RPM to low

[Ron570]

Need some help with my boat. I have a 17-ft aluminum Bluefin with an 85 HP Johnson OB. It has a 13 ? inch diameter 17 p three blade aluminum prop, Hustler. At WOT I only get 4400rpm and 35 mph. I?m ok with the speed but I know the rpm is to low. I was going to get a prop with less pitch. On this forum I read that for every inch of pitch you can change engine rpm about 200 rpm. That would mean I need to go down to about a 10 p. I was told that I should shoot for a WOT rpm of 5800. I?m also wondering if I should change the height of the engine. With the engine at full down trim the centerline of the prop is 8 inches below the keel. This is the same height the previous motor was at. I swap out a 50 hp Force. I can raise it 2 inches without drilling new holes. The engine is tuned and I also rebuilt the carburetors this year to make sure the engine was running it's best. That didn?t change the engine speed. But it starts and it doesn?t stumble during off idle accelerations anymore. The engine is newly rebuilt and broken in. Any insight you can lend would be greatly appreciated. Other than the engine rpm it runs great. Quick acceleration. Comes up on plane quickly. Big performance improvement over the 50 hp. I'm not sure what rpm the 50 hp ran at. I didn't have a tach on back then.

Thanks

 

[Outsider]

Re: RPM to low

Raise it up one hole and check performance, then two holes. Let that be your guide to prop changes. I think 8" sounds too low, cavitation plate should be level with the transom line as a starting point ...

 

[Silvertip]

Re: RPM to low

It's the position of the AV plate that should be flush or slightly above the keel. That then determines the position of the prop position in the water. You need to raise the engine, adjust the load to balance the boat, remove and replace any water saturated foam and anything else that may be wrong before spending money on props. A good tune up on the engine to ensure it is making full power is also necessary. That would include making sure the throttle butterflies are perfectly horizontal in the carb bores at wide open throttle.

 

[steelespike]

Re: RPM to low

Need some help with my boat. I have a 17-ft aluminum Bluefin with an 85 HP Johnson OB. It has a 13 ? inch diameter 17 p three blade aluminum prop, Hustler. At WOT I only get 4400rpm and 35 mph. I?m ok with the speed but I know the rpm is to low. I was going to get a prop with less pitch. On this forum I read that for every inch of pitch you can change engine rpm about 200 rpm. That would mean I need to go down to about a 10 p. I was told that I should shoot for a WOT rpm of 5800. I?m also wondering if I should change the height of the engine. With the engine at full down trim the centerline of the prop is 8 inches below the keel. This is the same height the previous motor was at. I swap out a 50 hp Force. I can raise it 2 inches without drilling new holes. The engine is tuned and I also rebuilt the carburetors this year to make sure the engine was running it's best. That didn?t change the engine speed. But it starts and it doesn?t stumble during off idle accelerations anymore. The engine is newly rebuilt and broken in. Any insight you can lend would be greatly appreciated. Other than the engine rpm it runs great. Quick acceleration. Comes up on plane quickly. Big performance improvement over the 50 hp. I'm not sure what rpm the 50 hp ran at. I didn't have a tach on back then.

Thanks

If that is a V4 it developes its power at 5,000 rpm as most V4s do from 69 to 84. 5,800 sounds a little high to me I would shoot for something in the 5,400 range so you are close to max rpm with a load.With an ideal setup and a sound well tuned motor I think you should approach 40 mph. at about 5,200 rpm.Check everthing out then make your prop selection at that point.
10" pitch on that boat is way low.Closer to that for a pontoon.

 

[Ron570]

Re: RPM to low

Thanks for all the great advise. I'll raise the motor and see what it does for me. The engine is tuned and the butterflies are horiziontal an WOT. I check that and tuned it before I ran it the last time. Just to make sure the engine wasn't part of the problem. According to the manual I have it shows 5000 rpm as the WOT rpm. Someone told me that I should go higher (5800 rpm) because of today's gasoline (not being as clean burning as that in 1978) and the higher rpm would prevent piston ring deposits.

I thought a 10p was low also.

Only problem, it's getting cold here in Michigan, but the water is still soft, so I should be able to get it out.

Thanks

Ron

 

[Ranger 330V]

Re: RPM to low

ron, that someone is right...just because you prop them to 5800 at wot doesn't mean you'll always be there. As long as you prop to 5500 at wot you won't have any issue with deposits. Those are notorious for coking or getting carbon deposits. if you've run it long with your setup do a decarb (search FAQ in johnson/evinvrude fourum) as soon as you can. Carbon will accumulate on rings and eventually break a piece of the ring off and ruin that engine. Lugging is really bad for these engines or any 2stroke for that matter. it may sound like its okay running but its getting deposits. raise the engine, do a GOOD decarb, and prop to 5500 at WOT and your set. Your engine may have powerpacks with rev limiters, crossreference your numbers on the powerpacks to make sure...I think they are limited to 5000 rpm if I'm not mistaken...if you have that you won't get over that rpm. hope this helps.

 

[Ron570]

Re: RPM to low

Thanks. I'll check the numbers on the power packs.

I don't have allot of time on this engine. Summer 2006 was the first year I ran it and I didn't go to WOT the whole season since I was breaking it in. I might have 80 hours on it. I didn't get out that much this summer.

I looked at the engine height again and even though the prop centerline is 8 inches below the keel the cavitation plate is about even with the keel. I'll still try raising it and see what it does for me.

Thanks

Ron

 

[72SideWinderSS]

Re: RPM to low

Ron,
Did you check your MAX Advance at 5000 rpm's?
Check your manual for the correct setting, should be part of a link and sync.
Joe Reeves has a post on here that explains how to set it without having to run down the river WFO hanging off the back of the boat.

 

[Texasmark]

Re: RPM to low

8" prop centerline to bottom of boat running a 13+ prop with a little room between it and the av plate?

14/2 = 7 so it looks like you need to come up a couple of holes beins they are usually 3/4" apart. That'll put you just above which is much better than below. Engine might want to ventilate in hard turns, but your Hustler is cupped, so it should reduce the tendency.

Then readdress whether or not you need a prop.

My experience with hustler and SS is that Hustler will cost you rpm's for the same size/pitch.....did me. In alum to alum probably THE way to go.

Mark

 

[Ranger 330V]

Re: RPM to low

ron, sidwinder is right...link and sync and make sure you are at the right advance...that engine should be 26 degree total advance I think. The Joe Reeves way is a great way and the way I time mine...alot easier than hanging off the back of the boat..lol. Just remember to time it 4 degrees retarded at max when you are setting it by mearly turning the engine over. These engines are set at 5000 rpm from the factory and will gain about 4 degrees once you are up and running.

 

[Ron570]

Re: RPM to low

I didn't think of the spark advance at WOT. I'll check this. Thanks. I'll look for the post from Joe Reeves. I went through a bunch of his posts but didn't find the one you're refering to yet. My manual shows 28 degrees max advance. Will also raise it.

I won't be going to a SS prop unless I find one as cheap as an Alum.

Thanks

Ron

 

[72SideWinderSS]

Re: RPM to low

Ron,
Here is Joe Reeves method:

(Timing At Cranking Speed 4?)
(J. Reeves)

NOTE: If your engine has the "Fast Start" feature", you must disconnect/eliminate that feature in order to use the following method. The "Fast Start" automatically advances the spark electronically when the engine first starts, dropping it to normal when the engine reaches a certain temperture.

The full spark advance can be adjusted without have the engine running at near full throttle as follows.

To set the timing on that engine, have the s/plugs out, and have the throttle at full, set that timer base under the flywheel tight against the rubber stop on the end of the full spark timer advance stop screw (wire it against that stop if necessary).

Rig a spark tester and have the gap set to 7/16". Hook up the timing light to the #1 plug wire. Crank the engine over and set the spark advance to 4? less than what the engine calls for.

I don't know the full spark advance setting your engine calls for, but to pick a figure, say your engine calls for 28?, set the timing at 24?. The reasoning for the 4? difference is that when the engine is actually running, due to the nature of the solid state ignition componets, the engine gains the extra 4?.

If you set the engine to its true setting at cranking speed, when running it will advance beyond its limit by 4? which will set up pre-ignition causing guaranteed piston damage! You don't want that to take place.

No need to be concerned about the idle timing as that will take care of itself. The main concern is the full advance setting.

Be sure to use your own engines spark advance settings, not the one I picked out of the air here in my notes.

http://stores.ebay.com/Evinrude-John...tc?refid=store

Also check that your butterflies are perfectly horizontal when at full throttle.

Good Luck,