...well at least I hope so. I'm building an intake muffler for my 1965 OMC DU.
So far it involves 2 flexible rubber 2-1/4" radiator hoses and a large PVC, pipe in pipe, muffler.
I took the Aluminum spark arrestor off the parts boat and drilled 2 large holes in it. I capped off the top with an aluminum sheet. The two intake nipples on the spark arrestor box were made from toilet tank flapper valve seats.
I have 2 different methods I plan to try. One is my custom made PVC muffler. The other is a stock air box out of a 1990 Jeep Grand Cherokee. I figure I can place soft rubber balls, foam rubber, and fluffy white kittens, no, just kidding, in the air box to absorb intake noise.
I decided to go the easy way first. I just piped the carb intake to the rear engine compartment vent with flexible rubber radiator hoses. They are corrugated both inside and out, so they should really knock the noise down.
However: It did capture all the noise and aim it out the back. Plan B will be to incorporate the muffler and air box.
Also you can see along the side where I added a toggle switch for the fuel pump. It has a valve to shut off the gas to run it out, but the pump runs dry then. I figured, not only will it save the pump, but if I forget, and leave the key on after the gas runs out, it won't kill the battery.
Like I said before, those DUs are just loud. I never thought about an intake silencer. My concern is that you may cause a restriction & it may run richer. Is it possible to have any more wasted fuel than you currently have? I don't know.
Best idea would be to test the boat for top RPM then install it & test it again on the same day to keep any variable as close as possible.
It is rather odd. You stick your head down in there with the engine and it is pretty quiet. All the noise that was in there before is out by the ends of the intake pipes. I'd say the intake noise is about 1/4 as loud as the exhaust when the drive is tilted. Yes, like you said they are loud....I'm going to correct that. It is as much a game as a desire to make boating more enjoyable.
I've run it quite a bit and it seems about average fuel consumption for an old V4. I have considered adding a blower fan to offset the elongated intake resistance. In addition, the cage and the snail-shaped body of the fan would further disrupt the sound waves attempting to leave the intake. Obviously, I need to find a fan that doesn't add to the noise problem.
I'll have mine out tomorrow. I will just tolerate the noise as I have done for the past 17 years.
As for fuel burn, my 155 V-6 pushes a bigger boat with less noise on less fuel. But, when I only run maybe 2 days a year, the cost isn't a big deal. I'm on my way now to go buy fuel, I'm using XD 50 for oil at about 40:1
Yaa, I've never had a newer, more efficient motor. My desire to get new stuff ended when they quit taking time to make it look cool. Unlike you, the Playmate/DU will be my main boat this summer and hopefully for many more. I can't believe how well it handles. Almost like a car.
Have you had your tach worked on at all? Mine sticks. It will eventually zero in on the correct RPM but sometimes I need to tap it to get it to what would seem a rational reading for the amount of sound/speed. I have not tried to remove it from the dash. Do they come apart? Or are they crimp sealed shut like most gauges. I also need to replace the light.
If you ever watched "The Great Race" starring Jack Lemon, Now, with my "cold air" intake mod, my DU now sounds like the Hannibal 8.
I have one of the aqua metet inductive tachs that has a transmitter inline inb one spark plug lead. It has always read about 1/2 speed & I have always ignored its inaccuracy.
All I can tell you is that the engine would barely idle and that opening the rear seat to allow the heat out would stabilize the idle. Also, adjusting the "gang bar" that connects all 4 idle mixture screws to its full lean position would also increase and stabilize things. We had an ambient temnp of 99 degrees and I can only imagine how hot it was under the deck.
OK here is the next phase.
I'm using ABS drain pipe for the manifold. The rubber radiator hoses attach to the custom spark arrestor box as shown above. The 3" elbow connects into a Chevy V8 pickup intake silencer. It comes out as a 2" elbow and adds back into the main 3" intake pipe. From there it will go to big cone shaped intake silencer off a Ford. I'll pack the 8" Ford air cone with coarse stainless wool. I capped that with a ABS plastic floor drain with a 3"pipe as the intake. There it will elbow up to a 4" in line bilge blower fan (to offset all this pipe resistance) and then to an intake port in the hood.
I'm having fun. I should be able to test next week. I'm not gluing anything until I hear how it works. There sure is a lot of space back there for all this pipe.
I know no one cares, but this is just too cool not to share. Long story short, I did not get out to test yet.
However I was concerned that I did not want my boost blower running at idle. Then I noticed the mercury switch that cuts out the starter when the throttle it too far advanced, has 2 other contacts. I tested the other contacts and sure as shootin' they connect as the throttle moves ahead of the start position. So I ran my wires to the mercury safety switch. Now my boost blower comes on as I open the throttle past the start position but not at idle.
You don't know the half of it. I'm always doing the MacGyver finesse on may levels. It must be a Scottish thing. Accept for me it is McGonagle not MacGyver.
Oddly that goofy little bilge fan does boost power. As I advance the throttle to the point where the fan starts, the motor speeds up. Everybody pooh poohs the thought of an electric blower on car forums, but here in the real world it does actually effect performance. Obviously not like a real super charger, but as an offset to intake resistance it does work very well.
OK everybody....Who has experience filling in "previous owner holes?
The previous owner added a horn in the i1980's to my Playmate. (see above) I need to fill in 3 holes. 2 1/8" holes and one 5/16" hole. Filling the hole is simple. Any number of compounds can do that. However the deck has a leather like pattern, non slip surface and I also need to match the color.
Any tips? If not, I plan to fill it in level with body filler and carefully carve the pattern into the spots where the holes are/were, then coat with paint.