Mercruiser 470, hero to zero. Timing chain jumped?

[Oshawapilot]

Starboard engine in a 30' express cruiser twin.

This engine has run like a champ for years. Currently shows 145 to 155 PSI across all 4 cylinders. Lots of oil pressure, runs in a comfortable temp range and not really abused.

From one day to the next last fall this engine went from running beautifully to suddenly refusing to do much past idle - it immediately starts to backfire in short order once you put it under some load. Runs OK at idle for maneuvering.

Happened last fall, but it was left as is. The boat just went back in the water last week (port engine received a rebuild that ran long over the winter) and it's time to tackle this engine now.

Is it possible the timing chain jumped? Plugs, wires, cap, points and condensor are on their way and will be swapped out shortly, but I'm left wondering if the problem is beyond that given as how it came on so suddenly.

Thougths?

 

[Eric 3.7LX Alpha 1]

These 470 based engines do have a timing chain tensioner. I don't know how common they are for failure. Do you have room to pull the harmonic balancer off with the engine in the boat.

 

[Oshawapilot]

Probably. There is an engine mount on the front on the 470 but it curves around the harmonic balancer and such, so it should be fairly straightforward. And on this boat in particular the stbd engine is the easier / more spacious one to work on, so that's workable. Just not sure if there's enough space to get the timing cover off after the fact however, but willing to wrench on things.

I'll get the timing light on it first and see how completely out of whack the timing is to start. If it's out of whack at all. It might very well be something completely different.

 

[jhamblett]

Timing advance mechanism stuck? Check under the points

 

[jhamblett]

I picked up a 470 this fall and while doing the cam shaft sleeve I noticed mine was missing. Found a few bits floating in the pan. I read on a few forums this engine could almost go without one. I couldn't find one for while and finally one popped up on ebay

These 470 based engines do have a timing chain tensioner. I don't know how common they are for failure. Do you have room to pull the harmonic balancer off with the engine in the boat.

 

[Oshawapilot]

Timing advance is definitely on the radar - will be checking it when we do the ignition work.

 

[Scott Danforth]

I would look at fuel system first.

its not probable, however it is possible that the timing chain is worn enough to jump

you can verify the timing chain with both engines there by simply pulling the valve covers and putting a dial indicator on the #1 rockers.

using a tape measure, start at the TDC mark on the motor and use a silver sharpie, put a tick mark ever 1" around the damper on each engine

then start at TDC and slowly rotate the motor and compare movement of the rocker arms on each engine and compare dial indicator reading for each tick mark

 

[kenny nunez]

When you are checking the ignition remove the 2 screws holding the breaker plate to inspect the centrifugal advance weights and spring mounts.
Also make sure that the rotor shaft does not have any excessive play. I have seen distributors worn to the point that the engine would not go over 2k rpm without backfiring through the carburetor.

 

[Scott Danforth]

good point, napa has distributor bushings for about $5. usually in stock.