mercruiser 2004 3.0 rough idle

[gizmolvr]

2004 mercruiser 3.0 with alpha 1 gen2. Motor usual takes a couple of tries to start when cold, once it warms up it will run fine. The issue is after it is run for an hour or so( runs fine in idle before that) and I try to run it at idle speed the engine stalls and bogs out(higher rpm's it is fine). I have replaced the plugs, water seperating fuel filter,and cleaned spark arrestor. Im not sure if its related but each time i have taken boat out I have filled up the fuel tank with 93 octane. Any ideas on what to look for? thanks John

[kainon]

It's great that you are using 93 octane, dont think its necessary in that engine, but it wont hurt it in the least. Sounds to me that you need your idle mixture adjusted and possibly your choke if you have one ( TKS Carbs ) dont have chokes.

Before Starting the first time for the day, do you pump the throttle ? how many times ? I would say that if the carb is properly set (choke or not) 1 time is enough to start the engine, it should then idle without issue. This sets the choke if equipped and puts a bit of fuel into the intake manifold so the engine can fire.

If the idle mixture setting isn't right, you will have 2 basic things that will happen, it wont be "easy" to start and it wont idle properly, could be either in gear or in neutral depends on how off the adjustment is. The best way to set the idle mixture is in the water with the engine warmed up and in gear. with the boat tied to a dock or trailer backed into the water is best. adjusting it with the boat on land an runnign form a garden hose is a little more difficult though it can be done, but puttng in gear isn't helpful. You do want to have your drive lowered as much as possible.

It could be time for a carb cleaning & rebuilding.

[chiefalen]

If it were my motor i would start at the carb, the fitting that enters the carb has a filter in it clean it, work your way back to the tank, next the fuel pump giving you between 4-7 psi no less. Next follow the line , have a fuel water sep. ? Dump it into a clear container. Let it sit have water in it ? Follow the line to the tank remove the anti-syphin valve it working good ? blow thru it gunked up ? when you pull the valve the fuel pickup tube will come up with it.

There a screen on the bottom of the tube clogged up ?

Next with the valve out blow air thru the vent it clear? Next i would pump the bottom of the gas tank to remove all the junk and water off the bottom.

Tilt the bow up ,

Using a cheap auto 12v pump and 5 feet of hose on the in side and a few feet of hose on the other stick the hose into the back corner on the bottom, and put the hose into a clear container pump till you see clear gas.

Put the valve and hose hooked back up and run the boat if fixed great nope rebuild the carb.

Before anything your choke is opening all the way when running.

Your timing is good and you have good plugs in short you tuned the boat up when , and the plugs look like what, heres how to read the plugs.
Spark Plugs

[mongo]

Or it could be something really simple like the idle speed adjustment screw. Runs fine when cold because the choke is operating. When choke closes your are back to normal fuel flow. Where the throttle cable attaches to the carb there is a small screw that contacts a stop when the throttle is closed. The screw can be turned to increase the RPM at idle. Should be about 650 RPM in forward gear.

[chiefalen]

Idles fine at first runs it for a few hours won't idle? Sounds like a filter is clogged as he running it then not enough fuel, then sits over night or longer dirt and junk fall off the filter, or screen. Starts it the next day and idles good.

Try all suggestions.

[gizmolvr]

Thanks for all the advise. Going to start working on it tomorrow and will post what I find

[gizmolvr]

Had a few minutes to look at it today. The filter in the fitting going into the carb(TKS carb) had a little bit of what looked like sand, cleaned it and reinstalled. Removed the fitting(im guessing the anti-syphon valve?) that goes into fuel tank along with the pickup tube and there was nothing clogging the pickup tube. I did break the part of the fitting where the fuel line connects when removing fitting so that will be replaced. Pumped some gas out of the bottom corner of tank and it was clean and no indication of water(I let it sit in clear container). I have not checked the fuel pump yet, is there a filter in there as well? The water/fuel seperator is brand new. Would a small bit of sand in the filter on the carb fitting cause the idle issues? I figure I will also get some new sparkplug wires as well to rule that out, does this engine have a distributor cap and if so should i also replace that? Another thing i was thinking of was to shorten the pickup tube about 1/2 inch so that if there ever is anything on bottom of tank it wont get picked up, is this a bad idea?(i never let the tank get below 1/4 anyway). Thanks for all the great advice

[chiefalen]

bell type pump has a filter.

Did you try it on the water yet?

The sand might be enough to restrict the flow.

Vent is clear, tank is clear.

Still doing it then rebuild the carb

[despy]

Was the filter in the carb inlet a ceramic filter? If so, they are designed to allow gas to pass but water won`t go thru them so it only takes a couple drops to restrict flow thru them.

[gizmolvr]

Does not have a ceramic filter, standard carb filter. Im going to clean the fuel pump filter as well and then take out on water to test. thanks for the help

[gizmolvr]

Engine is still acting up so I guess it has to be the carb. Just ordered the quicksilver carb rebuild kit today (part#804844002). Does the kit come with detailed instructions or does anyone know of a manual online that details the rebuild process? The one i have does not cover the carb . Thanks for the help

[gizmolvr]

Going to clean/rebuild the carb this weekend. The rebuild kit does not have any instructions on the process. Can anyone point me to some directions or manual on cleaning/rebuilding this carb? Thanks for the help

[budgie14]

Take a good couple of photos first,then remove carb putting nuts and bolts etc somewhere safe.put some clean rag in the hole left so you dont drop anything down it.clear your bench,put heaps of clean newspaper down,tip the fuel into container,then dismantle carefully putting each set of screws and or jets in seoparate piles.if its twin barrel,dont mix up one side with the other.may be different.use a good quality screw driver to remove jets,match the width of the jet with the width of the screwdriver.dont blow out galleries until you are sure theres no springs etc etc down the holes.have a lip on your bench,so that balls or jets wont roll onto the floor.feel free to take progression photos.match all the new gaskets to the carb before scraping old ones off[if you have to].carefully use tip cleaners or fine wire to check for dirt in the jets[honda are terrible for fine orifices being blocked,as they run so lean]Be careful to cover everything if you have to leave it.try that

[gizmolvr]

Rebuild went smooth and engine is now running great. I have the idle now set at about 750. What should the correct idle be set at. Thanks everyone for all the help with this

[chiefalen]

650 is perfect if you can get it to idle good at 600 even better the less rpm you shift at makes your gears last longer.

[kghost]

650 to 700 in gear at idle speed, no additional throttle.

If it seems to clunk to hard when shifting into gear turn it down just a bit.

Good work!!