Timming Advance/preignition combustion?

sabastianunf · Jul 20, 2007

I have a 2.5L 120 hp 4 cyl mercruiser sterndrive. It starts, idles and launches great. However when running at WOT or High RPM's under heavy load the valves start making a strange noise and loosing some horse power. It can be best described as sounding like a marble spinning around in the bottom of a tin can. If I back off the throttle a little it will go away. My father says that I may have my timming advanced too much. My motor still has good compression but is so old that you can not see the timming marks anymore. I have set the timming by ajusting the distributor until the engine sounds the best at idle or midrange rpm but with no load. Next time I have it in the water I will ajust while under load but I was just wondering if anyone else has had a simmilar situation or can confirm this diagnosis? Thanks for any good advice.

Don S · Jul 20, 2007

Re: Timming Advance/preignition combustion?

Get a timing light and see if the advance is working properly.

Robj · Jul 20, 2007

Re: Timming Advance/preignition combustion?

The timing marks on the pulley should be notched, so even if it is rusty and dirty you should still be able to see them. Check and adjust your timing with a light to the correct spec. Adjusting by sound is not a good way to do it. Another thing you may want to try is to run premium fuel once you adjusted your timing. You must resolve your pinging problem or it will destroy your motor.

Good luck and have a great day,

Rob.

thumpar · Jul 20, 2007

Re: Timming Advance/preignition combustion?

I am 99% sure with a timing light you will see the timing mark.

Zero Balance · Jul 21, 2007

Re: Timming Advance/preignition combustion?

I agree, you should be able to see the marks. All the same you can just bring it up to TDC, mark the old marks or make a new one with a soap stone or white out. You should be able to put a timing light on it. my light has the degree knob adjustment. If your light doesnt have this feature timing tape on the balancer may be necessary.

FreeBeeTony · Jul 21, 2007

Re: Timming Advance/preignition combustion?

I had the same problem with my IL6.........

He may be able to see the mark on the damper but the tab may be corroded and he can't see how to time it.

When I did my IL6 bondo actually took a pic of one he had around and was able to help me..........I thought that was great.

It does sound like it's too far advanced though. You really need to time it acturately so it doesn't damage the pistons.........

Good luck.

FreeBeeTony · Jul 21, 2007

Re: Timming Advance/preignition combustion?

Just found this old post..........unfortunately I don't have the pic bondo sent me.

http://forums.iboats.com/showthread.php?t=45403

sabastianunf · Aug 6, 2007

Re: Timming Advance/preignition combustion?

Don S said:

I found this pic from another thread. Is the tab the same on my merc 120 or do the peaks/valleys represent different timming on mine? Also I know you set the base timing first then put the light in advance mode. Also wondering 1) does the engine need to be under load and at wot to checking the advance. 2) Can I check the advance with the boat out of the water and on muffs? 3)where should I be as far as the tab when checking the advance - hopefully while on the trailer?

Don S · Aug 6, 2007

Re: Timming Advance/preignition combustion?

I don't have a clue what your 120 looks like, don't even know the year. But one thing I can tell you for sure is that there is no BASE timing mode on any 120's with the OEM ignition system, You just set the timing, then rev it up and see how many degrees the advance moves it. (Yes, you use a timing light, but how you adjust it depends on the type timing light you have.)

sabastianunf · Aug 7, 2007

Re: Timming Advance/preignition combustion?

sorry I thought I had listed the year above. Mine is a 1978 120 hp mercruiser 4 cylinder sterndrive. I can post some pics if needed. There just aren't any visible marks on my tab so I am just trying to figure out where my timing mark should line up on the tab. I also want to make sure that my timing is advancing all the way since the valves/motor only makes odd noises at WOT under load. My timing light is not working so I am going to buy another one - which is easiest/best type for checking these things? Does the pic above show timing marks for a 1978 120hp mercruiser sterndirve? Thank you for any help on these issues.

bruceb58 · Aug 7, 2007

Re: Timming Advance/preignition combustion?

When you buy your new timing light, get an "advance" type so that you can measure the advance at high RPM.

If you take a picture and post it looking down at your crankshaft, we might be able to help you with your timing tab.

Don't ever ever set your timing the way you did it...good way of ruining your engine.

Bondo · Aug 7, 2007

Re: Timming Advance/preignition combustion?

Does the pic above show timing marks for a 1978 120hp mercruiser sterndirve?

Ayuh,....... Notice that the 0* slot is Deeper than the others.......

I also want to make sure that my timing is advancing all the way since the valves/motor only makes odd noises at WOT under load.

Ayuh,....... It's Advancing Alright,........ That's Why your motor is Destroying itself.........
It's Timed Way Too Hot Now......

Those "odd noises at WOT under load" are your pistons being Consumed by Detonation..................

sabastianunf · Aug 7, 2007

Re: Timming Advance/preignition combustion?

Thanks for the help y'all. I agree that my timing was too advanced and I will time it with an advanced light tommarow. However, I never really had this problem before I put a new prop on my boat and the last time I had it out I ran it at WOT and retarded the timing just a little until the valve noise went away. It ran good for a little while then it started backfire/pop when running a straight line at WOT but as I would load the motor going through a turn I would start to hear the valve noise again. I know the bolt was tight so could this be a sign that my distributor is not advancing properly or could this have anything to do with the smaller pitched prop? The boat perfoms well with the new prop - gets a great hole shot and runs 30 MPH at 4200 RPM at WOT (rec range is 3900-4300).

Bondo · Aug 7, 2007

Re: Timming Advance/preignition combustion?

Not only are you running an Unknown Timing setting,.......

then it started backfire/pop when running a straight line at WOT

It sounds like your running Lean as Well..........

Change the Fuel Filters,+ set the timing with a Light...........

You're Lucky to not have Holes in the tops of your Pistons by now..........

Timming Advance/preignition combustion?

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