05-31-2015 02:39 - edited 05-31-2015 03:28
This is my first time posting so bare with me.
1992 Thundercraft 17ft Bowrider
I purchased a boat last week off a friend of mine, it has a 1980 70hp Evinrude Model#E70ELCSA. He had owned it for 2 years and always ran well and was able to achieve 55-60KPH. This year he thought it may be a good idea to run a tank of fuel with seafoam to clean out the carbs. Obviously ran a little rougher but was still able to achieve his top end power.
After I purchased it i noticed a small problem with its idle, so we made the appropriate adjustment and now idles perfect. But still can only achieve about 30-40KPH on the water under perfect conditions. It just feels like its not getting enough juice to go Wide open. It is also sluggish out of the hole.
Due to the specs I've looked up engine is designed to operate around 5800RPM at WOT
I currently can't exceed 4100 at very best or less
Other than the lack of power at the top end, engine runs like a dream. What could we be missing?
Things we have tried.
New NGK spark plugs
Remove and clean all 3 carbs.
Fresh tank of fuel 50:1 mix using Amsoil Synthetic
All 3 Coil packs were replaced last year
Thanks in Advance!!!!
Solved! Go to Solution.
06-01-2015 05:10 - edited 06-01-2015 05:12
We arre talking about a 35 year old engine here, so anything is possible.
You may want to switch to the factory recommended Champion QL77JC4 plugs gapped to .030"
The ignition system should be strong enough to spark across a 7/16" gap at cranking speed
check compression for all cylinders to be within 15% of each other
inspect the throttle arm to spark advance plate (timer base) link and be sure the plate rotates smoothly in conjunction with the movement of the throttle arm.
All three throttle plates have to be closed at ilde and open fully at WOT. Remove the air silencer to inspect. The three throttle plates should be completely horizontal.
Check fuel pressure at all speeds.
Remove and inspect each high speed jet from the carb float bowls and compare the sizes with the one in the parts book.
Replace the nozzle well gasket and float bowl gasket in each carb as any air leakage around the nozzle well leans out the fuel flow and loses power.
Refer to the service manual for proper synchronization of the linkage adjustments,timing and throttle pickup, and the high speed timing.
The engine may need a 15" propeller to push a 17" runabout to acheive the proper rpms - 5000-5500
06-19-2015 07:15 - edited 06-19-2015 07:16
To see if it is an ignition problem, first make sure that the system is in good enough shape to jump a 7/16" air gap at cranking speed. For a running test, use a clip-on timing light on #1 and monitor its flashes as the motor operates. See if the timing light flashes disappear when the cylinder no long fires.
If it does this at low speeds, watch the timing marks on the flywheel to see if they jump around or if another cylinder is firing at the wrong time occasionally.