1GM Performance after new head

[Dick Kobayashi]

I replaced the head - injector, valves, exhaust elbow, etc.
The enginge runs with less smoke and much more smoothly than before but here is the problem. When I bought the boat five years ago the engine ran at 3300 rpms - the max rated speed, but after a few years top rpm was about 2800 with poor operating characteristics.(smoke, soot, noise, etc.) After the new head (1500 bucks) the engine runs fine but only turns at 2800 according to the tach. As far as I can tell and I have not performed a formal test, the engine pushes the boat along just fine. The few extra revs would be helpful in a headwind. No other changes were made to prop, engine, fuel, etc. btw the spec sheet on the engine specifies 33/3400 as top speed. I welcome any comments on this.

[Dan & Pat]

Dick,

The other issues that affect the RPM's of your motor include, but are not limited to:

1. Prop - If your prop is not tuned and balanced, this will affect the efficiency of grabbing and throwing water. A simple thing like a 'ding' which looks like a little dent will drop several hundred RPMs from the performance. Another issue could be marine growth or even electrolosis eating away portions of the blades.

2. Shaft alignment - after your motor settles into place, the shaft-to-transmission alignment can also create issues. When anything occurs which might move the motor even the slightest amount, (such as major overhauling), the alignment can be off by as little as .004" (that's 4 thousandths!). This will create unnecessary friction in your drive train.

Dan

[Oswego John]

Dick,

Some questions.
All things being equal, is your operating temperature about the same now as before the head change?

Was your old head cast iron or aluminum?

Is your new head cast iron or aluminum?

Are you now using the original head after it has been milled down?

By any chance did you take a before and after compression check?

Was there any carpentry or other changes made by anyone before your old setup began to fail?

The angle I'm driving at is that if the engine is deprived of adequate air for combustion, the result is, especially at higher RPMs, incomplete combustion which results in excessive smoke, soot, lower top end RPMs, sounds of engine laboring and/or lugging. Not all of the incompletely burned fuel goes out the exhaust pipe. Eventually, you get carbon buildup internally, which diminishes total engine performance.

Check your engine vitals, check engine ventilation, check your prop, shaft alignment and check marine growth, as others advised.

Good luck,
O J

[rtbates]

how often and for how long do you run your engine?

NOT using a diesel enough can cause all kinds of problems. Biggest issue is running it for 5 min to clear the dock and shutting it off. Do this enough and your exhaust system will be become clogged with carbon. Get that puppy up to temp and keep it there for at least 1/2 hour.

Case in point. When I bought Seraph, our 25D, the 1GM would not get to 3k and smoked like crazy. I envisioned all kinds of problems, from wrong prop to fouled injector to plugged exhaust. Then one day while talking with the PO he mentioned how he loved his slip because he could shut the engine down after 5min and sail. To make a long story short I motored sailed for 4 hours twice and now she hits 6.1kts @ 3.5k rpm, NO smoke.

Use it or lose it!!

[Dick Kobayashi]

Thanks all. I'run it for a while and report back.

[Glen C]

your tachometer may not be reporting the correct speed...most are off by a few hundred rpm's, sometimes even on a brand new engine with a new control panel.

[Dean Abramson]

After it's warmed up, rev it briefly in neutral and see if you get more revs. If so, maybe the prop is fouled. I once cast off of a mooring in my 25D w/ that engine, and found I could only make 1100 rpm! I had to go into the water and de-kelp.

Dean

[Boyd]

Clogged fuel filters will hold the max rpm back. Also stale fuel will cause smoking and poor top end performance.

Boyd
s/v Tern
CD30 MkII
Fort Lauderdale, Fla.