'93 25hp 2 stroke Spark Plug Issue??

[Jason2tpa]

Prop update....they did a great job reconditioning it. Looked brand new. They added 15 degrees of cup. Did it help with keeping bow up? Nope. If anything it did the opposite. It did however reduce RPMs at WOT by about 400.

It occurred to me last night that a major contributing factor with not being able to keep the bow up might be the jack plate. The boat is 13' and is rated for max 25hp, which is what I have. The jack plate adds a 4" horizontal offset from transom. Although 4" adds a small amount of length, it still is adding length to a small hull. I dunno im just spit balling.

[zenoahphobic]

Prop update....they did a great job reconditioning it. Looked brand new. They added 15 degrees of cup. Did it help with keeping bow up? Nope. If anything it did the opposite. It did however reduce RPMs at WOT by about 400.

It occurred to me last night that a major contributing factor with not being able to keep the bow up might be the jack plate. The boat is 13' and is rated for max 25hp, which is what I have. The jack plate adds a 4" horizontal offset from transom. Although 4" adds a small amount of length, it still is adding length to a small hull. I dunno im just spit balling.

If you now have reduced WOT RPM you will also have less acceleration and momentary bow lift, so this work would not met your requirement.

If you have a jack plate extending 4 inches, then the rule of thumb the engine should be raised half to one inch. This is because the water level rises the further back from stern bottom.

Now this "too deep" of engine gives you more leverage lifting the bow, however the power (torque) is reduced by having to push harder through the water. Given only 25 horses, I believe your setup is more critical, if at all possible. I.e. A 35 might point you easily to the moon.

[zenoahphobic]

Remember also a jack plate extending 4inches may make the boat appear by that much longer, however the planning surface has not been extend.
This leaves less leverage for the engine to tilt the boat on the surface.
If not already critically examined the hull might have a slight hook in it which acts like a trim tab, keeping the bow low.

[Jason2tpa]

Half to one inch above the bottom of boat? Or half to one inch above standard height? Because the majority of articles Ive read, including my owners manual, state the AV plate should be half to one above bottom. So if the jack plate extends 4" then AV plate would be 2in above. Which is where it is now.

Also, let me be sure Im measuring correctly. Do I take this measurement with the engine all the way down? As in the trim rod would be through the 1st hole (closest to transom)? If so ive been gauging it wrong. Ive been measuring this height when the AV plate is parallel with bottom. Usually trim rod would be in the 3rd or 4th knotch up.

If you now have reduced WOT RPM you will also have less acceleration and momentary bow lift, so this work would not met your requirement.

If you have a jack plate extending 4 inches, then the rule of thumb the engine should be raised half to one inch. This is because the water level rises the further back from stern bottom.

Now this "too deep" of engine gives you more leverage lifting the bow, however the power (torque) is reduced by having to push harder through the water. Given only 25 horses, I believe your setup is more critical, if at all possible. I.e. A 35 might point you easily to the moon.

[zenoahphobic]

Half to one inch above the bottom of boat? Or half to one inch above standard height? Because the majority of articles Ive read, including my owners manual, state the AV plate should be half to one above bottom. So if the jack plate extends 4" then AV plate would be 2in above. Which is where it is now.

Also, let me be sure Im measuring correctly. Do I take this measurement with the engine all the way down? As in the trim rod would be through the 1st hole (closest to transom)? If so ive been gauging it wrong. Ive been measuring this height when the AV plate is parallel with bottom. Usually trim rod would be in the 3rd or 4th knotch up.

The answer is "at the ideal position"; but where is this?
It is one of these things where you try all position without missing any.
This means sometimes setting to maximum then setting to minimum May miss the sweet spot.
Place a long straight (very straight) preferably steel along your hull. You can use a jack or soft wedges on trailer members, but don't distort. Check for straightness of hull(small air gaps) at as many points possible.
Do this first, because if your hull (planning surface) is not absolutely flat, you're trying to solve the wrong problem.

[zenoahphobic]

And to add to the above, every change has an effect ( sometimes small).
The cupping of your prop generally would allow it to ride a bit higher( less ventilation but notably more able to hold the water in its grasp, and throw it out the back( thrust).
So after every change like this you really need to do all the other possible adjustments( this is tedious because say you had 4 holes up and 4 holes trim then that is 16 combinations.
Just reading from a far I can't direct you precisely.
So if you still persist that bow up more paramount, if at all possible, mount the engine without the jack plate.
But I still think looking carefully at hull.
There are other routes such trim tab to extend planning surface, or even a wing on your cab plate. Only suggesting these much much further down the road. Acedemically both these options give you better ability to raise the hull generally by giving your engine a better pivot nearer the engine(did not explain this principle well previously.)