Engine RPMS Pulsing at half throttle

[areoseek]

Alas, Everything this summer was going smooth, but here I am on a boat forum. Never good.

I have a 1986 Sea ray sundancer with a mercruiser 260 under the hood. This winter we had the gimbal bearing, water pump, shift cable, and bellows replaced.
took it out for the first long drive and all was well. That was in may.

Jump to last week, on a 20 mile cruise, I noticed around 2,500rpm that I got a significant "pulse " from the engine. RPM's would go from 2,500 to 3000, to 2000, and back again. I could feel this in the boat as well. Kind of felt like the prop hub would slip an inch or two and catch and repeat. HOWEVER, at WOT she's Purrffect. no slip. not even loaded up with seven people. this issue is only present from slightly above 1,500rpm, to 3,000rpm.

Help? Marine Mechanic said I was crazy. I have never seen this before.

going to go down tomorrow and do the ol' white sharpie on the prop hub trick just to eliminate that.

Thanks, -Devin

 

[achris]

Replace the spark plugs. My current and previous engines did the same thing when they want new plugs. And my engine is but 6 of your 8 cylinders.

Chris. ....

 

[areoseek]

Replace the spark plugs. My current and previous engines did the same thing when they want new plugs. And my engine is but 6 of your 8 cylinders.

Chris. ....

Will do tonight and report back! Thanks for the suggestion!

 

[Bondo]

but here I am on a boat forum. Never good.

Ayuh,..... I wish somebody had told me that, Years ago,....
I've been here 'bout everyday since I found the place,....

My 1st thoughton yer problem is possible crud in the carb hamperin' in mid-range circuit,....

 

[areoseek]

Replace the spark plugs. My current and previous engines did the same thing when they want new plugs. And my engine is but 6 of your 8 cylinders.

Chris. ....

Good news! Put a fresh set of mr43t's in gapped at .035 and That motor is running better than ever! I had no idea spark plugs went so quick on a boat motor! (changed two years ago).

Made a noticeable difference in my time-to-plane.

I appreciate the advice and will store it away in my head for others!

 

[areoseek]

alas, the issue has again surfaced. It seems it only rears it's ugly head when I have a full load on the boat. (5+ people). Under light load, it's not as bad. thoughts?

 

[NHGuy]

Check the carb for leaks around the throttle and choke shafts, and the carb base. Check the intake manifold too. Spray around those places with carb cleaner. If the engine speeds up you have a vacuum leak. The carb torque is just wrist tight with a small socket and ratchet. 15 inch lbs.
The intake manifold torque is 30 FOOT lbs.

The carburetor primary metering is dependent on vacuum.

If those things are good try to clean the primary metering by putting seafoam in the fuel and running at those loads. Also run at those loads and shoot bursts of carb cleaner into the carburetor while at that part throttle and at throttle values just above and below it. You are trying to replicate the vacuum load and add some cleaner then.

 

[achris]

An engine under higher load creates higher cylinder temperatures and pressures. That requires a higher voltage to fire the spark plugs. I would be looking very closely at everything in the HT side of the ignition; coil, distributor cap, rotor, plug leads (and don't forget the lead from coil to cap). Check the coil for corrosion and oil leaks. A coil that has lost its oil will exhibit the exact symptom you have.

Chris......

 

[NHGuy]

There is a piston that sits on a spring which is pulled down by engine vacuum. At light throttle there is lots of vacuum, it pulls harder on the piston and pulls the piston further down.
This part throttle metering piston is between 2 rods that extend down into your primary fuel jets. The rods are stepped but picture them as conical, with the tips narrower and the tops fatter. When there's a lot of vacuum at light throttle the vacuum sucks the piston/rod assembly down so the fatter part of the rod is in the jet thereby reducing the fuel that gets to the outlet and into the engine. That's a lean mixture. Perfect for light throttle!
When you let it rip you allow a lot of air which reduces vacuum, the spring pushes up the piston, the rod rises to it's skinny part and the carb gets more fuel. That makes more rpm and power.

Then further on as the engine picks up more rpm your secondary circuit adds more fuel and air.

 

[NHGuy]

But Chris is really good. He did this for a living for a good long time. I'm just a boat loving hot rodder kinda guy. His ideas are easy checks and things that need doing every so often.
Maybe just do the cap, rotor,and wires. And inspect the other stuff.

 

[areoseek]

An engine under higher load creates higher cylinder temperatures and pressures. That requires a higher voltage to fire the spark plugs. I would be looking very closely at everything in the HT side of the ignition; coil, distributor cap, rotor, plug leads (and don't forget the lead from coil to cap). Check the coil for corrosion and oil leaks. A coil that has lost its oil will exhibit the exact symptom you have.

Chris......

I know uscg is picky, Do I need "marine" cap, rotor, wires and coil? Or can I any automotive small block set?

 

[NHGuy]

The wires need to reach, so marine. TThe cap and rotor should be original equipment. Aftermarket iisn't so good.

 

[areoseek]

The wires need to reach, so marine. TThe cap and rotor should be original equipment. Aftermarket iisn't so good.

copy. thank you. will report back after pay day.

 

[Bondo]

I know uscg is picky, Do I need "marine" cap, rotor, wires and coil? Or can I any automotive small block set?

Ayuh,..... Ain't many SBCs in things other than Merc powered boats, with Thunderbolt IV ignition,.....

The cap, 'n rotor gotta fit the distributor used,.....

Marine spark plug wires are Much Longer than the automotive equivalents,....
Automotive wires go over the top of the motor, marine wires run back, 'n up along under the manifolds,....