3.7 Motor skipping under load

[Pst76]

I have a 1984 ,470 Mercruiser that Has a stator set up for charging. The motor will idle fine and I can rev the engine and Amp gauge will float between 11 to 13 volts.
Under a load after running about 15min at half throttle the enging started to sound irratic, kind of a slight skip but more of a change in RPM sound.First thought was a fuel issue but that seems OK.
I am running with a new battery and at a lower throttle setting motor runs fine and can rev without a load and maintain steady rpms.
I am thinking the irratic motion on the gauge may indicate a charging issue, under a load could stator output not be enough to maintain voltage at Higher rpm?

On the other hand if rpm are changing gauge would react the same.Motion on gauge is floating as opposed to a jerky motion.
Carb has been rebuilt so I think I am ok there.
Just looking to figure out a starting point with out starting to just randomly replace item's.
THanks for any feedback

 

[KaGee]

Re: 3.7 Motor skipping under load

When was the last tune-up?

 

[Don S]

Re: 3.7 Motor skipping under load

When doing your tuneup, check the distributor shaft bushings. I've replaced a LOT of the 470 Delco distributors. Merc has been using the Prestolite as a replacement for many years.

 

[Pst76]

Re: 3.7 Motor skipping under load

Tune up end of last season, Before winterized,Plugs, points, oil, filter, water seperator. Distr cap and wires looked relatively new. When I bought the boat last yr was told they had just had it tuned up begining of 2005 season. Plugs and points when I changed looked new but had purchased parts so I changed. I am looking at picking up coil, rotor, cap and new wire's

 

[Pst76]

Re: 3.7 Motor skipping under load

Thanks, Don, Would you think more a good tune up over Stator problem?
The main reason I questioned ststor id 2 weeks back we were out and after about 45 min cruising, if I hit the trim button the motor would miss fire. After looking everything over I found that my bilge pump switch had been on for that wholetime. From what I could see the pump had drained the battery voltage way down. When I hit the trim the draw from the trim motor dropped voltage.

ON the rest of the trip, now that pump was off amp meter showed a charge and motor ran fine.
I bought a new battery as original one as 2 yrs old and figured I had fixed any issue.
From the pump issue I am guessing that the pump drew more power than what the charging system could replace and still keep voltage to the motor circuit.
Pump ran a good 45 min while under way.
I will do a full tune up as well as check dist bearings, What is the best way to confirm good or bad bearings on the dist, and I'm guessing these would be internal to the dist assy.

Thanks

 

[Pst76]

Re: 3.7 Motor skipping under load

Don, Is there an equivilent Rotor, cap and wire set up I can get at a local NAPA or auto parts store?
I know the Mercrusier is similar to the old 460 ford just cut in half but other than going to a dealer are there other options for the parts.
I had replaced points but they were in a new box with the boat when I bought it so did not have to source them.
Thanks

 

[Pst76]

8/21 Update on fix.Re: 3.7 Motor skipping under load

8/21 Update on fix.Re: 3.7 Motor skipping under load

Did a complete tune up on the boat last week and still ran into a problem with throttling up. Did some futher probing and found the culprit. The anti syphon valve at the tank was stuck. I removed and found that the ball was stuck about mid way open. I sprayed it down with carb cleaner and was able to shake the fixture and have the ball an spring move freely. Re installed and took the boat out over the weekend and she is running like a top.