Carburator problem - I think

[jimrockireland]

A 1983 Mercruiser MCM228 4 barrel carburetor (engine s/n 6285627). Engine runs fine in my driveway on muffs. Idles about 900 RPM's and goes for at least a half hour before I shut it off. On the lake the engine starts right up and after cruising around for a while with throttle high-low and in between stalls when I shift into idle. I have to wait several minutes before it will re-start. Once it re-starts I can cruise up ad down the lake with high and low throttle, but if I put it into idle it stalls and must wait several minutes again before it re-starts. It does not re-start easily as it does when I first start up the engine. It takes some cranking before it sputters to life and begins working again. How should I proceed to debug the problem and can I do it in my driveway without going on the lake?

 

[shaw520]

Several things come to mind,.... a hot engine can "vapor lock" not allowing fuel to return to the intake,..usually combined with an overheating situation. or if its a motor with alot of hrs the timing chain can be stretched and as the motor warms the timing becomes too advance,...usually indicated by the engine struggling to turn over on restart. ..and theres the simple idea that its a bad coil acting up when warm.

 

[jimrockireland]

Engine temperature was solid at about 140 degrees.

 

[shaw520]

Engine temperature was solid at about 140 degrees.

Then I would lean towards one of the latter,.... a stretched timing chain will show signs of the engine struggling to turn over when warm,..(timing is too advanced),..
I would try the simplest first,..swap out another ignition coil,..cheap and easy to do.

 

[jimrockireland]

On re-start the engine doesn't really struggle to turn over. It turns over fine, but just doesn't catch. The engine is a re-build with about 50 hours on it. I was wondering if it could be a stuck float or something of that nature.

 

[shaw520]

Possible,....A stuck float would cause a flooding condition,.. the way to clear a flooded engine would be to open throttle to full open,... after just a few cranks engine should clear and start. You could try that,..just keep in mind a flooded engine can be potentially dangerous in a boat, (excess fuel and fumes in engine compartment)...if you suspect a flooding condition open engine hatch and run blowers for several mins before cranking.

 

[jimrockireland]

I'll give that a try. I was thinking along those same lines. When running the engine at high RPM's a high volume of fuel must be flowing into the carburetor. Then if the float is stuck open when I switch to idle the fuel keeps flowing into the carb and floods the engine. I do smell some gas when the engine stalls. Is there a way to unstick the float if it is stuck open and would it help to spray some carburetor cleaner into the carburetor when engine is running?

 

[shaw520]

Is there a way to unstick the float if it is stuck open and would it help to spray some carburetor cleaner into the carburetor when engine is running?

Probably not,.... its most likely a Rochester Quadra Jet,...without removing the carb from the intake, you can just remove the top of the carb,...check floats and needle assembly. Floats could fuel fouled,...or needle and seat could be clogged or obstructed.

 

[Bondo]

I have to wait several minutes before it will re-start.

Ayuh,..... Yer idle is to high, 'n that's hard on the drive, should be 650/ 700 at idle, in gear, in the water,.....

Are ya advancin' the throttle when ya restart it,..?? if not, why not,..??
It's a Carb, not Efi,.....

 

[jimrockireland]

I was not advancing the throttle when re-starting since I don't advance the throttle when I first start it up. I usually just crank the shift lever (Morse type) 3 or 4 times - return the lever to neutral position - then turn the key to start. I think I read somewhere in the manual that's the procedure for starting. Sometimes when I had the restarting problem I would refrain from the cranking since I thought the engine might be flooded.

 

[Bondo]

Ayuh,.... Yer startin' will be Much easier if ya advance the throttle 1/2 or so,....
Just be prepared to pull it back to near idle, or idle when it lights off,....
Cold or hot,...

Ya gotta go to Wot atleast once on a cold motor, to set the auto-choke,....

Unfortunately,... People have forgotten how to run a carbed motor anymore,....
Back when cars had carbs, this was common knowledge,....

 

[jimrockireland]

Thanks for the advice. I'm going to try and reproduce the problem with boat out of the water. If I can reproduce it I will give your suggestion a try. We had cycled several times with WOT before the stalling and re-start problem emerged. I was also going to try and crank over the engine with the choke wide open. If the engine was flooded this should help clear it.

 

[Blueghost924]

Ayuh,.... Yer startin' will be Much easier if ya advance the throttle 1/2 or so,....
Just be prepared to pull it back to near idle, or idle when it lights off,....
Cold or hot,...

Ya gotta go to Wot atleast once on a cold motor, to set the auto-choke,....

Unfortunately,... People have forgotten how to run a carbed motor anymore,....
Back when cars had carbs, this was common knowledge,....

I really thought I was the only weird person in the world that does this too Bondo LOL. I do this with my 1986 5.7 with the quad-jet bypassing the shift cable to about 1/4 throttle....it light up right away to about 1100 RPMs, and I back it down right away to about 700.

 

[biggjimm]

+1 on advancing the throttle. On my 260, if I don't, it will fire every time after about 3 revolutions, but if I push the throttle to about 1/3 it will fire before I can turn loose of the key. Every time. it's amazing what a little bit of air in there with the fuel will do.

Good luck. Jim.

 

[wahlejim]

...On the lake the engine starts right up and after cruising around for a while with throttle high-low and in between stalls when I shift into idle. I have to wait several minutes before it will re-start. Once it re-starts I can cruise up ad down the lake with high and low throttle, but if I put it into idle it stalls and must wait several minutes again before it re-starts. It does not re-start easily as it does when I first start up the engine. It takes some cranking before it sputters to life and begins working again.

This is all screaming sticky shift interrupt switch. Activates when going back to neutral, sticks for a couple of minutes, releases after the engine cranks for awhile and jogs it loose. I would start there before digging into any carb issues.

 

[achris]

.... Is there a way to unstick the float if it is stuck open and would it help to spray some carburetor cleaner into the carburetor when engine is running?

All that does is put carb cleaner into the fuel stream running into the cylinders. Not one single drop will enter the carb internals.
A stuck float is usually caused by either worn parts or gum from the engine sitting around for a long time (12 months plus, depending on the weather, and the grade of fuel) with fuel in the carb without being used...

The only way to properly fix that is to remove, strip and clean the carb.

Chris..........

 

[bspeth]

Chris wouldn't it be safe to assume a QJet bowl would leak down before giving the fuel a chance to gum up in there. I think I have read that they are notorious for leaking down, I realize conditions and circumstances vary.

 

[achris]

Chris wouldn't it be safe to assume a QJet bowl would leak down before giving the fuel a chance to gum up in there. I think I have read that they are notorious for leaking down, I realize conditions and circumstances vary.

No, it's the Webers that leakdown. The Q-Jets hold the fuel very well....

 

[bspeth]

Roger that thanks.