Soot on transom - bellows or prop problem?

[radiojon]

Newbie here; first-time boat owner as of late last year.

Late last summer, I had an exhaust bellows completely tear apart on the '06 Tahoe Q4 (MerCruiser 3.0L 135 HP) I'd purchased a few months prior. Since the other two bellows were still in okay shape, the mechanic suggested that I simply replace with an exhaust "boot" as a cost-savings measure until the other bellows needed to be replaced.

At the same time, they replaced my dinged-up, 3-blade prop with a brand-new 4-blade prop ... his suggestion was that the 4-prop would be better and smoother for me, since I mostly did a lot of tubing with my kids. They also did a routine engine service -- oil change and such.

I only got to take it out a couple of times before the end of the season last year, but I noticed a couple of issues -- which also happened today when I took her out for the first time this year:

1) I have a black soot residue on the back of the transom and partially around to the sides (which I believe also happened a time or two before the switch -- which led me to check the bellows in the first place), and

2) The top speed is about 10 mph lower.

Is it possible that both of these are related to the new prop? Or could they somehow be related to having an exhaust "boot" instead of the full bellows?

Thanks in advance for the help!

 

[Don S]

Re: Soot on transom - bellows or prop problem?

The black soot is not a problem of your bellows or prop, it's a problem of your carb, fuel pump, choke (or TKS system), ignition or who knows what else, that is causing an excessivily rich air fuel mixture.

 

[JustJason]

Re: Soot on transom - bellows or prop problem?

And the boat is slower because you changed props, you should have better bottem end and the holeshot should be better, and you should still hit peak rpms if the prop was pitched correctly.

 

[Gary H NC]

Re: Soot on transom - bellows or prop problem?

A 4 blade sounds like overkill on a 3.0 bowrider. What pitch is it and what is top rpm?

Agree with the rich condition...seen it many times and was usually carb related issues.

 

[180shabah]

Re: Soot on transom - bellows or prop problem?

10mph is a huge difference, was this gps measured? What was the pitch of the old 3blade, what is the pitch of the new 4blade?

 

[ken_23434]

Re: Soot on transom - bellows or prop problem?

10mph is a huge difference, was this gps measured? What was the pitch of the old 3blade, what is the pitch of the new 4blade?

Well, he also has a carb issue that might have gotten worse over the winter, too.

 

[Don S]

Re: Soot on transom - bellows or prop problem?

Well, he also has a carb issue that might have gotten worse over the winter, too.

And he has to get that carb problem fixed BEFORE he starts playing around with props, or he will be changing props again after the repairs to the fuel system.

 

[radiojon]

Re: Soot on transom - bellows or prop problem?

Thanks so much for the feedback, folks. Again, I'm a newbie, so apologies for asking so many questions...

* Could the excessively rich mixture have contributed to the exhaust bellows needing replacement? When I purchased the boat, it only had about 7 hours on it (so says the previous owner) and almost all of that was river fishing. The mechanic thought some trash or something might have sat over the impeller on one of his trips, causing it to run hot and char out the exhaust bellows. But if the mixture's been out of whack all along, could that have contributed? I used the boat about 9-10 hours without the soot problem ... it seemed to happen just before I noticed the bad bellows, but I can't be sure that the bellows weren't in bad shape all along and I just didn't take notice. I know that those rubber tubes and seals go bad over time (and this is an '06 boat), but it wasn't used that much and there's really low time on the engine (again, assuming that the previous owner was telling me the truth, which the mechanics believe to be so).

* Is correcting the fuel mixture problem a relatively easy fix? I certainly am not qualified to do self-maintenance (yet, but I'm learning), so will it be a costly repair at my local shop?

* What are your thoughts on using the exhaust "boot"? Is it a capable stop-gap until the other bellows need to be replaced?

Re: the prop - I do notice that the boat runs smoother, albeit slower. It planes out quickly with the 4-blade prop and I hit about the same max RPM. I just want to make sure it's not doing something bad to the engine that I have.

I'm not sure about the pitches on the blades -- are they stamped so I can find that info? I know the 3-blade was the factory-issued one, so I can probably find that in my owner's manual.

Again, please accept my thanks in advance for your advice, and for showing a new guy "the ropes."

 

[Don S]

Re: Soot on transom - bellows or prop problem?

* Could the excessively rich mixture have contributed to the exhaust bellows needing replacement?

No

* Is correcting the fuel mixture problem a relatively easy fix?

Without knowing what the problem is, it's impossible to say what the fix will be. Let alone if it will be easy or not.
If it's a TKS problem and your mechanic doesn't know anything about that system, then yes, it could be hard to fix.

The only damage the prop will do is if you are over propped and lugging the engine. Then you will have problems for sure. But until you get the fuel problem fixed, wasting time on the prop is just that. A waste of time.

 

[radiojon]

Re: Soot on transom - bellows or prop problem?

Okay, I'm going to go at solving this one small step at a time. I'm going to use up all the remaining fuel (a lot of which I think is still left over from last summer), then fill up with a fresh batch of gas. I'm hoping that's the inexpensive fix that does it.

A friend of mine recommends using premium unleaded (93 octane) -- says he uses it all the time in his boat. Also suggested putting in a bottle of fuel system cleaner (to help clean all the parts) with the next fill-up.

Agreed this is a good first step?

Thanks again for the help!

 

[180shabah]

Re: Soot on transom - bellows or prop problem?

Your friend is wasting his money on premium, and you would be doing the same. Running out the old gas and fillng with fresh is always good, just don't expect it to solve yourt running rich issue. Ditto the fuel additive.

 

[Bronc Rider]

Re: Soot on transom - bellows or prop problem?

Premium gas will actually leave even more soot. Premium doesn't burn as well as 87 octane. Its a waste of money unless the engine was built to run on it. If your tachometer is accurate and you can still hit the recommended wot rpm you can rule out the prop as an issue too. Propeller shouldnt cause an issue like that anyway. Have you removed any spark plugs to see their condition? If you got something in the fuel system changing the gas out wont fix it. You would need to change filters or service the carburetor or both.

 

[radiojon]

Re: Soot on transom - bellows or prop problem?

Progress to report! With just less than 1/4 tank of the suspected bad gas remaining, I added 4 oz. of fuel system cleaner and then it filled up with a fresh tank of 87 octane from the nearby Shell station. After an hour criss-crossing the lake, she sounded fine the whole time -- no missing or sputtering at all, and started right up both times when I'd stopped to sit for a bit with the engine off. When she came out of the water, there was only a much, much smaller amount of soot on the transom this time, so I'm hopeful whatever was in the system is pretty much flushed out.

If the problem returns, I'll report back. Thanks again for the help!