Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

17082403 Marine: idle circuit problem

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BarryK3:
Hi all,

I am having trouble getting a smooth idle on one of my engines: 1990 Crusader GM 454. This is one of the same carbs described in an earlier thread that was rebuilt recently by a local shop. It is the OEM marine carb.

I can't get a smooth idle below about 850-900 rpm. The carb works perfectly except for the idle. I get a very stable vacuum reading of about 18 inches at 1000 rpm. It stays pretty stable down to 850 rpm with about 17 inches vacuum. If I turn the idle speed screw down further, just slightly, it will get very rough and drop to about 650-700 rpm. The vacuum needle fluctuates between about 10 and 15 inches. I get the highest vacuum with the idle mixture screws completely closed (lean).

I removed the mixture screws and sprayed carb cleaner inside, also sprayed down into the brass idle tubes, then blew out these passages with compressed air. This did not help the problem at all.

I then tried closing the throttle plate further. The idle speed screw setting for 850 rpm was 3 full turns in from the stop. I backed the speed screw out all the way so the throttle was on the stop, fully closed. I then turned the mixture screws out about 6 turns each to allow more fuel. The engine started and ran very rough at about 450 rpm. I adjusted the mixture screws but could not get stable vacuum. The screws had little to no effect. I ended up going back to fully closed mixture screws and speed screw set for about 850 rpm.

I can live with an idle of 850 and a bit rough but would prefer 800 and smooth. The manual calls for 650-700 in forward gear which should be about 750-800 in neutral.

The other engine has a smooth idle of 800.

I set ignition timing the other day carefully to 10 deg BTDC per the shop manual.

I ordered a marine rebuild kit from Cliff today.

Any idea what is causing this behavior?

Thanks,
Barry

Kenth:
Clogged idle fuel tubes. Did the shop specify the idle tubes size?

tayto:
i rebuilt a marine carb last year. I tried using thin wire to unclog idle tubes but ended up pulling them and they were PACKED with crud.

BarryK3:
Thanks guys!

Did you remove them as shown in this video? He uses a small punch to push the tube down, then a drywall screw in the collar to pull the idle tube and collar out.
https://youtu.be/_UsWCvs7mO4

No, I don't know the tube size. I doubt the local shop touched them. I assume they are the originals.

I will wait until I receive the rebuild kit, then go through the whole carb.

Appreciate the help!
Barry

Cliff Ruggles:
Good move ordering one of my Marine rebuild kits.   It will have the correct Marine parts not automotive parts as are used in every other kit I've seen in at least the last 10 years.

Marine carburetors lead a rough life, little use and long periods of being dried out.  This "varnishes" up the idle tubes.  At a minimum they need "rodded" out to clean the debris out of them. 

I see EXACTLY what you are experiencing all the time and as Kenth mentioned it's almost always due to plugged up idle tubes combined with the wrong parts used to build the carburetor.

I'd also add here that real Marine units are NOT emission calibrated, so use tiny upper idle bleeds and tiny idle tubes, making them even easier to plug up and not delivery adequate fuel to the mixture screws. 

When I build them here in the shop I find them difficult if not near impossible to remove without smashing the ends over as Marine castings don't allow much room below them.  It is best if you ar able to leave them in place and clean them out with a piece of spring wire.  You will get a new accl pump return spring in your kit.  Straighten out the old one and shove it down thru the idle tubes.  Make sure it goes clear thru them as then are only a few thousanths bigger on the small end than the diameter of the wire.

You will run into a few where it's impossible to get anything down thru them and they will need removed and manually cleaned with a precision drill bit or replaced.......Cliff

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