Setting idle and timing

[Frank.O]

I have to set my idle and check the timing. As per Volvo book all running test have to be done under load with correct test propeller so how do you guys do it, how do marine shops do it (some marine shops in my area are nowhere near the water)

 

[Saline Marina]

What are we talking about, a GL carb motor?

If you have a small-cap distributor with remote coil, similar to a distributor from an early 90s TBI GM truck (don't get confused here because its also distributor used on a Volvo carb motor), you have to put the ignition module in base-timing mode with a jumper harness that plugs in and clips to B+. Using a timing light you adjust the distributor base to get the crank pulley lined up with the stationary timing mark. 0 deg BTDC is a good place to start. Then unjumper the module (remove temporary harness) then your ignition is good to go.

 

[Frank.O]

The engine is 4.3gl and I'm familiar how to perform all this stuff but my question is does the boat have to be in the water as Volvo book says is so how do you do it. Is there a special prop just for testing since you can't be moving while working on the engine.
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[Scott Danforth]

Yes, in the water, in gear, and at idle.

Many dealers have a test tank / pond or are on a body of water

 

[Frank.O]

So is there a specific prop for testing. If the idle is set with regular prop but boat is stationery the load will be higher than when boat is moving resulting in higher idle RPM
This is from Volvo book:
"All running tests must be conducted in water with CORRECT TEST PROPELLER to properly load engine."
is there a chart with correct test propeller size for any given engine or a formula to figure it out.

 

[jimmbo]

Idle speed needs to be set with the prop being used. A higher pitch prop puts more load on an Engine, even at Idle, and will require a slightly different setting than if a lower pitch prop is used. As with outboards, the Throttle cable will need to be disconnected while setting the idle speed, and then the Cable length readjusted to place a bit of Preload against the idle stop. Too little Preload, and the idle speed may not be consistent, too much and it will be hard to shift to Neutral.

Running tests are done to check an engines condition, however for the pasts few Decades, a lot, if not most Service Shops used a Dynamometer(Oil Pump with a variable flow) to load the engine, instead of Test Wheels(props). Test Wheels are often specific to a particular HP, or group of Engines. If a Test Wheel is used on Various HP, then each HP will have its own RPM benchmark. Using a Dyno, it is adjusted to provide a specific RPM and hopefully the Specified Oil Pressure is reached, or the Reverse is sought, a Specific Pressure, and hopefully the specifed RPM is attained.

 

[Saline Marina]

You can set the ignition base timing in the driveway. In my opinion there's no way you'd ever keep the engine running, under load of the prop in water, in the base timing mode. If you've done this, the engine is barely able to turn over with zero load on it, sounds about to stall, etc. But once you enable the module again (remove temp jumper harness) to adjust the timing on its own, it will run very well and be able to take on the load of turning the prop. If you looked at the timing light with the ignition module in full control (normal running) you'd note that the timing mark jumps around quite a bit, to the point where one could never accurately set the distributor base to achieve a specific BTDC value.

The vehicle analogy would be having to set base timing with the transmission in D with the parking brake set. It doesn't work like that, its set at a zero-load idle in neutral or park and when you put it into "D" the load of the torque converter forces the ignition module to make changes to adapt to the load, just like it does at any other RPM.

 

[WorkingFromHome]

The vehicle analogy would be having to set base timing with the transmission in D with the parking brake set. It doesn't work like that, its set at a zero-load idle in neutral or park and when you put it into "D" the load of the torque converter forces the ignition module to make changes to adapt to the load, just like it does at any other RPM.

Could you tie off to a piling or dock and set it that way? Seems like the closest thing to a vehicle where the parking brake is set and the transmission is in 'D'

 

[Saline Marina]

What I was attempting to state was that if one did set the base timing with the prop being driven, I think the engine will stall long before the mechanic ever gets the distributor base to the 0 deg BTDC (or whatever) mark, it is seriously just crawling along the more advance you twist in. It seems like the engine is idling about 400 rpms and the exhaust pulses seem to have dead noise between them.

The good news is that the ignition module is adaptive and it will run very smoothly throughout the RPM range and an operator would probably only know it when trying for peak power. I've purposefully set the ignition retarded quite a bit and the symptom is it sort of flat-spots when under a sudden load or accel and then after about 1 second there's this dramatic RPM gain almost like a turbo-boost kicked in. That's the seat-of-the-pants sign the base timing is retarded from where it should be.

Its entirely possible I don't know what I'm talking about...but...I have the 7.4L, 5.7L, 4.3L (sold the S10) truck engines, 3 tractor engines adapted to run the 2.5L version of this same ignition system, and the 4.3L GL motor, all of where I've had to mess with the base timing.

 

[Frank.O]

Well explained, fully agree with you
I'll set my idle while docked with the smallest prop I have and will check timing at home
thank you