tewks

I've got a 1997 searay with stock 330 inboards, weber carbs, and the Thunderbolt V ignition. The problem I'm having is the port engine is weak, only spins up to about 3800 rpm (compared to 4400 rpm on the starboard side), and uses about 20% more fuel than the starboard engine. Also, the plugs in the port engine look black after only a few hours use. Compression and vacuum tests are good. The engine runs smooth as silk, no coughing, missing, nothing, just no power! I've swapped the carbs and ignition modules from one engine to the other, but no difference. The chokes open fully. About the only thing I'm left with is ignition timing. The static timing on both engines is set to 10btdc (which has been checked about 10 times now...). I swapped the ignition modules... What's left that could be wrong? The distributor itself? Any clues appreciated!

T-N-T Sales

I believe the advance is built into the modual. The distributor is fixed and has no advance. Use your timing light and watch to see if the timing advances at the harmonic balancer, while you increase the RPM. That will tell you if it's advancing.

Gary Anderson

What do your props look like?

robyw1

I believe that fine piece of engineering known as a Webber carburetor might have an internal leak. Just after a run where you experience this look through the carb into the manifold and see if there are any puddles of fuel. If it is you need to move your boat into the deepest water you can find, remove the carb, hold it firmly in both hands, then heave it far as you can and don't look at the splash.

Roby

tewks

Thanks for responses!

The props were both reconditioned last season, and were in excellent condition when I launched this spring. Could a drag in the drive train cause that kind of rpm loss (600+) without any other symptoms, noise, vibration, etc?

The carbs had replaced last season due to a rpm loss on the starboard side, and replacing the carb did fix it. This time though, the first thing I did was swap the carbs from one side to the other, and no effect. (The old carb btw has been very useful as an dingy anchor, perhaps the best use for a weber 4bbl yet. Still looking for suggestions on how those belt driven fuel pumps could be put to good use!).

Tomorrow heading down to the marina with my timing light (and hammer) still convinced its a problem with the total advance, either because of the knock sensor or perhaps the pickup coil? From reading other threads seems the best approach is to skip the 'initial timing' stuff and just rev up to 3000rpm or so and see what advance I'm getting on each engine.

Thanks again!

Madmax

Thunderbolt V ignitions are very weird. They actually use the knock sensor to determine the rate of advance and total advance. How are you checking the idle degrees? You have to take the white/purple wire that's sticking off the distributor and ground it (it should have a plug in the end of it). At that point, you can set your timing for the recommended degrees. Checking it ungrounded is worthless. The module messes with the advance constantly to hold the idle rpm at 650. You will get readings all over the place...

Try that and see whatcha got.

tewks

I tried some things over the weekend, but to no avail. Sitting at the dock, total advance is 34 degrees, which sounds correct. I was kind of surprised how quickly it came in though, it was 34 at 2500rpm and stayed there.
The mercruiser book says that the purple/white is the signal from the knock module to the ignition module, and should be between 8-10 volts (when not intentionally grounded). I had a meter on it at the dock, and it held in that range. Was planning to go out on the water with the meter hooked up, but when I closed the engine hatch, it neatly clipped off the wires to the meter ! That pretty much put an end to my experiments for the day.
I'm still convinced its ignition. Maybe the knock sensor hears something when the engine's under load. I didn't know the knock sensor was used to control idle speed, though it was apparent that the timing was all over the place at idle when the test lead was not grounded.
Any other ideas? I haven't tried swapping the coils, but that might be the next thing to try (after I get a new meter...).

Scott

As stated above you must ground out the puple/white wire to set timing.