Rpm's and speed

[lennygilbert]

I just got a 78 or 79 chrysler 75hp outboard.It was turning about 2800 rpms and running around 25 at wot.I thought it might be because the oil mix was off.The guy told me that he fouls plugs at least once a yr because he runs the mixture oil rich just to be safe.when we got back on the trailer and got home i noticed that the middle plug wire ws just barely hanging on.Im asuming that is the reason for the missing and low rpms and speed???Boat had no pblm starting up but didnt have the umph i expected.<br /><br />What should the normal rpm's be at wot?<br />and can anybody give me a guess as to what the top speed should be?boat is 16.5ft,75hp,about 300lbs of gear inc people in the boat..Thanks

 

[byacey]

Re: Rpm's and speed

2800 seems awful low, should be around 4500 RPM at least, wide open throttle. This boat should stand up and move at a pretty good clip.

 

[b.gagnon]

Re: Rpm's and speed

You probably have a 1978..in79 they changed to a 70hp. I have the same size boat with a 79 70hp and with full gear gas and two people I can hit 35mph @5100rpm. I am turning a 17p prop. My speed does not seem to change much weather I have one or three people in the boat (including myself).<br />In my boat I have to run about 3800-4100rpm to get 25-27 mph.

 

[lennygilbert]

Re: Rpm's and speed

Did a compression check and All 3 cylinders were almost the same. I believe they were 120 but i cant remember exactly.I think i may have also found the issue with my low speed adn rpms.After the boat was pulled out of the water we removed the cover and found the middle spark plug wire just barely hanging on.So there was at least a .5-1inch gap between the plug tip and the spark plug connection..

 

[byacey]

Re: Rpm's and speed

Chances are it was arcing. Once this happens inside the boot it causes a carbon path that creates leakage. Replace the rubber boot.

 

[speeddemon1452]

Re: Rpm's and speed

my 95 70 hp force was fouling plugs alot and rpms were only hitting 4000 wot had to change prop pitch was too steep now its is running 5000 wot and not fouling plugs anymore.guess it was boging it down and not alowing it to burn off all that extra fuel.