1973 70hp chrysler runs great at idle but has no top end

[Ron Merriman]

Sorry if I ask stupid questions, but I am new at this. this the first boat i have owned, it set in a barn for 15 years, when I got it the motor ran good after I put in new plugs , and would do a respectiable speed. But later would only run at a slow speed even with the throttle pushed all the way forward. I replaced the magnaforce II ignation box with an aftermarket system i bought off the internet from a parts supply house that replaced the box and coil. But still no luck. I would be grateful for any suggestions. Thanks

 

[pnwboat]

Re: 1973 70hp chrysler runs great at idle but has no top end

Have you replaced the fuel pump diaphragm? Also check compression.

 

[Ron Merriman]

Re: 1973 70hp chrysler runs great at idle but has no top end

Thanks, I have not replaced the diaphragm, but it appeared to have no holes or tears, the compression is good. Do you think I should replace the diaphragm anyway?

 

[jerryjerry05]

Re: 1973 70hp chrysler runs great at idle but has no top end

Need to do a comp and spark test.
What's the comp readings??
No top end coulkd be anything from no spark on 1 cylinder to a broken connector on the linkage.
Start with spark??Then comp and then fuel system.
Ethanol is messing with older hoses and making them melt and clog the carb and pump.
top of the Force forum.First tread.From FrankA a link called a linc and sinc.Do this.
Post the results and then we can give more.J

 

[pnwboat]

Re: 1973 70hp chrysler runs great at idle but has no top end

It's a good idea to change the diaphragm when you don't know when it was changed last. The rubber has a tendency to get stiff and looses some of it's flex so it doesn't pump as well as it should. You can test to see if you pump is working like it should by squeezing the primer bulb when you're at full throttle to see if this has any affect. If it does, then most likely the diaphragm needs to be changed.

Also like Jerry says, check the spark at the plugs.