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Author Topic:   Quadrajet dripping after engine shut off
Dirt Full Roller

Total posts: 55
posted February 24, 2003 09:17 PM
sbc 355 with 750 quad. Engine idles fine
and runs fine, no problems, never misses a beat. However, about 1 minute after I shut off the motor the carb starts dripping fuel from the right side primary discharge down
onto the butterflies. It will drip for 2-3 minutes and when i restart i get the black smoke effect.

I have set the float level really low, 1/2 inch and replaced fuel inlet needle and set.

Also, after I shut motor off, if I quickly disconnect incoming fuel line hose, I
don't get the drip.

Any ideas on what the problem might be?

RC Racing
Dirt Maniac

Total posts: 144
posted February 24, 2003 10:03 PM
You may already be aware of this but just in case .....

Try to get forum member KPLugnut's attention on this. He's the resident SME (Subject Matter Expert) on carbs.

You might want to run a search (keyword carberator) and look for his name and profile in the posts.


Dirt Freak

Total posts: 408
posted February 25, 2003 10:01 AM
Sounds like a classic case of quadrajet vapor lock to me. The fuel bowl vent might be partially restricted enough that when the heat from the intake boils the fuel in the bowl enough, that it will force it out the nearest route.
If it's only coming from ONE discharge nozzle, then you make have the other one plugged and not know it, too.
I'd warn you also that setting your float THAT low is going to burn a piston when racing since the carb will be emptied completely of fuel when those secondaries kick in.
And trying to correct it that way is a crutch and not the real fix to the problem.

Your best bet might be to find a way to insulate the carb from that intake heat (spacer, etc), as well as making sure your bowl vent system is functioning properly. Some quads have adjustable deals there where you can help fix that. It was an attempt by Rochester to help fix the vapor lock and fuel boilover problems some cars had.
Also, it wouldn't hurt to verify your fuel pressure. Quads do NOT like more than about 6lbs of pressure.

Hope that helps,

Dirt Full Roller

Total posts: 55
posted February 25, 2003 10:23 AM
The fuel bowl vent has a thin piece of metal
inserted vertically in it, so not to allow
something to enter, for example a small ***** . I guess i could remove that. None
of the discharge nozzles are plugged as they
squirt much fuel when engine is reved.

About the float, what should the float level be set at? as I only set it this low to rule out a float level problems.

KP, even though I have the float level set so low, there still seems to be a lot more
gas than expected in the fuel bowl when i
remove the air horn. could i be having a
problem with needle and seat, as fuel presure is lower than 6psi?

Dirt Freak

Total posts: 408
posted February 25, 2003 11:32 AM
If the fuel pressure is lower than 6lbs, you won't have a problem with the needle/seat unless of course the float itself is either not allowing the needle to close properly (a bind of some sort) or if the float is junk and is taking on fuel allowing a much higher than normal fuel level in the bowl.
On float levels, since this is a racing application and not street, generally starting with a measurement somewhere around 1/4" or so will be a good starting point.
Again, some quads had very different fuel venting systems than others, especially the ones around the years where they changed to the "smog" type electronic quads. Right before that, they started using things like power piston limiting screws, and other weird stuff that has no place in a racing carb application.
Another thing that comes to mind is to make sure you have exactly the right gasket on there. Some carb tops had slight variations in the booster/squirter areas where you could get a real bad leak like you're describing if the gasket isn't exactly right for that carb.....just a thought anyway.

Hope that helps,
Dayton Umholtz, owner
Kinetic Performance

Dirt Forum Racer

Total posts: 82
posted February 25, 2003 07:00 PM
I agree, it sounds like heat boiling fuel out of the bowl. A phenolic spacer would help this problem. In regards towards your statement about fuel level in bowl still high than what you thought it would be, after you lowered the float level. I have seen many times where the wire clip that holds the needle to float is ran through one of the holes in the float arm. This will cause the needle to hang-up. The needle sould hang freely from the tab of the float.

Dirt Full Roller

Total posts: 42
posted February 25, 2003 07:53 PM
I have seen the 1/2 moon shaped bail(which the float pivots on) not be opened up enough.
This prevents the float from being held down properly...Also NEVER touch the needle with your fingers..

Dirt Full Roller

Total posts: 42
posted February 26, 2003 08:11 PM
After rereading post I noticed he said the drip was from one side....This may indicate a cloged air bleed causing a siphon effect..
...Once in a can free up a passage by reving the engine and covering the opening of the carb with your hand to the point of "near stall" and then givin' it a good rev (to clear out the rich condition)

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