Exhaust Bellows, '88 4.3L

[PITBoat]

Mine is broken in half in the middle; how important is that? I just bought the boat - my first one - a few weeks ago, and the PO pointed it out to me, but said his mechanic pretty much convinced him not to worry about it.

I know the exhaust is still escaping, but is there any thrust produced at all by the exhaust being directed out the prop where it's supposed to? IOW, am I losing some power because of that?

 

[fishrdan]

Exhaust bellows being torn won't hurt anything, except more noise. But...... if the exhaust bellows are torn, the U-joint and shift cable bellows are not far behind. When those go bad, things get expensive, quickly.

 

[tpenfield]

+1 on the bellows tear not a big deal.

 

[Lou C]

Yep just tilt it up and carefully check the drive shaft bellows above the exhaust bellows carefully for cracks. If you find any replace it.

 

[Scott Danforth]

my recommendation, pull the drive, replace all the bellows, inspect the gimbal, inspect the alignment, inspect the u-joints.

the inspection should be part of the normal annual maintenance.

while at it, install a new impeller (on a cobra, impeller change is extremely easy)

 

[Lou C]

Agreeing with Scott here, a new to you boat should have an outdrive/transom mount service no telling when it was last done. Including lubing the u joints and gimble bearing. The bellows on the Cobra is pretty easy to replace compared to the Merc Alpha. Also it’s a good idea to clean out the bell crank in the pivot housing and pack that area with marine grease. This will help keep the shift system working smoothly.

 

[PITBoat]

Besides anything that's broken already (bellows), there are a number of gaskets and so forth that have to be replaced when taking stuff off/apart back there, aren't there? How easy is it to acquire all those for equipment this old? Ah yes, a question I should have asked before buying a 30-year old boat...

 

[Lou C]

For the Cobra all u need is the transom service kit (transom to drive gasket; water passage o ring). And a can of OMC gasket sealer.

 

[HT32BSX115]

Exhaust bellows being torn won't hurt anything, except more noise. But...... if the exhaust bellows are torn, the U-joint and shift cable bellows are not far behind. When those go bad, things get expensive, quickly.

Some of those exhaust bellows have hole in them from new (on purpose.

My previously installed OMG 460 King Kobra operated without an exhaust bellows for about 15 years before I got it and discovered it was missing.

 

[PITBoat]

What about ventilation? I know when the drive is down, the two pieces of bellows are probably pretty much touching, but I am having a high rpm issue also (like, 5500 at WOT instead of the book 4,500 or so), and was wondering about maybe some of the exhaust gas being pulled down into the prop.

I have a new tach that I'm going to install before the next outing to see how the rpm readings compare to the factory one (which was only operating intermittently, and which I really am not trusting).

I plan to do a service on all that either at the end of this season, or the beginning of next... I want to at least get my money's worth out of this boat. And I really like it!

 

[HT32BSX115]

What about ventilation? I know when the drive is down, the two pieces of bellows are probably pretty much touching, but I am having a high rpm issue also (like, 5500 at WOT instead of the book 4,500 or so), and was wondering about maybe some of the exhaust gas being pulled down into the prop.

I have a new tach that I'm going to install before the next outing to see how the rpm readings compare to the factory one (which was only operating intermittently, and which I really am not trusting).

I plan to do a service on all that either at the end of this season, or the beginning of next... I want to at least get my money's worth out of this boat. And I really like it!

Don't worry about high RPM until you determine you really have it. RPM problems are frequently defective tachometers.

More info might be helpful too. What size boat is it, what is your drive ratio, what prop are you running (pitch and diameter)

It's possible you simply have the wrong pitch or drive ratio. If you have a drive that was originally on the back of a 2.3 or 3.0L engine, it would be a lower ratio. 2.3L engines had 2.00:1 ratio drives.

Your 1988 boat could have had the drive replaced many times in the last 30 odd years.....

The easiest way to determine your ratio is to pull plugs and hand turn the engine (drive in gear) and count engine & prop revolutions................... You turn the engine and count turns.............. your helper counts prop blades etc..........

I.E, 10 engine turns and 5 full prop revolutions = 2.00:1 ratio.........

The 4.3L engine is usually paired with a 1.68:1 drive ---> roughly 17 (16.8) engine turns equals 10 prop turns......

1 turn is a full 360 degrees of rotation and 0.8 turn is 0.8 x 360-----> 288 degrees of rotation

(if you don't want to measure.......288 isa tad more than 3/4 turn)

The 3.0L engine should have a 1.86:1 ratio drive. 18.6 engine turns = 18 + 0.6 x 360 turns...... equals 10 prop turns

(0.6 turn is an additional 216 degrees of turn) etc etc etc.

Cheers,

Rick

 

[PITBoat]

The name plate MOD number on the drive is 983842; there's a serial # too.

I wired up the new tach this afternoon and it seems to work correctly, but I haven't actually checked the ratio yet. If I take it out again before I do that, I should have more trustworthy rpm #s (which I'm hoping are different than what I saw with the original tach). If it is the wrong drive, I'm kind of stuck anyway, right, short of replacing that?

The boat is a 19.5 or 20' Renken Classic 2000.
Prop is a 14 1/2 x 19, #389924, 3-bladed
Top speed is a little over 40mph as currently set up. I woulda thought a V6 would do a little better than that, but again, maybe I've got the wrong gears?, and PO said the boat is kind of heavy. Gets up on plane easily.

Thanks for the detailed info.

 

[PITBoat]

UPDATE: The Model # I listed above translates to a 1986 1.68:1 paired with a 4.3, per my SELOC book. Unless the innards can be changed...

 

[HT32BSX115]

UPDATE: The Model # I listed above translates to a 1986 1.68:1 paired with a 4.3, per my SELOC book. Unless the innards can be changed...

One of the few things that SELOC may have gotten right! Usually only good as knee pads or for starting wood stoves....


If it's a 2.00:1, you might be able to change the prop to a slightly "taller" pitch 4 blade (higher pitch) and get away with it. You will probably lose some top speed though. The serial number of the drive doesn't help much since many drives have been rebuilt more than once in 30 years.

Once you verify you do indeed have a 1.68 drive, and your tach is verified, you can play with prop pitch a little ....What do you have on it now?

 

[PITBoat]

It's a 14 1/2 x 19 with top speed a little over 40mph.

 

[HT32BSX115]

It's a 14 1/2 x 19 with top speed a little over 40mph.

All you have to do now is verify your tach.

Using http://www.go-fast.com/Prop_Slip_Calculator.htm you can determine where everything is. Verify your speedometer too (phone GPS works fine btw)

If you're really doing 40 mph at WOT, your max RPM (if your drive is indeed 1.68) is probably near 4800 RPM or so, which is about where your RPM should be.

AND, if your WOT RPM is REALLY 5500, your drive might actually be a 2.00:1 ratio.