11 replies to this topic

[mingy]

A couple of weeks ago me and my beloved went for a tootle in the mini down a few country lanes ( no smutty replies please ).

We decided to go home via a three mile stretch of the M6 and as the motorway was fairly empty my wife said ( honestly ), why don,t you give it a blast..... SO I DID.  

We hit the magic 3 digits  confirmed via satnav but,  we suddenly slowed down and trudged home at 40mph. 

1 slightly bent valve was found as, i took the head off thinking it was a head gasket failure.

What i want to know is why, how, and whats the answer to stop it happening again. i was told my valve springs are of the double type and are rated at 180lbs if this helps.

[jt19]

I used to hear it in my 998 and back off. I always thought it was due to the springs being too weak.

I could be wrong though

[mingy]

I used to hear it in my 998 and back off. I always thought it was due to the springs being too weak.

I could be wrong though

I couldn,t hear myself think at a ton let alone a bouncing valve......

[jt19]

I used to hear a strange noise. A bit like a fizzing noise. It might of not of even been valve bounce lol. Just always thought it was as I was reving the nuts off it! Lol

[HarrysMini]

What was the engine speed at this? You would have to have a FDR ratio of lower than 3.44 to be doing more than 6000 RPM at 100 (rough calculations). 

 

What engine is it?

Edited by HarrysMini, 20 September 2014 - 09:10 PM.

[69k1100]

He didn't say what gear he was in. Valve bounce (or float) is caused by the cam lobes spinning too rapidly (over revving the engine) basically the lobe makes a full revolution before the spring can lift the valve out of the bore, never giving the valve a chance to close. Higher poundage springs will go some way to alleviating this but you might also look at lighter valves and rocker gear so the springs don't have to work as hard.

I would have thought a rod would have let go before the valves started floating.

Is it a possibility the guide grabbed the valve?

Edited by 69k1100, 21 September 2014 - 06:52 AM.

[Spud_133]

I've had valve bounce on a ministox for an entire meeting, which was due to a weak spring, replaced the springs and all was well

[Ethel]

Firstly the spring has to provide enough force to accelerate the the valve train fast enough to keep the follower in touch with the downhill side of the cam as the valve closes, 180lb springs ought to be plenty for an engine in top gear.

All springs have a natural frequency, think of tuning forks. The stiffer the spring, the higher the frequency. If that frequency harmonises with cam the vibration will increase, the same as pushing your mate on the swings in the park when you were a kid. Double springs each have different frequencies, so one will damp the other when it tries to vibrate.

I reckon your bent valve is more likely to be down to a binding valve guide or cam follower*, unless you over revved it changing down.

*Sustained max power will have generated more heat and expansion than it may have seen before.

[mingy]

Thanks for your replies and thoughts on the matter, my engine spec is: 1380cc, 286 Kent cam, Rimflow valves on modified head, with 1.5 high lift roller rockers set at 0.014 and rocker pedestals have 1.1mm shims underneath, and a HIF44 carb, i do not know what the FDR is but i can find that out, but i think it is a standard rebuilt box.

Rev counter was showing over 6 thou and satnav did show 100, i also forgot to mention i had to replace two pushrods to same cylinder.

[HarrysMini]

You need to rethink some things if you are getting valve bounce at 6K RPM with a 286. These should be able to rev to 8k RPM. 

 

What is the valve spring situation?

[Cooperman]

I also think it may have been valves getting squeezed in the valve guides.

What guides are fitted? If they are manganese-bronze there is a need for good valve stem to guide clearance, which is why many people will only use iron guides for very high-revving engines, then ream the guides to 9/32". I once had exactly that problem with a 970 Cooper 'S' with manganese-bronze guides, so I just fitted iron ones - problem solved.

Those double springs should keep the followers on the cam at up to at least 7000 rpm with a 286 cam.

[mingy]

I am sure the valve guides are of the bronze type but, the double valve springs are the original ones that were on the head before it was modified, there was a very worn 286 cam in the block which was replaced with a new 286 when the block was rebored and rebuilt.

When i replaced the valve and pushrods i was advised to put some 1.1mm shims under the rocker pedestals to account for the head being skimmed on various occasions.

I have not taken the car much above 70mph since but it will be having a rolling road session at Minisport in the next two weeks so i hope it can be diagnosed if it occurs.

Thanks for all your advise and comments, it helps me to learn more about the A series.