1950s Rochester rough idle and idle by-pass screw

[Lexi]

Hello Rick. I took some photos which I will post later. Looks like the "flat end" of the idle by-pass screw does not press up against anything, but it does enter a round port. Clay/Lexi

[Lexi]

As it has come up before and will come up again, for future reference here is the idle by-pass screw out of a 1956 Rochester 4GC carburetor from a '56 Cadillac. First shot shows a near pristine example. Second shot shows another with wear near tip. Perhaps a source of leaks if the circumfrence is out of round? May explain why tapered ones followed later. Next shot shows carb upside down with tip of the idle by-pass screw showing in it's passage, and also the round port it enters, (there but dark to see), highlighted by red arrows. Last shot shows same, but where I also inserted a pick to probe the round port that the screw goes into, see red arrows for 1) probe 2) tip of probe and 3) screw flat end. By filtered air I meant air that has passed through the air cleaner. That Rochester booklet says the air comes from the "primary bore". I take it that is the "horn" of the carburetor, so air first passes through the air cleaner to get there. Could have interpreted it wrong though. Will leave that up to the carb experts. But this is the screw that many say they have problems with. Clay/Lexi

[David King (kz78hy)]

I took some photos of the Rochester Carb catalog I have.  These pages are for 55 and 56 Rochester carbs, not dual.

Maybe this will assist.

[Lexi]

While I do have this same catalog, it is good that you posted these images for reference. Hope to look at my car this coming week. Thanks for helping out. Clay/Lexi

[Cadman-iac]

  Clay,
 This is what I was thinking about when you asked your question. On the early small bore Rochester 2bbl carbs, they did a couple things to vent the fumes after the engine shut down.


 This is the back of one with a hole in each side that is just vented to the engine bay. No filter, screen, nothing, so when the engine is running it can draw in air, and dirt, through these.


 Another method they used was to cut slots in the gasket between the base and body as you can see in the top two gaskets. The bottom one doesn't have any of that.


 Here's the paperwork from a rebuild kit for one.

 You are probably right about that air bleed screw drawing air from inside the air filter.
 When I read your question the first thing I thought about was this 2bbl design.
 Sorry for the misdirection.

 Rick

[Lexi]

That makes sense. With the Rochester 4GC, the service training booklet identified drilled holes in the casting also as vapour vent holes. I think if you go back to Jose's posted image file of the diagram, these were in the diagram. Clay/Lexi

[Lexi]

Well, I swapped carbs today and had a better look around under the hood. Aside from the secondary throttle shaft and idle by-pass screw vacuum leaks, I found a small chunk missing from my base gasket when I removed the carbureteor. When I had sprayed that area earlier, I also noted a vacuum leak there and I thought it might have come from the idle speed up control valve. They are vacuum connected and have a diaphram in them. With the engine running and the fan blowing the spray back, it was difficult to determine what area had the leak. The idle speed up unit is right where the leaky gasket is. Fortunatly, I had a replacement gasket ready to go. Attached image shows the damaged gasket area. Steel showing through with a portion of the gasket material missing. I have no idea what happened to it. Car now is running but difficult to keep idle down. Idle-by-pass screw not working the best, but at least the car is mobile again. Might have questions regarding idle later on. Clay/Lexi

Note: That damaged area is adjascent to the choke warming area in the intake manifold casting, (blocked in by a previous mechanic). I would think this would not be an area prone to vacuum leaks. Although the gasket looked a little rough, I did not notice any spikes in RPMs when checked, just a tiny rise in this area. Will have to check the idle speed up valve more closely.

[Cadman-iac]

  Clay,
Just a quick observation here. That missing section on your gasket is in the exhaust channel that runs under the carb. You said the passage is blocked off, so nothing really runs through that section of the gasket, which would include vacuum.
Unless there is another area of the gasket that is missing that connects to an actual vacuum source, the missing section you show should not affect how your engine runs.
Since the exhaust was blocked, you could actually cut away that front section of gasket and it wouldn't cause any issues, except for maybe not supporting the front of the carburetor.

  Rick

[Lexi]

That's what I thought. There should be no vacuum source there. Going to have to look at the idle speed up unit to determine whether it's diaphram is leaking. It is mounted in the same area. Clay/Lexi

[Cadman-iac]

 Yeah, it's tough trying to locate a vacuum leak with carb spray, with the fan blowing air all around.
 Are you using a straw on the spray nozzle to help pinpoint the spray?
 I hate having to even do that on a clean, painted engine because it can remove your paint.
 Does that idle speed up unit use a hose to supply the vacuum, or is there a gasket over a passage? If it's a hose, remove it and cap the source and spray again and see what happens.

 Rick

[Lexi]

Cadman-iac, yes I use an extra long plastic nozzle-say a foot to a foot and a half long, to better direct the spray to target areas with more precision. Reduces blow back from the fan. The idle speed up unit receives vacuum from a port in the middle of the cast iron base at the firewall side. Metal tubing connects it to the idle speed up unit. There are also a pair of wires that fire the solenoid. Have removed this unit for now. Tough to get this carb to idle at a proper curb idle. Not sure what the problem is, but way off. Partly caused by the carpet jaming the pedal, with perhaps a wonky pivot point there. PIA. Clay/Lexi

Note: Filled threaded carb base hole with a blind plug for now.

[Cadman-iac]

  Clay,

If you're unsure about if the carpet is interfering with the throttle linkage, you could temporarily disconnect your linkage from the carburetor and see if you can get the engine to idle correctly. If it does, then you know for sure the carpet needs attention.
If it doesn't make any difference, you know the carpet doesn't matter and it's definitely a carburetor issue.
Just a thought.

Rick

[The Tassie Devil(le)]

Don't forget that petrol these days is not as good as when these engines were built, and their carburetors calibrated.

I remember in the old days, one could get their engines to idle down to ridiculously slow speeds, and still run perfectly.

Bruce.

[Lexi]

Clay,

If you're unsure about if the carpet is interfering with the throttle linkage, you could temporarily disconnect your linkage from the carburetor and see if you can get the engine to idle correctly. If it does, then you know for sure the carpet needs attention.
If it doesn't make any difference, you know the carpet doesn't matter and it's definitely a carburetor issue.
Just a thought.

Rick

Good point. Sometimes I over think things. why didn't I think of this? Yes, the new gas is not as friendly to our cars. Rebuilder said he opens up venturi slightly to help compensate. I also have a damp looking carb bowl gasket. After running, if you wipe your finger along the gasket edge, traces of fuel will be seen on your finger tip. Hoping the gasket will swell given time and stop this, as this carburetor has sat for about 5 years or longer. Any suggestions? Clay/Lexi

[Cadman-iac]

  Something you may want to double check is your float setting. If the gasket is getting soaked like that, it's possible that the float is either set slightly too high, or it's not able to ride as high as it should due to other problems. What comes to mind is a small leak, or a build-up on the exterior adding weight to it which will cause it to sit lower thus raising your fuel level in the bowl. Either one of these will cause you problems with how it runs.
What I would do in this case is to check the setting as it is first, then drop the setting a small amount and test it to see if it makes any difference.
One consequence of a too high of a fuel level is not being able to adjust the idle properly.
As I phrase it, "The float don't".
The reason I suspect this is from what you've said about it, you haven't taken it out on the road yet, just testing the idle in your driveway or garage, so you're not getting any fuel "slosh" that would account for the wet gasket.
Just another thought. Hope it helps.

Rick

 Edit:
 One thing I would like to add, check to see if the float may be binding at any point in it's travel. This too can cause issues.

[Lexi]

Thank you Rick. I did not know that a high float level could affect idle. And yes, no "slosh", just idling in the driveway as no motion. More stuff to check with little time as our driving season whittles away,   Clay/Lexi

[Cadman-iac]

  A sure sign that the float is too high is if you can close off your idle mixture screws and the engine will continue to run.
The higher fuel level allows the engine to draw fuel easier from the idle circuit and it can also just spill over into the engine in what I would call a controlled flood.

You can weigh the float, but unless you know what it's supposed to weigh, that doesn't do much good. A build-up of minerals or chemicals on the outside of the float or any fuel that might have seeped into it makes it sit lower in the fuel, letting the level go too high.
Unfortunately there's no way to actually see what level the fuel is inside the bowl, all you have is the specs for the float setting,  so unless you have a brand new float, testing various settings is the only way to check.
On some Holley carbs, they have a removable plug in the side of the float bowl to visually check the setting, and some have external adjustments for the float as well. Both innovations stemming from just this sort of situation.
Good luck with your carburetor, I hope you find the problem soon. Gotta get Lexi back out and feed her for her upcoming hibernation, lol!!

Rick

[Lexi]

Good luck with your carburetor, I hope you find the problem soon. Gotta get Lexi back out and feed her for her upcoming hibernation, lol!!

Rick

Feed for hibernation. LMFAO! These Rochesters actually have threaded plugs on the sides of the fuel bowl. I believe they are there to measure float height. Will have to look into this. Clay/Lexi

[Cadman-iac]

 You got me thinking about those bowl sight screws and I thought I might have one of the 4GC carbs that had them, so I dug through what I have tonight.
 Apparently this was not very common, out of 11 carburetors I had one with the sight screws. It's got one for each float, front and rear on the driver's side.


 It's nice to be able to check the fuel levels, but you still have to remove the dozen or so screws and a pair of linkages to make any changes.
 The engineers were on the right track, they just needed to go a bit further with this idea. It would have made carb adjustments so much easier.
 Holley took it and ran with it and made it their own.

 A few of my other 4GC carburetors had the bosses on the side of the bowl but weren't drilled out for it. I suppose it would be easy enough to drill and tap the bowl, but you would still need to locate the correct pair of screws for it.
 This is also something that would only work on the Rochester 2bbl and the 4GC because of the bowl design. You couldn't do that with a Quadrajet.
 The 4GC is also the only carburetor that can hold 2 quarts of fuel in the bowls. Kidding!!
 When you pulled into the filling station back then you would tell the attendant to fill up the carb and check the tank. Lol!! I slay me!!

 But seriously folks, I've always admired the 4GC carbs, they were impressive.
 They probably were the reason why the government limited the size of the fuel bowl on later carburetor designs. The Quadrajet holds less than a 4th of the fuel that a 4GC does.
 Can you imagine the size of the fireball you would have if you had a backfire with a 4GC?
 If your fuel pump failed while driving you could still drive 8 miles on the fuel in the carburetor.
 You could get home and park the car, come out the next morning and get half way to work before the carburetor ran out of gas!!

 On that note I'll stop now.

 Rick

[Lexi]

Here is a shot of my carburetor with the 2 fuel sight plugs clearly visible. So they are there for float examination only and not to facilitate any float adjustment? Clay/Lexi