1980/81 70HP Chrystler speed issues

[jm]

Ok, got the motor up and running however I can only push my 15.5' tri-hull about 20MPH. I was doing this with my 40HP I thought the 70 would at least push me about 30-35MPH. The compression is 150, 150, and 155. The motor seems to rev higher when in neutral. I don't have a tach attached yet so I don't know exactly what the rpm difference is but it sure sounds like less. I need ideas..... Could it be the prop that came with it? What should I use as far as size and pitch for lake use? Could a mismatched prop cause the engine to sound like it is not pushing the higher RPM?

 

[Terry H]

Re: 1980/81 70HP Chrystler speed issues

You really need a tach to work this out properly, but you should be able to push a 13x19 prop to well above 30mph. You need a prop that can accomodat about 5000rpms at wot...You need to check the speed with a gps to be sure...just a Thought

 

[oldboat1]

Re: 1980/81 70HP Chrystler speed issues

kind of conflicting information with respect to lack of speed, rpms and prop pitch. A high pitched prop will lower top end rpms but push the boat faster than a lower pitched prop -- takes a bigger "bite". On the other hand, a prop with too low a pitch will give high rpms but lower top end speed. If both rpms and boat speed are too low, it's probably not prop pitch. Maybe it's carburetor adjustment or oil to fuel ratio -- too rich at high speeds, so not getting the rpms it should?

 

[Franki]

Re: 1980/81 70HP Chrystler speed issues

yep, and don't be reving the motor like that in neutral,, thats a good way of destroying it.<br />(it will definatly overrev) all it takes sometimes is a few seconds at 10,000 rpm to hand grenade.. they are not designed for it.<br /><br />hook your tach up, take it out.. see what rpm its really doing.. if its getting up above 5500 ,then you need to look at your prob, if it only gets to 3500 - 4000 then, again you need to look at your prob.. (but it may be a motor issue too. eliminate the prop first as its the most obvious possibility...)<br /><br />what was this motor on previous to your current rig? <br /><br />rgds<br /><br />frank

 

[jm]

Re: 1980/81 70HP Chrystler speed issues

I have checked the speed with GPS 18 - 20MPH. I will try and get the TACH running over the weekend. I think I need to pick up a new one. I wired the "S" on the tach to the 3rd connector labeled "TACH" on the motor and the other connector to "ACCESSORY" 12+V. Is that correct? The tach would not move. Checked the tach circut and I am getting power to it. Is there a liquid I could soak it in to try and release the tech's needle?<br /><br />Running 50 to 1 oil/gas mixture.