What's that sound?

[whalerjet]

Motor type OMC 115 with VRO. It is a 1994 engine and is called the turbo jet. Engine compression is 120 in all 4 cyl.<br /><br />At idle, and infrequently, there is what sounds like a clunk. It is as though the motor skips just the smallest amount. The sound does not persist but it will do it randomly. I indexed the flywheel and checked it out with a timing light. Each cyl. fired when it was supposed to at idle and at 1500. Any ideas what that sound might be???? Also, the motor will loose RPMS (800) off of W.O.T after about 20min. The power pack will be replaced just because, but I wonder if the two are related?

 

[sloopy]

Re: What's that sound?

Do you hear any thunking sounds when you decrease throttle?

 

[whalerjet]

Re: What's that sound?

No sounds "thunking" heard when throttle is increased. Checked the connecting rods today as suggested in this forum. Removed the plugs and pushed gently against piston after TDC - No movement what so ever.

 

[whalerjet]

Re: What's that sound?

?

 

[aeronutt29]

Re: What's that sound?

Keep in mind that I've never actually seen your motor....<br /><br />I will venture to guess that what you are hearing is caused by a slightly lean or rich mixture setting at idle that makes a cylinder miss every now and then. When that cylinder doesn't contribute it's fair share of power on one rotation and then catches up on the next one it might be just enough of a tortional upset to cause that clunking. Keep in mind that the crankshaft is always flexing like a coil spring from each piston's jolt of power. The trick is to have these rotational twistings cancel each other out by having a firing order other than 1-2-3-4-etc...

 

[whalerjet]

Re: What's that sound?

Thanks!<br /><br />That theory makes a lot sense as it does seem like the engine is trying to catch up. I was going to have the Carbs rebuilt. Is that something I should try myself or simply pay the mechanic? I have done a auto. rochester before, but I wonder if Outboard Carbs are more complex?

 

[fireman 757]

Re: What's that sound?

I think that they are actually easier than auto carbs. Sometimes it's tough getting them off without taking apart half your engine. Do them yourself. Just make sure that you set the float level with the carb turned upside down.

 

[clanton]

Re: What's that sound?

Some of the clunking is comming from the gear drive and prop. If you are dropping 500 rpm after 20 minutes, may be stator. Check stator output to pack, and pack output to coil with peak reading meter after the rpms drop. Voltage at cranking rpms 150 for stator and pack. Pack voltage to coil running 230 volts. May be a little higher at 5000 rpms.

 

[ICEMAN]

Re: What's that sound?

Hi, first questio to you, are you running this out of the water and getting the noise, or is it in the water? If it's out of the water, you will get noise from the gears in the gear box under the power head. The first time I started one of these, it was out of the water. It made an aweful noise, I shut it down immediately. I called the factory and told them what I found. The tech told me that noise was normal gear lash in the gear box. I told him at was very noisey and he assured me that it was normal. I was nervous when I started it again, but figured it was under warranty at the time, so let's see what happens. The unit is now about six years old and has yet to give a problem in the gearcase. I'm not saying this is your noise, but it could be.

 

[whalerjet]

Re: What's that sound?

Thanks for the advice on the gearbox. I did here the noise with it out of the water. I don't have a Peak reading volt meter. I read an earlier post that an ohm meter could be used, set to voltage, and then use a multiplier of 1.1414. Is that so? I don't recall who posted it - maybe schematic??