17082403 Marine: idle circuit problem

[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

[BarryK3]

Thanks so much Cliff!!

Barry

[Walleye4Days]

Another thing I'll add, is in regards to your engine. Even though the SELOC manuals and OEM guides suggest 10 degrees BTDC for timing, keep in mind that that is for a brand new - off the line - engine,  or at least a rather fresh engine with next to zero wear in it... Meaning, 10 degrees BTDC isn't accounting for stretch in the timing chain/timing gear wear, pushrod and lifter wear, cam lube wear, rocker arm wear, and all else that ends up causing the timing to actually need a little more advance than OEM original spec. Usually needing a couple, to a few, more degrees of advance than listed in the original manuals/oem specs to account for the wear of the engine and components.

This is ESPECIALLY true for Marine Engines, and other high utility use motors like in Tow trucks,  RV's, and other high torque application use. In a boat, any time the vessel is underway, there is full load/strain being put on the engine full time. Unlike a truck, car, or other land vehicle where there's coasting, varying degrees of load through acceleration and cruise on flat surfaces coasting down hills and etc (which is why highway miles are easier in a car) a boat engine is under full load at all times the vessel is moving, like its climbing up a hill nonstop.

Think of a truck with a trailer behind it going uphill. That engine is taking the full brunt of that load to keep pushing the weight uphill. That's what it's like for our marine engines anytime our boat is moving at all. Thus, the timing chains/gears and all else that is within the engine that gets rather used on anything past a couple hundred hours, more often than not ends up requiring a tad more initial advance than normal (by a couple, to a few, degrees more than OEM spec).

The reason I bring this up, is because that can have an impact on your idle not being able to hold steady on lower RPMS, because when you think there spark is igniting per intended, (because you have it set at OEM spec) it's actually a little delayed than OEM spec actually intends. This results in subpar idling at regularly low RPMs in addition to the carb issues that can cause it as well. Not saying this is your problem, but just something to keep in mind and for others that see this in the future to be aware of.

I had this EXACT issue on my SBC 350 as well as my BBC 454. I kept having a rough idle around 650 to 850, but around 875 and up, it purred like a kitten. Just ended up being that because I had around 300hrs on the 350 and around 1100hrs on the 454, I ended up having to advance my initial/base timing to around 14 BTDC on the 454 (up 4deg from Mercruiser's 10 degrees) and had the 350 at 11 BTDC (up 3deg from Volvo Penta's 8 degrees). All because those timing specs are based on engines with little to no wear within the cam, valvetrain, and related components and pieces that makeup the crucial flame/ignition timing.

Sorry for the book, just figured I'd offer the info in the rare case that this may help you out like it did me.

[Kenth]

I have found no reason to alter the factory ignition specs with the idle circuit adjusted for today ethanolblended fuels.
The same goes for hotter than standard camshafts with less pressure difference between atmosphere and intake vacuum.
Feed the engine with proper amounts of combustible mixture and the factory ignition specs will do just fine.
Bandaid a too lean mixture by setting ignition out of the scale will not make the full potential of the engine.

FWIW