too much or not enough prop

[beeracuda1]

i have a 16 ft alum.sylvan with a 50 hp johnson,i have a 12 1/4 x15 ss prop.wot im only getting 3800 rpms.motor runs good and speed is anywhere from 20 - 25 mph.book says i should be runnin 4500-5500 rpms.do i have the wrong prop? drinks gas bad,maybe another problem.any help would be greatly appreciated.thx.

 

[Texasmark]

Re: too much or not enough prop

Having been associated with 60 to 70 hp Johnsons on 15 ft fiberglass tri hulls years ago, running 15P props one could pull a pair of kids on waterskiis at 30 mph with the 60 and 35 with the 70. I have run 70's near 40 mph on 15' fiberglass boats.

I think you have other problems and they could be varied; bad gas a place to start, crudded up carbs next for starters.

HTH,

Mark

 

[cdn-pk]

Re: too much or not enough prop

GEEZ that is really low, i have more experience with the bigger V4 but 5300 - 5700 is where you want to be. and yes not allowing the motor to get to it's power band will consume more fuel.

but your problem is your already running a 15p drop to a 13 should increase rpms by 300-400 but your still way off. is your motor mounted too low? the cavitation plate should be level with the keel. if it's too low it will decrease rpms (more drag). check that out but you may have other problems....... at WOT are your carb butterflys wide open? and timing fully advanced?

 

[beeracuda1]

Re: too much or not enough prop

ok,i have put alittle stp water treatment in with fresh gas,would sprayin carb cleaner while runnin clean them out of should i take them off to do it?thx very much for ur help,much appreciated.will get them cleaned out n c what happens.

 

[corvette-old]

Re: too much or not enough prop

add a little 108 octane booster,but dont overwind it.

 

[beeracuda1]

Re: too much or not enough prop

i will check in the am to c where cavitation plate sits,i did take off the stablizer fin to c if it helped and it did help , before when hittin a boat wake the boat would slow down n plow,now with out fin it doesnt slow at all n goes right over the wakes. maybe it is sat too low.i,ll take front cover off carbs n run it n c if butterflys r open fully.thx u guys for the help,still learnin.will ck it all out in the am.oh ,if too low,would movin the pin up on the motor stop help any? thx again.

 

[beeracuda1]

Re: too much or not enough prop

will that clean carbs out runnin that threw it? im over lookin somethin,it runs good idles jus under a grand,doesnt skip a beat runnin wot,but jus doesnt get the rpms n speed.thx will give it a boost.

 

[cdn-pk]

Re: too much or not enough prop

Don't add anymore octane boost. octane increase the fuel ignition point, it's not gonna help a carbed up motor burn anything off. Outboard already operate cool (hence the TCW3 oil rating). You could do a decarb (see here: http://forums.iboats.com/showthread.php?p=2648076#post2648076). i do a decarb with seafoam once a season, always notice a difference after.

moving the pin may help you trim better but it won't help.... sounds like you may be sitting too low. i had to raise my 120HP, once i got the cavitation plate 1" above keel i jumped up 400rpms.

other good maintenance would be to pull off the carbs disassemble and spray carb cleaner then reassemble (or do a carb kit).

you never mentioned what year is the motor?

 

[glengold]

Re: too much or not enough prop

+1 for a decarb, just did it with a gallon of fuel plus a 16 oz seafoam dose and gained 600 rpm. I've also experienced a gain of rpm by raising the motor a hole. Did it ever run with in operating range?????

 

[Texasmark]

Re: too much or not enough prop

Your test is to take something like spray Sea Foam and give the carb throat a shot while you are running. If the engine takes off, it says that you are low on fuel for that carb. Sea Foam is perfect as you get a cleaner/combustion agent and some light oil. A lot of other sprays may not contain oil and your 2 stroke needs it.

Spraying through the throat does nothing to clean out the carb. Carb problems are usually a gummed up needle seat which keeps the float from working properly...i.e. carb bowl doesn't stay full of fuel and second clogged venturi ports. When air goes through the venturi of the carb, the air passage creates a lower pressure (very light vacuum) than is in the carb and it sucks the fuel up from the carb bowl. If they are clogged (varnished up) the fuel passage is restricted.

Best bet is to do a carb kit. There are other "decarb" procedures outlined within this site's forum, but they are primarily intended to clean up the cylinders/rings and all, not "necessarily" the carb.

HTH

Mark

 

[beeracuda1]

Re: too much or not enough prop

the motors a 1990 , i looked at it this mornin and wouldnt ya know it the cav. plate is settin about 4-5 inches below the keel.thing is theres no room to move it up any on the transom.so here i sit wondering with trim n tilt a transom rebuilt to accommodate raisin the motor any.the boat sat only acouple months before i bought it,ive put fresh gas n have a water seperator on board,dont know when service has been done on carbs but im thinkin its time,so thats next.will try sea foam n c if it helps.thanks so much for all ur help.will keep an up date.

 

[beeracuda1]

Re: too much or not enough prop

glengold,i will raise it a hole and try decarb.ive only had the boat a couple months but its never ran above 3800,was thinkin the 12 1/4x 15 was too much,it comes up on plane quickly but 25mph alone n all seats out is all she,ll do,rather slow i thought for a alum.50 hp.im bein told the motors sittin too low,which i ckecked n it is 4-5" too low so that also is hinderin the rpm,s.i will give all this a try,n carb kits,shes old but she runs with out a hiccup.thx so much for ur input will keep an update.

 

[cdn-pk]

Re: too much or not enough prop

Yeah it's way too low if you can raise it so the cavitation plate is +/- one inch from keel (thats about 4 inch rise x 400rpm per inch thats an extra 1600 rpms.... rough estimate you should gain) so that would put you 3800+1600=5400rpm..... Almost exactly where you want to be.

you must have some room to adjust..... unless you have an extra long shaft (25") on a 20" transom?? whats the full model number? you can measure your transom length by measuring from the keel to the top of the transom. and the motor shaft length by measuring from the top of the cavitation plate to the top of the mounting bracket.

But i'd definitly raise that motor.... that low you could plough rice fields


heres some more information:
Hold on hold on your jumping ahead here.

first determine where your cavitation plate is now:

1) out on level surface motor down
2) long straight edge under boat on keel (6' at least)
3) move straight edge to side of lower unit and determine if you cavitation plate is above or below keel
4) take tape measure and measure distance from cavitation plate to keel (top of straight edge) +/- 1" is the zone we want to be in


if you below and have holes left on your mounting bracket to adjust to then heres a good procedure

1. Park on a level, paved surface
2. Block car wheels so it can't move
3. Motor vertical
4. Place wood blocks under skeg until only small space
5. Remove upper engine to transom bolts
6. Raise tongue jack on trailer until slight load on skeg blocking
7. Loosen, but DO NOT REMOVE lower engine mounting bolts in slotted part of motor bracket
8. Continue extending tongue jack until hole you want lines up with bolt hole
9. Replace upper bolts
10. Tighten all bolts
11. Retract tongue jack
DONE

Credit to wilde1j for this


also don't forget to seal ALL 4 bolts with adhesive sealer 3m 4200 (i think.... can't remember)

 

[Texasmark]

Re: too much or not enough prop

If you are that low, you have a 20 inch engine on a 15" transom. You will not get there moving up a hole or two.................geez, we are fed what we are fed and none of it is relevant till you get to the bottom line..........why am I on this forum????????????

 

[Mi duckdown]

Re: too much or not enough prop

If you are 4 to5 inches to low in the water, the carb kit will do nothing.

Send some pics of your setup, motor/ transom etc.

That would be more help for you.

 

[Mi duckdown]

Re: too much or not enough prop

Easy there Tex. But I understand.
Tex cause your a nice Guy.

 

[steelespike]

Re: too much or not enough prop

If you are that low, you have a 20 inch engine on a 15" transom. You will not get there moving up a hole or two.................geez, we are fed what we are fed and none of it is relevant till you get to the bottom line..........why am I on this forum????????????

I don't understand; an inexperienced boater might not have a clue about engine height or different leg lengths.I think a suggestion about ways to solve the problem would be more constructive than wondering why you are here.

 

[Texasmark]

Re: too much or not enough prop

I don't understand; an inexperienced boater might not have a clue about engine height or different leg lengths.I think a suggestion about ways to solve the problem would be more constructive than wondering why you are here.

Okay, Okay my man, I blew it....I'm sorry, but we are trying to second guess problems and are spoon fed information without the luxury of being there at the site and experiencing all the additional information.....and we all (you included, I'm sure) pride ourselves in being able to help folks in solving their problems. Peace man!

Mark

 

[Mi duckdown]

Re: too much or not enough prop

beeracuda where are the pics? Can't help you without them.