1984 5.7L idle problem

[rocstar]

I have a 1984 Sea Ray with a Mercruiser 5.7L engine with the Thunderbolt IV ignition. I had the shop time it so idle was at 800RPM (and I think 8 deg. advance). For some reason, after idling for a while (10 min.+), the rpm starts to get erratic. It will go from 800 to 1200, and then sometimes from 800 to 600. The longer it runs, the more erratic it gets. I can't figure out the cause. I got a another used ignition module, but the exact same thing happens. I have replaced the coil and spark plugs and wires.
If I cruise at 3000 rpm, it runs great for about 45 minutes, and then it starts to run rough, like it is missing.
Any ideas?

 

[Don S]

Re: 1984 5.7L idle problem

There is a lot more to an engine idling properly than just the timing.
However, the timing should be set with the rpm at idle, which is 650 to 700 rpm, not 800.
Then, you need to make sure the rest of the engine is ok. Tuneup, compression test, then the carb idle mixure has to be adjusted. When was that last done?

 

[rocstar]

Re: 1984 5.7L idle problem

The compression was tested in June, and it was fine, 115-135 on all 8 cylinders. The carbs were tuned at that time also. Distributor pickup was inspected when the coil and plugs were replaced earlier this year.

 

[Don S]

Re: 1984 5.7L idle problem

Why don't you do a full tuneup, cap, rotor, sparkplugs, ALL AT ONE TIME, rechech the compression, check the static timing, and advance timing. Use a vacuum gauge and tach and readjust the carb after everything else is set.
Don't do it all piecemeal, with months between each. You never will get it running right.

 

[rocstar]

Re: 1984 5.7L idle problem

All of the things you mention have been done within a reasonably close time frame.
For the sake of brain storming a little, let's assume that the problem is not the compression, spark plugs, wires, coil, carbs, distributor, and that a Mercruiser technician did the timing with the appropriate equipment.
The tech. noticed that the timing was almost impossible to set because it became erratic after running for a long while. He suggested buying a new Thunderbolt IV ignition module. Those cost $400 new.
I did a little research and read that the TB IV don't often go bad, and when they do, they typically don't exhibit erratic timing, but fail all together. I picked up another TB IV used, and installed it. I get the exact same symptoms.
So now I am trying to figure what else could cause the problem, or if I could have purchased a second bad ignition module with the exact same symptoms. I would hate to buy a $400 new module and find that the problem hasn't gone away.
Hence, I was looking for some braining storming of other things that could cause erratic timing. Can ignition modules fail in the way I have described?