Author Topic: QJ wanting to stall at idle - marine application  (Read 1417 times)

Offline todhunter

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QJ wanting to stall at idle - marine application
« on: May 19, 2022, 07:36:06 AM »
I've got a QJ on a 1987 Mercruiser 350 engine - casting number on the carb is 17080561.  The boat/engine/carb is new-to-me, so I'm not sure of it's history.  I rebuilt the carb over the winter using a kit sourced from Cliff and followed his book and a thread on a boat forum that was put up by a guy who also followed Cliff's book.  I did pull the idle air tubes out and cleaned under them, then put them back.

The engine has all fresh ignition components and the timing is correct.  It has a new water/fuel separator filter, new fuel lines from tank to fuel pump, and before starting I removed as much of the old fuel as possible and have run at least 2 full tanks (37 gallons) through the engine.

For a description of what it's doing:  on cold start it fires right up with throttle about 15-20%...if it's sat more than a couple of days, I give it one full throttle pump before cranking and that seems like enough. I can let it run a few minutes then pull the throttle back to idle and it is happy at about 600 RPM. After driving the boat for 30 minutes or so (at roughly 4,000 RPM) and pulling back to idle, the engine wants to die. If I get back into the throttle, it's fine (maybe some very minor lower RPM hesitation that it works through) and it will run for as long as I want it, but coming back down to idle, it wants to die again. If it dies, it becomes difficult to get started again - sometimes holding the key over for a long period (10 seconds) can get it to gradually catch and start running again, but I have to get off idle quickly to keep it from dying again. It did this last weekend when putting the boat back on the trailer and while attempting to re-crank it, it smelled like it was flooded (I did not pump the throttle during this attempt).

The next day I ran it on the hose in the driveway and started it with the spark arrestor off, and it started fine. While messing with it, I was moving the choke plate by hand and if I started closing it, I got the same symptom where the engine would want to die. When I did the carb rebuild, I did not replace the divorced choke thermostat, and honestly it looks kind of rough - wondering if I should replace it?  Anyhow, I zip-tied the choke linkage for the primaries open on the carb and ran it the next day. It maybe ran better (maybe it was in my head), but when putting the boat back on the trailer again, I got the same symptoms of it wanting to die.  Another thing I noticed when running it in the driveway was that with the boat at idle, holding the choke open and looking down in the carb with a flashlight, some fuel would dribble out of the center of the venturis.  Guys on the boat forum said this is likely what is causing my issue - what are your thoughts?

I've got tomorrow off work and plan on pulling the carb off the boat and opening it back up.  Based on the boat forum's suggestions, I'm going to check the float height and also look for any trash that may be preventing the needle from sealing on the seat.  Appreciate any guidance or thoughts you may have to help me solve the issue.

Offline Kenth

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Re: QJ wanting to stall at idle - marine application
« Reply #1 on: May 19, 2022, 09:16:01 AM »
Mounting the needle lift clip in one of the holes on float arm will make the needle bind in seat causing flooding/nozzle drip.
Clip shold be mounted from behind of float arm.

Offline todhunter

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Re: QJ wanting to stall at idle - marine application
« Reply #2 on: May 19, 2022, 01:30:16 PM »
Thanks Kenth.  Someone on the boat forum mentioned that, and that's one of the things I'll check for when I open it up tomorrow.

Offline Cliff Ruggles

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Re: QJ wanting to stall at idle - marine application
« Reply #3 on: May 20, 2022, 03:04:14 AM »
Float height is another critical point for Marine units. 

Also make sure that the fuel inlet seat isn't leaking at the gasket under it.  I get  a lot of carburetors in here leaking under the fuel inlet seat.  A small leak there will cause all sort of issues at idle speed, but fine past idle because the fuel is leaving the carb faster than the leak. 

Either way you have a fuel control problem which will be related to the float, needle seat assembly or fuel pressure it too high........Cliff

Offline todhunter

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Re: QJ wanting to stall at idle - marine application
« Reply #4 on: May 20, 2022, 12:07:53 PM »
Thanks Cliff.  I pulled the carb apart today.  Now I recall when I rebuilt it the first time I took the book's advice and got rid of the little clip on the needle valve, so that wasn't the issue.  There was a small amount of sediment in the bowl and the inlet filter had some in it too, so I tore it all the way back down (even pulling the idle tubes) and cleaned everything and put it back together.  Float height checks out at 1/4" inch.

The fuel inlet seat is the new one that came in the kit, and has the crush washer underneath it.  How do I test this?  Hand operated vacuum pump?

Here's a short video I took after putting it back together and firing the engine up in the driveway on the garden hose:  https://www.youtube.com/shorts/s1D82uQabrw

If I'm understanding correctly, what I'm seeing is that the nozzles are still dripping, correct?

Offline Kenth

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Re: QJ wanting to stall at idle - marine application
« Reply #5 on: May 21, 2022, 10:13:56 AM »
Check for 4-6 psi of fuel pressure.
More than 6 psi rises the fuel level to the point of nozzle drip, idle instability, over-rich low-end mixtures and hot-start problems and even flooding can occur during idle and slow-speed driving.
One cure for too high fuel pressure is to lower float settings some.

Offline Cliff Ruggles

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Re: QJ wanting to stall at idle - marine application
« Reply #6 on: May 23, 2022, 05:43:16 AM »
Did you try backing out the idle mixture screws to stop the nozzle drip? 

Offline todhunter

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Re: QJ wanting to stall at idle - marine application
« Reply #7 on: May 24, 2022, 05:40:04 PM »
Cliff - I haven't tried that yet.  How should I do that - evenly on both sides, 1/4 turn at a time and see if the drip stops?  How many turns is too many?

Offline todhunter

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Re: QJ wanting to stall at idle - marine application
« Reply #8 on: May 24, 2022, 05:44:01 PM »
Also, I haven't had the chance to check fuel pressure yet.  But, I did run the boat last weekend and it had the same symptoms after slowing down from a long-ish run.  Also it did something new that I haven't experienced before - as we were running the boat at one point I could feel it start to loose power.  I backed the throttle down some, and it seemed to steady up, then when I tried increasing throttle, it was unresponsive.  Ultimately it just bogged itself down and died.  I let it sit for about a minute and it fired right back up, then we slowly finished our trip back to the ramp at a slower speed and it did fine.  I'm guessing it's all related, and all symptoms of the flooding.

Offline Cliff Ruggles

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Re: QJ wanting to stall at idle - marine application
« Reply #9 on: May 25, 2022, 03:51:28 AM »
Loosing power on a hard steady run also indicates a fuel delivery or fuel control issue with the float/needle/seat assembly.  This assumes the fuel filter is installed correctly and not plugged up.....