Engine knocking on low rpm

[Liviu B]

Hello. I'm new on this forum but I read and learned a lot from here. First of all, sorry for my English.
I need some help from you.
I have a Maxum 1900 SC with Mercruiser 3.0 (1993) engine with Est Ignition.
When I start it cold, run very rough and after warming run better but has a lot hesitations and start to knock. When I rev it, seems to stop knocking. The engine was rebuilt 3 weeks ago and never on water. Started only on mufs.
- lifters checked
- valve adjusted
- timing 1 btdc
- Carb rebuilted
- Ignition changed
- compression 125 to 135
​​Any suggestion is welcome. Thank you very much.
I'm desperate. I don't know what to try anymore.
Here is the video.

 

[Bondo]

The engine was rebuilt 3 weeks ago and never on water.

Ayuh,..... Welcome Aboard,..... rebuilt by whom,..??..??

That's who you should be asking,......

 

[Scott Danforth]

if it was just rebuilt 3 weeks ago, your compression should be closer to 150 psi and there should be no knocking

agree with Bondo, talk to whomever did the rebuild.....

 

[Liviu B]

Engine was rebuilt by me. I don't have much experience but I read a lot. I used Service manual 26 step by step.
​- New cam, crank and rod bearings
- Checked clearence everywhere with Plastigauge
- New rings
- Checked gap of the rings in bores
- Bores honing with that tool with 3 arms
- all the screws were tightened with the torque wrench
- head machined and new seals and seats
With the compression is a mistery for me. Someone told me that before install old lifters must pull off oil through them. (are in good shape) After I did that and install them again and readjusted the valves, compression on cold was 140-150 at all. After runing engine 20 minute and check again, the compression droped to 125-130. The valves were adjusted with 3/4 down from 0 lash (no up and down move).

 

[Scott Danforth]

the service manuals are intended to be used by those with experience. in this case, with rebuilding an engine.

those floppy spring loaded hones do not straighten out the cylinders, or do any real honing. they tend to do more harm than good

your compression should go up after running for 20 minutes, not down. that indicates a problem.

with a flat tappet cam, if your lifters are riding on a different lobe (if you didnt number them), they will have eaten them selves running in the first 20 minutes. flat tappet cams and lifters must be broken in together when new, and then are a mated pair until death.

if your motor needed new pistons, it needed to be brought to a machine shop, have the crank bore trued, then each piston hole would needed to be bored, then rigid honed.

what did each bore measure with the bore gauge?

this is an engine hone machine

and these are the rigid hones

 

[Liviu B]

The bores looked good with no major signd of wear so I didn't measured them. I checked only rings gap in the bores. I didn't want to spent more money for honing bores on machine to next size pistons.

 

[Scott Danforth]

The bores looked good with no major signd of wear so I didn't measured them. I checked only rings gap in the bores. I didn't want to spent more money for honing bores on machine to next size pistons.

then why was the rebuild needed if there was no major signs of wear? However unless you bore and hone to ensure the cylinders are round and true, you cant simply hone and go. the cylinders will taper and egg shape from normal running and not show any wear. then if there is an over-heat, its guaranteed that the bores are out of shape.

 

[Liviu B]

then why was the rebuild needed if there was no major signs of wear? However unless you bore and hone to ensure the cylinders are round and true, you cant simply hone and go. the cylinders will taper and egg shape from normal running and not show any wear. then if there is an over-heat, its guaranteed that the bores are out of shape.

The rebuild was needed because of a road bearing knock. Ok. So do you think the knocking is from piston? A slapping piston? I pulled off all wire plugs one by one and the knock is still there. The knock is with the same speed as camshaft spin and sparks for one cilinder.

 

[Bondo]

The rebuild was needed because of a road bearing knock.

Ayuh,..... So what did you do to fix this,..??

If it was just swap in new bearings, I'm afraid you've done a 1/2 azzed rebuild, 'n the results speak for themselves,....

Motors are like a finely tuned swiss watch, 'n require the same attention to details,....

 

[Liviu B]

Ayuh,..... So what did you do to fix this,..??

If it was just swap in new bearings, I'm afraid you've done a 1/2 azzed rebuild, 'n the results speak for themselves,....

No, the crankshaft was machined for +0.25 bearings and checked clearence.

 

[Bondo]

Was the rod replaced,..??

 

[Liviu B]

No. The rod was ok

 

[Scott Danforth]

rods need to be re-sized or replaced. they are never OK if there was a spun bearing.

if the bearing spun, usually the block needs to be line-honed to true up the crank bearing saddles as well.

 

[Liviu B]

I understand. A lot of work and money for nothing. It was a bad lesson for me. And the hesitation of the engine what could be?

 

[Scott Danforth]

without a proper tear down to find out what is causing the motor to shake like a dog passing a peach pit, not sure.

my guess, the bearing is spun again and its eating itself.