Mercruiser 5.7 wanting to stall at idle after driving around

[Lou C]

Cliffs is one of the well known Quadrajet experts, he sells better quality parts than some suppliers. Also has a rebuild/restoration program.
I bought an OMC/BRP rebuild kit and it was just a repackaged Sierra kit

 

[Lou C]

You can see the clip I was referring to in this pic....

 

[jimmbo]

Definitely a Non Emission Q-jet

 

[Lou C]

Yep when I start it up it smells just like a 1966 Chevy back there. Which, it kind of is!

 

[todhunter]

The Number that was stamped on to the Carb Body outside the Secondaries?
Being an 87, anyway to tell if it is the Original Carb?

Casting number is 17080561. Not sure if it's original, but doubt it, since that number comes back as a 1980 California Pontiac. It did have the aluminum Mercruiser tag on it, but that was under a bolt and could have been transferred.

 

[Lou C]

Mine's definitely original to the boat, a marine unit. A dead give away is the overflow connection nipple on the left side of the carb body. If the carb doesn't have this, it might not really be a marine unit.

 

[jimmbo]

I have a Q-jet I liberated from a 67 Olds Toronado 425. Back then they used a Clip that went around the Power Piston Tube, that had a Tang to keep the Power Piston from jumping out. A couple of Year later they went to the Phenolic press Washer shown in your pic. On my 67 O-jet, the Secondaries were locked out until engine warm, by preventing the Air Valve from opening until the Choke was fully open. Later ones like yours have the Lockout on the Secondary Shaft, and a Spring Loaded Linkage on Secondary Shaft to allow the Throttle Lever to move with the Shaft Locked.

 

[Lou C]

We had one on our 1975 Olds Delta 88 350. It had the hot air choke, don't think it had the electric heater though. Never had to have that one apart and that engine ran great the 15+ years we had it. I recall growing up in the '70s and hearing that Quadrajets and Autolite 4300s were too complex for beginners to rebuild. In the case of the QJet, that's not really true, there are a few tricks in getting the air horn off but other than that, they are not bad to work on at all. Cliff sells a modified accel pump linkage rod that allows you to take off the air horn without having to knock back the roll pin. Very convenient.

 

[jimmbo]

Casting number is 17080561. Not sure if it's original, but doubt it, since that number comes back as a 1980 California Pontiac. It did have the aluminum Mercruiser tag on it, but that was under a bolt and could have been transferred.

It might not be USCG legal. I doubt an 80s Pontiac Carb is calibrated for use in a Marine Application. Even if the Jets, Metering Rods, and Power Piston spring were changed, the Idle and Off Idle Fuel Circuits could not be easily recalibrated, and same goes for all the Air Bleeds that are part of the Fuel Delivery Calibration.

 

[todhunter]

Pretty sure mine is a marine unit - it has the overflow nipple.

 

[jimmbo]

We had one on our 1975 Olds Delta 88 350. It had the hot air choke, don't think it had the electric heater though. Never had to have that one apart and that engine ran great the 15+ years we had it. I recall growing up in the '70s and hearing that Quadrajets and Autolite 4300s were too complex for beginners to rebuild. In the case of the QJet, that's not really true, there are a few tricks in getting the air horn off but other than that, they are not bad to work on at all. Cliff sells a modified accel pump linkage rod that allows you to take off the air horn without having to knock back the roll pin. Very convenient.

A 75 Olds had no electric heating element on the choke. In 75 there was an Altitude Compensator in a Lot of Q-jets, some also had a Second Power Piston and Metering rod, on the Primary Circuit. In 76 the Altitude Compensator was removed, I know my 76 Olds 88 455 didn't have it, though there was a place for it, with a big hunk of Pheolic Plastic in its place

 

[jimmbo]

I can hear the Stones. I see a Zinc Carb, and I want it painted Black

 

[Lou C]

lol!

 

[jimmbo]

Pretty sure mine is a marine unit - it has the overflow nipple.
View attachment 360667

A 1980's Automotive Q-jet would look quite a bit different forward of the Primaries

 

[todhunter]

@jimmbo , ok so what do you suggest I look for as the issue causing the dribble and/or rough running at idle after running the engine for a while? Seems like you know a ton about these.

 

[dubs283]

what do you suggest I look for as the issue causing the dribble and/or rough running at idle after running the engine for a while? Seems like you know a ton about these.

Most likely clogged idle circuit. If the fuel can't make it through the idle circuit it is forced to travel up to the venturis and the result you see is the dripping. Since that area is not meant to control idle the result is a poor air/fuel mixture resulting in poor idling.

If you close the choke part/most of the way does the engine smooth out and pick up rpm slightly?

 

[Lou C]

Go on to Cliffs site & post up the question on his forum. It’s a pretty active forum.

 

[todhunter]

Most likely clogged idle circuit. If the fuel can't make it through the idle circuit it is forced to travel up to the venturis and the result you see is the dripping. Since that area is not meant to control idle the result is a poor air/fuel mixture resulting in poor idling.

If you close the choke part/most of the way does the engine smooth out and pick up rpm slightly?

Thanks for the explanation - makes sense. I'll mess around with that on Friday and report back. When I had it on muffs Saturday, closing the choke plate any more than it already was made it want to stall.

 

[jimmbo]

Fuel, normally only flows thru the Discharge Nozzles in the Venturies, when there is enough Airflow, to pull the Fuel out, from the proper Fuel Level in the Float Bowl. There is not enough Airflow at Idle, even several hundred RPM above Idle at low load to do that. If fuel is coming out of the Nozzles at idle, it is because the fuel level is too high.
One of the Best Books on Q-jet Theory, Operation, and Modification was Written by Doug Roe. The 1st Edition is better than later Versions.



Later version cover

 

[todhunter]

Pulled the QJ today and tore it all the way down - even pulled the idle air tubes again. I blew out all the passages and chased all the small orifices with wire. There was some crud in the bowl and in the inlet filter that I cleaned out. Someone mentioned the needle valve's wire clip and making sure it is oriented correctly - I remembered after getting it torn down again I took the clip off and tossed it out, per Cliff's book, so that wasn't the cause. I checked the float height - 1/4", per the spec. I also double checked the casting number - it's OEM Mercruiser.

I put it all back together, hooked up the hose, and the engine fired right up and ran fine. But I still think I may be getting nozzle drip. I took a video of what I'm seeing at idle - can someone confirm this is nozzle drip? If so, my next move will likely be checking fuel pressure. Here is the video: