Mercruiser 5.7 SBC Problem

[schwarz633]

I've looked for help on this over on a Sea Ray forum, but the technical knowledge is pretty thin over there.

I recently replaced the distributor caps and rotors on my 1989 300WE with twin 5.7 Mercruiser SBC inboards. The rotors were seized onto the distributor shafts and very difficult to remove. It was necessary to crank the engine to rotate the rotors in order to be able to pry up from opposite sides. This was done with the HT lead removed from the coil (bad idea, should have been grounded). On one of the engines I observed arcing from the HT tower on the coil to both of the small studs. Upon reassembly the engine wouldn't fire, no spark. The arcing had apparently taken out either the Thunderbolt IV module or possibly the hall effect sensor in the distributor. The TB module receives the signal from the distributor and provides the timing advance based on engine RPM.

After doing some research I decided on upgrading both distributors to DUI Marine HEI mechanical advance units from Performance Distributors. I liked the fact that the coil was incorporated into the cap and the installation would be clean and easy, eliminating the TB module and the coil. The only connections to the DUI distributor are +12V and the tachometer wire. The other option was the Delco EST which was ~ $100 more per distributor and not in stock.

My starboard engine is reverse rotation so I assumed that I needed a reverse rotation distributor which was +$70 for the DUI and +$300 for the EST. The only difference is that the gear of the distributor is cut in the opposite direction, both distributors turn CW.

After spending a couple of hours trying to get the reverse rotation DUI distributor to drop down into the starboard engine I discovered that my reverse rotation engine doesn't use a reverse rotation distributor. I have since found out that these marine SBC engines were produced in standard as well as reverse rotation with timing chain and timing gear versions of both.

I did however get the dead starboard engine running again using the standard rotation DUI distributor that was intended for the port standard rotation engine. The reverse rotation DUI distributor has been sent back to Performance Distributors to get converted to a standard rotation and I hope to be installing that on the port engine in the near future.

At this point I've only set the DUI distributor timing by ear, as both of the (2) digital variable advance timing lights that I have are telling me that the timing at idle was 39° advanced, with the engine starting and running normally. Performance Distributors advised me that a digital timing light won't work with the DUI distributor, something about their extended dwell period throws them off.

I recently located an old school non-digital timing light and printed out some timing tapes to attach to the harmonic balancer so I could check the full advance. The old school timing light is also telling me 39° advance at idle, it should be around 8° BTDC.

I'm absolutely sure that I'm clamping the timing light onto the #1 plug wire, been checked multiple times. And as I've said, the engines starts and runs great.

The only other thing I can think of is that the outer ring on the harmonic balancer has slipped, anyone have any experience with this happening?

Hopefully I'll gain some additional facts when I install the DUI distributor on the other engine and see what the timing lights tell me there.

[asphaltgambler]

I'm familiar with the DUI brand. Essentially slightly modified GM early HEI's. Do the new distributors have vacuum advance?

[schwarz633]

Originally Posted by asphaltgambler

I'm familiar with the DUI brand. Essentially slightly modified GM early HEI's. Do the new distributors have vacuum advance?

Mechanical advance only.

Does anyone know if the keyway on the harmonic balancer is in line with the timing mark on the outer ring?

[asphaltgambler]

So to find out where TDC is indicated, you can remove #1 spark plug, have someone bump the starter with your finger over the hole until you feel feel pressure. Then you put a long, skinny screwdriver down through the hole and holding vertically rotate the crank by hand in same direction slowly watching the screw driver rise.......until it doesn't. At t bgg at point you can reference the timing mark / balancer.

[schwarz633]

Originally Posted by asphaltgambler

So to find out where TDC is indicated, you can remove #1 spark plug, have someone bump the starter with your finger over the hole until you feel feel pressure. Then you put a long, skinny screwdriver down through the hole and holding vertically rotate the crank by hand in same direction slowly watching the screw driver rise.......until it doesn't. At t bgg at point you can reference the timing mark / balancer.

In the fiasco of attempting to fit the reverse (wrong) distributor, the engine got rotated so I did do the finger over the #1 trick so I could drop in the distributor. I didn't however put the screwdriver in and check TDC against the mark on the harmonic balancer. At the time there didn't seem to be a reason to do that, as I only had to get it close. I'll have to revisit that procedure to see if the harmonic balancer has slipped.

[asphaltgambler]

I'm pretty sure crank key way is 12:00 when #1 on TDC.

[schwarz633]

Originally Posted by asphaltgambler

I'm pretty sure crank key way is 12:00 when #1 on TDC.

I was looking at 5.7 harmonic balancers on Rock Auto and one of them said the TDC mark was 13° from the keyway. The pictures I've looked at on eBay seem to indicate a similar offset.

[A930Rocket]

Are you using an advanced timing light? Make sure it’s set at 0*. When I rebuilt my 930 engine, I set the advanced timing light to the total advanced timing. It idled great, but was retarded at speed. Everything got really hot and it wasn’t good...

[RANDY P]

To make sure everything is kosher I have a dizzy cap with a hole where #1 pin used to be. Feel for compression get to TDC, line up the oil pump tang, drop in dizzy and spin until tip of rotor is in the right spot.

[schwarz633]

Using a borescope in the spark plug hole and hand rotating the engine, it has been confirmed that the TDC line on the harmonic balancer it right where it should be. After putting it back together I was able to set the timing without further issue. After reading some things today I'm starting to think all my problems were caused by clamping the timing light pickup on the #1 wire at the cap instead of the plug. The timing light was probably getting false triggered by a different wire.

I do have some tach instability and inaccuracy, but I've ordered a Tach-Adapt from Bob at ashlocktech.com. He's confident that it will clean up the pulse from the DUI and make it compatible with the tachometer and synchronization gauge.

[asphaltgambler]

Ok good

[masraum]

Glad you got it running. 18436572!

[Scott Douglas]

Proud to say Bob is a personal friend. If he's involved, he'll help you get it straightened out if it still has any problems.