Rebuilding carb on a 3.0 liter mercruiser

lemichaelj · Sep 22, 2011

If anyone can help, I am going to rebuild my carb this weekend. when I shut the engine off I notice gas was still dripping constantly into the barrels. according to the repair manual this is called "warm engine perculation" (i still call it flooding) so what causes this? and do you think a rebuild will fix this? by the way I have to open the throttle almost all the way to start it up. so therefore I say its flooding. also no speed hardly at full throttle. Pulled spark plugs and number 1 was cracked. Dont know if I cracked it trying to get it out or if it was that way while trying to run it at the lake. but it was clean on the tip like it was firing, which makes me think I cracked it. also I see a barb like siphon valve on the fuel tank pickup line. Could this be a siphon valve as everyone talks about here? anyhelp would be appreciated.

mpdive · Sep 22, 2011

Re: Rebuilding carb on a 3.0 liter mercruiser

More than likely your needle is not seating fully in the seat, hence the overloading. An overhaul will cure that situation. Replace the float while your at it to eliminate a "wet float" condition. Every time I rebuild the rochester or mercarbs, the float levels have been insanely off. Years of fuel pressure has a way of slowly bending float tangs. The siphon valve will not be the problem as fuel delivery is ample.

lemichaelj · Sep 22, 2011

Re: Rebuilding carb on a 3.0 liter mercruiser

does the new float and needle combo need to be adjusted, for I dont have a way to measure to make sure? by the way the filter was extremely clean inside of the carb. havent torn it apart yet but plan to this weekend. so is that what causes the so called "warm engine perculation " the service book calls it?

mpdive · Sep 22, 2011

Re: Rebuilding carb on a 3.0 liter mercruiser

Percolation is a whole other animal. Many causes for it such as misrouting of fuel lines, too many bends, over insulated doghouses, etc... Your problem is plain out dumping. Needle and seat have no adjustment but the float does. You will have float level, which you will probably find to be way off, and float drop. A measuring guide will be included in the kit. The level will probably be around 9/16 and the drop 1 3/32. Be careful of the power piston and make sure you dont bend it. Also a word of advice, if it is the original carb, and is painted with the pretty black marine epoxy, the airhorn screws are most likely painted and your screwdriver will want to slip. Use a good screwdriver because if it dont fit the screw head and you round it off you could get aggravated. I use a pair of needle nose vice grips just to crack them loose if they look boogered up.

lemichaelj · Sep 22, 2011

Re: Rebuilding carb on a 3.0 liter mercruiser

now thats good advice. your right it does have a black varnish on it and I did think they might be hard to get off. A good screwdriver is good advice thank you.

lemichaelj · Sep 26, 2011

Re: Rebuilding carb on a 3.0 liter mercruiser

update, I took carb off, and wasnt able to do a complete rebuild. Turns out my local marine shop didnt have a kit, they wanted to sell me each individual part seperatly, which would have cost me close to $400 for each peice total. so I opted just for the gasket set for $43.00, and all I did was a thorough cleaning, found (not a whole lot) but some definate trash in the bowl and when i blew out the needle seat sure enough trash in there as well. What blows my mind the filter on the carb was clean. Go figure.
Boat runs excellent now, no problem with performance at all took it out on the water and ran like brand new again. Thanks to all for the help on this forum and I will be back to learn more from here. Thanks so much for the responses.
By the way, the carb kit, I found I could purchase for as little as $40.00. so keep that in mind I didnt know this until this weekend until I already purchased the gasket kit for the same price. hope that helps someone on here. looks like my local marine store wanted to rip me off. So I you can be assured I wont do business with them if I dont have too anymore.

dondi · Sep 26, 2011

Re: Rebuilding carb on a 3.0 liter mercruiser

i also have a merc 3.0 my local shop gave me a quote of $200.00 for rebuild on carb .pice you got was outragious

Oshkosh1 · Sep 26, 2011

Re: Rebuilding carb on a 3.0 liter mercruiser

I just bought a factory rebuilt Rochester 2bbl 2GCfrom Rockauto(Autoline was the supplier) for just over $180. After shipping the core back...the total price is around $120. It's already flow tested, painted, and all I had to do was bolt it in.

mpdive · Sep 26, 2011