force 125hp seems slow

[EverydayAngler]

ok guys here are my issues. i have a 83 125hp force on my 78 glastron hpv-170 17ft bass boat around 1100lbs with 2 180lb guys and 25gls fuel. 15p prop and im only seeing 35mph and only running 5000ish rpm wot, did a compression test tonight and #1-140 #2-140 #3 135 #4-135psi, i plan on doing a decarb on the motor tomorrow. my issue is i cant believe 35 is all this engine will push the boat. any ideas here guys.

 

[labawg]

Re: force 125hp seems slow

pitot or GPS?

 

[EverydayAngler]

Re: force 125hp seems slow

this is off the gps on my lowrance

 

[nickhaulotte]

Re: force 125hp seems slow

hmmm. this boat should be faster i have the same size boat with a 90hp force and am seeing 45mph. Clean carbs would be my guess. 2 stroke carbs dont seem to get near as dirty a 4 stroke in my opinion this is due to the oil. I would say to do the link and sync sticky that frank has on the first thread in this forum. What kinda gas octane and oil r u using? What about the lower unit oil? Is it dirty? What about water in the hull? Have you tried to trim up just a bit while on plane? Not too much tho it will reduce the amount of water going into the engine to keep it cool. I installed the dolphin fins on mine didnt notice a huge diferrance but... Quicker hole shot. Is the prop pitch right for this boat?

 

[labawg]

Re: force 125hp seems slow

This is the address for a Marine Propeller Calculator: http://www.csgnetwork.com/marinepropcalc.html

1983 125 hp Force has a 1.73:1 lower unit. At 5200 RPM your prop slip is 18%, at 5000 RPM it's 14.75%. 10%-12% is average for a new propeller sometimes a little more from what i read. If you were able to hit 10% slippage your speed at 5200 RPM would be 38 MPH, so you are close to max speed that 15p propeller can produce at 5200 RPM.

 

[EverydayAngler]

Re: force 125hp seems slow

i was really thinking this boat should be mid to high 40s. carbs are cleaned, i plan on synking the carbs when my dad gets back so he can help me with this on the water, just changed the l/u fluid out with syn oil, using fresh 87 oct i cant believe this engine would need any more octane then that, using pensoil syn oil mixed 50:1. i played with the trim till i saw the speed start to drop then lowered it a little. the hole shot is impressive comes out of the hole fast with very little throttle. i cant believe with a 15p prop im only seeing 5000rpm so im at a lose here. oh yeah it has a stainless prop on it. from my reading the compression seems of its well within the 10% range. im sure the carbs need to be synced it has a high idle around 1200rpm in neutral and that seems a little high to me. the boat is dry inside we just redid the transom, stringers and floatation foam in the boat this spring.

 

[labawg]

Re: force 125hp seems slow

My friend with a similar size boat as yours and 120 HP tops out a 46 MPH but has a 19p propeller. Do the calculations yourself. I myself have a 18' boat with a 1984 Force 125 HP, 17p prop and at 5200 RPM hit 42 MPH.

 

[EverydayAngler]

Re: force 125hp seems slow

ok now its starting to make sence. i think its time to find a 17p and 19p prop to play with.

 

[labawg]

Re: force 125hp seems slow

Beware, the higher your pitch the lower your RPM will be and this will reduce your holeshot. But I suspect your holeshot right now is very very strong. I'm sure one of the sages will chime in soon to explain better. I sure your motor could run a little better too. See if you can borrow a 17p from someone.

 

[EverydayAngler]

Re: force 125hp seems slow

it takes almost no throttle to get it out of the hole right now.