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454 stalling issue?

J

jerry and sons

Member
I have twin 454's 1991. Last seasons I was leaving my marina for the last day of fun for the season, when the motor stalled at low rpms exiting the slip. I did get the engine refired after some cranking, motor then ran fine. I was able to get on plane and rpms at wot were in the range. After returning to the slip I went to back in and the one motor dies again while being shifted in and out of reverse.
Unwinterized boat on Friday, motors start without much cranking, again while backing out of the storage marina to go to permanent slip the motor stalls again. Upon getting the motor refired black smoke came from the exhaust. Ran the boat about five miles all was well. Before entering my marina I played around with the motor going forward and reverse and no issue at all. I pulled into the marina and started to back into my slip and it stalled again. After tieing up I began to play around with the the motor.
It seems like the throttle when pulled all the way back and out of gear the idle drops to 400 rpm or lower. Is there a stop at the carb that could be broken or an adjustment to raise the idle on the carb? Why the balck smoke when refiring motor? (smoke does turn white after a few seconds after start up)
Thanks
 
There is an Idle stop you can adjust on the carb. BUT! Why does this suddenly need to be adjusted? The adjustment screw is usually very tight because of the spring that is on it along with some mild corrosion, I don't think it backed out. You can compare the idle stop on the problem engine to the other one. I think it may be time for a carb over haul. I am assuming this is a quadrajet carburetor.
 
Check the vacuum choke pull off. It should pull the choke butterfly open slightly when the engine starts. It could be binding or the diaphragm ruptured & its causing a vacuum leak.
 
Whoa boys! Before you do all of that stuff, check the carb to see if it's flooding (which it is):

1. Get the motor to idle, even if you have to bump the throttle a tiny bit.

2. Turn one (then the other) idle mixture screw(s) slowly in and observe the results.

3. If the motor speeds up and runs better, it's flooding; if it slows further and tries to die, it's not.

Guess which one it's gonna be!

Jeff
 
Jeff's diagnostic procedure may give misleading results, especially on an unmodified q-jet on a big block. the reason is that the idle speed stop usually is set to where the off-idle system is flowing most of the fuel (sometimes even the main system starts). With the Black Smoke description, I'm thinking flooding, too, but more likely to be from a dirty needle or a "wet" float. If the flooding is excessive, you can usually smell the extra fuel when your nose gets close to the carb.

If you wanna proceed down the adjustments path, this may help:
View attachment Idle adjustments.pdf
 
..."With the Black Smoke description, I'm thinking flooding, too, but more likely to be from a dirty needle or a "wet" float. "

That's still fooding, isn't it, and my test will indicate such.

Jeff
 
Black smoke usually indicates 'excess' flooding and I'd doubt the idle mixture adjustments screws would make a discernable difference with the mixture that rich.

Also, if the transfer slot is exposed to significant manifold vacuum, due to the throttle plates being opened by the screw, the slots will be the major source of fuel (vs the idle mixture ports) so the screws' contribution will be minimal and not likely to be noticed.

Now, if the engine is in good mechanical shape, and you want to back off the idle speed setting to close the transfer ports, then the mixture adjusting screws should do their thing.....just highly unlikely this is the case on an unmodified q-jet sitting atop a big block.
 
Wow. I had a flooding issue last year and never did track it down. I ended up putting an edelbrock on and it runs fine now but I wonder if mine is putting out high pressure as well. Obviously not as high as yours but high enough that the edelbrock works but the Qjet doesn't. I think I will put a pressure gauge on it and just check it out. Will any old fuel pressure gauge at autozone be good enough to check pressure?
 
Probably not too much pressure but a soggy float or worn needle & seat.

the AZ offered fuel pressure gauges should be fine - just be careful to ensure no leaks when you plumb the gauge in.
 
2 new floats and 2 needle and seats later still the same....not to hijack the thread. I would just put it on for diagnostic purposes only.
 
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