Mercruiser Timing Advance Problem

[Wet Wonder]

(Thanks everyone that helped me solve the no spark problem in my previous thread.)

1977 Merc 351 Windsor, 233hp, 2bbl, points ign, mechanical advance, inboard single screw.

Initial problem: Not reaching max rpm range at WOT. Maxing at 3,400, then starts to stutter and lose power.

Maintenance: All new distributor components from distributor housing, springs and weights, points, coil, cap/rotor, plugs and wires. I followed Don S.'s tests for fuel system up to carb and all passed. Merc-carb just rebuilt.

Where I am now: Timing problem.

---Tonight I hit the balancer with a timing light and adjusted it to about 10 degrees at idle, per specs in the manual.

Then I hit it with the light again while bringing rpm, while in neutral and docked, to 2000rpms. It advanced 7 degrees to a total of 17 degrees. If I'm reading the manual advance curve correctly, the spec advance for 2000rpm is about 20degrees, to a total of 30degrees on the balancer.

Then I brought it up to 3000rpms, again while docked, and it did not advance at all beyond the total of 17 degrees, whch it was at with 2000rpms. The spec advance curve for 3000rpms is about 22degrees, for a total of 32 degrees at the spark.

I realize that the timing not advancing may well be the cause of my WOT/rpm problem.

So is there a problem here from what I observed with the timing light? Or does the boat need to be under load to time the advance.

Thanks,
Dave

 

[rodbolt]

Re: Mercruiser Timing Advance Problem

ign timing with that point type distributer isnt load controled.
strictly RPM and the two advance springs and breaker plate.
sounds like either a spring is bad or the plate is binding.
check your fuel pressure at the problem RPM as well.

 

[jtybt]

Re: Mercruiser Timing Advance Problem

Exactly!

 

[Wet Wonder]

Re: Mercruiser Timing Advance Problem

ign timing with that point type distributer isnt load controled.
strictly RPM and the two advance springs and breaker plate.
sounds like either a spring is bad or the plate is binding.
check your fuel pressure at the problem RPM as well.

What does it mean? - "a plate is binding"

Thanks

 

[rodbolt]

Re: Mercruiser Timing Advance Problem

some distributers moved the breaker plate with centrifical force some had a more complex setup that moved the rotor assy.
if any of the advance mechanism is worn or binding the centrifcal advance wont work correctly.

 

[Wet Wonder]

Re: Mercruiser Timing Advance Problem

I replaced the distributor earlier this season. The springs on the older one were rusted out and the shaft was corroded.

So I picked up a new, I guess rebuilt, Prestolite fully assembled distributor. I just ordered a dwell meter so I can set the points on it. In getting a new distributor, would you normally have to adjust the spring weights for advance or should they be tuned in?

Thanks.

 

[Wet Wonder]

Re: Mercruiser Timing Advance Problem

The one I ordered is specific for the older Merc 351windsor right-hand motor, so would it come tuned in for that motor? - (understanding that the motor comes in a few different horsepowers, mine being the 233hp).

The new distributor didn't come with extra springs or bushings.

 

[Wet Wonder]

Re: Mercruiser Timing Advance Problem

Thanks, I'm going over there now to check the springs and weights.

 

[Wet Wonder]

Re: Mercruiser Timing Advance Problem, checked springs/weights

Re: Mercruiser Timing Advance Problem, checked springs/weights

I opened up the distributor and did the following:

- turned the rotor ccw and it snapped back
- pulled the plate
- was able to push the weights out with the springs expanding
- lubricated it a little

Then started the motor, hit it with the timing light. It was 10 degrees at idle as it should be, but would still only advance 7 degrees, even at 4000rpms.

I saw that there was one lighter spring and one heavier spring connected to the opposite weights. Then there were tabs on the weights that popped up through grooves in a plastic bar that crosses the distributor housing and through the shaft. It seems that the grooves limit how far the weights are permitted to swing out.

So is the very small timing advance I'm seeing related to the size of the springs/weights or range of their movement? Or should I be looking at something else? I feel like this is very close to being solved.

Thanks, Dave

 

[Wet Wonder]

Re: Mercruiser Timing Advance Problem

...

 

[Wet Wonder]

Re: Mercruiser Timing Advance Problem

Bump myself.

Anybody?

 

[Wet Wonder]

Re: Mercruiser Timing Advance Problem

Can anybody give me some advice on this?

 

[Wet Wonder]

Re: Mercruiser Timing Advance Problem

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Tonight I pulled the bigger of the two springs out, leaving one of the weights free. Started it up and hit it with the timing light and it advanced to 30 total degrees very quickly by 1,500rpms, and that advance remained as a straight line at 30 total all the way up to 4,200rpms.

The curve chart in the manual shows the spec advance as a straight line from 10degrees at idle to 30 total as it hits 2,000rpms. And then the line flattens out a little and gradually goes up to 34 degree total advance by 4000rpm.

So with only one spring right now, the advance is a little to quick up. I guess I need to get some different gauge springs and experiment to slow the rate of advance up to 2000 rpms.

But even with the heavier of the two springs out, the total advance I saw was 30 degrees at 4,200rpms. The spec is to achieve 34+ total degrees. Should I widen the grooves a touch, which limit the advance, so I can get it up to 34 degrees? The bar with the grooves that limits the movement is plastic, so I can push out each groove a little with a drill bit.