Honda 40 RPM Problems continued

[egboat]

I was running 3100 rpm at WOT with this motor on a 20 foot pontoon and 11 pitch prop. I bought a new four blade 8 pitch prop and now my tach shows 2500 rpm a WOT.

I have a dead spot in the throttle so I think the carbs must need cleaned. I changed the plugs already.

Is my tach messed up. I can see no way the RPM should go down. The boat seems to go about the same speed (slow). Unfortunately, I have nothing to gauge the speed against.

Can anyone help?

 

[pvanv]

Re: Honda 40 RPM Problems continued

Assuming your tach was working OK to begin with, you were over-propped to begin with.

IF your original prop was a "normal" 3-blade, then going with a 4-blade can knock the RPM down, even though you went from 11 pitch to 8. I would suggest a High Thrust (Solas Amita 4 or similar) prop to push the "barge". When I changed my sailboat OB prop from 5 pitch to 7 (4-blade HT units) my rpm went from 6200 down to 4800. Check the prop forum for size recommendations. Will probably be 7 or less.

You may have water in the 'toons as well. That aside, this rig isn't intended to plane. Heavy, displacement-type hulls call for HT props, and the RPM will be very sensitive to even small pitch changes. Again, the prop forum guys will be your friends on the sizing.

You can check speed with a cheap handheld GPS.

 

[steely51]

Re: Honda 40 RPM Problems continued

If you have a manual choke try pulling it on to somewhere near 1/2 to 3/4 choke. if you only have electric choke try intermitten activation of the switch. if the rpm picks up during this action you likley have restricted jets or an air leak more likley the first .