i need help - loud knock slow to reach top speed?

[clasavi]

all,

350 magnum 1998 efi mpi mercruiser with bravo drive

yesterday the mechanic adjusted the timing on the boat. the boat ran great when he ran it, however today i take the boat out and i have the following issues:

1) occasional knock from the engine room
2)hard to plan out
3)would not come out of the hole
4)struggled to reach top speed

since the mechanic test drove it, i did the following things:
1) changed fuel/water filter. (should i have filled the filter with fresh fuel?)
2) hooked up two batteries to a perko switch. (accessory cable is hooked to comm on the perk)

today, i also added more fuel as well as fuel stabilizer but nothing changed. what could it be? should i just sink it? should i change the spark plug wires?

additional facts, the boat cranks just fine and sounds great, she does well at low speed....

 

[Don S]

Re: i need help - loud knock slow to reach top speed?

What's the serial number of your engine, boat year doesn't really mean much.

I would suggest you pull the spark plugs and look at them, do they all look the same?
I would also suggest you do a proper compression test and post your results.

Not sure how your mechanic set the timing, that's done by the ECM.

 

[clasavi]

Re: i need help - loud knock slow to reach top speed?

Engine Serial OL067176
Sterndrive OL153367

the sparkplugs are new

 

[Bondo]

Re: i need help - loud knock slow to reach top speed?

Engine Serial OLO 67176
Sterndrive OL153367

the sparkplugs are new

Ayuh,... But ya still need to read their Colors to see what's gettin' fuel or what ain't...

 

[gm280]

Re: i need help - loud knock slow to reach top speed?

Since you stated your mechanic changed the timing, he could have easily advanced it too far which would account for the knocking especially during acceleration. And if he didn't tighten the timing down properly he could be moving on its own and that would account for the sluggish hole-shot and top end speed. I’d look real close to that first. Any time you have something changed and then it starts acting weird that is a first issue to suspect... JMHO!

 

[clasavi]

Re: i need help - loud knock slow to reach top speed?

i went down to the boat to look at the plugs. i noticed cracks on the white part of 3 plugs. replaced them and she is running fine.

thanks don for the suggestions

c

 

[Bondo]

Re: i need help - loud knock slow to reach top speed?

i went down to the boat to look at the plugs. i noticed cracks on the white part of 3 plugs. replaced them and she is running fine.

thanks don for the suggestions

c

Ayuh,.... Were they literally, White,..??

If so, it Ain't fixed, yer still diagnosin'....

Sounds like Detonation,....
Prime causes are a Lean condition, check the fuel filter for Water...
Bad timin',... That's still in question,...
or over propped, but that ain't changed...

Don't think it's Fixed, 'n run it,... It'll hole a piston in No time...

Did ya pour the contents of the old filter into something, 'n Look for water,..??
If not, pull the filter on it, 'n pour It into a glass jar, 'n Look...

 

[clasavi]

Re: i need help - loud knock slow to reach top speed?

Don,
what i meant was, the ceramic part of the spark plug was cracked or broken on three of them. i replaced them with new ones. i think the mechanic must have cracked them when he installed them back after he timed it.

 

[Don S]

Re: i need help - loud knock slow to reach top speed?

Which "white part" had the cracks, A or B?

 

[Augie56]

Re: i need help - loud knock slow to reach top speed?

I'm no rocket scientist, but if your mechanic cracked the external ceramic insulator on 3 of 8 new spark plugs he installed and didn't know he did it OR failed to replace them with new ones for you, you probably should be looking for another mechanic.

 

[cr2k]

Re: i need help - loud knock slow to reach top speed?

If your mechanic sold you a timing BELT for a 350 you got screwed, they use a timing chain.

 

[clasavi]

Re: i need help - loud knock slow to reach top speed?

to make matters worst here's the story, and no, i will not share what i was charged by the mechanic to set the timing... its embarrassing

last time i went for a ride she was fine, (after i changed the three "outer ceramic" broken spark plugs).
today, i ran the boat and she knocked again and she had a hard time getting out of the hole. when i got home i checked the other spark plugs and they appeared fine.

what's causing the loud "one time knock" only when under heavy load. also not getting out of the hole as fast as usual, any more ideas?

some more facts, she idles great and starts right up. i did notice some black smoke/black build up on the rear hull by the exhaust. could it be that the mechanic did not set the timing right?

 

[Bt Doctur]

Re: i need help - loud knock slow to reach top speed?

water, incorrect plugs, advanced timing,computer malfunction ,loose /corroded injector plugs, in short
A Lot of Things cause knocking/detonating.

 

[Bt Doctur]

Re: i need help - loud knock slow to reach top speed?

I think what he meant to say was if your having a problem it`s kinda dorky to see how fast it will go with the knocking.
If it was working properly before your mechanic worked on it , then ask your mechanic to recheck his work.
At the very least try to restore it to "pre-mechaninc status" to verify the troubles.

 

[Don S]

Re: i need help - loud knock slow to reach top speed?

Find a new mechanic because the one you have is taking your money for nothing.
Or, get the manuals, tools and equipment and do your own troubleshooting.
It's very hard to diagnose a sound without hearing it for real, and knowing if it's actually a knock, ping, rattle, or what.
What you may consider a knock, many may call a ping due to detonation.

PS: I also removed a couple of rude posts in this thread.

 

[achris]

Re: i need help - loud knock slow to reach top speed?

...yesterday the mechanic adjusted the timing on the boat.....

Engine Serial OL067176
Sterndrive OL153367

Not possible. This engine has an ECM and the timing is adjusted electronically by the ECM in response to signals from the knock sensor (and a bunch of other inputs).

i went down to the boat to look at the plugs. i noticed cracks on the white part of 3 plugs. replaced them and she is running fine.

I would be bolting that mechanic to a chair and asking him EXACTLY what he did, because he sure as hell, is feeding you pack of porkies....

Chris.......

 

[clasavi]

Re: i need help - loud knock slow to reach top speed?

Don,
your first statement said it all. i try to do it myself but my 8-5 job limits me a bit. the worst part is that i pay the mechanic and i am still on iboats asking questions. its very tiring...

because of it, she is on the trailer at my house and not at the marina tonight. Come monday, we are taking a road trip to a different mechanic.

don, i thank you for your professionalism and advises. i will post once my issue gets resolved

 

[clasavi]

Re: i need help - loud knock slow to reach top speed?

Update- I had the wrong plugs in the boat that auto zone crossed referenced for me. Not auto zones fault. I take full responsibility as I should have gotten marine plugs.
Lesson learned.

 

[bonzoscott]

Re: i need help - loud knock slow to reach top speed?

Does that mean all is well and no knock? A loud "one time" knock sounds like a broken gear tooth to me....