3.0 Mercruiser Timing Problem

[KCBINWV]

There seems to be valve ping/knocking at high rpm's. I have the timing set to 6 degrees btdc at around 700 rpm. When I increase the rpm to 1000, the timing jumps to about 12 degrees btdc, and then jumps off the scale as the rpms are increased. I have checked the points gap and they are within spec. The timing advance mechanism seems to be okay. Is this timing change normal for this motor? If not, what could be causing the problem? I only have 140 hours on the engine.

 

[akriverrat]

Re: 3.0 Mercruiser Timing Problem

how high rpms,what year engine?

 

[KCBINWV]

Re: 3.0 Mercruiser Timing Problem

The knocking starts after the engine is warm and turning 2300 rpm. The boat/motor is a 1984 model.<br />I don't know if this change in timing vs. rpm is normal.

 

[Trent]

Re: 3.0 Mercruiser Timing Problem

Iv not seen this question in awhile. <br /><br />Ok the timing is 6 degrees at 650-700 rpm. If you would look at the distributor advance curve you would see the RPM's will peak close to 28 degrees at WOT. The noise you are hearing..Does it sound like a real rapid tap? If so its normal for the 3.0 Its the exhaust shutter.

 

[Boatist]

Re: 3.0 Mercruiser Timing Problem

Yes the timing advance is normal but take your timing light on the water and adjust to zero and make a run. If noise goes away you are on the right track. My OMC 1980 3.0 did the same thing for a while and I had to run 91 octain to get it to stop. My onwers manual says run leaded gas 87 octain or higher. After I pulled the head off to replace a bad valve no longer pings on 87 octain. I not sure if head gasket thicker or carbon I cleaned out of head fixed the problem. Anyway when your fuel is low try some 91 octain and see if it helps. This way will not have to put in a lot of fuel to see if makes a difference.<br />Good luck and please come come back to this thread and let us know how you make out.

 

[Weasel6]

Re: 3.0 Mercruiser Timing Problem

Been there,done that and pulled out all my hair over the same thing. I did it all. I checked and adjusted my timing until I was blue in the face then Viola' !! I found it. Check the shaft on the distributor. Mine had a bushing that was worn out and the shaft would wobble about and give various reading at various times. Pull the distributer and spin and play with the shaft and see if it isn't loose. I bet it is. Now the bad news. I had to replace the dist. because kits are obsolete. Have to look about and find a used one. I did and it made all the difference in the world.

 

[KCBINWV]

Re: 3.0 Mercruiser Timing Problem

I don't think it is the shaft under the distributor. The engine only has 140 +or- hours, and everything seems feel like new. I have thought it may be the flapper in the exhaust. I can here it at idle speed. Could carbon build up cause this noise? I hate to pull the head to look. I'm going to try high grade fuel next. Then, maybe an additive for carbon. Any suggestion on what kind?<br /><br />Thanks, and I'll keep you posted. More later...

 

[KCBINWV]

Re: 3.0 Mercruiser Timing Problem

Took it out this past weekend and ran 93 octane. It didn't seem to make any difference. The engine really runs smooth and has plenty of torque. I am going to try checking into the exhaust flapper valve. Anybody try removing one. I understand this flapper prevents water from entering the cylinder through the exhaust valve if the engine would stall while backing up hard. Should I remove it or replace it?

 

[Trent]

Re: 3.0 Mercruiser Timing Problem

As I said before.........Tapping of the 3.0 (in the area of the exhaust shutter)is NORMAL!! If you replace it. It will still tap. No worries mate! Go have fun!!