cavitation?

S

stang32

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Joined
Jul 17, 2004
RO Number
14425
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56
I am running a 4.3 vortec motor in my 22' deckboat.
every other time I go to take off, it feels like I am dragging a trailer under my boat. then it will gradually clear up & run strong. it does nt seem like it is slipping, the RPMS do not take off like it is in neutral or anything like that but more like it is being held back..
I was thinking cavitation but do not know what the indications are for cavitation. any other thoughts on this are appreciated.
it is a mercruiser, 4.3 vortec, alpha 1 gen II & 2 barrel carb.
 
Do you have the outdrive trimmed all the way in while taking off?
 
Need a little more info:
-What propeller is on it now (stainless steel / aluminum?, pitch?)
A damaged hub is a possibility, but that is usually an all or nothing thing.
Is there any damage to the propeller? Vibration?

-What rpm is it turning at WOT?
 
I've usually discovered that this type of an issue was due to ventilation ( ie: prop sucks in surface air then spins inside the bubble ). Assuming that the engine is mounted properly, the cause is normally poor outdrive trim. Try "sucking the drive in" towards the transom ( or forward ).

One other possibility is that your weight distribution is too far forward, which will make it more difficult to progress from displacement mode, through "the hump" and onto a plane. This can also introduce ventilation.

I agree w/ others that a spun hub is possible, but unlikely.
 
...If.... RPM's aren't elevated when it is being "held back", there is little likelihood of either prop ventilation or spun hub.

You may have a semi-bad sp.plug or some other reason 1 or more cylinders are not combusting right away or carb may just need cleaning or..?.
Once it runs right will it accelerate normally after that ? Does it only do it when you accelerate hard rather than more-gradually increase throttle?
 
An over-pitched prop will do that also. It's like starting out in 2nd or 3rd gear from a stop. (remember when you could do that?)
 
First, is the engine racing as you start off and it feels like it's being held back? This is the indicators of spun coupler, spun hub, cavitation or ventilation.

Second thought was you have a light boat and over pitched prop like timeout says.
 
ok a little background, it is a brand new remanufactured engine, all new ignition parts, ie.. cap rtr, wires, plugs.
just rebuilt the carb with excellent results, idles rb2@gr5t 550-600 w/ no stumble, had to bump it up to 650 to be in spec.
I am not sure of the prop size, & I had never driven the boat prior to the engine replacement. the hub seems fine, I could not spin the prop manually when in gear(engine off)
I start off with the drive trimmed up & lower it as I get on plane. the prop is aluminum & no damage on it.

I have tried taking off fast & gradual, & it happens both ways , it does not happen every time, but in 1 day it happened enough to notice.

thanks for all who gave some ideas, more are encouraged,
 
Outdrive should be trimmed in for hole shot to get it on plane faster. Trim out (up) after reaching plane for efficiency. Once on plane you can increase RPMs without more throttle by trimming up. If it starts porpoising you've trimmed out too much.
 
quote:
I start off with the drive trimmed up & lower it as I get on plane.
Click to expand...

Really should be opposite - drive trimmed down starting off and the trim up once on plane. At speeds below 25mph I don't trim much at all.
 
Paul I'm pretty sure it's your trim. My new-to-me boat (not in my sigline) was a dog the first time I took it out. I trimmed the motor down from towing trim to launching trim using the switch on the motor and was really disappointed in its performance getting on plane. It also porpoised wildly. Then I fiddled with the trim button on the throttle and it all became clear to me. Trim switch on the motor only got it down so far. Trim switch on the throttle was the fine tuning. It allowed me to trim down even farther than the switch on the motor. Hole shot improved immensely, porpoising stopped. Once on plane I can pick up 5-7 mph with the trim alone.
 
I have tried taking off with trim in & out, the problem occurs in each position,. all be it, as i said earlier, it does not happen every time, it does happen in each position. i have been a boater for 26 years & i know the effects of trimming, this issue does not appear to be trim related.
 
Could very well be cavitation or ventilarion. Have someone drive as you watch what happens to the prop.

How deep does the drive sit in the water when stopped.
 
quote:
Originally posted by stang32

I have tried taking off with trim in & out, the problem occurs in each position,. all be it, as i said earlier, it does not happen every time, it does happen in each position. i have been a boater for 26 years & i know the effects of trimming, this issue does not appear to be trim related.

Click to expand...
Keep an eye on water depth my 40 Mainship SB wouldn't get up on plane unless I was in 5 feet of water or more. Be careful modern engines if they sense something is wrong or if you start to get spark knock they will retard timing. If the timing starts retarding your not getting on plane. The computer wants to keep the spark plugs from cutting holeS in your pistons.
Bill
 
Would still like to know what WOT RPM is when it does get up on plane.

You say this is a new re manufactured motor and you rebuilt the carburetor. I would be looking at a few things in addition to making sure it has the correct propeller - you probably know this but it should be turn 4400-4800rpm at WOT

1. Spark plugs / wires. Make sure you are getting spark on all 6 cylinders. Check the spark plug wires against the firing order.
2. Check the timing - 10deg BTC.
3. When you rebuilt the carburetor are you sure the accellerator pump is the correct type and installed correctly. There are two types on the Mercarb 2bbl.

This motor with a carburetor is about as simple as it gets, there can't be much wrong.
 
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