Engine knock

[nateo]

It's idling on muffs at 1200 rpm and has a knock stardboard side of engine. Idle screw is turned all the way down and I adjusted timing at 8 btdc but at this high rpm. Any chance this is something simple?

 

[Bt Doctur]

remove the control cable, adjust the idle rpm to its spec or find out why you cant, then check adjust the timing.

 

[Speakrdude]

Try saying, "who's there?"

 

[flipbro]

Its likly Mr rod bearing. And hes not happy. All joking aside.Try what Bt said then pull that over reved engine and replace. Seems like problem after prob

 

[nateo]

I got frustrated today and dropped it off at shop. Now that i thunk about it im wondering if i have that throttle cable routed correctly. Seemed to have some tension on it. I thon doc is right.

 

[Bt Doctur]

it wouldn't matter if you routed it thru the hull and tied a knot in it, you adjust idle speed without the cable attached, then attach the carb arm first, then adjust the barrel to put slight tension closing the throttle

 

[nateo]

Well I went and picked it up today and thought I would give it one last try. I adjusted throttle cable and did get the rpms down to 750 (in water), any lower then that and it sputtered and died. There was still a knock even at that rpm. I dropped it off at shop. I gave it my best but i'm done messing with it.

Just curious is it possible it could still be something relatively simple or does it likely need to be rebuilt?

 

[nateo]

and follow up question. Is it possible that engine could still need a rebuild even if compression test is all in spec?

 

[nateo]

I've been reading up a little and have a better description of the sound...Detonation...it kinda vibrates the whole motor a bit. It's not consistant though. Could this be timing still?

 

[skippy2235]

With all you have been through, the high revs, broken valve keepers, bent valves.
my guess is your rod is a telling you something, and that is fix me.
Knock knock, I am bad.
My suggestion, cheaper to rebuild / replace motor. sooner then later.
IF you decide to just run it, it gets worse, then you know for sure. cost more money.
take your time, do it right.
Mine did the same thing 2 years ago.
also make sure you use enough oil on new engine. That was my issue, internet was not correct. I was using 5 qts should have used 7.
been running 7 for 2 years now.

 

[81 Checkmate]

Got a question Skippy? Are you talking a big block with 7 quarts of oil. Is it a small block with a oversize oil pan? Im just asking becouse never heard of a avarage small block running 7 quarts of oil at least not in your run of the mill boat.

 

[skippy2235]

No small block 351W with a truck pan.
My issue was pan rusted through. Center sump.
Put on truck pan with rear sump. Yes replaced pickup tube.
5 qts oil lasted 15 hrs before rod knock.
Just saying when taking pan off measure where 5 qts put you at. In a boat all that bouncing and rocking at 3k RPM. Does not leave much oil left in sump.
As long as crank does not hit oil. Fill it up.

 

[nateo]

With all you have been through, the high revs, broken valve keepers, bent valves.
my guess is your rod is a telling you something, and that is fix me.
Knock knock, I am bad.
My suggestion, cheaper to rebuild / replace motor. sooner then later.
IF you decide to just run it, it gets worse, then you know for sure. cost more money.
take your time, do it right.

Been thinking the same thing for a while. Good to hear another opinion. Wish I would have rebuilt that sucker at beginning of summer when I had the motor out. Thinking this might become a winter project. Best I've found on a replacement is $1800 shipped and still have to switch a lot of the components. I might just end up doing a partial rebuild myself this winter. I had 150 on all cyclinders so I suspect some components may be reusable.

 

[skippy2235]

Since you have good heads, and good compression. to get a few years, what I did the first time was:
took engine out, took off pan, took out crank, tuned crank and replaced the bearings, put back together.
less then a week.
DID NOT take off heads or manifolds, Just the bare min to get crank out.
lasted me untill the 5 qt thing happended. My guess it would still be working had I measured the oil height first.

 

[81 Checkmate]

Skippy - Thanks for the clairfication on motor size. Some people might have seen that and started dumping more oil in their motors than what they need.

 

[nateo]

Since you have good heads, and good compression. to get a few years, what I did the first time was:
took engine out, took off pan, took out crank, tuned crank and replaced the bearings, put back together.
less then a week.
DID NOT take off heads or manifolds, Just the bare min to get crank out.
lasted me untill the 5 qt thing happended. My guess it would still be working had I measured the oil height first.

Interesting about the bearings...Sometimes I hear a screeching noise. never knew if it was belt or something else

 

[DeepBlue2010]

In your first post, you said that the idle mixture screw is tighten all the way down. In a following post you mentioned that after reading a little, your better describer of the sound is detonation. If you tempered with the idle mixture,, the engine may be running lean and this will cause the detonation. Engines are not a through away components, if you don't have enough expertise to diagnose and fix an engine problem, you should leave it to the pros. You still could run into problems with a rebuilt engine or during swapping peripherals. I don?t think you will thought that one away or rebuild it as well.

Compression is not an indicator that the engine needs (or doesn?t need) a rebuild. It is a diagnostic tool that ? in the hands of a skilled technician ? can help him/her decide what needs to be done. A low compression can be due to an unadjusted or worn out valves. In this case, nothing need to be done to the block at all. Also an engine with perfect compression that spun a bearing will need rebuild.

 

[ricohman]

A knock is not going to go away. Well, I guess it will eventually when the motor is really done. Only by tearing it down will you be able to tell if the crank or offending rod can be saved. Perhaps you could get by with turning the crank and some oversize rod bearings.

 

[nateo]

In your first post, you said that the idle mixture screw is tighten all the way down.

I wish it was just an idle air fuel mixture screws but they are currently set two and half turns from the bottom (I learned how to adjust those with the help of individuals on this site last summer). Don't ever recall saying anything about them being tightened all the way. The only problem I have with taking it in is that I have no idea whether or not they are as good as some of the wizards that room these halls. lol..that and I know all the history. The detonation problem troubles me though due to the fact that particular problem just started recently, which makes me think it was something I did. Well anywho....I took it into the shop a couple of nights ago. I went to war with this boat this summer. It troubles me to completely give up. Looking forward to hear what the pros say and then go from there. If they say it is something internal to engine I might end up taking it on myself. I'm not a pro or a wizard but I like to learn and don't mind working hard. Reading and watching youtube videos is extremely helpful but we also learn by doing. and making mistakes, haha

 

[DeepBlue2010]

Your first post

It's idling on muffs at 1200 rpm and has a knock stardboard side of engine. Idle screw is turned all the way down and I adjusted timing at 8 btdc but at this high rpm. Any chance this is something simple?