Carburetor problem or something else???

I

imspacemancraig

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Joined
May 24, 2009
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31301
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OK, 1986 Crusader model 270 (350) engine. After running for about 30 minutes my port engine starts to "fall" and catch itself then goes away for a bit and then does it again so forth. Like it isn't getting enough gas and the catches itself back

When I push it above +3200 rpm, it does it "harder", more pronounced and more frequently. If I back down to 3000, it does it very slightly, but not as often.

Now, if I back all the way down to go through a no wake zone once this start happening and try to get back up on plane after, when I approach 3000 rpm, it has a big "fall", catches itself and then operates like above.

We have replaced the fuel pump twice, so pretty sure that isn't it. Replaced the fuel filter multiple times and nothing there indicating anything. Checked the pick up tubes in the tanks and replaced the screen on them, doesn't look like anything in there. The fuel lines from the filter to the pump and pump to carb have been replaced. Carb has been rebuilt a year ago (problem was then too and we thought this would do it).

My thinking is either there is something in the fuel lines somewhere between the tank to carb, or more likely, could it be something inside the carb? Here's where my knowledge of the carb is light, to say the least...

The carb is a 4 barrel quadrajet. So my thinking is, could it be something the way its tuned or possibly the accelerator pump or something along those lines? Because it only does it when I am accelerating or when I am running above 3000 rpm where I would expect more fuel to be pushed. It does take the initial 30 - 40 minutes to start, but once it does, it never really goes away... That is why I think either something in the line that takes time to clog or in the carb itself.

Any thoughts would be appreciated.
 
Have you cleaned the filter in the carb? Should be where the fuel line goes into the carb.
 
Yes, but not recently. May check this this weekend. I am almost thinking it may be running too lean and once the engine gets warmed up, its not getting enough fuel...
 
Check fuel volume output not pressure. Run the engine with the fuel input to the carb directed into a bucket. You should get a good stream. If there is a clog it will show a dribble. If you get a good stream look elsewhere, your fuel system is fine.
 
Where else would you look. My boat was doing same. Changed filter at the carb and the fuel filter. Also I changed fuel pump.So what is next step
 
Where would the anti siphon valve be located. I have two tanks if that makes a difference. Inboard engines
 
Craig...I had the same problem with my newer-to-me Santego. The problem was at the A/S valve-some crap made it past the screen on the pick-up tube to the A/S valve. Brian..the A/S valve is usually located on the top of the tank directly in front of the elbow connected to the tank-at least to my limited knowledge-your results may vary. Each tank should have its own A/S valve.
 
Ya, we replaced the screens on the pickup tubes and replaced the a/s valves too. The old ones, you couldn't even blow through without bursting a brain cell. I may pull them out again and see if anything got back in there. I am either thinking the carb needs to be tweaked, or there is something in the tank or lines.

Brian, the valve should be really close to the tank. Looks like a hose nozzle end that the fuel line slides over and has a hose clamp on the hose. Just has a small ball inside it so that if the fuel line breaks, the ball will slip back and prevent fuel from spilling out into your bilge.
 
Could also be a ignition coil that is on its way out or the electronics in the distributor
 
Check fuel flow/volume as mdoherty suggested. Then check coil like saber said. Get a spark indicator that you put on the top of the coil.

Carb problems are too much fuel or not enuff. Next time the engine starts running real bad, stop the engine and look down the primary side of the car and have someone pump the throttle. You should see a stream of gas from the accelerator pump. No stream means the float bowl is out of gas or accelerator pump is bad.

Too much fuel can come from the well plugs leaking. They are the tubes with solder you can see when you turn the carb upside down. Add some JB Weld to re-seal the plugs.

The other way of too much fuel is the secondary 'trap doors' tension spring is too loose and they're opening too soon and/or too much. The secondaries shouldn't be opening all the way. Have someone run the boat as you close the secondary plates by hand...just a little at a time. You'll be lowering the air mixture.
 
Not the ignition coil or distributors. Both are new full electronic replacements as of 4 weeks ago. This issue was happening prior to them being installed and continues, though not the reason these were put in. These fixed a different problem on the starboard side which I am happy to say is fixed.

My mechanic swears this issue on my port engine is fuel related. He put a new fuel pump on thinking that we weren't getting enough fuel volume, but that didn't fix it. He is going to look at the fuel lines and the float on the carb, as well as the filter on the carb (I won't get up there to see what he does).
 
My 2 cents worth. I am not hearing any other symptom but 'falling off'. Is there no back firing or miss firing? If not, I do not believe it would be a fuel supply issue. If you were reducing fuel but not closing the throttle, you would be running a very lean mixture which would cause back firing. I am not hearing any rough running engine. This would indicate possible fouled, miss firing spark plug, for what ever reason, or possible a stuck/hanging valve. So I am leaning towards an exhaust blockage issue or something is 'loading' down the engine
 
No back firing or miss firing. It just falls/stumbles and then catches itself back up. The spark plugs have all been replaced as of early June. At the time it was really running rough so we pulled them out and they were full of gas and black. Once we replaced them she ran smooth again, but then this problem came back. Had the issue last year too but it would take longer to appear.

She idles great, not rough at all. Never dies. It feels like it starves from no fuel or fuel blockage and then catches up. I am leaning towards the float not set properly. If it were a stuck valve, I would think it would happen consistently. I don't see any smoke or smell anything unusual when under way.

I am no expert, but I would think too much fuel would cause a back fire, not too lean or can that happen either way? I don't get any back fire from it. When it starts happening, if I back down on the throttle it lessens, if I push it, then it is more prevalent. Also, taking off from idle, when I get close to 3k rpm, it will stumble and then catch up. This is why I keep thinking the fuel is either blocked somewhat or the float isn't set that that it runs out of fuel and then stumbles. Its probably really close to being set right because of the way it stumbles and catches up...
 
Could very well be float too low or float valve sticking. Could still be secondary plates opening too soon. Running smooth at idle eliminates well plugs leaking.

Does it run worse as you give it gas and smooth out as throttle stays constant. That would be bad accelerator pump.
 
I would swap the carb to the other engine to see if the problem moves.
Most carb problems I ever had were from crap in the carb itself. While you have them off, try spraying the crap out of them with carb cleaner and turn it over. Sometimes that will do the trick.
If the problem moves with the carb and cleaning it doesn't do the trick it might be time for a rebuild.
 
quote:
Originally posted by imspacemancraig

.............. My mechanic swears this issue on my port engine is fuel related. He put a new fuel pump on thinking that we weren't getting enough fuel volume, but that didn't fix it. He is going to look at the fuel lines and the float on the carb, as well as the filter on the carb (I won't get up there to see what he does).
Click to expand...

Apparently, your "mechanic" is a person who replaces parts until the problem goes away rather than using tests to determine the problem.

A better mechanic can probably figure out and correct your problems.
 
Trouble now days with high cost of labor, it's often cheaper to buy new than go thru all the tedious test procedures.
 
I would take a good look at the exhaust as I just heard today about some boats that have problems with mufflers that internally fall apart after a number of years.
 
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