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Rochester carb issues

S

sammi

Regular Contributor
"So I have Crusader 454's

"So I have Crusader 454's with rochester carbs. Last fall I had the oil analyzed and found fuel dilution of 5% so I rebuilt 1 carb myself and bought a rebuilt carb from national carburetors in Jacksonville. I consider myself a better than average shadetree mechanic but have never rebuilt a carb. This way I would have one that I know(or should know) works and mine that I could just replace with the old one if my job fails. Well, this spring mine seems to run great but the carb professionally done seems to be dumping too much gas down the throat. At idle it is spraying gas like an aeresol can down the throat. The engine runs fine though. Not like a stuck float. Also no gas in the fuel pump line to the carb so that is not an issue.I did put a new fuel pump on that side as well but it is a mechanical fuel pump so I don't think pressure can be too high. Any ideas?"
 
"Sure. Rebuild that one as we

"Sure. Rebuild that one as well! You're on a roll.

Could be the "pros" set the float too high, or they didn't replace it. One of the Q-jet's failing was a float that absorbed gas and didn't float well enough. Millions of gallons of gas were wasted by them.

Jeff"
 
"Jim,
The fuel delivery at

"Jim,
The fuel delivery at idle is determined primarily by the idle jets, which are adjusted by screws on either side of the carburettor. As the throttle plates are opened, the main jets begin to flow fuel.
If the carb appears to be delivering excessive fuel at idle, this could indicate mis-adjustment of the idle jets. Although these jets are also affected by float level adjustment, the first step would be to attempt to adjust the jets before venturing further into the carburettor.
10 mechanics will suggest 10 different ways to adjust them, but here is what works for me:
1.) Start the engine and allow it to reach normal operating temperature. This will permit the choke plate to assume its open position.
2.) Slooowwwly turn one of the idle adjustment screws Clockwise, until the engine begins to run rough and mis-fire on the bank of cylinders that corresponds to that side of the carb. At this point, you have "Leaned" the idle mixture.
3.) Back the screw out (Counter-clockwise) in small increments, until the engine runs smoothly again on that bank.
4.) Repeat this process for the other idle adjustment screw.
I find it to be helpful to adjust the idle RPM via the (Idle Set Screw) down to about 600 RPM, prior to making the idle jet adjustments. This assists with determining the point at which the engine begins to "lean misfire". Once your adjustments are completed, return the Idle Set Screw to its former position by watching for the correct idle speed on your tachometer.
I hope this helps."
 
I turned those screws all the

I turned those screws all the way in and fuel still sprayed in. There was no change in flow.
 
Time to disassemble the carb a

Time to disassemble the carb and inspect. Do you still have the instructions that were included with the rebuild kit for the other carb? These will give you the correct specs for float level adjustment and such.
 
"Yes, I do. I bought another r

"Yes, I do. I bought another rebuild kit for the original carb as well. I guess I will take this one apart as well. Then I will have a spare, I guess."
 
.."I turned those screws a

.."I turned those screws all the way in and fuel still sprayed in."

Classic carb flooding! Time to show that you can do a second one better than those "pros".

Jeff
 
"At risk of stating the obviou

"At risk of stating the obvious, but make sure the throttle plates are completely closed."
 
"Jim:

I believe what you ar

"Jim:

I believe what you are seeing is termed nozzle drip. The characteristics of nozzle drip (vs flooding) include an aerosol like discharge from the main nozzles and little, if any, control of the idle quality from the idle mixture screws. If it is flooding, you will see large drops that flow from the nozzle (vs the finer aerosol spray). If its drip and not flooding, the idle mixture screws WON'T do anything because the throttle plates have exposed too much of the transfer slots - the idle mixture is exiting those slots vs the mixture screws' bores.

If you reduce the idle speed, the drip should go away and the idle mixture screws will provide some control. I'd first try lower the float level in the bowl. (Mine are at 3/8".) This reduces the "head" on the fuel in the idle circuit and makes the main nozzle a bit less sensitive. If the nozzle drip is still there, you may need to mod the carb to supply more "idle bypass air". There are two approaches, both requiring some minor "machining".

The concern over the float Jeff mentioned is valid. best to spend a few bucks and replace them while everything is apart, especially if their age isn't known.

The other 'dirty little secret' is that some of the car rebuilders 'undo' what RPD did. Instead of having dozens of uniquely calibrated items, they provide a 'generic' product for a very wide application. The end result is pretty evident. Another 'secret' is that the engine OEMs specified the carbs for a worst case scenario. Many times, what you have can be tuned to your specific install with much improvement over stock."
 
"Are the "correct" flo

"Are the "correct" floats for Rochester Q-jets metal or plastic, or does it make any difference?

Thanks,
Erich"
 
"The OEM material is Nitrophyl

"The OEM material is Nitrophyl, the black plastic. Brass units are also available in the aftermarket. Like most things, each has its pros and cons.

What is more important is to get the correct float for the casting that you have. Besides the shape, the fulcrum length is another discriminator. Making the appropriate float adjustment is another critical step is getting the carb dialed in."
 
Do you know what else causes a

Do you know what else causes a "nozzle drip"? I bet no clip that connects the needle valve to the float. I took the carb apart and nothing. I thought I'd find it at the bottom of the carb but no. They must have never put it on. Yiest. I'm a freaking Optometrist. I shouldn't know how to rebuild a carb better than "the professionals" Anyone can make a mistake I guess. Now I need to find a clip.
 
"afraid not, the clips purpose

"afraid not, the clips purpose in life is to ensure the needle is pulled open when the float drops."
 
"When the engine has not been

"When the engine has not been used for a period, the fuel can evaporate from the float bowl, via the vent tube. This can leave a varnish that tends to "glue" the float needle to its seat. As another informed member stated, the purpose of the clip is to pull the needle from its seat when the float travels downward in the bowl.
Neglecting to install this component is indicative of "sloppy work" (and dropping the little bugger under the work bench is not an acceptable excuse).
The "varnish" that forms as the fuel evaporates is caused, in part, by the waste products that the refineries dump into the pump fuel in an effort to "dispose" of it -at our expense. -But that's another topic.
Just apply the same attention to detail as you do when you are working on someone's eyeball; and you will have "two" carburettors that work perfectly."
 
"So help me out here. Is the f

"So help me out here. Is the float and needle's only job to make sure there is proper fuel level in the bowl? If the clip is off, the needle should be down; therefore closing the valve not allowing any fuel to enter the bowl, right? Mine doesn't have the clip yet the bowl is full of fuel. What is going on there."
 
The clip just helps the float

The clip just helps the float pull a stuck needle open. So does fuel pressure. How necessary is that clip? Since half the carbs built don't have one...


Jeff
 
"Jim:

Jeff explained why y

"Jim:

Jeff explained why your bowl has fuel. The needle is the control mechanism, the float is what makes it work. Marshall added the 'if the needle gets stuck' part that I omitted regarding the clips purpose.

I've shared the same thought Jeff did regarding the 'need' for the clip. I concluded if RPD put it there and it is available, I'll keep it. I'd say it isn't essential though I'd be inclined to make a note of it in my log book in case there was a dry carburetor after sitting over a winter..."
 
"I appreciate all the help! Un

"I appreciate all the help! Unfortunately, after setting the float to 3/8 inches I have the same issue. The rebuild kit and float(from this website)for my carb came today. I will try my hand at my carb and hopefully send this one back."
 
"Did you try backing out the i

"Did you try backing out the idle speed screw to close the throttle plates some? If not, then you may have enough air flowing thru the venturi to turn on the main nozzle. I've seen some BBCs that 'drip' with the idle above ~475rpm. With the 'drip' occuring, you won't see any difference in the idle mixture screw settings"
 
I did. I backed it down slowly

I did. I backed it down slowly until the engine just died out and still got the nozzle drip condition right until it died. I have to say it did seem to spray less after resetting the float but still there. Although maybe I'm just thinking that. I'll try mine and see what happens. This all started because of a fuel dilution problem and someone here suggested just leave it alone and change oil more frequently. I'm beginning to think that was the best advice yet.
 
"You didn't say what RPM y

"You didn't say what RPM you could get down to. If it died above 500 rpm, you can unhook to choke pull-off to provide some air that should let you reduce the rpm some more. Some of the qjets also had a manifold tap (PCV valve sized) that could be used to add some air, too. If it is the larger 3/8" hole, you may have to put your fnger over it a bit.

You did check the inlet seat and valve to ensure shut-off does occur? also, you didn't say if you put in a new float/kept and checked your old one."
 
"I would guess about 550 rpm i

"I would guess about 550 rpm it died but i don't know. I was down by the engine turning the screw and didn't note the tach.

"You did check the inlet seat and valve to ensure shut-off does occur? also, you didn't say if you put in a new float/kept and checked your old one."

I don't know how to do this. Remember this is the professionally rebuilt carb so I'm not sure i got a new float but i have to say it appears most everything is new inside. My old float will not fit into the new carb. It is a little different size."
 
"Mine used to die whenever the

"Mine used to die whenever the idle was adjusted below 500rpm (with a drip but reduced in spray from the 750 rpm idle). I use the "vacuum" method to test the needle and seat. Really easy if you have a hand-held vacuum pump.

Assemble needle into seat.
connect vacuum tester to threaded end of seat.
apply at least 10" vac.
reject needle and seat if it will not maintain vacuum for 5 minutes.

My source for parts says he sees a very high 'leak rate' in needles and seats, maybe one in ten. He also says you can reform the seat with an old accelerator pump checkball if you don't have a spare: insert checkball into seat. place seat on block of soft wood. apply flat punch to top of checkball and tap lightly with small hammer. Redo leakage test."
 
Thanks Mark. They might not ta

Thanks Mark. They might not take back this carb since I opened it up so if they don't I just may tinker with it. Now another question. I rebuilt the other carb last night and put it on tonight. It is alot better buuuut...At idle I have a slight drip(not the aeresol spray) from the right side primary. The left side doesn't drip. The left side isn't clogged because they both spray evenly when I go past 1000 rpm. Any thoughts here?
 
Are both idle and main air ble

Are both idle and main air bleeds open on both sides? Did both idle mixture screws control the idle quality? did it have the 70 main jets and the 42 primary rods?
 
"That's funny. Remember, y

"That's funny. Remember, you are talking to an Optometrist. I don't have a freaking idea what you just said. I guess I need a pro after all. It did appear like the mixture screw controlled the idle on the left but not the right."
 
"With no control on the "d

"With no control on the "dripping", I'd think that the Idle tube or the air bleeds on the right are clogged. The idle tube meters the fuel from the main jet into the idle circuit. The idle air bleed starts the emulsification process. The main air bleed works like a vacuum break for the idle circuit.

The air bleeds, upper, are most easily seen from the bottom of the air horn, looking from the gasket side towards the top. The main system has another air bleed usually around the cluster. The idle system also has a lower air bleed. the upper air bleeds are on the 'back wall' of the primary side of the airhorn. If you hold the airhorn between your eye and a light, that helps, too.


I'd bet most 'pros' aren't fluent with Q-jets; if they were, the 'nicknames' would have stopped a long time ago. I'd suggest spending $15-20 and get a good q-jet book. Ruggles or Roe for authors. I don't even think they teach carb theory i the tech schools any more."
 
Sorry if I'm saying this t

Sorry if I'm saying this twice but there is a weird little Quadrajet issue that may not have been covered. When you take the throttle plate off of the bottom of the carb turn it over and check the two little welch plugs that they put there under the main jets. I will always either JB weld them to seal them or you and get one of those little rubber seals that fit in the base plate.

If it is leaking there if will drain the carb into the engine when it sits around for a bit. If you seam to have to fill the carb by cranking it a bunch after a couple of weeks of sitting then it may be part of your problem.
 
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