View Full Version : Engine rpms randomly dip down severely

05-20-2012, 11:17 PM
I've got the 351w engine with the holley 4160 carb and I'm trying to figure out what's wrong. The engine starts fairly easily but it isn't like an EFI engine. I'll run it in neutral at about 1500 rpms for 30-45 seconds and then I can usually put it in gear without the engine trying to die. I figure this is normal but last year I could crank it up and it was instantly perfect.

I have my idle set to about 700rpms when in gear. The part that seems weird is after driving it around the lake and starting it back up, the engine sometimes hangs on by a thread at around 250-300rpms and I have to fight to keep it from stalling. Once I get the skier/wakeboarder up, everything is normal unless I have to keep circling back. This doesn't happen every time either but enough to be an annoyance. Sometimes it cranks up and idles normally while others it acts up.

The boat is an 86 and I have no idea if the carb has ever been rebuilt. Before I buy a rebuild kit though and hope that's the problem, I was hoping someone could confirm or deny if I'm on the right track or if there is something else to look at first.

05-21-2012, 04:42 PM
Just finished rebuilding the 4160 on my boat and can tell you that it is most likely the cause of your issue. There are 2 screws on the primary fuel bowl that control the air/fuel mixture at idle. You should start with them out at 1 1/2 turns and then adjust from there.

05-21-2012, 05:02 PM
Before you start adjusting the carb, ignition system basics should be covered. Spark plugs, distributor cap & rotor, plug wires all need to be up to par. I change plugs every year and cap & rotor every two years. Electronic ignition or points? Points should be changed every year.

As to adjusting the carb, they don't tend to just suddenly come out of adjustment where you can turn the idle mixture screw a half turn and solve an issue. If your carb has not been rebuilt in the last ten years, it is very likely that this is at least part of your problem.

05-21-2012, 05:17 PM
Along with all the other recommendation above you should also check the choke for proper operation.

05-21-2012, 05:35 PM
Put a timing light on #1 wire it and see if your timing is relatively stationary or fluctuates erratically.

05-21-2012, 05:42 PM
Thanks. I went ahead and ordered a rebuild kit along with the secondary diaphragm since Holley recommended it on their site and it was only another $10.

I don't really plan to adjust the carb settings until after it's been rebuilt (provided it's needed). I do have an electronic ignition but it's probably about time for a cap and rotor change. I will also admit that I've been lazy this year and didn't change the spark plugs but they do only have 1 year on them. The electric choke does function properly because that was one of the first things I checked when the problem started at the end of last year.

05-21-2012, 05:45 PM
Put a timing light on #1 wire it and see if your timing is relatively stationary or fluctuates erratically.

If it's erratic, what would cause that? I've only put a timing light on 2 strokes and if those are off, it's either the static timing or the ignition brain is shot.

05-21-2012, 05:59 PM
You could have worn out advance springs that occasionally allow the weights to get hung up. Not overly common, but it does happen.

05-21-2012, 07:40 PM
Fuel Quality / Flow sounds like an issue.
Low speed Idle circuits are always an issue stored over winter.
At least these carbs are very simply even if never explored before.
Do a Google search with model number, and should see video list for overall rebuild.

Low idle kill in gear is usually low fuel flow in idle circuit.
One can operate the choke by hand to augment this condition to make it recover smoothly, and verifies fuel starvation for normal operation.
Carb build required.

Air Compressor is simply the only cure to blast out pre-soaked assemblies.

Fuel filter / if it's just a simple inline, or filter insert at carb fitting, these just don't provide a quality result.
The very best are frame type canister that the can screws off to gather water and debris and simply dump out.
See through ones are major upgrade step with drain.

Fuel additive like Techron is a good one to add to fuel tank.
Moisture, is a huge concern, adding Isopropyl to fuel can help.
If fuel level is low, I'd just recycle it in lawn mower 50/50, and start with fresh.

I use a vacuum pump attached to refrigerant tank, and simply vacs out any fluid.
Attached to oil drain hose to violently drain the oil out at machII.
Now I know it's drained pulling out any particulate before it can settle out doing it normal forever slow mode even warmed up.
Never drain an engine cold. Everything you wanted out stayed behind.

Pilots of single/twin engine are the worst offenders of draining oil cold.
I tear off the drain pans, and 1-2" of solid grey cheese is left behind.
Engine warms up, and this cheese re-melts back into system, Nice job Captain.
$27,000 engine junk before its time. Or worse, forced, sitting in corn field for no reason.

Good point by MrMastercraft with advance weights.
They can get rusty, so move them manually lubed, not WD40.
You can clean with it, but it's not a lube.
Perfect lube is water proof LPS3 spray. Leaves a waxy coating forever.
I also use it on exposed circuit boxes of fuses. Never corrosion.

With Electronic Ignition, timing is rarely effected.

Timing is quite simple once distributor is loose enough to move only by hand.
Advancing it - RPM's go up.
Retard - RPM's go down.
Set by ear pretty much spot on. Instant throttle response.

Cap & Rotor - apply best grade silicone spray & let solvent flash off. Inside & out.
This will prevent high moisture from low temp evening leaving high voltage tracking internally in cap.
Repeated high voltage tracking, can leave a mark, and it will follow it the next time moisture allows it.
It will sound/feel like cross fire when it happens which it is, engine jerking, or quit running.
Dry/wipe it out will recover fine.

Spark plugs wires - With spray bottle of water with one drop of soap to make it wetter,
fog the wires lightly while running.
This will reveal where the weak points are now, vs out on the water.
Any arcing to ground along the wire, replace.
Don't fog the distributor cap during this test.

Also use the same high grade silicone, and highly spray them down, inside the boots on the plugs as well.
Now the ends won't pull off the wires, and will be easier to pull off the plugs vs glued on the plugs.

Spark Plugs - Good way to learn how well the engine is running looking at them.
Coat the threads with light smear of anti-seize, that will end the nonsense of stripped holes.
GOod LUck, Have FUn.

05-21-2012, 08:42 PM

05-23-2012, 07:24 PM
Carb rebuild and cleaning did the trick. The engine runs much smoother, more consistent, and has instant pep when I give it gas.