I know this has been discussed here but I wanted to share my experience. I changed over to a paper filter in my air cleaner. Mainly due to the few times oil has been sucked up into the mesh.
Now, a description of the problem. The car idled fine, ran fine. But from idle if I slowly depressed the accelerator there was always a slight stumble before the engine would pick back up again. Barely noticeable. Everything was as it should be, centrifugal advance working as it should. Timing dead on. Idled great. No amount of slight adjustments could get rid of it.
I decided that my paper filter was dirty and needed to be changed. My parts store had to order what I wanted so I switched back to oil in the cleaner.
That's when I realized that the stumble was gone. I then picked up the paper, put it in and THE STUMBLE IS BACK! Pulled it, refilled with oil and the stumble is gone. I swapped the cleaners back and forth and every time with the paper the stumble rears it's ugly head.
Both filters I have, the dirty and the clean are Fram.
I am not saying it is the paper filter, more so just sharing my experience and wondering if anyone has any thoughts on this.
Joe,
Hi. I converted my oil bath to a K & N about a week after I got the car. Looks factory, no cutting of the air cleaner, and fits perfect. Never had a problem, car ran great (when it ran…).The only thing I could possibly see it throwing off would be the air-fuel mixture at tip-in. Doesn't make sense that you can't adjust it enough to get rid of the stumble. You might have to experiment by upping the idle jet a size or two. Is it a Carter or Stromberg? We should look at the Carb and try to find out how the circuitry works off-idle. I can make a phone call to the guy who does my carbs and ask him if you can't get it right. He's great at the vintage stuff.
Bobby
Best leave it out then Joe. Problem solved. ;D
It's a Carter WD-O. Just came back from a cruise to see how everything is. Running 100% fine. I will leave it out. Enough other things to still deal with I don't want to introduce another odd problem that needs sorting. Not that it was much of a problem. Hardly even noticeable, only an obsessive owner would ever know.
Just a wild thought Joe, it could be something to do with the fact that you said you have a smaller La Salle carb on there thats working on its limits to supply enough air to the engine and the paper filter is just a tiny bit restrictive??? :o
I thought that, but all the claims on the interwebs say that the paper filter flows better than the oil bath. I guess I'd need to see some actual quantitative tests to really believe it.
I don't really understand why it would only manifest as the slight stumble. Otherwise the car runs fine and has great fuel economy. I have also thought about putting a camera inside the carb and watching what happens under the different conditions. Sounds risky but I might try it.
Hello Joe,
Before trying the camera, have you experimented with no filter? I would be curious to know if the off-idle stumble is present with no filter as well as with the paper filter. I concur with Bobby B.'s notion that the paper filter has slightly altered the relationship between airflow/density/fuel-ratio/vacuum, and the off-idle circuit of the carburetor. The design of the idle and off-idle circuits are such that it doesn't take much to throw them out of adjustment. A paper filter may flow better than an oil bath filter, but at what engine speed? And is a better flowing paper filter necessary at idle and off-idle engine speed? For perfection, you may have to retain the oil bath filter. Just a thought.
Christopher Winter
Maybe the oil filter is partially clogged and making it run a little richer? Try partially restricting the paper filter and see if that changes anything.
Are you putting the paper in the original housing so the weight and inlet and support is all the same? Or is the paper one a completely different setup? Just wondering if its possible you have a something rubbing on a linkage or pulling hotter air or something like that.
The entire housing is as original the only change being the swap from pot scrubber element to paper. The pot scrubber element was soaked in gasoline for more than a year :) I expect it is clean. The stumble is NOT there with no filter housing.
My suspicion.. based on no fact, would be small droplets of gasoline collecting at the jets due to slow air flow. The same as if the carb was just slightly over-choked. Upon slow opening of the throttle, from idle, you end up with a split second of overly rich mixture as the droplets are pulled into the intake. Hence my reasoning of sticking a tiny camera in there. I don't think I will bother with the camera.
I just went for a 25km run and the car is performing perfectly.
The stumble you describe indicates to me restricted air flow. Carburetors are designed to create a partial vacuum and draw the gas out of the bowl while the float insures the fuel pump can keep the bowl full. The partial vacuum can be restricted by partial blocking the airway or increasing the size of the airway.
I installed 750 CFM Carb in my 1979 Dodge Lil Red express which was the fastest vehicle made in the US for 1978 and 1979 due to government meddling with restrictions on horse power in cars (Dodge foiled them by producing several models of hot rod trucks.) The 360 Dodge really needed a 600 CFM Carburetor. The slower velocity allowed by enlarged air passage gives the truck the same stumble you describe but when it catches up in RPM's the truck really roars which is what it is supposed to do.
Bottom line to me is to probably trust Cadillac engineering and go back to the oil bath.
Maybe I should hack together a simple rig to test the difference in flow through the silencer with both different filters.
Joe: In the 12 plus years that I have used the paper filter in my 41 and 47, I have never experienced any hesitation or rough idle in either car. The only problem that I have had with carburation, while running the paper filter, was with the 47. The Stromberg I used on the 47, although the correct Stromberg, was more prone to develop vapor lock than the Carter for 47.Using a paper filter did not cause the vapor lock. The paper filters I use are Fram CA 148 or a black NAPA Gold 2055 as pictured below. I am surprised that you have had not had more feedback on the paper filter from other users. Bill