High RPM spark knock, merc 350

[tcgravy]

1987 merc 350

To give a quick recap.....engine had water in the oil, determined via leakdown and pressure test that cylinder #5 had a bad intake valve and #3 had a coolant passage leak. Found out there was a blown head gasket and one of the heads was cracked too. So got a set of remanufactured heads to replace the heads that were on there. Same cast heads, 193, got a good deal on them. bla bla bla

So put the new heads on.....compression is 150-170 in every cylinder.....engine seems to run and idle great.....except.....

At high rpm/high load there is a major SPARK KNOCK.

Symptoms are here, at maybe 3600 rpms when under high load it will do it, or at higher rpms, WOT around 4000 it will do it. It will not do it in neutral. RETARDING THE TIMING is the only thing we've found to make the noise go away.....but, the timing has to be retarded about 12 degrees beyond spec (to 4 degrees retarded, ATDC instead of 8 degrees advanced, BTDC)

So with the engine retarded this far, it only revs to about 4000 rpms.......it should rev to right around 4600 rpms.

So....anyone have any ideas???

 

[Don S]

Re: High RPM spark knock, merc 350

Have you checked to see exactly what the advance is doing?

 

[tcgravy]

Re: High RPM spark knock, merc 350

Have not checked to see what advance is doing.....the timing light I have can only set / show me idle timing, cannot check advanced timing. From what I've read, the timing should be fully advanced by 3200-3400 rpms....so I'm assuming that its fully advanced by the time this is happening.

Are you thinking this is happening but it just keeps advancing? and is advancing too far?


by the way....can i run it all day with the timing retarded?? as stated, retarded 4 degrees ATDC it will not spark knock at all.

 

[Don S]

Re: High RPM spark knock, merc 350

Do you have the Thunderbolt IV electronic ignition, or points distributor?

you don't need an advance timing light to see what it's doing. Just put some extra marks at say 5? intervals on the dampner. Get the spacing off the timing mark tab.

 

[tcgravy]

Re: High RPM spark knock, merc 350

Good point - I hadn't considered that. That's definitely worth a try. Is it OK to rev the engine up to 3500 rpms on the muffs? I can probably get to the lake to do that in the water.....thanks, thats a great idea.

It is TB IV, electronic ignition. There does appear to be a very small amount of corrosion inside the distributor (on the bottom of it) just from when it sat while doing the head swap. Very small amount of corrosion.

 

[Don S]

Re: High RPM spark knock, merc 350

3500 for short burst is ok on muffs. Just make sure the drive is down in the normal range and aimed straight, not turned full lock to one side.

 

[tcgravy]

Re: High RPM spark knock, merc 350

So....again, from what I have read, I should be looking for 22-24 degrees of advance, for 30-32 degrees total advance, for the 350. Is this correct? I'm going to try your method and see if I come up with.

If it advances too far, what does that mean? bad distributor?