94 Mercruiser 350 issue
 

I wanted to see if I can get some opinions on an issue that I am having with my 87 242ss with a 1994 Mercruiser 350.

Issue: I am unable to turn higher than 3200rpm. I purchased the boat about 2 months ago in Tampa Fl, when I went for the sea trial I was able to get 4200 rpm brought it home and changed the oil, water pump, thermostat and lower unit fluid. I put back in the water and was unable to turn higher than 3200 rpm. I checked the timing and it is spot on a 32 degrees total. I have changed the fuel line that runs from the tank to the fuel filter, I have replaced the filter 2x and both times there was no water or major debris in it, I changed plugs, wires, distributor cap and rotor and had the carb cleaned professionally. I am still unable to turn more than 3200. I was thinking of replacing the Fuel pump next what do you guys think.

Does it hold a steady 3200 RPM or does it stumble, skip, layover ect? Have you done a compressioin check and verified that you are getting WOT at the carbs when at WOT at the throttle? Does it have a vacume secondary that isn't opening due to a hose being unhooked or not pulling enough vacume do to a leak somewhere?

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I'm assuming it's a carbed motor? 3200 is pretty spot on for secondaries not opening on a mechanical carb. Like redwhite said, I'd go straight to the carb and start there.

In the unlikely event that it's an efi motor, it would most likely be a limp mode issue..

I verified and the carb is open at WOT. It stays at 3200 without any issue and it idles beautifully

I highly doubt it's a fuel pump then. If the pump was not supplying the bowls would run dry and you would notice the change in the way it runs. It would stumble like it is running out of gas because that's exatly what it would be doing. But, an easy way to check is to put a fuel pressure gague on it and make sure the line is holding constant pressure while the motor is running and at RPM.. I don't know the exact pressure the carb's needle and seat will hold but you should show around 5 lbs at least.

You didn't mention if it has mechanical or vacume secondaries.....or if it even has secondaries at all, so that may be a factor still. Other than that I would also check for signs in the drive that there may be a restriction there.

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If the motor isn't engaged will it rev past 3200RPM's? Also like mentioned about is it just sitting at 3200, I would think that if it was starving for gas you would here it in the motor breaking up.

Does the boat have any aftermarket ignition like a MSD box or something like that?

Couple comments, questions

1. Is the tach accurate? I know you said 4200 before and 3200 now but my 86 formula tach was off by 1000rpms and it took only a tiny adjustment of the screw on the back to make it right.

2. What prop and what speed GPS? We can look at these numbers combined with the rpms to see if something isn't adding up

3. You looked at the carb and said the blades are opening all the way, but that is the primary. Are the secondaries opening up?, if vacuum advance is it working? This is kind of a pain but you may have to look at the carb while underway to see. Depending on the load, the secondaries open up around 3500-4000 or so but yours could be 3200.

4. It probably wouldn't run good, but is the choke not opening or stuck closed?

5. Does it plane off smoothly or does it feel down on power? Maybe a bad plug wire/plug/ or weak cylinder but not bad enough to affect idle.

Just remembered you changed plug wires, check you didn't swap 5 and 7 by accident

Just some thoughts

I am sure the tach is right because it feels slow on a GPS i get at best 29mph at WOT and it takes forever to plane.

If it were me...I would re-think every thing that i did since I got it home. It seems odd that it ran fine in Tampa but after you did the things you di...it runs bad.

Any chance you bought some bad fuel? What is the thermostat that you bought rated at as compared to the one you replaced.

Something has changed...you just have to figure out what.

Something sounds carb related. Secondaries is where I'd be looking.