Rochester carb trouble

[Guest Mrutter59]

Hi.

have a Rochester 4 jet that gives my some trouble. Have it on a 1962 olds 98 with a 394 engine. It have a dash pot and automatic choke. I need help to adjust the fast idle screw/choke cam setting, idle mixture screws, float level. The car runs ok in speed up to 20-30 m/h, runs ruff and uneven in range of 30 - 60. Higher speed its ok. A have been trying to find adjustment specs for this carb but its not so easy...carb number is 7020950.

Any help is welcome.

/M

[Aaron65]

OK, first, I looked in my 1965 Motor Manual for your adjustments, but realize that modern gas has rendered these baselines just that, baselines.

 

Mixture screws--1 1/2 turns out

Primary float level (from heel of float)--1 15/32 in.

Secondary float level--1 3/8 in.

Primary float drop--1 1/2 in.

Secondary float drop--1 5/16 in.

The choke adjustment is the index mark.

 

If you're planning to work on your own car, a manual is a must, so it's good advice to pick one up.  With that being said, have you had the carb apart?  Have you blown out all the air bleeds and idle passages?  Up to 60 mph, most cars are running on the transition circuit, which is the little vertical line at the front of the throttle bore.  It bisects the throttle plate at low speeds.  If there's any crud in there, you could have problems.  Have you checked fuel pressure?

 

I'd check the usual tune-up stuff too.  Is the advance mechanism in the distributor moving freely?  Are your points/condenser in good shape?  Plug wires?  Does the engine run roughly when you hold it at the offending RPM in neutral?  Is it missing?

 

There are a lot of variables to this stuff, variables that aren't always obvious if you're not used to diagnosing old cars (and sometimes they're not even obvious to those who are!).  I think we're going to need more info. here if we're going to be of much service.

[Guest Mrutter59]

Thanks for your answer.

I have a service manual ( and a lot of questions) now and there is a little difference in setting the sec. float level. There is a Toe level and a have wedge type floats and i think it has a dimple...means that when i have measure the drop level 1-5/18 its nearly in bottom of he float bowl?

Slow idle adjustment: must i have a throttle return check Holding Fixture gauge in place...i dont have one of those so is there another way to do it? Do you have any numbers for the distance and picture of where it should be placed? 

Throttle return check: clearance between the contact screw and throttle lever contact..where is that located? 

A have a ac car and the ac is not installed for the moment. The carb dont have a idle compensator, and i dont know what that does?

We also installed a new vaccum standard advance, sens the old one was dead. Means that the other carb that was one the car, a Edelbrook 4 barrel, was tuned whit the engine in a different way. I see that when i check the ignition timing...there is 3 marks on the wheel next to the engine front and they are about 1in. before a setting "nob"? that sticks out from the engine front. (Its on the right side from engine front center) Is that the correct mark for setting the ign. timing? Cant see anything else that looks like marks..are they located under the waterpump so i dont see them?

I dont have points/condeser, but have a accell system. Its like  the MSD.

I am going to take the carb apart and look at everything...there is one thing whit the choke system...its divided in 3 parts...the upper rod one is not in a hole but is sliding in a "trac". The consequence of this is that 1. the fast idle screw dont end up on the lowest part of the idle cam and 2. the lok up system dont set the secondary throttle arm free....means the car is only running with 2 barrels active, not 4.. howe do i block the upper choke lever from sliding, `couse when i hold it in place with my finger and then press down the hole choke linkage, it works nice. 

Well, a lot of q:s but if you dont ask, you dont learn...it seams that the Rochester i a bit difficult to adjust and a hope its worth the effort to change to the original carb.

 

Again, thanks for your answers and help

 

M

 

 

[carbking]

The Rochester 4-GC is NOT a difficult carburetor to adjust, as carburetors go. And one of the better early 4-barrels. Virtually bullet-proof if properly restored.

 

You have the manual; study the information on the various circuits and what they do. Then you have an idea of what might be causing symptoms. Circuits in your carb:

 

(1) Choke - if properly adjusted, only functional for 4 ~ 8 minutes after startup

(2) Float(2) - keeps the fuel level in the bowls correct

(3) Pump - adds fuel during acceleration only

(4) Idle - basically only in play up to maybe 800 RPM

(5) Off-idle or transition - between idle and main, maybe 750 ~ 1200 RPM

(6) Main - above 1200 RPM

(7) Power - augments the main circuit when vacuum drops below a pre-set figure under WOT

 

Remember (and it is a proved fact, not just my opinion) - out of every 100 "carburetor problems", 3 are compression, 92 are ignition, and 5 are actually carburetor related. The ignition figure may be actually higher (opinion) for AFTERMARKET electronic ignitions.

 

And the "hot idle compensator" is a valve which is basically a metered vacuum leak at high temperature, slow driving. Purpose is to lean out the mixture in city driving with an ambient of maybe 90 degrees F. or above. Helps to prevent "hot soak" (often mislabeled vapor lock) stalling when stuck in traffic. Rochester made an aftermarket version to add if one was needed and not stock. Here is a picture -

 

It is called a Carb - airator. Has a 1/8 inch male pipe to screw into a vaccum source.

 

Jon.

Edited May 14, 2016 by carbking (see edit history)

[poci1957]

I have a 4GC on my 1957 Pontiac which is just now on the road after a long full restoration.  It is very hard starting, cranks and cranks when cold as if the gas has drained out of the fuel bowl, this can happen sitting overnight or sooner.  Will always catch immediately with starting fluid, although it will stall and demand another shot until (presumably) the gas gets to the carb.  Once started it idles well and when warm will restart easily.  No evidence of gas in the oil, no backfire from gas in the exhaust, carb and fuel pump are stock and rebuilt by me (with carb shop rebuild kit, thanks Jon).  New fuel lines throughout and good clean original fuel tank with no paint or treatment inside and gas is fresh with 10% ethanol.  I have seen it said that modern gas with ethanol evaporates faster than old gas but my other old Pontiacs (one with a QJet and one with a 2GC) do not have this problem.  Are 4GCs particularly susceptible to evaporation or "drain back" or is drain back even a real thing?  Any advice is appreciated, thanks, Todd C      

[joe_padavano]

49 minutes ago, poci1957 said:

Are 4GCs particularly susceptible to evaporation or "drain back" or is drain back even a real thing?  Any advice is appreciated, thanks, Todd C      

 

The fuel inlet on the carb is above the level in the float bowl, so the gas would need to flow uphill to "drain back".  Unless it is leaking out the bottom of the carb due to a crack in the fuel bowl, evaporation is the problem, possibly coupled with other misadjustments.  I do have this same problem with all of my carbureted cars that are not driven daily - Edelbrock (AFB) and Qjet carbs included.

[carbking]

As Joe stated - evaporation.

 

"Drain back" or "leak down" as more commonly called is a Holley issue with the type AA-1 carb; as the power (a.k.a. economiser) valve has a direct port from the valve into the intake manifold. Blow the valve, and an 1/8 inch hole drains fuel into the engine. The 4G power valve is a second passage to the primary main discharge nozzles.

 

An electric fuel pump, properly installed with the mandatory safety wiring, will solve the issue.

 

Otherwise, remove the air cleaner and fill the carburetor bowls through the bowl vents with a squeeze bottle (a re-purposed ketchup or mustard squeeze bottle works fine).

 

Jon.

[poci1957]

I guess those bowl vents are the rectangular slots seen here sticking up, one in front, one in back?

 

So does this seem like I may have caused a problem during the rebuild or is this fast evaporation within the norm for a 4GC with modern gas?  The only other variable is I have a modern open element air cleaner inside the original air filter housing (big oil bath seen below) and have not currently reinstalled the top of the housing.  Might having the housing in place enclosing the filter slow down the evaporation?  Thanks for your input Jon, I appreciate your trouble, Todd C

 

    

[carbking]

Todd, most styles of carburetors experience the evaporation problem. V-8's worse than inlines, as there is more heat in the manifold.

 

Interestingly (at least to me  ) is the Holley end bowl carbs, with an air gap between the bowl and the manifold, seem to have less evaporation problems than other carbs.

 

The dual AFB's on my 390 will be completely dry in an hour in the Missouri summer after running the truck. An electric pump (I run mine all the time instead of the mechanical pump) solved the hard start issue.

 

Jon.

[poci1957]

11 hours ago, carbking said:

The dual AFB's on my 390 will be completely dry in an hour in the Missouri summer after running the truck. An electric pump (I run mine all the time instead of the mechanical pump) solved the hard start issue.

 

Well at least it doesn't sound like something I did wrong then.  What kind of fuel pump and installation did you use, and did you eliminate the mechanical pump?  Thanks, Todd C

[D Yaros]

Can only add my name to the list confirming evap problem.  When either my '55 Cad with a Rochester 4-Jet, or my '62 Olds with a 2 bbl sits for any length of time I first refill the fuel bowl via the ketchup squeeze bottle technique for easy starting.