Valve bounce

Like others here I would question what the 'ell anyone was doing reving it to 5000rpm with no load anyway, these things weren't designed to do that, they are old and low reving lumps. I again like others would suspect a rev limiter being the cause.

The engine we had in our 2002 SeaRay 225 WE was a Merc 5.0L V8 and even with the MPI fuel injection it was only rated to 4800rpm WOT.

As the good advise already posted look out for service history, the condition of the risers (they always rust away) and the compression.

Our old SeaRay always ran a bit rough for the first tank of fuel of the season (petrol degrade over time) plus the winterising stuff takes a bit of burning off and then she picked up on the second tank full of fresh fuel, even when using fuel stabilisers etc through the winter.

Have you asked anyone at SeaRay if they are aware of this as a problem with the engine in general or specific to the installation in the 240Sundancer? I would call Marina Marbella in Hamble and speak to Garry in the Service Department, he has always been very helpful when I have had a problem.
 
If you're getting smooth performance at 4500rpm and 34kts you're doing okay, bearing in mind the props might not be perfect, and any hull fouling.

dv.
 
Joining this rather late I would question the horror that some feel at the thought of revving the engine without load.
Is that a slight confusion with the overevving of diesels without load, which can cause problems at MOT time.
The risk of revving without load is that you can easily over-rev the motor, but going to the normal rev limit shouldn't do harm, unless the motor is crap.
I'd be suprised if it was valve bounce, it's not something you come across much these days.
I would plump for a coil problem. That's indicated by the way you describe the revs rising and falling. Valve bounce would just prevent the motor running faster, as would an ignition rev limiter. It will show up under load as well, so try a run see if it raises it's head, provided you can reach those revs.
I had the same problem on the mini I built, on the rolling road.
The only other thing I can think of is sticking valves, but unless you've just had new guides fitted (or there has been some valve damage like bent stems) I can't see that, and it still wouldn't cause the varying rev range.
 
Why didn't the mechanic put a timing light on it & see if the ignition was cutting out at speed?

Certainly some rev-limiters can sound quite crude when they cut in - all those I've heard have not simply limited the speed, but made it jump up and down over a narrow range, kind of like a misfire.
 
No idea... Im pleading 'newbie' which is why I paid a surveyor to look at it all for me. I'm really confused now and dont know whether to carry on with the purchase or not. I guess I need to find out a) if there's a problem at all and b) what the problem is if there is one.
 
if those engines do have a rev limiter, then very likely that's what's causing the misfire at those revs.
 
A quick google looks like mercruiser 5.7s did have rev limiters on back in 98 - is there any easy visual way to tell (are they electronic devices that sit somewhere?) This would be good news!
 
.. and a tech page shows the full throttle range:

HP/kW 250/186
Displacement Litre/CID 5.7/350
Cylinders V-8
Bore & Stroke (mm) 101/88
Compression Ratio 9.4:1
Fuel System 2V Carb

Full Throttle RPM Range 4400-4800

Ignition Type Thunderbolt V
Alternator Amp. 65
Length (cm) 81.3
Width (cm) 74
Height (cm) 55.9
Weight (kg) Alpha 430
Weight (kg) Bravo One 449
Weight (kg) Bravo Two 456
Weight (kg) Bravo Three 460
 
Hmmm, but it's not really a "common fact", rahter it's a commonly made claim that's actually wrong and has no engineering basis at all :-)
 
Rev limiters If its got an engine management system then its probably "built into that software" if not then its probably a centrifugal device in the rotor arm that breaks contact at a certain rev - look for a bulky rotor arm. But you should be able to tell the difference between a rev limiter - the engine goes quiet and then stutters and the metalic jangle of valve bounce.
 
I agree that revving a cold engine is bad for all the reasons you say. But that's a different point guv. Just on the pure matter of whether revving a motor unloaded is bad whereas revving it loaded is ok, that is just plain wrong and has no proper basis in engineering etc

I dont buy your 4th para at all Matt - re higher acceleration of internals breaking down oil film. Two reasons. First, the compressive load put on the oil film by rapid accelaration of internal engine components is much much less than the compressive force exerted by running the engine under full load. Second and more important you are forgetting your calculus and geometry matt: the maximum acceleration of internal components occurs at max rpm. Frexample the max acceleration of the piston caused by the little end pushing it upwards occurs at max rpm and 1/2 way between TDC and BDC. (And the accelerative force is of course zero at TDC and BDC, even at 5000rpm). If you blip the throttle on unloaded engine so it rises from 500 to 5000rpm very quickly it does not materially alter any accelerative forces compared with rising from 500 to 5000 very slowly - those forces are still at their maximum when you are at max rpm.

(I'll come back to you on Swamp-age. Been busy but ont case now)
 
Hi just looked for Thunderbolt V ignition on google

It does have a rev limiter and it seems its set to around 5000rpm.

If the engine is missbehaving below this figure then I would suspect that something is wrong.

there is a tool that can be used to interrogate the managemnt system to give this information

Thunderbolt V ignition module support!
The MerCruiser Scan Tool now supports 1996 MerCruiser Thunderbolt V ignition modules. Technicians can now access engine operating hour data from the module and review the number of hours an engine has operated in several preset RPM ranges. The following information is accessible using the scan tool:
Key On / Engine Off Hours
Hours run under 1000 RPM
Hours run 1000-1999 RPM
Hours run 2000-2999 RPM
Hours run 3000-3999 RPM
Hours run 4000-4999 RPM
Hours run at Rev Limiter
Total hours operated

as you can see the impication is that the rev limit is greater than 4999.
 
re-reading your original thread about the sea trial..... no way do I see 'valve bounce' causing a drop from 5K to 3.5-4K rpm, not a chance.

So very likely the rev limiter, and yes, some do behave exactly as you described. My suzuki DF115 outboard for example, maybe less of a drop, but sounded real rough when the limiter cut in. Similarly on a renault petrol engine I had in a van. And a number of m/cycle engines too.

Still best if you get someone who knows what they're on about to take a look/listen. Since as you've gathered from this thread, revving a motor to 5K in neutral and not expecting a rev-limiter derived misfire, leaves little confidence in the guy who 'tested' it.

some more modern engines have limiter which cut in extra early in neutral, maybe 3000rpm for example.... so a cynic might wonder if your guy knows much about engines at all.
 
pure v applied engineering

your excellent rebuttal of para 4 noted. Cept er...why do well-revved motorbike zing zing engines last only 22k miles whereas zooming down the road at megaspeed is all fine? According to the pure analysis model it seems it's fine to razz the engine, and in fact finer than just sitting at light holding top revs due to lower amount of time attop loading. I must say this *feels* odd and wonder if there are oil pump-related issues, dunno.

But anyway, reverting back to temp point that escaped annhilation it's not realy a totally and utterly different issue - it's just that when saying "no-load" people really mean "cold", but er aren't much kop at the engineery analysis. Thus thrashing at start very bad cos it's cold, not cos no-load. Except no-load almost always is (practically tho not er pure engineeringly wherever they do that) also cold.

Also no-load warming up as per mole sud neighbours bad as well, as it prolongs the time whilst cold...

I did once invite the boat engine mechs round after a decent trash so they would be able to get all the oil out easily - but they immediately left to return next day and work on a cold engine.
 
Re: pure v applied engineering

main problem with no-load revving is ppl think it's big or clever to rev the socks off right from tickover with a great fistful of throttle, whereby w/o a decent limiter , the max revs can easily be exceeded in some cases.

Or at least that's the theory, tho of course a bit hard to prove. and with modern limiters on petrol engines, prob unlikely, or not. Hmmm


spect the short life of bike motors is down to higher state of tune of course, a MX two-stroke might get new rings every few hours running, and a new piston every 5 sets of rings say. Or if not thrashed, same engine rings'll last a seasons racing.


outboard wise , ppl seem to think motors last for ever, hmm, until they break down.
 
Re: pure v applied engineering

hmm, I wonder then if service schedules should be based on number of revolutions as opposed to engine hours

"check/adjust valves every 1x10^7 (can't do to superscript here) engine revolutions" /forums/images/graemlins/cool.gif
 
Re: pure v applied engineering

Yes, the idea that 200hrs is 200 hrs is pretty rubbish really, but makes money for the franchised dealers esp while in warranty, which therefore pleases the manufacturer too.
 
Re: pure v applied engineering

based on temperature & revs then, and thrashiness too. Also a weighting for boy-racer types who cane their cars & bikes, so likely boats too eh?
 
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