1987 454 RPM dropping in and out above 3000 RPM

[dpostman21]

I have a 1987 Sea Ray Sundancer with a 454 paired with a alpha 1.

Took it out for the first time (just bought a couple months ago) I had outdrive/motor serviced and ran great through no wake zone idling at 500 RPM smooth. Once out in the lake got up on plan ran fine @ 2800 RPM, gave it a little more throttle and notice the RPM were varying kind of fading in and out. When it what spinning up good it was smooth no back fire or anything of that nature would relatively smooth loose RPM. I did not press the envelope kept it under 3000 RPM for the next hour and a half with no problems at all never stalled.

I had the following done:

1. Bellow kit
2. lower end shift cable
3. water pump kit
4. gear lube
5. oil and filter
6. fuel filter
7. Cap & Rotor
8. Spark Plugs

Any ideas?

 

[Bondo]

Ayuh,..... Pull the fuel filter, 'n look for anything in it, but good fresh clean gasoline,.....

 

[dpostman21]

I will look at it. Boat had 1/8 tank from last year I put 40 gallons of 87 octane in it before I went out.

Thanks for the advice!!

 

[muskyfins]

That's a carbed motor, so could be fouling in the jets. I had this happen before. What I did is a little unconvential but worked. Add 2 cans sea foam to tank. Remove fuel filter and discard. Fill half way with seafoam. Remove coil wire from dist cap. Crank motor a few times. Wait 10 minutes. Replace coil wire. Drive boat at varying speeds and loads. Problem resolved after some time.

 

[dpostman21]

That's a great idea thanks!!

 

[Scott Danforth]

If it is crud in the carb, i have found that no magic snake oil works as good as a tub if carb cleaner and elbow grease....and sometimes pin drills

 

[HT32BSX115]

If it is crud in the carb, i have found that no magic snake oil works

Yeah. I have never understood how naptha and mineral oil (Sea-Foam, MMO, Owl-Snot, or any other Snake-oil) can "clean" or "fix" anything in a fuel system! And running any marine engine under load with a high concentration of any oil added to 4-stroke gasoline fuel risks detonation in a hot engine.

It sounds like a fuel delivery problem. You need to find the obstruction and remove it. That will entail manually cleaning the fuel screen, replacing filters and/or water separators, removing/replacing the anti-siphon check-valve and tank screen, carburetor filter (if installed) and finally, taking the carburetor apart and clearing the jets. It you still have a fuel delivery problem, you might even need to test/replace the fuel pump.

Unfortunately, there is no mechanic-in-a-can.....


Rick

 

[muskyfins]

All I can tell you is it worked for me. Was i just lucky? Maybe, but I've done it more than once successfuly in various applications. And isn't carb cleaner just another snake oil? Don't you use solvent to remove carbon fouling in a gun barrel? Varnished gasoline and carbon fouling are both disolved by solvents. And I acknowledge there are times when it's just too far gone and you must resort to pin drills. Especially if it's clogged with something that won't disolve like sand.

Twenty some odd years ago, when I worked at a car dealer, a chemical company gave us a seminar on their fuel injector cleaner product. The guy took a valve from a pushrod motor and poured his brand of cleaner on it. Waited 5 minutes and was able to wipe it clean.

 

[dpostman21]

No Title

Thanks for the suggestions all. Went to my marine mechanic today, he has serviced this boat for 18 years. He said these Rochester carbs can get like this after sitting. We reviewed the work I had done and it appeared his guy did not change fuel filter. He suggested I start there and add some mercruiser quickleen.

See picture of filter.

 

[dpostman21]

***UPDATE***

After replacing fuel filter (Didn't look that bad) and adding mercruiser quick clean to fuel in the correct concentration, I to boat out for a short run last wednesday, and it ran great hit around 4000 RPM with 39 mph on GPS. I thought sweet problem solved. Took boat out this past saturday and problem returned.

Here is whats happening. Start boat up let warm up to 145 then idle about 15 min to get in open water. Gradually get in to throttle slowly increasing until boat reaches plan, run like that (about 2800-3000 RPM) for a minute or so then increase throttle. All seemed well on the way down the river. Ran it WOT for brief periods when I could here a variation in the motor. I backed down to more of a cruising speed of 2800 RMS and don't seem to have the issue.

On the way back I wanted to troubleshoot the issue so I spun it back up and could hear it just wasn't right. At one point it would not go beyond 3000 RPM just for like 20-30 seconds then it would spin up perfectly (without throttle adjustment) like nothing was wrong not skipping a beat. All the while there is not back fire or stalling just hesitation to spin up.

To me it feels like almost like an on off switch when its fading in RPM as quickly as it fades in it fades out and runs perfectly. Could this be something electrical like an ignition coil? I really dont think its fuel related but it certainly could be.

ANY THOUGHTS?????

 

[HT32BSX115]

***UPDATE***

After replacing fuel filter (Didn't look that bad) and adding mercruiser quick clean to fuel in the correct concentration, I to boat out for a short run last wednesday, and it ran great hit around 4000 RPM with 39 mph on GPS. I thought sweet problem solved. Took boat out this past saturday and problem returned.

Here is whats happening. Start boat up let warm up to 145 then idle about 15 min to get in open water. Gradually get in to throttle slowly increasing until boat reaches plan, run like that (about 2800-3000 RPM) for a minute or so then increase throttle. All seemed well on the way down the river. Ran it WOT for brief periods when I could here a variation in the motor. I backed down to more of a cruising speed of 2800 RMS and don't seem to have the issue.

On the way back I wanted to troubleshoot the issue so I spun it back up and could hear it just wasn't right. At one point it would not go beyond 3000 RPM just for like 20-30 seconds then it would spin up perfectly (without throttle adjustment) like nothing was wrong not skipping a beat. All the while there is not back fire or stalling just hesitation to spin up.

To me it feels like almost like an on off switch when its fading in RPM as quickly as it fades in it fades out and runs perfectly. Could this be something electrical like an ignition coil? I really dont think its fuel related but it certainly could be.

ANY THOUGHTS?????

Well, first of all, verify your rated (max) RPM is in fact 4600. Then get a different TACH to verify it's working properly.

Timing will influence "top" RPM as will any ignition "problems" to include spark plug wiring, coil, distributor etc.

There's a bunch of things you can also check, click on the following link, focus particularly on #15 http://forums.iboats.com/forum/engin...rch-version-of

Since the boat is "new-to-you" it's possible that it NEVER was able to make rated RPM. It may be overpropped or the engine might just be "tired", or one or more of the problems in #15 might be occurring