Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

The real truth about fixing 'nozzle drip'

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makomark:
I have a pair of stock marine 454's in my boat. Both topped w/ Qjets which have varied over the years. When set to spec idle, 700rpm-in fwd, they seem to continually exhibit "nozzle drip." Q-Jet casting is 17080560

After getting Cliif's book and reading it, I'm having a bit of trouble in formulation a way to 'fix' this issue. (to be honest, it has only become an issue since the gas dock prices peaked this year.) My original plan, based on many postings and several published books, was to 'drill the butterflies'. Based on my new recent expanded education, I don't see how this will fix the drip.

This is based on the fact that the volume of air required to maintain idle, though flowing thru the throttle plates, vs around them, still needs to flow thru the venturi area. The revised plan says to check the old cores in the basement (much closer than the ones in use) to see how much of the idle bypass circuitry is there and size it to make it work for my setup.

I can see where resetting the butterflies to their optimal position will help make the off-idle transition better and that should happen going down either path.

So, has anybody ever solved their nozzle drip by drilling holes in the primaries? Other thoughts on this topic?

tnx    mark

Cliff Ruggles:
Drilling the throttle plates is often done to help reduce the angle of the plates at idle.  Although not quite as effective as adding idle bypass air, it can help in some situations. 

It will not help if their is not enough idle fuel to the engine from the idle system.  This is a common problem with Q-jets, as the idle tubes are often either partially or fully blocked/restricted with dirt/debris from many years of use.

Insufficient idle fuel is also caused by a "low" vacuum situation at idle speed, from either not enough ignition timing, installing a larger than stock camshaft, lowering the static compression ratio, or simply not a high enough compression ratio for the combination of parts.

Before adding any holes to the throttle plates, I would try removing a small manifold vacuum hose and seeing if it helps the situation?  The hose that supplies vacuum to the choke pull-offs is easily accessable on Marine carburetors.

If the additional air allows immediately causes the engine to speed up, and the throttle plates can be lowered enough to stop the "nozzle drip", then adding a couple of small holes to the throttle plates will help.  I would make sure that the idle mixture screws still have full control of the idle A/F ratio from rich to lean before making the modification.  It is common to have to increase the size of the idle down channel restrictions, and/or open up the idle tubes some, in conjunction with adding more idle bypass air.  Hope this helps some.....Cliff

makomark:
Thanks for the feedback, Cliff!

There's plenty of vacuum at idle, even though the CR was lowered a bit when the engines were rebuilt. The cams are the 'standard' marine cam made by many sources; 214/218 deg duration @ 0.050" with valve lift of 0.461/0.480". Based on memory, at idle, vacuum is less than 16" .

I do believe the base plates are set up for bypass air, or at least enough to warrant finishing the job. I'll take your suggestions and thoroughly go thru the idle circuits this winter. There may be some crud down there as I've never had the tubes out (though they have been poke out with copper wire and blown out afterwards.

Is there a recommendation for a receipe you'd offer for the marine 454's; I'm looking for economy, not performance. Three mods from stock have been done: 1) idle channel restriction opened to 0.055" per OEM service bulletin, 2) secondary rod hanger changed from F hangers to P hangers, 3) power piston spring is now edelbrock gold (starts at 6", full up at 4").

Finally, does installing the bushing kit on the primary throttle shaft require any 'extra' steps to maintain the USCG approval on use of these carbs?

Cliff Ruggles:
Nothing at all wrong with installing bronze shaft bushings.  They are self lubricating, and much less likely to seize up in the Marine environment than not having them installed.

Based on the CID, lowered compression and cam specs I would add some idle bypass air.  As with any other modification, start out small, you can always go back and open it up some.  If pulling off a small manifold vacuum hose helps, then start out with about .080" bypass air on both sides.  You only need enough to lower the throttle angle enough to stop the nozzle drip.

It would be a good idea to clean out the idle tubes first, to make sure that they aren't adding to the problem......Cliff

makomark:
Regarding the primary shaft bushings...

My limited understanding is they go thru a process called "stabbed and slabbed" (could be backwards) to ensure the leak rate is within the USCG specs. I can't say I have every seen a shaft that has had this procedure done to it. this got me to thinking its end result may be in the throttle plate where the bushings would go, hence the question.

Thanks again, not only for the response but in establishing this forum.

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