Burying the Exhaust Relief Holes

[imtwink]

I wanna see what you guys think about the problem I am having.
I have a '86 20' Bass Boat with a 2.4 Merc 200.
When I am about to blast off, I trim the motor all the way down and the exhaust relief holes are under water. The motor will stutter and stumble before taking off... sometimes for quite a while. Is this normal??? If I don't lower the trim all the way, the prop will cavitate and lose all bite. If I raise the motor it will lose its bite also.
Is the sputtering because of too much backpressure??? After it gets out of the hole, it runs pretty clean and strong.
I don't want to have to change to a 25" mid, so if anyone has any suggestions, please let me know. Thanks, Twink.

[Raceman]

Without knowing a little more about your boat it's hard to say exactly, but there are some things you could consider.

I would think there are two categories: setup, and engine condition.

As far as condition, has the boat always acted this way or has the condition worsened? If it's worsened I'd look at any changes in setup, like different prop, or significant change in weight or weight location. There's always the possibility of a boat getting heavier from trapping water somewhere or waterlogging. If it's always been that way a different prop, either over hub if you can live with the slip gettin' on top OR one with relief holes behind the blades may solve it. Also, if it's worsened engine condition could be a factor. If the low speed pickups aren't set right the engine can be lazy off bottom. If a needle is struggling to seat and there's some extra richness the engine can be lazy on bottom.

If the boat/engine combination has always been this way and setup or prop change won't fix it, it could be exhaust restriction as you suggested. Since bassboats, moreso with smaller ones, tend to be loaded very back heavy the exhaust is jambed in the water. If the tuner is completely submerged it'll be so restrictive that it's harder for the engine to clear out. I had this difficulty more with early 2 liter 175 Mercs than with the newer stuff. The larger engines seem to have the grunt to clear the exhaust better. Anyway, on a smaller engine I drilled a half inch hole in both sides of the tuner at the top on the sides and it fixed the problem I was having. This was on a 15 Ally that floated REAL low in the back and the exhaust was buried. I have seen a boat that was drilled all the way through the two vent holes at the top of the mid, through the liner and into the tuner also, and the owner felt that this helped his boat get on top, but it's been a long time and I don't remember any more details.

In any case, going to a 25 inch mid won't help your problem. It'll only bury the prop farther and compromise the speed and handling.

I think I would try some different blades and make sure everything is right with the engine first before doing any more exhaust mods.

[pyro]

Sounds like a tired motor and a crappy prop.

[Raceman]

Sounds like a tired motor and a crappy prop.

Damn Chad, you said in one sentence what I wasted 3 paragraphs on.:D :D :D

[T-REX]

I've noticed that a lot ov bass boats set reel low in the water, and when trimm'd down to take off, the trimm attitude iz such that if the float leves are wher they poz'd ta be, they will spill over at tha angle the mota iz!!!....

Try anuther prop, to let tha mota spin up quicker, or trim ya mota up more while take'in off...I have noticed in sum cases this will help git the rpm up, and it will catch it'z breath...

I had this same problem with a Basscat, panteraII(no bota bury's deeper than a basscat) wit a 2.4, and I had to lower tha float level, AND leen the bottom end, AFTER we found a prop that would allow the mota to spin up quicker...The prop that worked best for Jay's bote wuz a 4 blade, with 13"s diameter(kan't remember the name, but it wuz origianaly a OMC prop!)...

Juss sum suggestions to be add'd to Racemanz an Pyro's

[j_martin]

My bass bote does the same thing. High five prop lets it spin up, and then it bites in and goes. When I'm idling, I move the trim out till the motor is level, and can idle for hours. When I want to blast off, I mash the down trim, mash the throttle, then mash the up trim. Took me awhile to learn how to drive it, and also how to land it without taking on a lot of water.

hope it helps
John

[mr fun]

low octaine? or dirty plugs and oil rich mix? :rolleyes:

[imtwink]

Thanks for all the reply's.
Little background... I just bought the boat about a year ago, the motor had 2 bad cylinders. I bought a remanufactured powerhead and installed it. I shaved the heads to get between 135 to 140 lbs. compression. The carbs (wh 31's) were rebuilt with new needles and seats. The jetting is 82/84 for the mains, 61's in the idles, and no jets in the vents. I have not yet done a good check on the pistons for jetting. The can is out, and 5 holes are drilled on each side of the lower secton of the mid. Oil injection is off and using premix 40/1, new plugs, and 91 octane.
Now for the prop... the prop it came with is a 3 blade thru hub exhaust that looks to be modified. I can find no numbers or letters except for "M 24" stamped on the inside of the hub... I would guess that would make it a 24 pitch, but I have put on a 23 pitch trophy that turns less RPM at WOT. Top speed by GPS is 56 mph at about 5300 RPM with the 3 blade, and 56 mph at about 5100 RPM with the trophy. That seems slow to me, but I am sure it's a pretty heavy boat. Could I possibly need less than a 23 pitch prop??? I would like to have a boat that would turn 6500 RPM and at least break 60 mph!!!
Please throw any ideas or suggestions my way. Twink.

[Raceman]

First of all, you need to turn the engine faster, and too much pitch is most likely the cause of the bogginess on bottom, whether due to a weak powerhead, or just too much pitch for a healthy one.

Even so, it's unlikely that you'll increase top end speed by pitching it where it'll turn 6K, which is about what I'd be shootin' for. If the props you have don't have vent holes in em, one that does will help your low end also.

Also, as I suggested before, I'd want to eliminate excess weight from trapped water as a cause before I looked too hard at the engine.

[tlwjkw]

Sounds like tha boat/motor set up really sucks!!!!!!!!!!!!

[T-REX]

I'd want to eliminate excess weight from trapped water as a cause before I looked too hard at the engine.


Fill ya jack plate with floatation...Them floaties that kids use in a pool call'd "Noodles" works better than anythang by displace'in the most water!!...You'll be suprized what that lil bit ov floatation will do...

Now after here'in yo exsplination ov what ya have, I thank ya prop selection SUKKZ....U gunna need a 4 or 5 blade prop wit relief holes to git dat tug out tha water....Git tha rpm's to 6k or better......U may see sum speed increese(depending on prop), but juss take'in that load off tha mota will be in yo bess behalf...

I say ya biggest prollem iz propeller.......dats my story, an I'm stick'in to it!!:D

[Superdave]

From my little experience I would say the prop with only an "M 24" is most likely a Michigan prop. Find someone that has a few props you could try out.

[SatisfAction]

I know every motor runs different and so on and so on... but on my 2.4, when I switched to 2 liter heads, velocity stacks, AND went from a vertical to a horizontal front, I knew I would have to jet the motor richer. I started with some jets in the 80 range and they were way too rich even for my application.. bogged and ran like ape ass. I went down to 74's in the mains, 60's in the idles and no vents and that seemed to be the magic number. I can get 7200-7300rpm and instant 5000rpm when I floor it from idle. My idle is a little rich sometimes... I can feel/hear it at times when I blast out of the hole.. but after that there is no problem. I have WH-34's. Might wanna play with the jets a little. Just my 02.

[pyro]

I once purchased a freshly re-built powerhead that had the timing set at 15 degrees after top dead center. :rolleyes:

[imtwink]

I do feel like my prop has too much pitch, but reading what other people have on their boats makes me think that a 23 pitch is not that much. (guess I'm wrong). The Trophy prop I have does have 4 blades and vent holes, it will still bog, then once the engine cleans out, it will rev to about 4000 RPM until it gets going pretty good. I do have a 13.5" x 21 pitch aluminum prop that I haven't tried yet. That will be next.
Raceman, what do you mean about trapped water?? I have removed the rear tanks to look and of course the rear plug to drain all water... do you mean trapped inside the hull, and if you do, how would you check that and remove trapped water??
The timing I have adjusted through link & sync as best as I can. I have the timing set @ 24 degrees BTDC WOT, and the primary timing @ 8 degrees ATDC to get it to idle at about 1000 RPM in neutral. I have had the powerhead off and pulled the adapter plate to see the tuner, it seems okay and has no blockage through the mid.
I have read Jay Smiths way to check jetting by running it wide open then cutting it off clean, but I haven't ever done it yet. It may be running a bit fat.
Rex I have a hydraulic jack plate, if I can get some of those "noodles" in there I'll do it.
Thanks guys, Twink.