1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

avpaulb · Apr 10, 2013

Picked up this boat a couple of weeks ago. Great condition overall for it's age. I checked out the motor to the best of my knowledge and all seems pretty good.

I'm planning on doing some fairly extensive mechanical checking on the motor before she hits the water. One thing that I noticed right away was that the carb was in good shape, but needs some adjusting to make it to full open. It appears that the accelerator pump rod (?) might be wrong.

The carb is a Rochester 2G, or something like that, and the rod in place is strait rather than having some bends in it, which the Mercruiser manual shows. I think this has something to do with the throttling issue?

Short story: I have a mechanic friend who is willing to GIVE me a new Holley carb to replace this one. Also a 2 barrel. He also claims that it should work perfect and improve performance, just might need a little tweaking.

I'm well aware of what a performance carb can do for an automobile... just very curious as to what may happen in a marine application. Any thoughts? No worries? Don't do it?

bonzoscott · Apr 11, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

Has to be a marine carb to prevent fire / explosion. I can't picture an accelerator pump shaft anything other than straight. You're not speaking about the "arm" that depresses the accelerator pump shaft - are you? I think it would be best to figure out what your OEM carb should be and go from there.

avpaulb · Apr 11, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

I know Holley makes Marine carbs, just not sure if this is one or not. I'll see if I can find a pic of the Rochester compared to mine so you can see the "rod" in question.

Don S · Apr 11, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

I don't think the Holley carb has the same bolt pattern for your manifold. Even if it did, how would you hook up the linkage from the throttle cable?
You would be a lot better off to rebuild your OEM carb and adjust it to the manual.

avpaulb · Apr 11, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

The rod that is not bent is what I speak of. When you push throttle forward, this assembly only opens the intake about 1/4 inch. Result: full throttle not achieved.

Just wasn't sure if there is some other adjustment that needs to occur.

Pardon the dust... also rebuilding some decking portions before I was smart enough to cover the mechanical stuff.

matt167 · Apr 12, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

Don is right, The Rochester 2G uses 2 versions of a an odd bolt pattern, a large and small base. The marine 2G likely has the later large base. The Holley 2bbl's have a rectangle. It would require an adapter to use a Holley on that manifold, and the adapter would increase the chamber volume of the intake and would only hinder the performance of the engine.

fishrdan · Apr 12, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

avpaulb said:
The rod that is not bent is what I speak of. When you push throttle forward, this assembly only opens the intake about 1/4 inch. Result: full throttle not achieved.

Got pics of what you're referring to, assembly?

I agree with sticking to the Rochester carb, fitting the Holley would be a pain. Quicker and easier to fix the Rochester than Rube-Goldberg the Holley. And, you might get better performance out of the stock carb, if the Holley isn't setup for a small displacement engine.

avpaulb · Apr 14, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

fishrdan said:
Got pics of what you're referring to, assembly?

I will get you some as soon as I fix my camera! Thanks for the carb thoughts. I've informed my mechanic co-worker that he will not be "frankensteining" my boat motor.

Side note: I have two air intakes on the engine compartment, each connected to some "dryer-vent-style" conduits. These conduits are run strait to the floor in the engine compartment. I'm curious about this placement given that the air intake is obviously on top of the carb. Thoughts?

avpaulb · Apr 14, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

Another side note: this boat has a 100% functioning gauge cluster with everything you could think of, other than a speedometer. Anyone have any idea if this motor/harness has that capability?

matt167 · Apr 14, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

The speedometer is an air speed gauge rigged up to read water pressure. They work OK to a point. If you buy 1, it will not use any electrical connections beyond 12v lighting that you can hook to your Nav light switch.

The dryer ducting that you see runs to the bilge blowers. Blowers have been a USCG requirement on inboard and I/O boats for a long time. It's recomended to run them 5 mins before starting so that any fumes are evacuated. gasoline fumes are heavier than air and will always settle in the bilge. You probably have a placard near the key that says it.

avpaulb · Apr 25, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

OK, finally got some carb pics...

This pic shows how far the throttle plate opens at full throttle position:

As you can see... not far at all.

What it should look like:

All of the mechanical parts are in good shape. The question is: how to fix?

fishrdan · Apr 25, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

The top plate you're opening with your finger is the choke, look down the carb when the choke is opened and you'll see the throttle plates. On a cold engine the choke plate should be open about that far, after the engine warms up a couple minutes the choke plate should be opened all the way. Top pic shows a cold engine, bottom pic shows a warmed engine. (I'm guessing everything is normal.)

avpaulb · Apr 25, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

Both pics cold.

I guess I'll have to get this thing put back together and run it for a bit to see how it behaves. I just happened to notice when I "test ran" the motor that it seemed to top out at a low RPM level. 3000-3500 or so.

I'm doing some cosmetic work currently. I'll report in once done.

old islander · Apr 25, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

That's the choke plate in your pics. Run the motor till it's warmed up completely and check that plate. It should be 90 to 95% open. If not, adjust it to the manual specs.

fishrdan · Apr 25, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

fishrdan said:
bottom pic shows a warmed engine.

avpaulb said:
Both pics cold.

I didn't say that right, warmed up engine should look like bottom pic (choke completely open).

avpaulb · Apr 25, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

will do

avpaulb · Apr 29, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

Getting the gas can back in...

One thing I noticed when I removed it was that the fuel line from the can to the pump ran UNDER the motor, probably touching the oil pan.

Is this possibly correct? Or should I run in somewhere else, if possible?

avpaulb · May 5, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

Finally got the boat on the water today for a trial run. Ran well all day, but:

Stopped for about 20 minutes to fish, with motor off. When I tried to restart... nothing. No click, no turn, nothing.

Still have power to all gauges, trim motor, and lights. Just no start.

I haven't been able to throw a voltmeter on the battery yet, but any other thoughts?

bonzoscott · May 6, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

Being that it ran great all day, I would suspect poor ground cable or connection. Wire brush very clean both ends of both cables. Steel wool battery terminals, engine block ground and copper terminal for the hot cable at the starter.
When you check for voltage at battery (v12.5+), also check for volt drop while someone turns the key to start. Should be no less than v10.5. If so, you may have a charging problem.

avpaulb · May 6, 2013

Re: 1975 Crestliner Trihull w/ 3.0L Mercruiser - New Project

bonzoscott said:
Being that it ran great all day, I would suspect poor ground cable or connection. Wire brush very clean both ends of both cables. Steel wool battery terminals, engine block ground and copper terminal for the hot cable at the starter.
When you check for voltage at battery (v12.5+), also check for volt drop while someone turns the key to start. Should be no less than v10.5. If so, you may have a charging problem.

I'm going to check all of that. My fish finder has a voltmeter on it. I did notice that when I tried to start it the voltage dropped from about 12.8 to way low. While running it seemed to stay around 14.5 volts or so. (Never higher than that)

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