Carb Adjust on 1990 Mercruiser 5.0

Hey everyone, just installed a new (Remanufactured) Mercarb 2bbl carburetor on my 1990 5.0L Mercruiser I/O. The boat starts and runs 100% better than it did last season. No more hard starts, no more hesitation when I goose the throttle, thanks to your advice on here you helped duplicate and eliminate the problems I had. Now, here is my next question. The only thing that was adjusted on this carburetor was the idle. I took my boat to a local lake last weekend and trotted around for about an hour, verifying everything is operating correctly before the boating season begins. When I pulled the boat out, I noticed behind the exhaust tips (boat has side exhaust) Everything behind the tips, including the transom were practically covered in carbon build-up... or "soot". I also noticed while in the water and the motor is revved, it puffs black smoke. The local marine center believes the carb is running too rich, and told me to tighten the mixture screw at the base of the carburetor. Sounds easy enough, but how do I know when to stop turning the screw? Is this something I test while the boat is under load? Any help on this would be greatly appreciated.<br /><br />Thank you all very much...

Re: Carb Adjust on 1990 Mercruiser 5.0

Check some sites on carb adj. while using a vacum gauge, this is how I adj. my 2bbl carb.

Re: Carb Adjust on 1990 Mercruiser 5.0

Or you can just turn the screws in, clockwise, until the engine reaches it's highest idle rpm. You can use a tach, vacuum gauge or just listen to the motor. Do one then the other then repeat. At this point you can back them out, counterclockwise, a half turn or so & you should be about where you want to be. You'll probably have to reset your idle because it will most likely be to high after you do this as it does sound like your running a tad rich.

Re: Carb Adjust on 1990 Mercruiser 5.0

Hello, River rat...Go back to the first post where you asked us about this carb (Topic: looking for replacement carb, any ideas?) I listed the exact proceedure you need to follow to get this carb tuned properly. Let us know how it goes!

Re: Carb Adjust on 1990 Mercruiser 5.0

Fireship- took your advice and finally purchased a vacuum gauge. Now heres a stupid question. Once again, I'm used to automotive engines which have vacuum lines running every which way. I cannot find any vacuum lines to tap into. Theres an elbow fitting on the back of the carburetor which points down towards the intake manifold. Can I use this port to measure the amount of vacuum? <br /><br />Thanks again!