|
 Sound
Tips For Noisy Engines
by
Steve Horsfall
Last year I purchased a 1999 18-foot
Bayliner Capri, powered with a 3L 135 hp MerCruiser. We love the
boat but have one complaint. I am quite disappointed in the engine
noise. It idles quiet but is quite loud when cruising anywhere past
25 mph. I'm not sure if it is the engine or drive unit that is loud.
I have talked to one owner who has purchased
the exact same boat and he said he was also disappointed with the
louder-than-expected engine noise. I have also ridden in other boats
with inboard engines that are much quieter than this one. The dealer
told me this is "the nature of the beast," and there is nothing
I can do. I have put extra insulation on the inside of the engine
cover but it didn't make much difference. Do you have any suggestions
as to how I can reduce the noise level?
B. McLaren
E-mail
This is not the first time I've come across this
problem. The reason the insulation made little difference is that
there are lots of places around the box and the rear jump seats
where the sound can escape into the cockpit. Also, there is little
insulation in the hull fibreglass structure, so the boat suffers
from a lot of water noise created by the water against the hull
while underway. Large amounts of spray foam is one option. It is
available in large aerosol cans at building supply stores.
Another thing that makes any boat seem loud is
running with the top up. A lot of people like to remove the back
curtain and the side windows when they go for a ride, but they leave
the main top up to protect themselves from the sun. If you put the
main top down, the boat will be significantly quieter.
Throttle Advice
I have a 1975, 135 hp, V-4 Evinrude
motor. The problem is when you hit the throttle it dies out. If
I feed the throttle slowly it will eventually get going. Once it
does it runs fine. At wide open throttle it runs about 5,000 RPM.
The recommended range is 4,800 to 5,500 RPM.
The second problem is cold starting. When the
electric choke plate closes (which is in the specified clearance
of .015 inches) it won't start, but if I get someone to hold it
down with a little bit of pressure, it starts within two cranks.
I have changed plugs, plug wires, coils, needle
& seats, in carb and also the high and low speed jets. The carb
float is fine too. The power pack and the timer base have been changed.
After doing all this, it still does the same thing.
My question is, could it be the crankshaft
seals are leaking? That is the only thing I can think of. One more
thing, a compression test was done and was very good.
S.H.C.S.
E-mail
Your engine is definitely suffering from a lean
condition at idle. Either not enough fuel or too much air is getting
into the engine. Check the color of the spark plugs at the spark
area. They should all be the same tan to light gray color. If they
are not, which ones are different? If it is the top or bottom cylinder,
crankshaft seals could be leaking air. If two upper cylinders or
two bottom cylinders are the same, it is likely a carburetor problem.
I suggest trying richer idle jets or turning adjustment
screws out one half turn (if low speed mixture is adjustable). Internal
crank seals are rarely the problem. Also, with the engine idling
and the air box cover removed, check if there is any gas spitting
out the throat of either of the carbs. If there is, this indicates
chipped or broken reed valves. These are located between the carb
intake manifold and the block.
If none of the above, I would check for broken
bleed hoses or vacuum lines or leaking intake manifold gaskets.
The hoses are a common problem on all engines, the intake gaskets
are a rarer problem. Please note that if you find chipped or broken
reeds, you should check maximum RPM at wide open throttle after
the engine has been repaired. The most common cause of reed breakage
is over-revving and one of the symptoms of broken reeds is a poor
idle and a loss of top speed. Where over-revving is the cause, a
larger propeller will need to be fitted to bring the RPM down to
the 4,800 to 5,500 range.
If your engine has adjustable low speed mixture
screws on the carbs, it is important to set them just a bit richer
than a smooth idle for best all around performance. If you go 1/4
to 1/2 turn out from smooth idle this should be about right. I suspect
you have a lean mixture problem at slow speed only because a high
speed lean-out condition usually causes piston failure if it is
left too long. And as your compression test readings are okay, I
don't think your problem is in the main jet area. When you correct
the low speed mixture problem, your hard starting problem will be
corrected also.
|
