'87 48spl bogging on throttle up (Solution!)

[philobite]

My '87 48spl bogs down when I advance the throttle handle, and seems to hit on only one cylinder when the throttles open up. Here's the relevant info:

- When I disconnect the link between the carbs and advance one throttle at a time by hand, the top carb accelerates the engine fine by itself. When I advance the bottom throttle alone, however, that one bogs, especially if I advance it slowly. If I advance it rapidly is stumbles and then catches up. (under no-load conditions)
- In actual use on the water (loaded conditions), the engine bogs on throttle advance.
- Idles somewhat rough. When trolling at idle, it'll run for 5 minutes and then stumble and die and need to restart. Troll for 5, stumble... restart, and so forth.
- I swapped the top and bottom carbs and the bogging problem moved from the bottom to the top... so it moved with the one carb. This seems to indicate to me that the problem is in that one carb.
- I've had both carbs off and removed the jets (there are no adjustables on these carbs) and everything in both carbs looks exceptionally clean in and out. I've pulled the jets and blown them and the carb out with air, and runs some spray carb cleaner through everything. Also, both floats are adjusted so they are parallel to the carb body when inverted.
- I've pulled the reed box off and everything looks well there (no warps, cracks or misadjustments).
- New plugs.
- New fuel filter.
- Fuel bulb pumps up hard and remains reasonably so when running.
- All spark advance linkage is fine, tight, operational.

I'm baffled and would appreciate input. Bogging seems to be associated with that one carb. I don't see how doing a carb rebuild would help (please do correct me if I am wrong here) since the carb and both jets are spotless clean, floats seem to be adjusted, I can blow air through all of the orifices. Could this still be any ignition issue? Any advice would be appreciated.

 

[tashasdaddy]

Re: '87 48spl bogging on throttle up

Re: '87 48spl bogging on throttle up

there is something wrong in the one carb. it may be that the float needle is not seating, and allowing the fuel pump to force fuel thru the carb, it bogs on faster revolutions, which is a when the pump increases pressure.

 

[philobite]

Re: '87 48spl bogging on throttle up

Re: '87 48spl bogging on throttle up

there is something wrong in the one carb. it may be that the float needle is not seating, and allowing the fuel pump to force fuel thru the carb, it bogs on faster revolutions, which is a when the pump increases pressure.

Thanks for the idea. In theory that makes sense. What I'll do is inspect the float needle for damage/wear, and then swap the needle valve and seat between the carbs and see if the problem moves between carbs, and report back here.

Any other ideas in the meantime?

 

[philobite]

Re: '87 48spl bogging on throttle up (Solution!)

Solution to the problem: I found two things that, when corrected, fixed the bogging problem on the lower carb.

1) The lower carb fuel inlet needle valve didn't close off fuel as crisply or as soon as the one on the upper carb. I thoroughly cleaned the needle and seat, and then I adjusted the float arm so that the needle valved closed sooner. This adjustment brought the closing point at about the same as the upper carb. BTW, it did mean that after adjustment the float did not end up parallel to the carb body when carb was inverted. It ended up about 1/8" away from parallel

2) The vent screen that lets air into the fuel bowl looked very clean from the outside, but when I sprayed carb cleaner into the screen (from outside the carb) a bunch of crud bits blew down into the vent tube. So the vent was substantially blocked, even though the carb looked perfectly clean in every other respect.

Put it back together and... vroooom! Both carbs accelerate smoothly and in unision. Kudos to "tashasdaddy". Thanks for the advice above.