Detonation issues...rebuilt mercruiser 165

[WOEISMEIGOTTA470]

Hello all you mercruiser experts...

I have a currently active thread in the restoration area, a 19' cuddy cabin with a rebuilt 165 mercruiser. I recently had the boat running in the water again which is really awesome, except for that it seems I am experiencing detonation between 2-3,000 rpm. Basically under more engine stress when the boat it trying to get on plane, I am getting a pretty bad rattling noise. I don't seem to get this noise running it in the driveway or put-putting around in the channel. I have my initial timing set to 6?btdc, and am running 91 octane, so I'm a bit puzzled as to why I'm having this issue at all. Only thing I can think of is the advance curve is off, and the total timing is too far advanced, or possibly the fuel mixture is too lean??Otherwise engine idles great, has excellent throttle response and seems to have good power.

Other thoughts:
The Merc 165 engine originally was manufactured in a time where leaded fuels were around. I bought the longblock already rebuilt, so no idea what the compression ratio is. My compression test yields about 160psi, if that helps any.

Possibly unrelated, but there seems to be a more than average amount of crankcase vapors venting from the breather. Is that something typical since it's a fresh engine, and is not fully broken in yet?

I recently adjusted valves and will try putting on a less aggressive prop, 17p vs. a 21p and see if it makes any difference on my next water test.

Any other ideas or suggestions on diagnosing my problem? Thanks in advance! (Pun intended, ha!)

Video below @ 1:03 you can hear the bad sound, before I back off the throttle.
https://youtu.be/IIrNUYzUMd8

 

[Scott Danforth]

I would start with the prop.

 

[harleyman1975]

Yup the 17 pitch prop will most likely fix your problem.

 

[Bondo]

Possibly unrelated, but there seems to be a more than average amount of crankcase vapors venting from the breather. Is that something typical since it's a fresh engine, and is not fully broken in yet?

Ayuh,.... I agree, droppin' to the 17" prop will be a wise choice, til it's broken in, 'n you can run some Wot rpm tests,.....

I'd expect blow-by, til it's broken in,...

A few hours under a good load oughta do it,.....

If ya hear it pingin', make it Stop,....
Detonation is Bad, the piston is tryin' to go up, 'n the combustion slams it down, before it hits Tdc,...

 

[WOEISMEIGOTTA470]

Yeah, I definitely will try the prop swap, possibly tomorrow. I soon as I heard the noise I backed off right away so. Hoping the prop makes the difference. Thanks guys.

 

[alldodge]

initial timing set to 6?btdc, and am running 91 octane,

Manual 3 states timing should be set at 8 BTDC
Plugs MR43T
Point gap 16-19 and Dwell 28 to 34 degrees
http://boatinfo.no/lib/mercruiser/manuals/mercruiser3.html#/34

Running 91 octane will not anything other then empty you wallet faster.

Are you setting the timing correctly

 

[harleyman1975]

"Running 91 octane will not anything other then empty you wallet faster."
I know my 383 in my plowtruck pings on pump gas (87) but not if I run 91...Think what you will. It starts better too.

 

[WOEISMEIGOTTA470]

Thanks for the timing info Alldodge. The fact that I'm using 91 and timing is set to 6*, less adv. than the manual spec, is peculiar IMO. I'm sure dwell is fine cause I'm not running points, as stated I'm using a pertronix kit.

Here are my plugs, 1-6 from left to right. They are NGK's BR6ES

 

[alldodge]

Thinking their new plugs the white may just be not enough time, otherwise I would be thinking its running lean

 

[Bondo]

Ayuh,.... Agreed, looks like a fuel delivery issue to me,...

 

[NHGuy]

Put on that shorter prop and drive it. Plugs don't get black that fast with good ignition. And I can say for sure that high octane resists preignition and detonation better that regular. That's what it is for.
Once your engine is propped so it will make the correct full throttle rpm you should not have any more pinging. And it should not need the premium gas or reduced timing either. Switch back to regular gas, then set the timing so it has the correct all-in advance. See what Mercruiser designed in. Since you have an aftermarket ignition it may require a different idle setting than OE. If the ignition kit was designed for your engine it probably has a recommended timing setting in the setup sheet.

 

[WOEISMEIGOTTA470]

Thanks, I'm definitely swapping props, going to head out tomorrow for sure and will report back. I'm also looking into colder plugs if necessary, learned a few pointers from NGK's website https://www.ngksparkplugs.com/about-ngk/tech-talk/spark-plug-basics

Hoping for a good smooth WOT run and no noise. 17 is a pretty significant drop from a 21, same quicksilver aluminum black diamond brand too. If my memory serves me right, when I had the 470 in it (180hp, 4bbl), the 17p wasn't enough prop. Alpha drive is a 1.84 ratio. I figured with thay drive ratio the 6 cylinder would need more prop. Anyways well see what happens tomorrow!

 

[harleyman1975]

Looking fwd to the results!

 

[WOEISMEIGOTTA470]

Dammit....made no noticeable difference. I'm out here now on the water, anchored and letting the engine cool down. Then after its cool I'll quickly start her and run up the throttle and see if it's any different. Only thing I can think of is it's jetted too lean.

 

[achris]

Have you run a timing light on it right through the rev range? You need to make sure the curve isn't adding too much advance too early.

Chris......

 

[WOEISMEIGOTTA470]

Have you run a timing light on it right through the rev range? You need to make sure the curve isn't adding too much advance too early.

Chris......

Yeah, I checked the timing up to where it stopped advancing, all in its short a few degrees of 30.
I dropped the carb off at the carb doctor by me. He's gonna open it up and find out what jets are in there and get back to me. I researched and did find out the mercury part number for the 2bbl Rochester, as well as the jet that is supposed to be in there, a .082" orifice. The carb shop is closed on weekends, so won't hear anything till next week :/

Carb assembly diagram:
https://www.perfprotech.com/mercury-...5/6?model=1159

 

[NHGuy]

​Also make certain it's not getting too hot. Have you got a laser thermometer? Might run it up to speed then quick take a bunch of temps. When you run it on muffs in the driveway do you get tons of cooling water? It should flow a lot of water so the thermostat housing can allow as much through the engine as the stat requires.
This is tougher to test, because you can't really see how much cooling water is flowing under load. But heat can play a part.
I just watched the first video. I heard the point where your engine rattled, good going to pull back quick.

 

[NHGuy]

Cold plugs are normally only needed for high compression. If it is high compression they can be a help against your problem.
You should not be needing them unless the head and/or block were decked so far that the compression ratio has gone up by around a point.
Pistons cause a smaller chamber due to less dish, were they changed?. And thin head gaskets raise compression too.
I'd confirm top dead center and be certain your timing mark is right. You can do that while the carb is away.

 

[jimbo_jwc]

Having just read your post and rebuild thread n being a 1987 Thompson 196CC Sea Rage owner I noticed one thing that concerned me . You mentioned that you got a heat exchanger from a 3.0L Mercruiser and I think that is way to undersized for your six cylinder . Heat could be your problem .

 

[RobertThoreson]

I think NH guy is on to something. Double check your timing marks vs TDC. Then if that's all good, manual back the timing off more to see if it goes away. Changing props would have just bandaided the problem. If you can't dial it in with manually retarding the timing, you will need to find out your compression ratio. Sometimes when putting an engine together one may put parts from a donor motor on thinking they are the same but not realizing domed pistons with smaller cc heads plus a copper shim head gasket they found cheap makes a screamer of a motor but a nightmare to tune.

Not saying that's exactly what you did but the manual is just a starting point where it comes to timing.