boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

[jasonmd13]

I got a 1996 rinker 18' bowrider ski boat this winter. It has a mercruiser 4.3l 205 hp 4 bbl. It came with a alpha one gen 2 outdrive, but was replaced with a alpha one gen one outdrive from a 4 cylinder by the decals on it. It has a 17 pitch prop on it. With 6 people on it, and ski gear, it will only reach about 30 mph. That is with 2 adults, 2 teenagers and 2 kids. This seems slow. I redid the boat this winter. New floor, foam, seats , carpets ect. I did a prop check and one site said it was right. Not sure. Any ideas? Thanks

 

[steelespike]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

We need the wot rpm and speed by gps. Depending on the outdrive gear ratio; with the right prop you should be good for over 50.
The 4 cylinder gear ratio is probably about 1.8 or 2.0 may be marked on the outdrive. may call for strange propping.

 

[jasonmd13]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

The wot with the whole family was 4000 and the speed about 27-28 by gps. The speedometer was right on. I though the gps was going to be faster. This was our 2nd trip out. The first trip on had 5 people, plus no bimini top on yet and it went 35 wot and 4600 rpm.

 

[steelespike]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

You definitely have a performance issue.A test with a light load would be better to evaluate.
But I think a motor issue.The motor should overev with a 17 even with the family aboard.
With the 205hp on a 4 cylinder leg 2.00 gear ratio you probably would need a 23 to 26" prop
and would run speeds around 50+The 2.00 may limit top speed.
For example the 205 at 4800 with a 1.7 ratio and 23" prop speed is around 55 assuming motor can make 4800.
a 2.00 ratio at 4800 with a 23" prop makes about 47.
Does the boat respond to up trim?
Does the throttle open all the way?
Have you checked compression?

 

[jasonmd13]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

Our first trip out, we was turning 4600 rpm and traveling 35 by speedometer. I replaced the bottom of speedometer because I thought it was wrong. After replacing it; by GPS it was accurate. I had more people bit only 4000 rpms and 29-30 MPH after trimming up and Bimini top upand lots of wake board stuff. I am not sure how a 4 cylinder outdrive would effect the v6. The motor runs smooth and has a hydrofoil. I went to a turning point prop selector and it said I should have a 17 pitch for watersports. I haven't checked compression. Thanks for your input.

 

[steelespike]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

With a 17" prop and your 4.3 on a 4 cylinder leg the holeshot should just about snap your head off.Even with the family aboard
hole shot should be really strong.With the foil planing should be just about instant.
The 4 cylinder will have a gear ratio of about 2: 1 prop turns once for each 2 revolutions of the motor,The 4.3 would have
about 1.7:1 prop turns once for reach 1.7 turns of the motor.So the prop on a 4.3 can be turned faster with the motor at 4,800
than the prop on the 4 cylinder leg with a 2 to 1 ratio.The faster the prop turns the faster the boat goes.
You probably can make up for the 2 to 1 ratio with the right prop.
Your hydrofoil will help your hole shoot but may slow your top speed.
When you trim up does the boat respond ? Bow should come up and speed increase sharply.

 

[jasonmd13]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

Thanks, that makes since with the 2 to 1 ratio. I wish they would have put on the same outdrive. It did good planning out and ok on the take off. A little slow trying to pull a wakeboarder with the family aboard. I called 2 prop dealers today. It currently has a mercury 15x 17 pitch on it. Both said 15 diameter was too big, but one said i need a 15 pitch and the other said I needed a four blade 14.25 x 19. Seems like the 4 blade might keep the take off strong and the 19 help with some speed. This is my first inboard. Growing up we always had a fish and ski. The trim does help the topend speed, but does purpoise easily. Which prop idea seems right?

 

[jasonmd13]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

I also have to work on the take off with a skier. I gunned it too quick and it would have a 2 second delay. Sometimes, it seem to stall around 10 mph then pickup from there. It wouldnt lose speed. Maybe just a 4.3 lx inboard thing.

 

[steelespike]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

If it stumbles when you advance the throttle to quickly it could be could be the accelerator pump in the carb.
Also if the timing is off it may stumble.Diameter can affect rpm a little not to the extent of your problems.
As pitch goes down diameter tends to go up. and vise versa.
A 17" pitch is typically used on 4 cylinder 140 hp I/Os for skiing with a load.Your boat should be screaming and over revving with a 17" prop.I think you can see that a 17 doesn't even make sense with 205 hp on a 4 cylinder leg. A 15" prop is very close to what you might used on a good sized pontoon boat
with a 140hp. we shouldn't be picking props until you solve your motor problem.
Be assured that with that leg your prop is going to be in the 20"+ range.
Check that the throttle opens all the way and that then secondaries in the carb work.
Check timing is right and advancing as it should.
Be sure its running on all 6 cylinders.
Look on the leg for the gear ratio.
Check the compression.
While the plugs are out you can varify the ratio by turning the motor by hand and count rotations for one revolution of the prop.
To be accurate you will need to mark pulley and prop.

 

[bnicov]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

You've got a couple of things going on. You are never in a million years going to get a lot of speed and get up and go for wakeboarding with a boatload of people. Deal with that. That said, you have the wrong leg on for your motor. Prop should help but ultimately you are trying to run the hundred yard dash on crutches. Try to find the correct leg and swap them out, it'll cost you a few bucks but you will have a proper starting point to prop your boat. Next, you need to figure out why your motor is stumbling, it could be a carb issue OR a timing issue. WITH THE ENGINE OFF, have someone give the throttle a shot and you look down the throat of the carb, did you see a healthy squirt of fuel from all the jets?? If not, the carb's accelerator pump is not working right. You may as well rebuild the carb, it's not rocket science and if you are good with a wrench, you can do it yourself for under 50 bucks. Next, check the timing according to the procedure in the manual which can be downloaded here.
Good luck and enjoy the boat. I personally don't do watersports with more than 4-5 people on the boat incl. kids. It's too hard on it. 6-7 adults + coolers, gear, etc... adds up to a lot of weight, just go for a cruise with a gang.

 

[MRCLEAN92626]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

I'll chime in here with my experience with the same size boat and 4.3L w/ 4bbl. I have at this time the following props, and I'll explain why I have each of them and what details I can give you about each.

18P four blade WOT will easily over-rev the motor at 5K+, so I use this prop for waveboarding ONLY and keep the RPM's around 3200. Great holeshot, and will pull a boarder out of the water quickly even with six more people and gear onboard. Max speed is so slow I never GPS'ed it, but I think 30mph is about accurate.
19P three blade Just got this one so I haven't gotten a chance to test it yet.
21P three blade Great all around prop. WOT sits right around 4600-4800 with a boatload of people and gear. Max speed with 6 people is 41 MPH on GPS
23P three blade The 23 pitch blade isn't great with a bunch of people and weight, it takes too long to plane and WOT is too low. GPS showed 44MPH with TWO people and trimmed way up, 4500RPM's.

I wouldn't go with anything higher than a 23, or a 21. I can't see how you could get 50+ mph out of this motor in an 18ft open bow runabout. If I was in your shoes I would go buy a 21P three blade prop, and see how it does. I bet you would find that made a world of a difference and put your WOT right where it's supposed to be.

 

[Maclin]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

Based on the description of what happens when you gun it, it sounds like the carb does need some dialing in. The accelerator pump shot seems off, should be no delay when gunned. The delay and poor top RPM could also be the secondaries are opening up too soon and bogging the whole time after that.

Could also be the timing advance mechanism in the distributor, or simply incorrect static timing.

In any event, even with just a 2bbl carb that engine with that drive and a 17p should just blow right on by the rpm limit and overrev easily. Sort out the engine tuning problems and I bet a 21 or 23 pitch prop will be the ticket.

 

[jasonmd13]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

Y'all are awesome. I had asked 2 different people with similar but new boats, and they said the delay from takeoff was normal. I took off the breather and turned on the key start the fuel pump. The accelerator pump jets on one side nothing came out and the other was a small trickle. I tried it several times and always got similar results. I have a service manual, but kind of scared to mess with a carb rebuild. My local shop want 250-300 to rebuild. Does anyone know of a how to video or should i just send it to the shop? Plus, how long does it take to repair? Also, looks like a complete rebuild kit with the accelerator pump is about 130 plus cleaner. Thank you for everyone's knowledge, this site is awesome.

 

[Bondo]

Re: boat doesnt seem to have much speed for a 4.3l mercriuser 4bbl

Y'all are awesome. I had asked 2 different people with similar but new boats, and they said the delay from takeoff was normal. I took off the breather and turned on the key start the fuel pump. The accelerator pump jets on one side nothing came out and the other was a small trickle. I tried it several times and always got similar results. I have a service manual, but kind of scared to mess with a carb rebuild. My local shop want 250-300 to rebuild. Does anyone know of a how to video or should i just send it to the shop? Plus, how long does it take to repair? Also, looks like a complete rebuild kit with the accelerator pump is about 130 plus cleaner. Thank you for everyone's knowledge, this site is awesome.

Ayuh,... A Brand New Edlebrock, is 'bout that same price,... 'n it's a carbon copy of the Merc/ Weber you've got...