Carb problems

[04Bayliner]

2004 Bayliner 175 with a 3.0 Mercruiser. Carb is a 2 Burrell Mercruiser not a Rochester. Initial problem was run-on and bogging down under acceleration. Initially verified engine timing was correct and rebuilt carb. Used a kit from MikesCarburetor.com, soaked everything in B-12 Chem tool for about 45 minutes, assembled with new parts and verified all adjustments with spec sheet. Engine cranks easily and runs great. It idles well and accelerated without bogging down or stumbling.

Now for the problem. There is a loud hissing coming from around the idle enrichment screw. I cranked the engine and allowed it to run for a second, then sprayed WD-40 all over the area. Spraying it produced no change in engine sound or speed. It runs too good to have a large vacuum leak, but still hisses loudly. This particular model does not have the TKS system on it. Enrichment screw is set about 2 1/8 out from seated. Do you have any suggestions?

 

[Grub54891]

Did you hook up the small safety hose from the fuel pump?

 

[04Bayliner]

I did hook the safety hose back up. The sound is coming from around the idle enrichment screw. If I place my fingers around the base of it, the hissing stops. My question was poorly stated. What would cause the idle enrichment screw to hiss.

 

[Grub54891]

I'd have a look at the gasket myself. You sprayed wd but still check it. I've seen old gaskets stuck to the manifold, even though it looks clean, don't mean its clean. Just something else to check.

 

[04Bayliner]

The gasket is suspect number 1 for sure. The new idle enrichment screw seemed a little loose to me when I installed it, but this is my first marine carb so I didn’t have much to compare it with. Anyone ever gotten a defective screw that is loose in the hole?

 

[harringtondav]

2 1/8 turns out from seat on your idle mixture screw caught my eye. I've had a lot of carbed motorcycle, small engine, auto and marine carbs. The starting point for idle mixture setting is normally seat, then 1.5 turns back. My 4.3LX 4bbl Weber quit dieseling at 1 1/4 - 1 3/4 turns out from seat. W/O the Merc manual, I guess the hissing is air entering around the idle mixture screw/needle. I'd set it to the screw 1 1/2 turns out. This could cause your idle RPM to drop since a lean mixture (2 1/8 turns out) tends to raise idle RPM. Tweak this screw and your carb idle screw until you get a good idle at 650 - 700 rpm.

 

[Scott06]

You mention the inew dle screw felt loose, I would try putting the old one back in and see if the hissing goes away.

 

[04Bayliner]

Thanks for the replies. I found the issue. I didn’t get all of the old gasket material off the intake/exhaust manifold and it wasn’t sealing. The leak was directly below the idle enrichment screw.