1983 mercruiser 260 350cid Low RPM issue

If its still got the original Mallory distributor I would throw it and get any other marine rated distributor.
Those Mallory ones are absolute crap

 

Ok, I will start with that stuff first. I am fairly new to distributers, do you know of any good websites I can read through? I'm having a hard time finding good info.

 

Let me see if I have this right...the engine was rebuilt four years ago and you're just now noticing this lack of RPM?

Guys...I wouldn't be so quick to blame the mechanic for a condition that might not have existed at the time he completed his job. It sounds as though the engine runs fine except for the lack of RPM. Why not start with some of the basics like throttle control, fuel filters, or sparkplugs?

Before I tell this owner to start throwing parts overboard I'd recommend that he have a real mechanic take a look?

When you think about it this is a twenty year old 350 Chevy Mercruiser...if you start listing all the junk parts used in this set-up he won't have anything left in the engine room. Hmmm.. come to think of it that might not be such a bad thing.

MM

 

I wonder if the mechanic ever ran the boat or just rebuilt a long block?

Other than the distributor ( it still worked) I cant think of anything sub standard on that model, what do you refer to?

 

It had low RPMs 4 years ago. The mechanic told me it had low RPMs and he could not figure out why. The block is not the original in the boat, just found this out. He put in a rebuild 350 engine.

 

what kind of boat is it with what size prop etc?
When you owned it did it ever go properly i.e. what speed did it do?

 

It's been topping out at 3,000 RPMs for 4 years in a row. I think that would change if the fuel filter was clogging up. New plugs make no difference(replaced them twice). There is some kind of problem that doesn't get better or worse as the years pass by.

 

Is the tach accurate?

 

Good question.

The idle RPMs seem reasonable to me. I can't recall off hand exactly what it is, but definitely above 600 and below 800.

 

That was my bad..sorry! I'm going to blame not enough coffee, too much coffee, and that guy over there for my misreading of your original post.

Let me see if I can get this right... we have a rebuilt 350 (of unknown origin) that from the time of it's installation (four years ago) could not run up to it's rated RPM. The original mechanic said it had a low RPM condition and he didn't know why. In the intervening years since it's installation the circumstances haven't changed.

Ok there's a short list of possibilities for you to check.

1) With the gear shift in Neutral open the throttle and record the max RPM.
2) If the engine does not rev past 3000 rpm.
a) Check throttle cable for full movement.
b) Disconnect cable and operate linkage manually...record max RPM
c) Run a compression test wet and dry... record psi.
1) Run a leak down test
d) Install vacuum gauge and record readings under load and no load conditions.
e) Check ignition timing
f) Check fuel pump pressure
g) Check valve lash

Those are a few I can think of off the top of my head.

If the engine does reach full throttle while in Neutral but not under a load.
The above still apply except for the throttle cable. In addition you'll want to check for water migration, correct prop pitch and diameter, and fuel system restrictions. Another thought occurs to me...I wonder if this rebuilt engine is fitted with the proper cam for marine use. Aside from the Mallory ignition that Quadrajet is also prime suspect. Some mechanics might suggest you run a float test on that model of carb.

In the end you'll need to find yourself a reputable mechanic and begin some serious troubleshooting to track this one down.

Again I'm sorry for my previous misread of your post.

MM

 

Well Im totally and utterly confused now. Too many cooks spoil the broth.

As I see it you have never seen this engine reach full RPM?

If that is the case then all previous diagnosis is usless.

I and others were led to believe that you had your motor overhauled and NOW it wont reach RPM.

It is NOW reasonable to assume that the 3 times you have needed an overhaul is BECAUSE of low RPM and slugging the motor with an oversized prop causing engine damage.

 

In the time you have owned this boat have you ever seen it rev up to full throttle? When you are running at 3000 RPM how does the boat feel, how does the motor sound? Can you provide any speed data (GPS, not pitot) at the tach's 3000 RPM. You can tell if a 350 is really at 3000 by the sound. That is a real comfortable RPM for that motor to cruise at, and it would sound as such. If the tach is reading 3,000, but the engine sounds like it's screaming check the tach. If the tach was replaced, or if someone messed with it, it is entirely possible that the selector switch on the back of the tach is in the wrong position i.e. thinking it's a 4 cyl... Just a hunch.

 

Hire a MECHANIC,

 

Did any one get the final word on this...
I have the exact problem....was producing 4200-4400...blown head gasket and required new motor... buy a rebuilt long block Vortec engine by reputable re-builder....all work done by marine mechanics (not me) and the new rebuild engine in boat is maxed out at 3200 rpms...THEY are stumped!!! Suggesting something like wrong camshaft for boat engine perhaps???...$$$$

 

I'm still stumped. I haven't investigated all the possible options yet. When I am in idle, the engine will rev really high so that tach isn't messed up. It's definitely load dependent for me because I will get a different max RPM depending on how many fat chicks are on the boat. Just me = 3,300. 4 fatties =3,000.