Slight bog when accerating

[88Skier]

I got the Sporty out over the weekend, and put new plugs, cap and rotor in it, hoping that a slight bog that developed last summer would go away. The engine runs great at all speeds, except it bogs slightly when WOT from idle. The bog is much worse if the engine has not been running hard. There is no backfiring, just hesitation. Is this a carb issue, maybe accelerator pump? Anyone know how to adjust it?

[Baddog]

I am no expert here 88, but my guess, and it's only a guess based on working on my motorcycle, is a partially clogged jet. I've seen weird things happen on carbs with only a teeny tiny spec of something in or near the jets. Clean out the mystery thingy and all runs well again.

Easy enough to clean in any event.

[Woodski]

What brand of carb is on that boat? Ones equipped with the Weber carb (Mercruiser) do exhibit a bog when cold, that is the fact that the fuel is not atomizing in the intake manifold. If persistent, then a potential accelerator pump replacement might be in order (regardless of brand) as that gives a squirt of fuel to allow the boat to accelerate.

[electricjohn]

I agree with both above. Go with some carb cleaner, then look into the accelerator pump. You should get a shot of fuel down the throat of the carb if you (quickly) open the throttle by hand right at the carb. Engine does not need to be running being it is a mechanical pump. Look for a steady stream.

[88Skier]

I'll try running some carb cleaner through it. Sounds like you all think it's not getting enough gas, rather than too much. It only hesitates when the throttle is opened up from idle. If you nail it to 3/4s there is no hesitation.

Holley carb

Edited June 19, 2007 by 88Skier

[Baddog]

I'll try running some carb cleaner through it. Sounds like you all think it's not getting enough gas, rather than too much. It only hesitates when the throttle is opened up from idle. If you nail it to 3/4s there is no hesitation.

Holley carb

Oh. Well. Now that symptom could be too much gas. No clue what you would do about that if that is the case.

[Woodski]

If still persistent, and it should be repairable since a new problem, you could increase the size of the accel pump to get more fuel at smaller throttle openings. Have you changed from regular type gas to oxygenated fuel, that might have an effect since the air/fuel mixture might be different.

A couple of other items, check the seal of the base of the carb to the intake manifold and proably all the intake gasket seals (allows too much air) and perhaps the throttle shaft seals or looseness in the throttle shaft (again, allows air to enter).

[wienrdog]

Could be a mis-adjustment on your accel pump & it's pumping too much gas or has a slight leak and isn't pumping enough or is pumping too much....

" The correct accelerator pump clearance is necessary to prevent hesitation during acceleration and to ensure max pump output. If clearance is insufficient, the throttle valves may not open fully and the accerating pump diaphram bay be damaged by overextension."

Do take a look at you accel pump. Mine was pretty mangled after my first season with the boat & I needed to replace it because it wouldn't fully fill. I fully rebuild the carb as the only accel pump diaphram I could find was in a rebuild kit. Only took an hour or so longer.

On the holley I used to have.

1) Hold the throttle in wide open position while manually depressing the pump lever.

2) Measure clearance between the pump lever and the head of the screw on the pump arm.

3) Turn the adjusting nut as required to get the recommended clearance.

The clearance for some likely Holley carbs - don't know which one you have

500CFM 2bbl 2300-2v (80402-1) = .25-.38mm (1/8-9/64")

650 cfm 4175 Holley 4175 Spreadbore (80390) = same

650 cfm 4160 4v (80383) = same

Most engines listed were the same clearance (except 1 5.0L 1 1 5.7L with 2bbl) They were .254-.381mm (,010 -> .015")

If you can find an engine specific recommendation for clearance you'll be better off, but it looks like these numbers should be close.

Edited June 19, 2007 by weinrdog