Mercruiser 305 - Quadrajet Rich Idle

[ddkjunior]

Hello,

I have a Cobalt boat with an early 90's Mercruiser 305.  I tried rebuilding the original carb, and 2 rebuilt units...all of which had a variety of issues.  I found a brand new Quadrajet for this motor and everything is now working perfectly except at idle.  It runs very rich at idle and will usually die.  It starts hard after being shut down when warm and is clearly too rich and loading up with fuel.  The idle mixture screws have no effect and can be completely turned in and the mixture is still rich.

All carbs I tried had this same issue to various degrees.  The engine itself is in great shape with very low hours.

Any suggestions would be appreciated.  Thanks.

[Shark Racer]

Do you have nozzle drip?

What's the float level?

Define "brand new". Who rebuilt it?

[ddkjunior]

I'm not sure how to check for nozzle drip...but after reading the section on idle circuits in Cliff's book this sounds like a possibility.

The carb is new old stock 17086069.  Knowing it probably sat for some time I took off the air horn and inspected everything...changed the accelerator pump, air horn gasket, float valve and seat, etc.  It's been a while since I went through it so I can't remember all I did.  The float was at the recommended setting so I left it alone.

Choke seems to be working fine, fuel pump is brand new Mercruiser...

If it is nozzle drip...how is this fixed?

Thanks

[Shark Racer]

Two things immediately come to mind that cause nozzle drip:

1. Not enough fuel at idle.
This is caused by the idle circuit not having enough fuel flow (Idle DCR + idle tube restriction) or the mixture screws set too lean. What happens is that the engine starts to pull fuel through the main circuit to keep itself running. The fuel comes out in big drips, not particularly boosted as the main circuit is designed to do, so it will run rough and very rich, even though the idle is actually lean.

2. Idle throttle angle too high.
Similar to above, but the idle speed setting has gotten the carb to transition to the main circuit and is pulling some fuel. This is corrected by adding bypass air to achieve the desired idle speed.

To check for nozzle drip, look down the primaries (with a mirror if need be to keep your face from going straight above it), and see if fuel is dripping out of the main boosters like a sink that's just not quite off.

If this is the case, check your idle speed and idle speed screw. If it's really far out, you need idle bypass air.

If that's not the case, try backing the idle mixture screws out a bit. I think the ones on my 78 are around 3.5 turns out, it'll definitely get nozzle drip if they're in too far.

[TommyK]

Although the two scenarios that Shark Racer outlined above are the most likely culprits if nozzle drip is in fact the problem,  I would add one other possibility that is easy to test.

If the initial timing is low the engine will require more primary throttle opening to get the desired idle speed which as Shark Racer has stated can result in nozzle drip. Sometimes adding a little initial timing will raise the idle sufficiently to allow closing of the primaries enough to eliminate the nozzle drip.

[ddkjunior]

Thanks for the advice - it appears to be more than nozzle drip.  I adjusted the idle speed screw so the primaries would close and the mixture screws were still ineffective from fully closed to 3.5 turns out.  I advanced the timing a while back so I could get decent vacuum...about 15 inches was the most I could get...so I think the timing should be good and it runs awesome above idle.

At idle its loading up and flooding quite a bit and when it stalled out I could see fuel seeping out of the base and a little out of the accelerator pump.  Not good on a boat.  Looks like I'll have to pull it and check the float, needle and seat.  Could be getting too much fuel pressure as well but I don't have an easy way to check it.  It has a brand new (not rebuilt) Mercruiser fuel pump and I'm hoping I don't need a regulator or need to try another fuel pump.

Thanks again.

[Shark Racer]

Did you look down the airhorn and verify the boosters were not dripping?

What kind of fuel pump is it? Electrical or mechanical?

[von]

Some aftermarket replacement fuel pumps have to much pressure for a Q jet to handle. Ones I  had were around 9 psi. I've had two of them and finally got an AC Delco pump that was OK at 5 psi. A Q jet usually can only handle about 6 psi depending on needle seat size. The bigger the less pressure they can withstand. That said, if you have a leak between the needle seat and main body, that will cause flooding. BTDT. Sometimes the body seat gasket sealing surface gets eroded or boogered up or sometimes the old gasket doesn't get removed and stays in place then a new gets placed on top of it. Of course a defective float or needle and seat or wrong float setting could cause it too. One other thing; many times people incorrectly install the tiny clip on the needle through a hole in the float arm. That could possible cause a problem with the needle getting hung up and not seating completely. It should hang from the outer edge of the float arm (hard to explain without a pic). Many people including me leave the clip off entirely.

[ddkjunior]

This was a brand new Quadrajet but I knew it probably sat for quite some time.  I tried to leave it alone as much as possible but had replaced the Nitrophyl float with a brass one along with the accelerator pump.  I put the original float back in it and adjusted the level from the recommended 9/32" to about 11/32" just to be safe.  It does not flood now and runs great!  It idles fine but the mixture screws are still pretty ineffective.  I'm guessing it's still a bit on the rich side but starts and runs excellent so I'm going to leave it alone for now.  Thanks for the help.