1987 Merc 350 noise at high RPM, timing related

[thedillybar]

I have a completely stock 1987 Merc 350. Compression 150-180psi all cylinders. Recently replaced heads after discovered water in the oil, low compression in cylinder #5. Found leak in head gasket between water jacket & cylinder #3. Found crack in head on #5.

The timing is controlled by an electronic Thunderbolt IV. Stock timing is 8* BTDC (at idle).

With the stock timing and 87 octane, the engine starts to make noise (call it a knock, ping, whatever) around 3000rpm (under load) up to WOT. As the timing is retarded, it doesn't make noise until higher and higher rpms. (for example, 6* BTDC 3200 rpm, 4* BTDC 3400 rpm, etc. [made-up #s])

In order to make the noise go away at WOT, I have to set the timing (at idle) to 4* ATDC. At this point, the engine runs ok (no noises, starts good, runs good) but only revs 4000rpm when it previously rev'd 4600rpm in this boat. Clean bottom. 4600rpm before was in freshwater; now in saltwater.

Previously blamed this on bad timing marks, but this weekend verified that the timing marks are pretty darn close. Stuck pushrod in #1 at different crank positions. Goes in the least distance near 0*, and approx. the same distance at 16* BTDC and 16* ATDC. It might be off a few *, but not 12*.

 

[chiefalen]

Re: 1987 Merc 350 noise at high RPM, timing related

Who adjusted the valves on the new heads?

 

[thedillybar]

Re: 1987 Merc 350 noise at high RPM, timing related

Thanks for the response.

I adjusted the valves. 3/4 turn past zero lash I believe (zero lash determined by can't spin the pushrod by hand). After I heard the noise I went another 1/4 turn and it didn't go away.

I would expect to hear valves at idle, and not expect them to be affected by changing the timing.

 

[Maclin]

Re: 1987 Merc 350 noise at high RPM, timing related

You may have more than one problem.

To me "Can't turn the pushrod" is past zero lash some. "Slight resistance while turning" is more like zero lash in my thinking. If you can't turn the push rod then the lifter has already compressed some.

That 4,000 rpm "limit" sure sounds like the valves are too tight on the lash.

I am not sure what would make it knock like that, just some guesses. It could be improper heat range plugs or incorrect gap. Fuel quality comes to mind as well. Could just be one cylinder pre-igniting due to a sharp edge somewhere or problematic spark plug.

 

[chiefalen]

Re: 1987 Merc 350 noise at high RPM, timing related

My thinking is they are to tight.

Had that problem on a friends boat.

Same symptoms.

Drove us nuts, till we figured it out.

Was on a new motor with new heads, straight from Michigan motors.

We went after the timing also.

I tell you my friend the owner of the boat, was gonna throw me overboard, cause i yanked the valve cover off when he was in the head on the boat.

Than on the water with all our tools i went after the valves.

Couldn't be anything else.

A year latter and his boat is running like a top.

 

[thedillybar]

Re: 1987 Merc 350 noise at high RPM, timing related

Thanks for the response Maclin.

To me "Can't turn the pushrod" is past zero lash some. "Slight resistance while turning" is more like zero lash in my thinking. If you can't turn the push rod then the lifter has already compressed some.

I rotated the pushrod constantly while tightening the rocker nut. As soon as I felt any difference, I'd consider that "slight resistance while turning". About 1/16 of a turn later was "can't turn the pushrod."

That 4,000 rpm "limit" sure sounds like the valves are too tight on the lash.

Not sure why valve lash too tight would limit the rpms. It seems to me that if the valve lash is overtightened, the valve will never close all the way. This would result in power loss, but I suspect would be evident in a compression check.

I am not sure what would make it knock like that, just some guesses. It could be improper heat range plugs or incorrect gap. Fuel quality comes to mind as well. Could just be one cylinder pre-igniting due to a sharp edge somewhere or problematic spark plug.

Appreciate the ideas, I think I've covered these though. Brand new set of MR43T gapped .035" (recommended). Haven't tried a 2nd set of plugs with the new heads, but looking at them...no reason to suspect an issue. Been through a full tank of gas, 3 different gas stations.

 

[thedillybar]

Re: 1987 Merc 350 noise at high RPM, timing related

Thanks for the followup.

My thinking is they are to tight.
Had that problem on a friends boat.
Same symptoms.
Drove us nuts, till we figured it out.
Was on a new motor with new heads, straight from Michigan motors.
We went after the timing also.

I guess I have to try it if you had the same symptoms. Did retarding the timing fix the noise?

Wouldn't the compression suffer if the valve lash was overtightened?

 

[chiefalen]

Re: 1987 Merc 350 noise at high RPM, timing related

Maclin beat me to the punch, was probably, posting while i was still typing.

And he is good.

Listen to him, re-adjust the valves run it and post the results.

You just might be surprised.

Good luck !

 

[Maclin]

Re: 1987 Merc 350 noise at high RPM, timing related

Thanks chief, just followin' your gut feel on the valve adjustments.....

 

[chiefalen]

Re: 1987 Merc 350 noise at high RPM, timing related

Listen if i were you, would i do what a beaten up, old ,half a dead man tells me to do, would i listen to him?

Up to you.

Your boat.

Just keep us posted how it works out.

 

[Bondo]

Re: 1987 Merc 350 noise at high RPM, timing related

Ayuh,...

#1,... I'm pretty Sure you've got the valves, Way Too Tight....
0 lash is when there is NO, Up, Down movement left...
If you tighten til the pushrod doesn't turn,... That's WAY Too Tight...

2ndly,...
Detonation is caused by several things,....
1 of which I don't see mentioned anywhere here....
A Lean condition....
Are the Fuel Filters Clean,..??
How about the Carb,..??

 

[chiefalen]

Re: 1987 Merc 350 noise at high RPM, timing related

The op seems a very capable wrencher.

He already knew to check all that.

 

[Maclin]

Re: 1987 Merc 350 noise at high RPM, timing related

Hey DillyBar,

After checking the lash and all I would put in new plugs and so some WOT runs then look at them to see if a lean condition jumps out at you as Bond-o suggests. I did have that in mind but the valve lash kind of took over, not as good at multi-task-typing as I used to be

 

[KJSmitty]

Re: 1987 Merc 350 noise at high RPM, timing related

Thanks for the followup.



I guess I have to try it if you had the same symptoms. Did retarding the timing fix the noise?

Wouldn't the compression suffer if the valve lash was overtightened?

Call me offsides yet I've never had out of adjustment rockers-hydraulic lifters give this symptom either - but obviously some of the resident experts have so that very well could be it.

I don't believe it was brought up but did you check the max timing at say 3000-4000 rpm?? I know nothing about your specific distributor workings or timing within the "Thunderbolt IV", but could you have a centrifugal-mechanical (or possibly vacuum, ie from wrong vacuum source) advance issue that is throwing in too much advance at higher RPM's etc.?

Just a thought yet could be mute given your specific setup etc..

Good luck

 

[meesh]

Re: 1987 Merc 350 noise at high RPM, timing related

Just a quick question, and not to be insulting. When you set the valves at zero plus 3/4 did you make sure each one of them was on the flat side of the cam lobe? Hope you find the answer.

 

[chiefalen]

Re: 1987 Merc 350 noise at high RPM, timing related

I'm gonna wait till he does whatever he choses to do.

And post what he did and results, till i comment again.

 

[mthieme]

Re: 1987 Merc 350 noise at high RPM, timing related

They sound tight to me too.
Did you perform the valve adjustment with the engine running at operating temperature?

 

[mthieme]

Re: 1987 Merc 350 noise at high RPM, timing related

They sound tight to me too...by at least 1/4 turn.
Did you perform the valve adjustment with the engine running at operating temperature?

 

[180shabah]

Re: 1987 Merc 350 noise at high RPM, timing related

....I know nothing about your specific distributor workings or timing within the "Thunderbolt IV", but could you have a centrifugal-mechanical (or possibly vacuum, ie from wrong vacuum source) advance issue that is throwing in too much advance at higher RPM's etc.?

Thunderbolt IV is electronic.

 

[JustJason]

Re: 1987 Merc 350 noise at high RPM, timing related

If the valves were to tight it wouldn't cause detonation. If the valves are to tight, the valve timing essentially changes, and your going to lose some compression. On top of that if the valves are to tight they are going to float at a lower RPM.
I would double check the lash first, then be looking for a lean condition as Bondo said. The other question is how big a boat are we pushing with this engine?