prop questions

[jdman]

I have a '79 Aristocraft 19' with 140 OMC. While reading through the original owners manual, I found out that my engine should be turning 4200-4800 rpm. Well, by my stock tach, I am only turning about 2500. The engine sounds like it is wound up pretty good at WOT, could it be that my tach is not accurate? or am I severely under propped? hard to believe my engine should be turning twice as fast as it is! I have noticed that it spark knocks at WOT no matter what octane I burn. I don't run at WOT, just bump it back till it stops rattlin'. could this be a symptom of the wrong prop? My grandfather had this boat since it was new, don't know if he ever changed the prop, but i do have two spares that came with the boat when i got it. only one of the three has a marking, 13x19, it is one of the spares, so i don't know what the one currently on the boat is. Any suggestions? thanks, Jdman

 

[Blue Thunder]

Re: prop questions

Check on the back of the tach and make sure the "dipswitch" is in the 4 cylinder setting. If that checks out, get an alternate tach and check the dash tach for accuracy. It is possible the engine is just getting tired and cannot reach the proper rpm any longer. To get to the proper rpm by changing props, you can expect about 150-200rpm change for every inch of pitch you go down. In you case, you would need a 10" pitch prop to get to the 4200rpm level.... which is not right, and leads back to engine trouble.<br /><br />BT

 

[jdman]

Re: prop questions

I think you're right, blue thunder. there's too much difference there to be prop related. As I stated earlier, the engine seems to be wound up pretty good, by the sound. I'm thinking more and more it's a tac problem. Thanks, jdman

 

[Boatist]

Re: prop questions

Jdman<br />What is the speed of your boat?? With that engine and boat 17 or 19 pitch prop is about right. Your speed should be in 35 to 40 MPH area. I think on the back of your Tac you will find a lever to set it to 4 cylinder, 6 or 8. Since you are also having spark plug problem I think you should do a compression check and check your timing.