Inboard Replacement

Off in the Bushes · #1 07-31-2017, 09:59 AM

I am thinking of replacing my inboard motor, right now it is a carbureted GMC 6 cylinder.
Was thinking of a fuel injected 6 cylinder.
Is it plug and play to swap out?
Right now I can't troll off the main, I think that the cylinder rings pass as I am getting gas in the oil.
Could you troll off a new motor or would Idle speed be too fast.
Are you better off with a kicker motor.
Thoughts?

Ken07AOVette · #2 07-31-2017, 10:03 AM

Always better with a kicker. Use a fraction of the fuel. I had my kicker in the shop to get the carb cleaned, they ran it for 2.5 hours mid throttle. Used less than 6 Litres. Imagine that on the main.
As for plug and play, the engine is looking for a constant 12v circuit for the ecm, a constant ground and power to the starter. If the computer is on the engine, and it should be, it will be 'mostly' plug and play other than making sure all your guages read correctly. I would look into a NMEA system and run them on your fishfinder.

ROA · #3 07-31-2017, 10:33 AM

Don't know for sure so just spit balling here but a Fuel injected replacement engine is going to require a oxygen sensor in your existing exhaust manifold. Probably also require a different fuel pump and possibly return line to tank. Will also need a power supply to the new computer. A call to a marine shop is needed to see if you can even get a fuel injected crate engine with the same bolt up on the bell housing, engine mounts and heads.

MSN, Holly and Edlebrock make self learning throttle body fuel injection kits to convert a carb engine if you wanted to go that route one a new carb engine too

big zeke · #4 07-31-2017, 12:27 PM

I have a 4.3 in my boat (might be the same as yours), several years ago I looked into the swap over, would have been several grand for essentially the same HP. The inputs that an EFI are looking for don't exist in a carb job. A swap to breakerless ignition is fairly easy but ditching the carb is not.

If you are getting blowby, you'll probably be looking at 5k+ for the rebuild including R&R depending if they can reuse crank, cam, manifolds etc. Many parts are specific to marine engines so an auto crate engine should not be used.

Keith at Calgary Power Boat can work you thru the details (or someone local to you). A properly rebuilt carb can run a long time, trouble free.

Off in the Bushes · #5 07-31-2017, 06:42 PM

So I talked to my local boat mechanic and he thinks the gas in the oil is from the carburetor. That doing a replacement should solve the gas in the oil issue.
I only have limited knowledge on the working of a internal combustion engine. Anyone explain that to me?
Was the fuel too rich and washing down the cylinder walls?
But the rings on the cylinder must be shot to get the fuel into the oil?
Would a compression test prove that?
Do people rebiuld these motors?

pikeman06 · #6 07-31-2017, 07:47 PM

Yep gas in the oil is a hanging valve or way too rich on the carb or a dead cylinder but it woulda run real crappie. You don't need to be a mechanic to pull your plugs and if you got one that's abnormal then you know you probably looking at a rebuild but if it is a dead plug because of a bad wire or plug boot you might be OK. Check for spark change that oil. If you got spark on all cylinders run it for a bit and check the plugs. If you still have an oddball plug do a compression check...pulling an inboard is major. They are well built. Check those wires and distributor.

-JR- · #7 07-31-2017, 09:23 PM

Does the engine sound like it has a miss ?
If yes look at the plugs .

If No .
While running .remove the oil fill cap and see if exhaust is coming out ...this is BlowBy . needs rebuild

If you are just getting fuel in the oil .
remove carb and install a needle and seat ,,better yet get a carb kit .
may be float just needs adjusting

Did you get a new fuel pump that is over fueling.

Might only be getting fuel in oil when engine is off .
Might want to get a fuel shut off valve.
Sometimes just pulling your boat on a gravel road will make your float in your carb jump up and down letting fuel in ,but this is very rare.

If the plugs are all dark and wet ,might be running rich ,may be hole in float.

pikeman06 · #8 07-31-2017, 09:59 PM

What this guy just said too. I love trouble shooting. So you don't like to troll with it cuz it won't run at low idle or it runs at low idle but just too fast. If it runs decent at low idle you probly got a good engine. In that case get a kicker mount and look around for a 2 horse or maybe a bit bigger. 500 bux all in and you troll like there's no tomorrow. Or many thousand to try and get a 6 cylinder inboard to troll slow which isn't what they are made for. You can buy 3 good fishing boats for the same price of the modifications you speak of. There are cruiser/ski boats then there are fishing boats.

KegRiver · #9 08-01-2017, 02:36 AM

I have a chev four cylinder stern drive. I looked into a swap too.

First, you have to have exhaust manifolds to fit the new motor. There is no guarantee the old manifolds will fit even if the new motor is the same displacement and manufacturer.

Second, the motor mounts have to match or you have to find or build adapter plates.

Finally the output shaft has to match your motor and again, there is no guarantee it will. It has to be measured or cross referenced.

Beyond that, there will be new wiring to figure out and throttle connections to figure out. They will not match but they can be adapted.

Can you troll with a stern drive? Yes, I've done it a lot. But a outboard is a better bet for trolling.

What makes you think you are getting fuel in the oil?

I ask because there is only two possible points in the motor where fuel could enter the crankcase and that is past the compression rings or through the fuel pump diaphragm on a mechanical fuel pump.

The problem with that is, if the motor was ran long enough to reach temperature, it would be very unlikely that you would detect any fuel that may get past the rings as most of it would evaporate faster then it could get past the rings. Even with a scored cylinder or a cracked pump diaphragm.

No rings seal 100 percent, so if your float is sticking, the carb would leak into the manifold and from there down past the rings, when the engine is off and you could very well detect the smell of fuel in the oil before startup.

If that is what is happening, do not run the motor until the problem is corrected. You could experience a catastrophic fire or even an explosion if that is happening.

Off in the Bushes · #10 08-01-2017, 03:55 AM

So I am leaning on the fact that the carb is defective. As I had not used the boat in years and when I went to use it, it ran really rough so I took it in for servicing and they the carb was plugged from old gas and they got operational.
Then I took the boat and was trolling on the stern drive 3-4 hrs. Took the boat out again and that is when the tail bearing had seized. (that was their diagnoses, but I had changed all the oils in the tail and engine before I winterized the previous fall, so I have no idea where the oil went none on the ground.) So back to the shop they said the freed it from the bearing and all is good. Sounds odd but I'm not a mechanic. When trouble shooting the stall and seized up on the lake I checked the oil that is when I noticed the cream color and that it stunk like gasoline.

KegRiver · #11 08-01-2017, 09:03 AM

Quote:

Originally Posted by Off in the Bushes

So I am leaning on the fact that the carb is defective. As I had not used the boat in years and when I went to use it, it ran really rough so I took it in for servicing and they the carb was plugged from old gas and they got operational.
Then I took the boat and was trolling on the stern drive 3-4 hrs. Took the boat out again and that is when the tail bearing had seized. (that was their diagnoses, but I had changed all the oils in the tail and engine before I winterized the previous fall, so I have no idea where the oil went none on the ground.) So back to the shop they said the freed it from the bearing and all is good. Sounds odd but I'm not a mechanic. When trouble shooting the stall and seized up on the lake I checked the oil that is when I noticed the cream color and that it stunk like gasoline.

Rough running would indicate an over rich mixture or ignition problems.

I'm not sure what you mean by tail bearing. Pilot bearing perhaps?
Changed all the oil in the tail? Do you mean leg?

It does sound odd, them saying they freed it and all is good. A seized bearing must be replaced or it will fail again very soon.

Cream colored oil indicates water in the oil. Not good!
If it's oil in the leg, you have a leaky seal, if it's in the motor, you have a blown gasket. Either way, you have big problems.

big zeke · #12 08-01-2017, 09:18 AM

I tend to agree with Keg, water in a leg is bad, might be best to plan on a new leg as the teardown labor is a killer. Water in the engine is no better, rebuild the existing block or get a new marine one, the labour from fiddling with an engine like that will also kill you. A carb rebuild is cheap but I doubt that's your only problem.

If the entire power pack is gone, you might want to look at podding with an outboard. There are several guys on the coast that do this and a lease return engine might be a better fit esp if use is low. I'm guessing the entire repair (no matter which way you go) will be in the low 5 figures.

Maintenance and winterizing is very important on any engine, long storage periods, without proper prep are tough on anything.