the engine/trans mounts loosened - are they now tight?

Was there considerable crap and corruption on the cylinder head or piston crowns that was not removed and is now causing detonation?

The only thing engine wise that would make a clunking under load is a main bearing or crankshaft failure. I doubt (hope) this is not your case but perhaps with the one cylinder not firing much the main bearing was not hitting hard on that side.

You mentioned replacing the head gasket - same thickness?
Ignition set correctly?

I hope you do not mind the brain dump but my gut feeling is that it is something simple like a component was not put back correctly or is fretting (ex fuel tank to frame).

As others say, need to isolate the source. But easy to do is make sure your tank is not slapping on the frame anywhere. You can tell by just taking it off and you will see paint rubbed off real quick. I know. I found Zoris (flip-flops) a good material for padding the tank to frame.

Dave
69S

Dances with Shrapnel said:

Headsteady set up correctly?

Click to expand...

+1

grantb said:

she started first kick but now has a very annoying "clunk" tractor like sound that you can only hear when you twist the throttle under way, very hard to hear at idle and revving whilst in neutral. Only appears under engine load

Click to expand...

MkIIIs can make some odd clunking noises if the hydraulic primary chain tensioner is not operating properly or is not fully primed. Or the primary chain could be worn out?

What type of oil is in the primary case?

As LAB said, chain. Final drive chain can make funny noises too, tighten it as a test. You dead sure we're NOT hearing spark knock, right?

Thanks for all the replies.
I did notice a noise prior to the loss of compression, but not quite as bad.

Lots of ideas there for me to try and eliminate, I accidently left out one of the tank isolating rubber squares, now back in, however I will run it without the tank to see.
The hydraulic tensioner looked good when I checked the stator, it was extended and taking up the slack, how do you prime it? I run 300ml of SAE 20, the only SAE 20 I could get is heavy duty fork oil, should I go back to engine oil 20W-50?


The head steady prior to removal was not tensioned at all, I have since tensioned it only to 1.25", book says 1.45 but I am nearly out of adustment, is the spring supposed to sit between the 2 ridges? the bike has the usual low rpm vibration but smoothes out nicely above 2000. Isolastices appear to be good.
I found lots of engine too mounting plates bolts loose and 2 nuts missing, I have now been over them and all tighten up. Exhaust re tightened with a C spanner after first run, nice and tight.
The piston and head had very little carbon build up, it does not sound like detonation but I am new to british bikes. The local expert here in Perth has had a listen and his comments were "It does not sound like bearing failure top or bottom."
I will check the to see if the kick start drops,
You are right I also have to isolate where the sound is coming from, I have a set of rollers and with some mates will try that, as it comes on under load in gear, something is taking up tension and banging away, very fustrating, but then it is a brit bike, they do keep you busy, a very steep learning curve.they are not meant to be easy like my jap and italian bikes, but then it rides the best!!!
I have a new HDpro camera, if all fails I will make a video over the week end.
Thanks again for taking the time to respond.

One more thing....

grantb said:

The hydraulic tensioner looked good when I checked the stator, it was extended and taking up the slack, how do you prime it?

Click to expand...

The plunger springs take up the initial chain slack, however correct chain tension also relies on the hydralic action of the plungers. If it is working correctly, the tensioner should self-prime as hopper fills with oil that drops from the upper run of the primary chain.

grantb said:

I run 300ml of SAE 20, the only SAE 20 I could get is heavy duty fork oil, should I go back to engine oil 20W-50?

Click to expand...

Why 300cc (when the book capacity is 200cc)? Excess oil is likely to cause clutch slip.
Why SAE 20 (when the book recommendation is 20w/50)?

Others may suggest you use ATF in the primary, however from personal experience I prefer not to use ATF in a MkIII primary case as the hydraulic chain tensioner seems to work better with 20w/50.

grantb said:

...
The head steady prior to removal was not tensioned at all, I have since tensioned it only to 1.25", book says 1.45 but I am nearly out of adustment, is the spring supposed to sit between the 2 ridges? ...

Click to expand...

The two positions are there so you can get 1.5" (plus or minus 0.05), so it can be moved to the forward position with no issues (mine was). However, should be noted the overall purpose is to help with vibration, so if going to 1.5" makes it worse, then I'd go back to 1.25.

Because the MK3 hydraulic tensioners in the primary are different from all the other years, I would advise you to go with a 20W-50. Two items to note:

1. I have read in several places that fork oil tends to run a bit light in grade (saw a test on like 10+ brands and only one had viscosity that matched up to its advertised weight, many were half - 20W really more like 10W).

2. I've run fully synthetics in the transmission (Redline) and in the engine (Mobil 1), but I try not to use a synthetic in the primary - may be just my prejudices, but I'm hoping it helps with any oil slung onto the clutch plates is I use less slippery "regular" dino oil.

grantb said:

I did notice a noise prior to the loss of compression, but not quite as bad.

Click to expand...

and

grantb said:

as it comes on under load in gear, something is taking up tension and banging away,...

Click to expand...

So with the above taken at face value:

Is the rear drive chain properly set.
Is the rear chain worn or have frozen links
Are either of the sprockets clapped out. I had an experience years ago where a chain drive was fine until you rolled on the power and it climbed the teeth and removed all slac in the chain - made an afwull sound and feel.
Do you have the rear cushions in place between the sprocket and wheel hub; maybe one or more of them are shot or missing and you have metal to metal under load.
Look for fretting between the fuel tank and frame tube. As you say when you run and load the bike w/o a fuel tank you will know one way or another.

Back to the gear box; I do recall the gear box making a crucnch chugging sound when the layshaft bear gave up the ghost. In my case it did not last long making that sound and I have a hunch that is typically how they go out, fast and furious.
Does the noise occur in all gears?
Does the noise cease in fourth gear?

Can you feel or sense it in the vibration or ride or is it just a noise?

Lot of help I am, more questions than answers.

As Concours said, check final drive chain as i had problems with clunking under accelleration from 1st to 2nd only to find drive chain needed adjusting because it was hitting the chain guard.
Tim

Another possible source of the clunk under acceleration?

Given the right set of conditions, if the Isolastic rubbers are shot, then the chaincase could be coming into contact with the Z-plate?

After a ride today, I had riders along side, as the noise only appears under load,who identified the noise being on the left side and more than likely the head, sadly some white smoke came out the left exhaust pipe, this was intermittant, so off with the head something weird is going on in there.
Maybe a valve guide is loose? is that even possible? white smoke, could this be caused by oil leaking past in to the exhaust port area?
Sadly the wife has doing other things this weekend, hopefully during the week I will get the head off.
Thanks to all I will update as I go.

Well grantb, that is the shits but on the bright side, at least you are homing in on it.


Was the smoke under load or off throttle? Did the smoke correspond to the tractor sounds?

Before you pull the head, have a look at the spark plugs for excessive oil and do a proper plug chop and check again. My point being is pick up as much evidence along the way.

Don't assume the smoke and the noise are related. Case in point. The noise existed before and is now more apparent BUT you pulled the head and reinstalled it. If upon reinstall you inadvertently plugged off the oil return for the intake valve pockets with too much gasket compound or debris then flooding in the intake valve pockets could result in smoking and have nothing to do with the noise. And yes, you can get smoke out of one pipe only with this scenario. And yes a fretting gas tank or something in and around the outside of the cylinder head can sound like a cylinder head noise.

White smoke can also indicate an upset head gasket, typically draing oil from the pushrod tubes. Did you retorque the head at least once after a heat cycle?

As stated above, since you really don't know what is going on yet, take it a step at a time and gather all the evidence.

Ugh, I've been trying to avoid putting myself in your mystery stressing, hoping the brain bank here would list the cause. The intermittent part is extra maddening with a wife that takes time away from an active ailing family member : )
Noise sources I've had, push rod steel cap loose, Stellite lifter bottom let go, flattened cam lobe, cam tensioner let go, oil pump snout fracture loose, thumb nail piece of carb slide inhaled then processed in chamber on valve seats and bore walls till out spit out, broken piston ring, detonation by sharpened edges or coking form above, loose nut in primary case, head steady rubbers or plate failure for tank impacting, headlamp ears o-rings cushions dissolved, rear hub bearing balls dissolved, intermittent conduction path that triggered Boyer to fire out of time on throttle. Noise Wesley's '71 had, rod shells dissolved.
Who knows what you may be able to add to the list we all left out in ignorance. The snits would be tearing down w/o finding it, a few times...with wife miffed at the waste of time and money diverted from you know who...

Could the clunk under aceleration be the forks topping out?