The real truth about fixing 'nozzle drip'

[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.

[Cliff Ruggles]

Marine carburetors are required to have an approved flame arrestor in place, that's about all that a USCG Boarding Officer is every going to look at on your engine.  They will check to make sure that you have a working ventilation fan in an enclosed engine compartment.  At least in regards to engine room complicance on small I/O watercraft.  Of course vessels are catagorized by length, and type, so make sure you check specific regulations that apply to your vessel.

In any case, we recomend to install bronze shaft bushings on all builds.  The factory clearances between the primary shaft and throttle body are relatively "loose", and they only get worse with time. 

There is no need to install bushings on the secondary shaft, as the throttle plates are designed to be self centering and seal off when closed.  The shaft must "float" slightly for this to happen.  If/when secondary shaft bushings are installed, this more often than not tightens up the shaft to a point where the throttle plates will NOT seal off effectvely.......Cliff

[makomark]

Actually, the knowledge of most of the 'kids' that perform this role is pretty high. My query was based more on USCG compliance (read this as insurance/investigator/surveyor)  than a boarding related check.

I know the carb has at least two requirements on it - the vents must 'drip ' internally (the infamous holley inverted J tubes) and if flooded, must not permit more than 5cc outflow in xx minutes (i can remember the specific value). The second item is the root for 'stabbing and slabbing'. BTW, these are independent of the vessel size; they are based solely on the fact that there is a permanently installed engine (inboard or I/O).

I'm sure you would have seen any difference between a true marine Qjet and the auto types when disassembling the base plate. If you haven't noted any, then I'd suspect my info doesn't apply to the qjets.

thanks, again,     mark

[makomark]

Correction - the phrase is "grooved and slabbed". I found this link:

http://www.boatsunlimitedny.com/tt_marinecarb.php

which has the same text as one of my old Holley catalogs.

If I get time, I'd take out the shaft from an old baseplate that is from the marine Qjet and post a picture. Hopefully, having a reference, one can determine if Rochester did anything different.

[bigvette1]

Like to comment and jump on the issue of "dripping fuel" at the boosters...
1st - Is this acceptable to drip at idle???

2nd - The carb is new.y rebuilt so that should not be an issue.

3rd - Vacuum at idle is 17", the CR is 9.25:1 an the cam is a Crane 218/218,  LCL is 112 and lift is .500 so that appears not to be very radical.

Next as a note -- the carb at idle is sucking enoromus amounts of air - loud to say the least.  As a point to prove, I installed an Edlebroch 1901 that I had on the shelf and the amount of sucking air is nothing compared to the carb we are questioning.  The vacuum at idle is the same 17-18". 

Last, if this is not a normal condition and assuming the float is set right - where do I go to approach this issue.

All comments are welcomed.  Thanks

[bigvette1]

Let me add something that maybe useful.  After running the car for sometime and pulling the carb off the floor of the intake manifold is wet with gas, maybe even some oil being sucked in.  I thought this was supporsed to be dry for all purposes. 

FYI

[Cliff Ruggles]

Nozzle drip is directly related to the throttle angle of the primary throttle plates at idle speed. 

Things that cause the throttle plates to be high enough to start pulling fuel off he boosters include low compression combined with big camshafts (low vacuum at idle).

Idle mixture set too lean. 

Retarded ignition timing, or at least not as much as the engine wants/needs for the combination of parts.

Not enough idle bypass air.

Combinations of all of the above.

The "sucking" sound at idle is often produced by the air traveling across the two slots in the airhorn that lead down to the idle bypass air holes in the baseplate.  Nothing abnormal with that, some will make a high pitched sound, and it varies some for different applications.

The first thing to determine with nozzle drip, is if you have control of the idle A/F ratio with the mixture screws? 

The mixtures screws should slow the engine down considerably when turned in the clockwise direction.  They may not kill it clear out, but going lean with them should show some control.

Make sure the engine is fully warmed up at idle.  Adjust the idle mixtures screws for the best idle quality.  We do this here by backing them out, 4-5 turns for the 10-32 screws and 8-9 turns for the fine pitched metric screws.  Most carburetors will end up about 2 1/2 to about 4 turns out with the 10-32 screws, and 4 1/2 to about 7 turns out with the fine pitch metric screws.   This "normal" range can be effected by the size of the holes under the mixture screws and how much taper and lead the idle mixture screws have.

Late model carburetors use small holes under the screws, and long/tapered mixture screws, so they require quite a few turns out from seated to get a lot of idle full flowing.

Slowly turn them in one at a time until the engine speed changes or slows, then back about 3/4 to 1 full turn.  Repeat for the other screw, then balance them out if/as needed.  Set the idle speed screw so the engine is around 700-750 rpm's. 

If the car is has an automatic transmission, hold the brake and place the transmission in gear, and observe the drop in rpm's.  If it drops more than about 200rpm's, raise the idle speed and add some more idle fuel, or both until you get the best idle quality in and out of gear.  Hope this helps some?......Cliff

[bigvette1]

Thx Cliff, explains the situation and where to go,  JP

[Cliff Ruggles]

In any and all cases, set the idle mixture screws for the best possible idle quality in and out of gear.  Lean settings will cause nozzle drip, and really "stinky" exhaust, which is more eye burning that a richer setting.

This often leads hobbyists to believe the mixtures are too rich.  Lean mixtures are simply very difficult to burn, and when we don't put in enough fuel in PPM for complete combustion, a LOT of stinky fumes exit the tailpipes.  A slighly richer mixture, in contrast will have a more complete burn, and a slightly "rich" smelling exhaust.  A slightly richer mixtures also requires less spark lead, and works better with blocked heat crossovers, or none at all.

A common mistake made when tuning, is not allowing the engine to complete heat-soak before setting the mixture screws.   The engine should be fully up to temp, and have been driven considerable before making final adjustments.

Ignition timing plays a BIG role for idle fuel requirements as well, just 1 or 2 degrees with some engine/cam combinations can make a tremendous difference in idle quality, in and out of gear, as each set-up has a "sweet spot" for the timing and fuel curves, expecially at idle speed.

Also keep in mind, that nearly an aftermarket camshaft is going to have different lobe positions, LSA, and duration than a factory camshaft.  They most often reduce idle quality, just a by-product of their designs, as it takes more duration and more off seat valve timing to make more power in the upper rpm ranges.  When doing so, idle quality and low speed power suffers, no way around it.  With this in mind, fully expect to have to increase ignition timing beyond factory specs, and add some idle fuel, or both, for best results in all areas........Cliff