Mercruiser FORD 351 loss of power and low WOT revs

[johnnymac59]

I have a Bertram powered by twin Ford V8s, and the starboard 351W will only rev to 2800rpm under load (it revs freely to 4800rpm in neutral), after this any more throttle causes a flat spot and drop in the vacuum gauge reading (15psi to 3psi or sometimes zero) with no more power. there is also unburnt fuel noticed out of the exhaust. I have cleaned fuel filters, water separators, replaced carburetor with a new Holley (old rochester was done). Compression tests are a little low-110-115psi but no major difference between all cylinders. I'm about to check timing, but any advice is greatly appreciated! Cheers John

[Haut Medoc]

You don't say whether they are sterndrives or MIE....
That sounds like a possible exhaust blockage/restriction....
Maybe a broken exhaust shutter?.....

[johnnymac59]

Hi Haut, thanks for your reply. The engines are inboards, shaft drives. There doesn't appear to be any exhaust blockage as the water flow is good both at idle and high revs. Where exactly are the exhaust shutters? so I can inspect them. Would they be inside the exhaust manifold? John

[bifflefan]

Maybe timing chain...?

or try here

http://forums.iboats.com/mercruiser-i-o-inboard-engines-outdrives/list-possible-causes-low-wot-295070.html

[johnnymac59]

Thanks Bifflefan, lots of info here, will take it onboard and keep inevstigating. Cheers, John

[Bond-o]

Ayuh,... Up in the Adults Only section, there's a list of things to Look at when WOT can't be achived,...
Have a Look...

[johnnymac59]

Hi Guys,

Have been through the top ten list and ticked them off as I go. Fuel system fine, new carby, clean filters, clean hull, porps ok, tested coils, new plugs...etc..etc...

Timing! Was running at 23deg BTDC...so slackened dizzy and altered timing to 12deg BTDC. Had to adjust idle and mixture, sounding good. Will take it for a test ride tomorrow...fingers crossed!

[a70eliminator]

You'll be back.

[johnnymac59]

Good call a70eliminator, I'm surely back!!! The timing did make a difference, will rev out to 3300rpm now and feels better but still not right....whats the chance that the timing chain has jumped? I keep reading people suggesting this and others saying it's bollocks and never happens. How can I check this? If I find TDC on no.1 cylinder then do I check to make sure the rotor arm is pointing directly at the no.1 position on the distributor or slightly past it? Its been a long time since I worked on engines so bear with me guys.

[a70eliminator]

The vacuum reading you describle are the result of restricted exhaust. Look at scenereo 14 of the interactive here ---

http://www.secondchancegarage.com/public/186.cfm

[Bond-o]

The vacuum reading you describle are the result of restricted exhaust.

Ayuh,... Melted shutters maybe,..??

[a70eliminator]

I don't know anything about inboard exhaust to even know if they have shutters? But if they do.....

[johnnymac59]

Hi Bond-o thanks for your reply. The engines are inboards and the only shutters I know off are on the rear on the boat. They flap freely. Are there shutters inside the manifolds/exhausts anywhere? When the engine idles it has a slight pause every now and then and more water empties out the back, its like a slight build up in exhaust pressure, but nothing that has concerned me.

[Bond-o]

Are there shutters inside the manifolds/exhausts anywhere?

Ayuh,... About every boat is different,... Donno...
If there are, they're probably somewhere, Just downstream of the risers...

[johnnymac59]

The vacuum reading you describle are the result of restricted exhaust. Look at scenereo 14 of the interactive here ---

http://www.secondchancegarage.com/public/186.cfm

Thanks mate, great link. I will pay strict attention to the vacum guage next time I take her out and in the meantime I will examine the exhaust system. Cheers guys.

[dakine]

Does it ever backfire when you try to get more RPM by advancing the throttle?

[johnnymac59]

Hi dakine, no mate, picks up well accelerates well till about 3000rpm then the rest of the throttle has no effect...could be described as flat. I have checked cables and there is full opening of the carby so its getting plenty of fuel. No backfiring or erratic revs, just flat similar to a bogging.

[johnnymac59]

Hi guys, sorry its been a while since my last post (been busy...and not fishin either!) Anyway, had an expert come see the boat and sounds like I have blown a head gasket (lack of compression, steam coming out of the exhaust) and possibly valve corrosion. However, no oil in water and no water in oil evident. So heads coming off soon. Will keep you updated on any movement. I had redone compression test and wet v dry showed little difference, hence I thought valves are shot and not sealing properly. probably could confirm this with a leakdown test but just as easy ripping off the heads. Will wait and see.

[Bond-o]

Ayuh,... Is this boat Moored,..??

If so,... How much Sea Life is living on the bottom,..??

What were the compression test #s,..??

[Lyle29464]

I have several inboards and have worked on several but never seen any flappers inside the systems.

I would swap carbs side to side. disregard if it did the same thing with the old carb.

[johnnymac59]

Hi Guys, thanks for the input. Compression tests ranged from 95-115psi dry and 105-125 wet, so low on compression but not pointing towards bad rings as the increase is minimal. So I started to think that the loss of compression is due to bad valve seating/corrosion. I now think there is no doubt the compression is the culprit of the lack of power at WOT, whats causing it? Steam from exhaust points towards blown head gasket, and this in conjunction with bad valves would give me the symptoms.

The boat is moored and has a little growth bu not much, I have a brand new carby on it and the problem still the same as with the old carby...so points towards engine. Will keep you informed of findings once I strip it down.

John

[Bond-o]

Compression tests ranged from 95-115psi dry and 105-125 wet, so low on compression but not pointing towards bad rings as the increase is minimal. So I started to think that the loss of compression is due to bad valve seating/corrosion. I now think there is no doubt the compression is the culprit of the lack of power at WOT, whats causing it? Steam from exhaust points towards blown head gasket, and this in conjunction with bad valves would give me the symptoms.

Ayuh,.... She sounds pretty Tired.... I'd think pulling the Heads is the logical next step...
With steam involved, follow the rust trails in the exhaust path,+/ or the Really Clean areas of the combustion chamber...

Btw,... Alittle marine growth is Too much....

[Lyle29464]

Good call a70eliminator, I'm surely back!!! The timing did make a difference, will rev out to 3300rpm now and feels better but still not right....whats the chance that the timing chain has jumped? I keep reading people suggesting this and others saying it's bollocks and never happens. How can I check this? If I find TDC on no.1 cylinder then do I check to make sure the rotor arm is pointing directly at the no.1 position on the distributor or slightly past it? Its been a long time since I worked on engines so bear with me guys.

starboard engine most likely does not have a chain. If its a reverse turn it will have gears.

[johnnymac59]

Thanks Bond-o for the heads up. Will pay attention to that when I strip her down.

[johnnymac59]

It is reverse rotation, and heard they were made with gears rather than a timing chain. Good call will check this out when I remove the cover. The rotor arm in the distributor rotates CCW the same as the standard rotation, so I guess the distributor has a reverse gear on the bottom of the shaft?