Carburetor

[reeleminjohn]

Hi, I have a 1986 Gw seafarer226 with the 5.0 liter v8. It's a gm motor. When I bought the bought someone must have put a mercruiser carb on it. This is my first fishing vessel so I'm a bit wet behind the ears. The motor runs great. No issues
My issue is that the boat is kinda slow to plane (15x17 prop.) At wot I'm pushing like 32- 3400 rpms with a top speed around 25. Does that sound right to you guys? I know the boat is 3700lbs but it seems to me it should go a bit faster?

 

[Tuna Man]

John, I'm not too familiar with i/o engines in boats so take this as a grain of salt....I would think a marinized GM small block should spin up to more like 4200-4500 rpm when properly trimmed, especially with a light load. At 4000rpm I would think your boat would hit about 35mph with a clean bottom (smooth paint, no growth, light load and the correct prop). Again I'm guessing here and trying to recall being on I/O boats as a kid, but I think they are set up to produce maximum horsepower/torque over 4000 rpm.

Are you trimming the bow up once underway? Is your boat bottom clean and smooth? Are you testing with a light load? Are you spinning an aluminum prop or a stainless steel prop?

Grady used to and perhaps still has archived test reports available, I would certainly reach out to them. Keep in mind there reports are often "the perfect world," but at least they will offer you some guidance and a reference point. Your title reads "Carburetor," this could be a problem, but I'm doubting that if it starts instantly and runs great.

 

[reeleminjohn]

Tuna man, I did a little research and discovered that my boat should have a Rochester 4 barrel not the mercruiser 2 barrel it has now. Hmmmm

 

[DennisG01]

Mercruiser, until a few years ago, used GM blocks - but then would 'marinize' it... things like cam shafts and carbs would be different, and of course bolt-on things. There were also options for 305's with either 2bbl or 4bbl's. I'm not sure exactly what year(s) those were options - but there were many years where they were.

Is speed verified by GPS? Assuming so, yes, it should go faster and your RPM's should easily be able to get into the mid, if not upper 4,000 range.

My first train of thought goes to what Tuna mentioned... making sure hull is spotless and proper trimming of the drive. Can you report on that? Either one of those things can absolutely account for your lack of performance.

The prop is not the issue (unless it's beaten up, of course) - there's no way that boat would need a 15" pitch prop. Even so, in theory, you'd have to drop to something like a 10" or 12" to gain 1,000 rpm.

 

[reeleminjohn]

DennisG01, thank you for responding. Yes I can verify a clean hull. It is painted. I took it out on the Hudson and worked the trim. Boat planed out nice but still only 32 - 3400 rpms.

 

[DennisG01]

Has it always been this way? I honestly can't offer any real workd advice on how much of a difference going from the 2bbl to a 4bbl would make as I've never done that exact swap. I find it hard to believe it would make that much of a difference, though. From what I remember, the rated HP difference between the two was about 20, maybe 30HP. That certainly won't account for what you're seeing.

FYI, just because it's painted doesn't mean it's clean. A painted hull that sits can still develop slime. Also, when you did the trim... could you have trimmed more? Meaning, how far did you trim? Did increased trim no longer cause speed increases?

Speed... GPS? That tach could be wrong and the pitot could be partially clogged.

Other things that come to mind that could explain it...

-- Fuel quality. Either old gas or water in the fuel. Possibly a clogged filter.
-- Plugs/cap/rotor. Possibly a plug wire not seated fully or even two wires mixed up. You could even pull the plugs and "read" them.

-- If you're confident on all of the above... next thing I would look at is the engine health. You may have one or more cylinders down on power. I'd do a compression test as a starting point. Possibly a leak down test.