Prop won't develop RPMs

seahawk

Hope someone might get me started in tracking down a problem. The boat is a 280 EC with the MAG 377  6.2L 320 HP Merc. This is our first season with it so how it should perform is largely unknown although we generally accepted the fact that a boat this big with a single engine was going to be marginally powered. We've had it out half a dozen times and usually been able to nurse it up on plane using the tabsangd have got it to hold us on plane at about 23 to 25 MPH.......topped out at about 33 MPH at 4000-ish RPMs. To make this happen I put the tabs down, leg down powered up and watched the speed and RPM creep up over the course of about 30 seconds until things stabilized. I would have thought the ops would have come up more quickly and the speed would have built up and near the end the rpms would have increased the last couple of hundred rpms. The last couple of times we've taken the boat out we've followed the same procedure and couldn't get the rpms to go over 3100 rpms and the speed wouldn't build over 16mph no matter what we did. We stopped the boat and inspected the props for debris and ran in reverse for a minute. Then ran in neutral and the RPMs came right sup to 3600 rpm at which point the rev limiter horn came on, and I backed it off......but it was on its way to coming up to full rpm. But once again leg down in the water, in gear .....couldnt get it to give me more than 3000 rpm and nowhere near getting on plane. Does anyone have any ideas..????????  they would sure be appreciated.    Thanx....... J.

YYZRC

I would start with new plugs and cap and rotor. How much slime on the bottom? Do you trim up after you get on plane?

skennelly

Agreed with ^^^^

Also as a side note a poorly running engine will still rev up in neutral when there's no load so that kind of test isn't valid.

seahawk

It  sure wont hurt to have the engine tuned up....God knows when the last time was that it was done so thats on the 'to-do' list for haul -out this fall.
We checked the throttle linkage and its giving full travel so the linkage isn't loose, or binding anywhere. 
The engine sounds smooth and strong though and delivers full power with no load.....no weird noises....not sluggish....nothing unusual. Even under load it "sounds fine, feels fine,.....no vibration". But when I put it in gear and load it up, the engine just doesnt seem to be able to get the power through the leg, to the prop. Just wont power up....

seahawk

BTW YYZ.   sorry ,....forgot to answer your questions. I dont know exactly how much slime is on the bottom, although its been a hot summer and I'm sure there some, but the bottom was freshly ;anti-fouled; in the spring. and......the times I have been able to get up on plane and 'cruise' I DO trim to get best performance using the tabs and the leg trim......

PickleRick

While changing plugs it would be a good idea to do a compression test.  Rule out internal issues. 

Post up pics of all the spark plug tips, they will give an indication of how each  cylinder is running.

Aqua_Aura

Give it a full tune up and then see where you're at after that. Plugs,wires,cap,rotor,fuel filter. If it still isn't running well you can check fuel pressure and also make sure you have good fuel and keep some stabilizer in the fuel as well. 

Checking the plugs should be your first step as this will be the best visual to see how things are firing in the cylinder. 

YYZRC

I had a bad spark plug and my boat ran exactly as you describe. Sounded fine but just wouldn’t develop full power. Could even be as simple as a plug wire has come off. 

seahawk

Thanx for the input so far.....and those are all things that were on my list for the fall haul-out......looks like I may be getting to them sooner rather than later. Cant do much more boating til this is resolved and the season is getting late......

YYZRC

I got plugs and cap/rotor on Amazon very quickly. Good luck!

PickleRick

Just make sure your cap is marine rated.  Lots of knock off items being sold on eBay/Amazon under an oem part number,  even oem knock off packaging.

I'm pretty sure the only difference is the vent is screened on the marine cap vs non screened on vehicle cap. 

I get most of my parts on ebay.  I love n.o.s. deals.  

seahawk

Thanx again to ya'll for the input ...........

seahawk

Just wanted to post a quick follow up to this post from last year. Thanx to everyone who commented,......you were right. We had a full electrical tune up done and the boat is running like new. on the delivery to our new marina last week it was such a difference getting up on plane and cruising almost effortlessly as compared to the struggle last year. The mechanic that did the work said that one of the plug wires had been rubbing against something and was worn almost halfway through. That alone will do it, I guess, but the tune up was definitely the difference in performance that we were looking (hoping) for. Thanx again everyone for the advice. 

Dream_Inn

Great to hear @seahawk !!  I like to hear the follow ups.