Engine Bogs down consistently at 3000 RPM but fluctuates when I leave it there.

[kcsguide]

I have a 1976 Fiberform Baja 24 CB with a Volvo Penta AQ240A engine with a 280 outdrive. It has a Holley 4160 4 barrel carburetor.

Last summer I had the engine overhauled by a professional machine shop and picked up an overhaul kit for the carb and overhauled the carb myself, carefully cleaning out every port etc. and replacing all gaskets, O-rings, diaphragms, power valve, etc.

The engine starts and runs fine up to 3000 RPM exactly, where it bogs down and seems like it will die if I don't pull the throttle back to 3000 RPM where it again runs fine. (This only happens under a load. ie; in the water and in gear) The strange part is that when I pull it back and leave it at 3000 RPM, it fluctuates in RPM from every few seconds to a couple of times per minute. Sometimes up to 3200, 3500, 3700 and even up to 4000. Up and down without any movement of the throttle on my part. And when it fluctuates up in RPM it is still running smoothly.

I'm guessing that the significance of the engine consistently bogging down at 3000 must be where the secondary barrels should be opening up but they are not.

Here is what I have done so far:
1. Replaced the fuel pump with a new one...No change.
2. Re-timed to 10 degrees BTDC...No change.
3. Checked that the accelerator pump lever clearance is at .015.
4. Tweeked the idle adjustment screws to best performance. About 1 turn out...No change.
5. Adjusted float levels on both the primary and secondary floats. (the secondary was way low)...Haven't tried on the water yet.
6. Removed the secondary throttle-opening diaphragm assembly and tested for leaks. No leaks.

I looked down the carb as I revved the throttle up to around 3600 RPM but the plates in the secondaries don't move and the secondary diaphragm rod does not move.

What am I missing?

 

[Saline Marina]

I don't know the drive or engine but check all spark plug wires for any signs of exterior chafing. Use your fingers with engine off and engine cool and work down every single wire between distributor and spark plug. Where wires naturally "cross" each other is a point ripe for chafing that is an unwanted outlet for the electical energy. An example would be a V8 engine running on 6 or 7 cylinders more or less

Also an ignition coil is a place where the insulated windings can break down to the point where one can get sort of a "half-coil" operation. It works sort-of but eventually the increasing difficulty in delivering enough energy to jump the spark plug gap finds a path of lesser resistance, and this is detrimental to combustion and thus limits max power.

The coil has a lot of work to do. At 3000 engine rpm, the crank rolls over at 50 times per second*, and the distributor is rotating 25 times per second. But the coil could be asked to deliver 200 individual sparks in 1 second in a V8 engine.

* when I think of this I am still amazed that an internal combustion engine does not destroy itself within a few seconds. Try to move your fist or even finger at 50 movements per second...

 

[alldodge]

The engine starts and runs fine up to 3000 RPM exactly, where it bogs down and seems like it will die if I don't pull the throttle back to 3000 RPM where it again runs fine. (This only happens under a load. ie; in the water and in gear)

Sounds like a carb issue and it will "only" happen under load. The motor is burning next to no gas when just reving up

5. Adjusted float levels on both the primary and secondary floats. (the secondary was way low)...Haven't tried on the water yet.

This may very well be your problem. If bowl is not a correct level the motor will not get enough fuel.

I looked down the carb as I revved the throttle up to around 3600 RPM but the plates in the secondaries don't move and the secondary diaphragm rod does not move.

They should not move with reving, only under load.

 

[kcsguide]

Thanks AllDodge, That's a valuable piece of information. Sounds like I need to put it in the water and find out if that's all it was. Also, Saline Marina, I've been itching to replace the coil anyway because it looks like it's a million years old.

Thanks and I'll let you know what happens.

 

[Scott Danforth]

bring your float up (sounds like your float level is off)
verify the spring in your vac secondary chamber. should be light yellow in color.


if your coils is working, no need to replace unless you like spending money. you could rattle can it black and make it look new

 

[kenny nunez]

Try the out board 6 gallon tank way. Hook one of them at the fuel filter and see if it will turn up. Go ahead and re check the float settings first. If the out board tank lets the engine turn up then pull the suction tube from the tank, there may be a restriction in the tube caused by the small filter that may be inside of the tube. If the anti siphon valve is aluminum replace it with a brass one along with new fuel hose.

 

[kcsguide]

OK, Took it to the lake today and problem solved. I'm just surprised that the manual didn't mention a word about adjusting the float in the secondary bowl. Thanks everyone for the help!

 

[bc2planker]

Hi folks- I am having this same issue. Hope I can piggy back on this thread...
VP 4.3Gl Engine. Bogging @ 3000rpm, only under load. 2 barrel Holly carb, no secondary, so what worked here won't be my fix.
First thing was I found water in the fuel. Dealt with that, confirmed good gas. Still bogging.
Then I rebuilt the carb using Holly rebuild kit. Replaced everything I could from the kit. Then it was stalling when put into gear. If I quickly moved into gear and more throttle, it would get past the stall and run well. I could run up to 4500rpm, no issue.
I tested vacuum and it was low, so adjusted the carb idle and both jet screws. This fixed the stall when putting into gear issue, but the bogging @ 3000 returned. Vacuum improved to about 14, so still a bit low.
It had a who knows what fuel pump, output 2psi at idle, 5-6psi higher throttle. I replaced with new fuel pump. Now its bogging @ 2000rpm!
Notice fuel lines to the pump were 5/16, rather than 3/8, so replaced those.... back to bogging at 3000rpm. Runs fine below that, but feels a little sluggish.

Next step I was looking at was adjusting the timing. Any other ideas??

Thanks!

 

[GA_Boater]

Hi folks- I am having this same issue. Hope I can piggy back on this thread...
VP 4.3Gl Engine. Bogging @ 3000rpm, only under load. 2 barrel Holly carb, no secondary, so what worked here won't be my fix.
First thing was I found water in the fuel. Dealt with that, confirmed good gas. Still bogging.
Then I rebuilt the carb using Holly rebuild kit. Replaced everything I could from the kit. Then it was stalling when put into gear. If I quickly moved into gear and more throttle, it would get past the stall and run well. I could run up to 4500rpm, no issue.
I tested vacuum and it was low, so adjusted the carb idle and both jet screws. This fixed the stall when putting into gear issue, but the bogging @ 3000 returned. Vacuum improved to about 14, so still a bit low.
It had a who knows what fuel pump, output 2psi at idle, 5-6psi higher throttle. I replaced with new fuel pump. Now its bogging @ 2000rpm!
Notice fuel lines to the pump were 5/16, rather than 3/8, so replaced those.... back to bogging at 3000rpm. Runs fine below that, but feels a little sluggish.

Next step I was looking at was adjusting the timing. Any other ideas??

Thanks!

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