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Rochester carb issues

"Tom makes a good point, regar

"Tom makes a good point, regarding the factory plugs leaking. This is indeed a common problem that affects the Rochester carbs and should be addressed when the rebuild is performed.
This does not result in the dripping fuel problem that has been reported, but could possibly add to the Oil/Fuel dilution issue that was mentioned.
Another problem that is sometimes seen is loosening of the Idle tubes. As was mentioned previously, these tubes meter the fuel that flows through the Idle Circuit. If one or both of the tubes is not firmly press-fit into the body, fuel is permitted to flow around the tube, thus circumventing the metering effect, and may result in excess flow and dripping.
It is important to note, that the Q-jet carbs do not operate like their (Holley, Edlebrock, ect.) counterparts. Their Air-demand design is, in many ways, backwards to most well-meaning mechanic's comprehension, and imparts a wide range of suggestions that may or may not be relevant to the solution. (Sorry, just telling it like it is.)
Unlike most carburettors, the idle circuit on Q-jets is adversely affected by a low or poor manifold vacuum. Thus, a weak vacuum strength will often cause an over-richening of the Idle Circuit and may result in the reported dripping problem. I read some helpful advice from another member that suggested using a vacuum gage to assist in diagnosing the issue. I am inclined to "second" that advice.
You have the advantage of one correctly running engine and carb that can be used to compare measurements with the ailing one.
Another important comparison is fuel pressure, as the discussion has touched upon differences in float size, length, and material -all of which affect the effective force applied to the Float Needle and Seat. Excessive fuel pressure will force fuel past the float assembly, regardless of height adjustment, and flood the carb at idle. These basic troubleshooting tests must be performed at the beginning of the diagnostic process, lest one should "chase tail" with a host of other "possibilities" that will manifest themselves as a result.
If you are able to confirm consistent vacuum and fuel-pressure levels between the two engines, it would be of (nearly indescribable) help to remove both carburettors and disassemble them (side by side) on a clean, well lighted workbench, to compare jet sizes, metering needles, ect. It is possible, that your "rebuild shop" has installed a kit that was tailored for an emissions friendly automobile, and the carb will never work correctly on your marine engine, without identifying the differing components. For example: did they "drill out" the threaded Float Needle Seat; and "Press in" a new assembly of a different size and type? Only a careful inspection and comparison can identify this type of unknown.
Lastly, while you have the two carbs off of the engines, try mounting your working carb on the ailing engine, and comparing its operation at idle. This may take an hour, but if it starts dripping fuel like the other carb, it could save you an entire summer of frustration! (Substitution is the easiest troubleshooting trick in the book.)"
 
"Marshall:
Good inputs!

C

"Marshall:
Good inputs!

Can you explain this a bit further:
"Thus, a weak vacuum strength will often cause an over-richening of the Idle Circuit and may result in the reported dripping problem."

That's a new thought to me and i'm alway willing to lessen my ignorance.

Also, on those well plugs, I've only seen two carbs that leaked (secondary wells) that used the modern sealing approach. I'm pretty sure both of those were due to water induced corrosion. The old brass plugs leaked quite frequently and established the epoxy procedure."
 
"I have a vacuum gauge that I

"I have a vacuum gauge that I had on an old truck. There is a plug behind the carb on the top of the intake manifold. Is that what I plug into? The plugs on the bottom of the carbs are all epoxied closed. Both engines run like new, burn no oil, start instantly hot or cold, so I don't think it is vacuum issue but easy enough to try. Swapping carbs may be my best bet. I'm getting pretty adept at removing and installing carbs."
 
"you can use the pipe plug - j

"you can use the pipe plug - just get one on top of one of the runners, not the cooling passages.

you can also pull the vacuum hose that feeds the choke pull off - it is supplied with manifold vacuum."
 
"MakoMark,
The Quadrajet uni

"MakoMark,
The Quadrajet units use a power enrichment circuit to supply additional fuel when responding to engine demand. As you open the throttle plates quickly,(such as when you realize that your pleasure-craft has drifted into the path of an on-coming supertanker) Air rushes into the intake manifold and decreases the vacuum signature of the charge. With a Holley carb, the corresponding momentary decrease of flow through the venturi causes a lean condition due to a simultaneous reduction in fuel flow. The same thing happens with the Quadrajet. However, the two carbs differ in the way that they compensate for this undesirable condition. The Holley sprays additional fuel into the venturi via the cam-actuated accelerator pump; whereas the Quadrajet further enriches the Primary circuit as the Power Piston senses the low/weak vacuum signal. In this respect, the Quadrajet is superior to the Holley, in that the Holley must use a relatively closely matched accelerator cam for the particular engine it is fitted to. The Quadrajet compensates for a wide variety of engine displacements and charactoristics, thus allowing it to be used on small blocks, big blocks, and even six cylinders with minimal adaptations.
One inherent drawback to the enrichment circuit design of the Quadrajet is that a weak vacuum signal occurs in situations other than WOT (wide open throttle) such as when your engine stalls or the idle speed is insufficient to maintain adequate vacuum. Likewise, mechanical problems that affect vacuum will also affect the Quadrajet as the Power Piston reacts inversely to the weak vacuum signal. The Idle circuit is very much affected when the Primary circuit is enriched, although its effect accounts for little in comparison to the flow of the main jets in a "full-on" acceleration event. Unfortunately, a substantial vacuum leak, stuck intake valve, torn or missing gasket, heavily warped carb plates (due to over-tightening) worn throttle shaft bushings, ect. can all result in a weak vacuum signal at idle. Insufficient idle speed will also reduce vacuum and prompt an enrichment response from the Quad, hence the occasional dripping at 500 rpm.
Simply put, the Quadrajet needs a stable and consistent vacuum at low engine speeds to enable its Idle Circuit to operate effectively.

Fuel for Thought:
We all know that a poorly functioning carb can dump excessive fuel into the engine. This can lead to rough running, misfiring, backfiring, plug fouling, hard starting and contamination of the Lube Oil, to name a few. It is also important to note that excess fuel "washes" the cylinder surfaces; effectively removing the lubrication film from between piston rings, skirts and cylinder walls. When the sparkplug fires the charge, the rings are "clamped" against the cylinder walls and scraped downward with amazing force. The engine needs lubrication in order to continue smiling under these conditions :eek:)"
 
"thanks for the input Marshall

"thanks for the input Marshall. I understand the power piston operation. Being as that occurs in the primary jets, upstream of the idle circuit, I don't see how that richens the idle. The other fuzzy area is where the power system starts to operate which is driven by the spring. The marine carbs i've seen had springs that start the power circuit at 8" of vacuum. that's pretty low at idle, even for a poorly tuned engine. What am I missing???"
 
"Good news, bad news. So I rem

"Good news, bad news. So I removed the carb again. Took it apart, cleaned it. This time I removed the primary jets and there was crud under the right side jet. I don't know if that was a problem but I put it back together and no more dripping out the top any more. The bad news is, if I turn the idle mixture screws all the way in, it then leaks out the top."
 
"you didn't take the jets

"you didn't take the jets out the first time? Did you blow compressed air thru all those little passages? Did you find all the air bleeds and make sure they were open? If not, there's probably more crud and that stuff (as you have found out) will do all kinds of bad things.

when you turned the screws all the way in, what rpm was the engine making? did you get any control with the idle screws? Whe you say leaks out the top are you referring to the main nozzle(s)? did yoou get a vacuum gauge reading at idle

If it kept running, slower and rougher, with the screws in, I'd think the idle air bleeds are clogged. With the drip (not the aerosol spray)out of the nozzle, the main air bleeds may be clogged, too.

The good news is its getting better and your best idle should be with the screws out a couple turns....at least its going in the correct direction.

Did you notice if the newer carb and the old carb have a different shape to main casting, between the primary bores and the inlet fittings?"
 
One thing you might want to tr

One thing you might want to try is running a can of Sea Foam through her. The stuff is great at cleaning the system out. Now that said make absolutly sure you have a good filter on because all the crud is going to get cleaned out and you don't want it to end up in your carb.
 
"MakoMark,
The fuel flow to t

"MakoMark,
The fuel flow to the Idle Tubes is supplied from the Mainwells, which in turn, are filled (ultimately) via the Mainjets. The main jets are composed of fixed orifices, which are varied in overall volume with the use of metering rods.
In a properly working Idle System, the Idle tubes deliver conditioned and metered fuel to the orifices located just below the Primary Throttle Plates when resting upon the Idle adjustment Stop Screw.
As the Throttle Plates are opened, airflow through the multi-venturi increases to the point where sufficient vacuum differential develops to overcome the fuel flow through the Idle Tubes and thus begins to flow to the Venturi orifices. Again, assuming proper operation, manifold vacuum steadily decreases in proportion to the opening of the plates, and the Power Piston lifts via spring-pressure to raise the metering rods. It is interesting to note the uniqueness of design of the Quadrajet, in that the fuel flows through the metered Mainjets prior to entering the Mainwells and the Idle circuit system. Unfortunately, poorly metered flow through the Mainjets results in problems "downstream" as the Idle Circuit depends upon the metering rods being positioned accurately as engine vacuum is applied to the Idle orifices.
In answer to your second question: (regarding the vacuum values at which the Power Piston begins operation) Again, we must think in terms of a malfunction in the system that varies from the norm. Many a "rebuilder" chooses to skip the meticulous aspects of the process, whether due to indifference or ignorance, and may neglect to fully remove all gasket material from sealing surfaces within the unit during reassembly, or may over-tighten fasteners with disregard for proper tightening sequence, or a host of other issues that may combine to cause internal vacuum leaks, which in turn, adversely affect the carefully designed operation of the unit.
There are several well-written articles that cover (in depth) the principles of operation of the Quadrajet, that can more completely answer questions that you may have.
My gut feeling is that you have offered sound advice, regarding the recent discovery of "crud" in the carburettor body, and that cleaning and flushing of all fuel passages is vital.
Another thought:
Once the carb is fully cleaned and sparkling, bear in mind that you have rendered the Throttle Plate bushings "Squeaky Clean" also. The use of some light lubricant on these bushings will curtail early wear of these surfaces, and prevent the resulting vacuum leaks at the bushings in the future."
 
"Jim,

I've been mean

"Jim,

I've been meaning to ask you if your engines have the thick spacer plates below the carbs, and if you have reinstalled them on both engines, or have deemed one or both of them to be not worthy of reusing?"
 
"Marshall, The spacers are bot

"Marshall, The spacers are both in place. You are referring to the triangle spacers to level the carb right? I soaked the body in carb cleaner flushed with hot water then blew out all passages with compressed air. Can I poke a wire down all those holes to be sure they are clear or is that a no no. I couldn't find my vacuum gauge. Still looking. Yes the drip was coming from the main nozzles."
 
"Jim,
Yes, if it will help yo

"Jim,
Yes, if it will help you to be certain that the passages are clear, you can poke wire through them. Just be careful not to mar or otherwise damage the jet orifices. Any alteration to their shape or surface will affect their performance. I prefer to use Fishing Line for this. If you have a can of spray carb cleaner that has the plastic tube that fits into the spray button, insert the tube into the passages and flush them out. Wear safety glasses when doing this, as it may splatter. Performing this outdoors is best, as good ventilation is needed. Most carb cleaners are quite flammable, and any vapours that you breathe will greatly tax your liver."
 
"At 600 rpm I get 13 inches of

"At 600 rpm I get 13 inches of vacuum; at 800, 15-16 and at 1000 18 inches. Steady needle through all ranges."
 
"After dinking around some mor

"After dinking around some more with the mixture screws I got it to 16@800 and almost 20 at 1000. Timing was dead on at 10 deg BTDC and I could detect no leaks with the starting fluid technique described in the webpage. Did compression test and got:
1. 155 2. 143
3. 155 4. 145
5. 150 6. 140
7. 140 8. 140

Ruggles book came today and I have the carb at home now. Tomorrow I will take it apart(again) and check those idle air tubes etc. Thanks all."
 
"Comp figures look great. Han

"Comp figures look great. Hang in there. You'll get.

I bought an Edelbrock 1409 used off eBay. Ran okay after a rebuilt, but it had a world class stumble off idle. Rebuilt it again--twice--with no change. My good drag racing buddy suggested drilling out the idle passages to see if something was stuck in there. It was! The factory seals the passages with lead and there was way too much in one of them that was blocking the flow. Runs fine now.

See: you gotta hang in there! Ironically, the swine I bought it from KNEW it was a dog when he sold it to me, but the joke's on him now.

Jeff"
 
"Did you change the baseplate

"Did you change the baseplate gaskets - one under the carb and the other below the wedge? a paper one is usually ok under the wedge. A thicker one is usually better under the carb. a good parts store may have it, if not, a 'hot rod' connection could help. Another area would be even and sufficient torque on the retaining nuts.

Also, did you check the primary throttle shaft for wear, where the shaft goes thru the plate? Wear there will cause that small leak Dave mentioned. Other suspects would include the choke pulloff and its hose and the fitting it connects to. The secondaries usually don't wear like the primaries.

When you check the shaft, it helps to remove the throttle cable. last item is to check any capped off fittings on the throttle plate - make sure the cap isn't cracked."
 
"New gaskets on the carb but n

"New gaskets on the carb but not in between the wedge and manifold. They look pretty good though. I took it apart again tonight and everything was clean and open. However,in the past I had a screwdriver barely big enough to span the needle seat and probably didn't get it as tight as it should have been. I found a bigger screwdriver and tightened it properly. I wonder if gas was getting through the threads and flooding the carb. I'm kinda hoping that because I just don't know what more to do on my own. I'll make those checks you describe. I did learn a ton from Ruggles book"
 
"I don't think flooding is

"I don't think flooding is the concern ....

Ruggles book is great; only "shortfall" is the focus on the 170 style units with APT. Cliff is also a very reasonable source for parts, including the most comprehensive kit you will ever find.

If the carb is still off, check the castings of warping."
 
"The carb went back on this am

"The carb went back on this am. Every single passage is open. confirmed with spray gumout. I have pretty good control with the mixture screws but when I completely close them I still can get the gas to come out the primaries. Maybe I'm not understanding the physics but this seems reasonable to me. According to Ruggles, at idle there is not enough vacuum to pull gas through the primaries so gas is pulled to the idle mixture passages and some to the transfer slots, as vacuum(and speed) increases, fuel continues to be pulled through the mixture, transfer AND primaries. So if I close one way for fuel to go wouldn't vacuum increase to the other 2 passages thereby pulling it through the primaries? Anyway, I ran it at idle for 10 minutes or so then ran 8 miles at 3000 rpm and it ran great. When I came back in I pulled a spark plug and it looked brand new. Last time I idled for 10 minutes the plug came out black. I also had a sheen of gas in my dock. That too has gone away. It may not be right but it is as right as I can get it. I may try to return my store bought carb(the real bad one) and see if they will credit me that and get a brand new Edlebrock 1409 for $100 more. If not, I think I will change the oil, watch the plugs for fouling and recheck at 25 hours for fuel dilution. I didn't hook up the vacuum gauge because the kids were getting anxious and it is a 25 year old stewart warner gauge so I'm not even sure of the accuracy of it."
 
"Jim:

Glad you have rid you

"Jim:

Glad you have rid yourself of some of those bugs.

It may well be that with the idle screws in you may get some minor flow from the main nozzle. Rest assured, it isn't from any vacuum change due to the position of the mixture screws.

If you will, all the vacuum is under the throttle plates. this draws the mixture from the mixture screw port and maybe a bit from the transfer slot. As the throttle plates open, more of the transfer slot opens and more mixture exits there. When enough of the throttle plates open, the airflow thru the venturi will provide enough vacuum for the main nozzles to flow.

If you take off the carb in the future, measure how much of the transfer slot is visible between its end (bottom) and the throttle plate. If that exceeds 0.040", the the throttle plate is open too far at idle. The fix is in Cliff's book. I believe the "adding bypass air" method superior to the "drilling the throttle plates"."
 
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