302 mercruiser not firing on all 8

74century

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Dec 23, 2015
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I have a 1974 mercruiser 302 which ran fine earlier this year.July 3 took it out and heard a noise from engine box (possibly backfire),motor never skipped a beat.next day took it out and couldn't get the boat on plane (no power) throddle all the way out.All plugs were carbon fouled.Mechanic has worked on the boat all summer.He has done,comp test,leak down test,which he says I have a tired motor.he has installed new plugs,wires,rotor,dist cap,just went thru the carb today per my request.timing is correct, firing order is correct,points set to spec.Everything seems fine but the plugs keep fouling up, wet carbon fouled and motor continues to run on 5 or 6 or 7 cyl.little hesitation on throttle. pull plugs out clean them up reinstall,run motor sounds good for 5 sec then back to running on 6 or 7 cyl.There is smoke coming from the exhaust (blowby) and gas or oil slick on the water after running. I will be pulling boat out for the summer soon.Any ideas would be great.mechanic says he will pull valve covers off next if I want him to.talking about cyl walls, valves, rods, blah blah.Idk. Thanks.
 

Bondo

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Mechanic has worked on the boat all summer.He has done,comp test,leak down test,which he says I have a tired motor.

Ayuh,.... Welcome Aboard,..... What are the Numbers,..??

Just how Tired is it,..??
 

74century

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I don't have the #s,i remember him saying the #s were good,consistent on all cyds for a 40yr old motor, no low #s on any certain cyd.(The bottom end is tired) I have heard that several times from him.Could the timing chain have jumped and causing a timing issue even when we believe the timing to be correct?
 

hemi rt

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Jumped timing chain usually means no start or popping out the carb or exhaust, if you could get us the compression figures it may be easier to help you. Wet fouled usually means it's running rich - high float level, choke sticking or a bad carb. pulling the valve covers will only tell you if you have broken valve springs, valves not opening or closing or bad rockers or rocker shaft.

try bring the engine up to operating temp, let it idle and take the oil filler cap off a valve cover - any blueish smoke?

The saying 'your engine is tired' is just saying 'I don't know what is wrong' - take it to another mechanic - even an automotive mechanic may be able to help you.
 

74century

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I think a different view may be a good idea.I am still not sure if he has the right firing order.The valve covers were never taken off to check.The manual says 18456273( and that is the one we are using) but who knows if any work has been done to this motor over the 40 yrs before me.I have heard standard ford firing order,also 351 cam being used in a 302.It might be possible that the rotation is wrong also.The motor is stamped 225R.He saysThe R is right hand (standard) i have also seen where left hand is standard.The basics here should be figured out first i would think.I have all winter to figure it out
 

harleyman1975

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First thing in my little walnut brain was "power valve". those Holley 2 bbls have a power valve that
can blow if engine backfires. A blown power valve will give the symptoms you seem to be describing.
 

74century

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he showed me the power valve on carb,but dont know if he checked to see if it was working,think all he did was clean the carb and reinstall it.THat is something to check.Thanks.I never hear the motor backfire when its running.
 

Fishermark

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I don't have the #s,i remember him saying the #s were good,consistent on all cyds for a 40yr old motor, no low #s on any certain cyd.(The bottom end is tired) I have heard that several times from him.Could the timing chain have jumped and causing a timing issue even when we believe the timing to be correct?


A "tired" engine has low compression. That fact that you have good compression numbers doesn't fit with a "tired" engine.

Now it might be that he heard main or rod bearing knocking - that might be considered a "tired" engine, but with worn mains you will experience low oil pressure. I agree that your mechanic is using the phrase "tired engine" as a way of saying he has no idea of what is really wrong.
 

74century

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I believe you are right. The first thing I am going to do is get that carb looked at.Thanks.
 

harleyman1975

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Super simple carb to work on. They make a replacement power valve that is more resistant to a backfire or "lean sneeze".
 

74century

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Dec 23, 2015
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The boat at times will work great,maybe not be as responsive as i would like.But you can get a good day on the water until it feels like a electric cutout while under 3/4 to full throttle (bucking , quick back to back hesitations, surges) not sure what you call it, then the next day, no power with throddle all the way out,just cant get on plane running bad.check plugs and most are fouled.One thing i dont notice is how the carb would make a hissing noise. It was the only thing you heard when the motor ran, the motor was always quite.I know the motor runs cause i have it running great at times.I am just throwing these things out there in case anyone has any ideas.idk
 

74century

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UPDATE!! Harleyman1975, you were right ,bad power valve.Replaced it , adjusted idle mix screws and exhaust smoke disappeared.Engine is back to normal.Thanks!!
 

Pete104

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UPDATE!! Harleyman1975, you were right ,bad power valve.Replaced it , adjusted idle mix screws and exhaust smoke disappeared.Engine is back to normal.Thanks!!

And the mechanic's response? By the way, thanks for the update! :)
 

harleyman1975

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Hey hey even a broken clock is right twice a day! Glad I could help! Now you learned something too. Win?Win.
 

KJM

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I love this site! For anyone willing to learn the experience and advice here is invaluable. Got to me a mechanics worst nightmare though....I'd wonder how much money in unnecessary repairs this has saved?
 
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