Valve bounce

youngr

youngr

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Following the Engine Misbehaviour we experienced on the sea trial for the searay 240 sundancer I'm trying to buy, the vendor agreed to get the problem rectified. I spoke to the engineer today (he has a mooring that might be available for me), and I asked him how the engine fix was going. He's cleaned up the distributor and says it runs much better now, but there's still a problem maintaining high (5k) rpm with no load on the engine. He doesnt seem to think it's a fuel or electrical problem now, but that it might be "valve bounce" and theres not a lot you can do about it. This doesnt sound right to me, and I'm not sure that if when the sea trial comes next week and the problem's still there I should take the boat on. Has anyone got experience of valve bounce on an engine - surely it's fixable with new springs or valves? (And thats if that's what the problem is). Typical of me to find a boat that doesnt work right.

Russell
 
Why on earth would he want to maintain a 5k rpm with no load?????
You are going to be running well below that under load!
My 5.7 will valve bounce (I would imagine) under those circumstances,remember the gm 5.7 is an old design and not meant to be revved the nuts off it.

Cheers
 
Well... it was a test the surveyor did; Im well out of my depth here being new to boats and boat engines, so Im trying to just work out whether the boat Im trying to buy is ok or not! The surveyor basically put the boat into neutral and then put the throttle up; as it got near 5k it started misfiring badly and jumping up and down between 3.5/4.5k.

Some people have suggested this is an ignition or a fuel problem, but the engineer thinks not and that it's valve bounce and thats all there is to it. I must admit the boat seemed to run ok - it did 30 kts with the trim tabs not working, so I assume it will do another 2-5kts with them in action, and didnt misfire.

What I dont want is to buy a boat thats got a knackered engine - are you saying that any 5.7 would start misbehaving at 5k in neutral like that, any my surveyor perhaps didnt know what he was doing?

Help!
 
OK, no problem - loan me the keys and tell me where she's moored Ill give it a go for you /forums/images/graemlins/smile.gif

Am I worrying about nothing then do you think? Is this a problem that the surveyor's found thats not really an issue in any case?
 
Agreed. Bloody stupid thing to do. That engine should redline at about 4500-4700 or so, from memory, and that's under load. Push it to 5000 with no load and it would be reasonable to expect it to self destruct.

We had a 5.7 V8 in a Maxum 2300, and that was good for about 30 knots. If the Searay can hit 30, then my guess is teh engine was probably sound before some pillock decided to rev the nuts off it...
 
A compression test is the norm on one of these lumps.....

However the critical things to check are the exhausts and risers i.e make sure they have been replaced at some time cos anything older than 10/12 yrs thats been running in salt water is due to expire at great expense!

PS,run it under load and also watch the temp,listen for any untoward noises etc!

Suggest you question your surveyors experience with petrol V8's also.

Good luck.
 
A comparable age Volvo (same base GM engine) has specs for max engine speed of 4800 if carb and 5000rpm if fuel injected. As others have stated these motors don't have a lot of rpm headroom so under no load quite likely it is hitting the rev limiter. Bear in mind that if you rev the engine under no load the rev counter will probably lag behind the motors actual speed.

Volvo Older engine specs can be found here
 
Thanks, the compression test was all ok, and it didnt misfire under load at 30kts, can't say I could hear anything untoward, it was a bit choppy, but I was at the back of the boat so wouldve heard anything noisily wrong.

Im going to go for another sea trial next week, Ill go without the surveyor this time. I guess if it makes 32/34 kts its probably safe to say it's ok, bearing in mind all the other tests that have been ok..
 
As stated if these engines red line under load at 4700 it may have ignition cut off at 5000 to prevent the engine over revving. Running an engine at WOT (wide open throttle) is not a problem as it is designed to run upto that speed.

Valve bounce on a modern day(ish) engine at 5000..............not in my experience! more likely ignition cut off which will sound like a misfire.

Tom
 
I'm confused by some of the replies. From an engineering point of view, why is it ok to run engine at 5k rpm under load but not under no load? Surely if it's ok to do it loaded, it must be fine to do it unloaded?

And what has the no-load got to do with valve bounce? If valves are bouncing at 5k rpm no load, they will bounce at 5k rpm loaded. Ok there are different cyl pressures loaded but valves should not rely on cyl pressure to close in time for the next thwack by the cam. They should close under force of the springs alone, else there's a fault.

Happy to be proved wrong on all this... just curious.

If valves are bouncing it needs new springs for starters which isn't a massive job
 
Technically you are right, i think.

But I would reckon it is worse to fully rev an engine off-load because "off-load" usually means "not hot". High revs on otherwise cool and hence easy-to-work-on (i.e not *yet* loaded) engine means tolerances are all wide open and hence risks greater damage than fully-hot engine with bearing etc tolerances all taken up. Oil will also be cooler less viscous and not working properly either.

This doesn't applyif engine given a nice run to warm it all up and *then* rev with no load - but i wd guess the mechanic never does this cos he couldn't touch a thing.

Also, no-load can of course involve dramatic screamy revving which could definitely cause damage, higher acceleration of internals breaking down oil film etc. Gentle rise to high revs means this doesn't apply, though the point above still does.

Pistons frinstance aren't circular but slightly oval to compensate for extra heat expanding them to snugger fit - on a cold engine they would be looser in the cylinder, hence more liable to cause damage at higher revs.

The valve bounce though, hum. Good mechy thing to say but must be incredibly hard to diagnose unless had 2 identical engines one with crummy springs one without. Swap springs as you say. Probly not that cos he didn't know about the engine having to be hot, really, before giving it some stick. Or er perhaps it is that but he can't bothered to change springs...

I reckon boat engines dramatically smoother /quieter after more than an hour of running, big lumps of metal etc. So bad idea to wham them whilst cool.

imho.
 
if you reved my engine up to 5000 rpm id kill you !!!

take it on sea trial if you get 30 odd knots - its fine
 
If the valves are bouncing, that could mean weak valve springs, but I've not heard of this being a common problem. Knackered valves due to water ingestion from leaking risers or manifolds, maybe.

As to running off load: Shoot me down if this is rubbish, as this is just logic+guesswork, but if you run max revs under load, then you are running at full throttle, hence lots of mixture to compress and resist the urge of the piston to leave via the cylinder head.

Under no load, you'll hit max revs at part throttle, less mixture to squash, so although the compressive loading on bearings & conrods will be lower, the stretching forces on those bearings & conrods will be higher, which might not be a good thing. Bouncing off the rev limiter isn't a good idea, either, as the rev limiter is often a bit heavy handed.

It's all mostly irrelevant anyway. The thing that normally goes bang in an over-rev is the valvetrain (springs not being able to get valves out of the way in time and oops, hit the piston).

dv.
 
valve bounce at 5000 rpm with no load sounds like your running on the rev limiter.
if you have mis fire on one of these lumps change plugs, leads,dizzy cap, rotor arm. make sure timing done in the correct way which is not just with a timing gun you have to earth out the module into base timing mode. if still running bad change the dist. module inside the distributor.
 
Its a common fact, official or not to anyone who has been in the motor/marine engine trade, that reving the nuts of any petrol engine whilst not under load is the best way to wreck them. Many engines now have a a revlimiter of one type or another fitted. designed to stop this type of abuse. The symptoms you describe would relate to one of these devices working in its correct manner.
I also suggest you question your surveyers methods.
 
In days of yore before rev limiters valve bounce prevented engines from over revving and destroying themselves, or at least that was the theory. As the revs rise the valves and springs resonate, at a certain frequency the valves rebound off the seats and “bounce”, this results in a loss of compression and means that the engine will not rev any faster – its not dependant on the load on the engine it’s a “feature” of the valve gear. That’s the good news, the bad news is that it can also be caused by weak or broken springs. More bad news is that while the valves are bouncing the collets that hold the springs on can pop out, and a valve thus free can drop into the cylinder – not good! British Leyland A and B series engine suffered from this and they fitted little springs to hold the collets on the valves while they bounced, then later they introduced double valve springs to change the harmonics and firm up the valve gear. The original Norton desmodromic valve gear had no springs and two cams to prevent bounce, one to open the valve and one to close it, but they eventually needed a light spring to account for wear.

In your V8 it sounds like the springs are shot, I would not recommend revving it to hard with or without load to listen to the valves - it could get expensive.
 
so... where does this leave me... should I insist that the problem (which I can only reproduce by revving it up to 5k) is fixed, or should I accept that it can run quite happily at 4-4.5k WOT at 30-34kts, and leave it at that? Am I just masking a problem that may cause other issues in the not too distant future? I'd be happy to walk away if thats the right thing, but I've already chucked about £500 at it with surveys and a boat lift. If I'm going to be saving myself repair bills of £1000s in the future it might be the best option though?
 
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