1991 70 hp low rpms at WOT

[latonkaboater]

Just got this 1991 70 HP Evinrude installed on my 20 pontoon. The best WOT rpm I can achieve is 4100 rpm running a 13X19 prop. I am only getting 1 mph more than with the previous 48SPL Johnson engine. Compression on the new motor is fair averaging around 105 psi. I have not checked the tach settings but they seem accurate since the motor is considerably quieter at WOT than the old engine which was turning about 5200 top end.

Should I be looking at throttle adjustments or a differnt prop to get better performance?

[Elton Smith]

May be carb adj.I have a 35 of that vin.so Im just guessing.See if your carb linkage has a rod from the carb to the throttle.MIne has a small br*** rod with a screw in it.That is your speed adj..It will be just under the flywheel.Open the throttle by hand and see where it stops.By moving the br*** stop you can let the butterfly in the carb fully open.

[latonkaboater]

Thanks for the input. I will be checking the throttle linkage this weekend. I had made an adjustment on the fly but it was hard to tell if the butterfly valves are full open with the boat out on the water. From what I read, a 19 pitch might be too much for a big heavy displacement hull, but I am still trying to understand the relationship between prop pitch, speed and engine torque. The theoretical speed vs pitch relationship is clear but how much will it make a difference if the engine output increases due to the higher rpms which should happen. Running the numbers it looks like if the slip doesn't change (mine seems to be running about 45% right now), then increasing rpms just offsets the pitch change and the theroetical speed doesn't change much.

[phillnjack]

OK
First of all the prop is way too much for that boat.
your revs should be 5500 and even 5000 is way too low for the engine as it don't get into power band until just over 5000.

compression at 105 is not good at all, it should be atleast 120 for that engine.
and when you say average, can you tell us exactly what each is saying on the guage.
you need them to be as close as possible to each other to have a good engine for power.
a 105 mid and 110 and 100 for example is not good at all, as the 100 to 110 is a bit too much difference when you already have low numbers.

but you could be checking the compression wrong , many people test it wrongly and get bad numbers.

you need to start the engine and let it warm up, then switch it off.
take all 3 plugs out straight away, ground them out or pull safety lanyard.
put the throttle lever to fully open or get someone to hold carbs fully open and then flick the switch to turn the engine over for 3 seconds.
do this to all 3 holes and write down exactly what each one is.

if you try it cold and with no fuel in the engine it will give low numbers, and will also give low numbers if the throttle is not fully open.

if still low then try a can of decarb (seafoam or similar) to see if there could be sticky rings etc due to carbon build up.

let us know the numbers your getting for each hole.


prop wise I would think a 15 pitch is more like what your wanting,or maybe even a 13 pitch if its a heavy pontoon or lots of weight onboard.

no way will that run a 19 pitch unless its realy lightweight boat.



phill