WOT rms seem low
 

Hello everyone, this is my first post and first visit to this forum. That being said, I just picked up my first real boat. It's a 1987 liberator 211 with a fresh 300hp 350 and a merc drive. My question is, I can only seem to get a maximum of 4500 rpm at WOT. It has a 19p stainless stilletto prop on it. No matter what I do with the trim thats the best I can get. Any input would be greatly appreciated!

Need to know the max rpm's your engine can handle. Make sure carb is going full throttle, with engine off push throttle all the way forward and take a look down the carb and see if butterflies are open all the way

Max rpm's should be in the 5300-5500 range possibly even a little more as it does have a roller setup. I did check the carb, but it has a vacuum actuated secondary so im going to check that this weekend. Otherwise it does seem to open all the way.

Rpm's are controlled by the prop,assuming the carb is big enough and working correctly. google search prop calculator and you can put in your info and see what your slip is and experiment a little. Going down in pitch will increase rpm's going up in pitch will decrease rpm's. Or try www.go-fast.com for a prop calculator. What kind of shape is the bottom of the boat in? Clean or dirty, dirty makes a huge difference.

Where did you come up with the rpm range you feel it should be running in? Stock 350 motor in that boat would have been 260hp. 300hp isn't that much more. Where did the motor come from? Here are the specs for merc's 300hp
350 mag which only shows 46-5000rpm max. I'm sure it has a roller set up in it also. You may be at your max now. I don't think you would want to go any smaller than a 19P prop to increase your rpm's. Have you checked the timing at full advance?
http://www.perfprotech.com/store/art...fications.aspx

No, I really dont want to go any lower on the prop. As for the rpm range the guy I bought the boat from had the motor installed and Im going off what I was told I kinda figured it would be more near the 5000 mark. The timing was off and it is now 8 degrees advanced, and it is still only getting near 4500. I am going to check the carb this weekend to make sure the secondaries are opening when they should. the carb is a holley 600cfm. how much more timing if any can i get out of it without to much detonation? Or am I right where I should be?

I was talking about checking it at full advance to see if it is advancing as far as it should at high rpms. 8 degrees is what they tell you to set it at idle but that is assuming the distributor is advancing correctly. You should have 30 minimum to 34 maximum degrees total and it should stop advancing there by 26-2800rpm. In order to check the timing this way your balancer must be marked in degrees up that high. If not you can get timing tape to put on the balancer which is marked in degrees. Have someone bring the engine up slowly to 3000rpm while you watch with the timing light. Timing should keep advancing as the rpms come up and should stop advancing at 34 degrees before 3000rpm's. This is where you want to set your timing, not at idle as long as it is resonably still close at idle. If your distributor has centrifugal advance weights in it be sure they move freely and aren't locked up from rust and such as this would be a cause for it not to advance correctly.

I will check the timing this weekend. Thanks for the input!

I'd also check the tach. It could easily be off 500 rpm. Do you have gps speeds at WFO and 4500rpm?

the tach is off, it actually reads around 3600. and i dont have a gps yet but should have one in the near future. on the speedo it only reads 42 though.

Use a good tach and GPS speed and report back. There is no way to make any suggestions without real info.

So here's my report WOT new tach read 4600, speed on gps was around 40-43. I am getting 34-35 degrees at full advance@ 3000 rpms. Carb seems to be functioning correctly, although I think it could use some tuning. So any thoughts?

I would try a 17 and 19 pitch Mirage Plus prop.
To calculate prop slip, you will need to know the gear ratio in the outdrive.

I think the prop looks a little suspect, the blades seem to be awful thin and the machining is poor at best it does'nt look like it was cleaned up at all when it was produced. So when I can get some funds together I am going to switch props out. Thanks for all the input everyone, I will let you all know how it works out later on.

You can use BBlades test program to try different props.

Thats where I figured I would go, they are just down the road from me. And I have heard nothing but good things about them. How much am I going to lose on the top end buy swithching to a 17p? As it sits the best I can get out of it is 45 mph. Also would trim tabs help me with any of this?

You're down on power. A 221 Lib with a 330hp 454 runs 55-57mph.

See thats where I'm at a loss, a friend of mine has the same boat with the same running gear and he is running in the low 50's. I'm not sure about the prop he's running though.

You should gain top end speed. You will get more rpms and the propslip should go down. Your drive is relatively deep by todays standards and the 4 blade prop is probably holding you back.

Griff, current setup is a 3 blade but like I said I dont think it is in very good shape. The blades seem thin and the hub surface is somewhat rough. So just to clarify, your saying that a good 17p 3 blade is the direction I should go? Also the carb is only 600cfm, would switching that out to a 650 with a mechanical secondary help at all?

That 330 H.P. big block your buddy's boat has will produce alot more torque than your 300 h.p. 350 small block will. He is runnin about 10 mph faster than you. I would say that is fairly close given the differences in the engines. You may gain a couple mph by the 17P prop..

I guess I should have said that it is the same boat as mine. My buddy is also running a small block chevy. And that is why I am wondering where I can make up the loss, I havent had a chance to see what he is running for a prop yet though

Yes...the stock secondary spring in the secondary diaphram is very stiff. Holley sells a spring assortment kit that lets you customize the secondary opening but I never have had any luck doing it with a marine application.

I have run 600 CFM Holley Double Pumpers on 350 Chevy's with great success. A 650 Marine is hard to find...good luck.

Before you buy another carb ($500 new :eek:) go back to basics. Make sure you have no blockage in your fuel system...I have seen as have others a dirty fuel filter or water separator restrict fuel pressure enough to cause a problem like yours. Double check all your lines for kinks or restrictions.

I still say you are down on power and you are trimming too high trying to get rpms, making excessive slip. Borrow your friend's prop and see what happens.

Not every 3 blade prop is the same. I think youy need to test a 17 and 19 Mirage Plus to see which works better and pulls the correct rpms. 600cfm should be enough carb. A 650 might help top end a little, but start with prop testing befroe you worry about that.

Thanks for all the input everyone it is greatly appreciated. I am going to give everything another once over and try to do some prop testing in the next couple of weeks. I'll keep you all posted, thanks again guys.