WOT RPMs not there

[houstoncajun]

Hi all,

I cant seem to get but 3800 RPM and not plane out.... I'm going to change the Cap, Rotor, Plugs and Wires this weekend but the prop is a little pitted and the tips seem ate up, can it be a major factor in there?

Thanks for any tips....

 

[a70eliminator]

Re: WOT RPMs not there

Pay attention to detail on that tune-up your doing, its usually the little things that get ya.
Even with your worn out prop the RPM should still be achievable.

 

[AZSenza]

Re: WOT RPMs not there

I agree, when I got my Larson, changing those three things netted me another 400 RPM top end and ran a lot smoother.

 

[boat1010]

Re: WOT RPMs not there

I guess my question is did it use to run better? The small pits on the prop shouldn't make that big of a difference.

 

[ziggy]

Re: WOT RPMs not there

have ya gone up to the adults only sticky and checked out #12? it's a pretty good list that ya might go through ea. item and give them a check..

pitting on the prop ay? like in cavitation burns? this was a quick google of cavitation.... http://en.mimi.hu/boating/cavitation.html.... food for thought anyways....

 

[picklenjim]

Re: WOT RPMs not there

What diameter and pitch is the prop.That's a pretty good size boat for a single small block.A prop that is to small in diameter for the boat and thin on the leading edge can have cavitation burn problems.Also makes it hard to get on plane.If it could get on plane it might pick up some more rpms.

 

[houstoncajun]

Re: WOT RPMs not there

Thanks for the info... i'll change the plugs, wires cap and rotor and check the timing this weekend. Then try again, i'm not real sure of the prop pitch but i have a spare and think its 16 x 16. I have a hard time quick hauling it to just change the prop (need to rebuild trailer because of rust). It did plane out before buying not reaching the WOT though. The surveyor did a compresson test (all within 10 %) and broke the plugs and installed some other ones ( I dont think he gapped them because he didnt know the gap size for one) so I bought the ac delco ones required and starting from there. I ran some sea foam through the gas tank, hopefully that helps "degum" it. The engine has 770 hours on it and from what I've seen around thats not too bad. I'll repost after i'm done...

 

[Don S]

Re: WOT RPMs not there

If you are at 3800 rpm and not on plane, then you are probably way bow down. Try trimming the outdrive up after you get to full rpms. Bet they come up the bow comes up and you start planing.

 

[houstoncajun]

Re: WOT RPMs not there

Ok... so i went and changed the plugs and distributor cap, (3 of the cap screws sheared off in the body of it so I clamped the cap back on, just to run until I can fix it) It now will get 4200 RPM but is not gaining any speed, about the same at 2500 RPM as 4200, it just churns water, the spare prop is a 48-78118-17, i need to do a quick haul and change it out and test again. Here is a pic of the prop on it now. I tried trimming everywhich way and moving the weight around still not getting on plane. any thoughts are appreciated

 

[m monk]

Re: WOT RPMs not there

Is the boats floatation waterloged

 

[Don S]

Re: WOT RPMs not there

You probably need to clean the bottom of the boat. If it's all covered with barnacles and other marine growth, it will kill speed and rpm.

 

[houstoncajun]

Re: WOT RPMs not there

bottom paint is still "active" and pretty clean. the drive needs a paint job though. Guess i'll change the prop out and go from there... dont think the floatation is wet? (dont know how to check without destroying something) no blisters or wet stringers found on the survey, a couple of scrathes and minor dings to the bow from trailering it. I will find the problem though, i am determined

 

[270Searay]

Re: WOT RPMs not there

[If you have a rubber hub in your prop it could be slipping

 

[houstoncajun]

Re: WOT RPMs not there

Thanks for the help guys,

I plan on doing the following:

first fix my distributor cap (just drill thought the broken screws and put a nut on them)
change the prop to the spare listd above
put a stingray plate on the cavitation plate, (it helped alot on my ski boat)

then take her for a spin again

problem I have is I have no way to pull her out the water unless I do a quick haul (cost $125 for 40 minutes) I'm trying to find a place near shore that I could back up to and do this work in the water. problem is water is dirty, salty and muddy bottom here. Is it recomended to change my prop in the water?(my first time too). And I need to see if I have long enough drill bits to put the plate on in the water for my cordless drill

Oh and my electric anchor rope doubled under the pully and got stuck... lol so i'll fix that too

Thanks again, think I'm on the right track with throwing my money in the hole in the water

 

[ziggy]

Re: WOT RPMs not there

i would not put a whale tail on it. you've got trim tabs.
certainly not do the whale tail and the prop at the same time. if it fixes the problem, you won't know which did the repair. also, you said this boat did plane. so. something is still amiss.
you can replace your prop in the water. my buddy who like to experiment with different props does his all the time at the beach. just don't drop any parts....

i don't suppose your tabs are all the way down are they? that'd keep the bow real down and perhaps not let it get up on plane..

 

[krisnowicki]

Re: WOT RPMs not there

change it in the water... Worst case it will cost you a new prop nut or wrench if you drop it. 10 bucks. I change mine int the water all the time . Wrech is either a 1 1/16 or 1 1/8 can't remember right now

 

[houstoncajun]

Re: WOT RPMs not there

Changed prop to the spare 48-78118-17 (aluminum), got it to plane and up to 25 MPH at one time. Lots of wind and current, and the waves were pretty heavy so kind of sporatic. I was pulling 3800 RPM tried bow up to gain some RPM but it started to back off MPH and looks like it went to 4000 RPM. looks like cruising speed would be around 12 or 15 MPH like this. Still not achieving WOT per spec though, not sure where to go on that

My trim tabs are small, is it wrong to put a stabalizer on the cavitation plate "whale tail"? It sure helps on my ski boat. This boats dry weight is 5500 LBS so I know it takes a minute to get up and going.

Should I try aother prop? will stainless steel make a big difference?

 

[mmcdjs]

Re: WOT RPMs not there

Hi. I like all the details that you are putting in here! I'm having a similar problem with my 1982 Century 5000 23' with 5.7L. When I go WOT I only get about 2400 RPM, 3000 at best, the stern is down hard and I'm churning lots of water but can't seem to get up on plane either. I'll keep watching your posts to look for idea's on my boats problems!

 

[houstoncajun]

Re: WOT RPMs not there

Glad I could help while trying to figure out mine.... Best improvement done so far is changing the prop... any info I find out I'll pass along. sounds like you might be having cavitation problems too.

 

[mmcdjs]

Re: WOT RPMs not there

Yeah. I'm looking at everything right now.. gonna start with the spark, then fuel, then prop. If it was cavitation, wouldn't my rpm go up though?