Mercruiser FORD 351 loss of power and low WOT revs

J

johnnymac59

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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

Haut Medoc

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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

johnnymac59

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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
 
J

johnnymac59

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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

Bondo

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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...
 
J

johnnymac59

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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!
 
J

johnnymac59

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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.
 
Bondo

Bondo

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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

Ayuh,... Melted shutters maybe,..??
 
a70eliminator

a70eliminator

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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

johnnymac59

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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.
 
Bondo

Bondo

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Re: Mercruiser FORD 351 loss of power and low WOT revs

Are there shutters inside the manifolds/exhausts anywhere?
Click to expand...

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

dakine

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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

johnnymac59

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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.
 
J

johnnymac59

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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.
 
Bondo

Bondo

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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

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

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

Lyle29464

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Re: Mercruiser FORD 351 loss of power and low WOT revs

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.
 
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