The Stovebolt Site Forums The Shop Area Engine Shop Changing idle rpm’s

I have recently swapped my Rochester out for a Carter 2100S, and I am having some issues with idle rpm after warmup. It starts fine and idles fine initially, but after a couple of miles, the idle rpm drops suddenly to where it is just barely running and has died on me a couple of times. I first tried adjusting the idle mixture, but that did not fix it (at cruise, everything seems fine). Yesterday it happened again so I stopped and adjusted the idle speed up some and started driving again. Then about a mile farther down the road, whatever happened before went away and the idle was now too high. My foot feeder linkage seems okay, once the choke is closed, the throttle appears to be at the stop, and I had not seen this issue with the Rochester, so I am thinking that maybe I have missed something in the overhaul that could be causing this.

Thanks for your help,

Lee

Make certain your throttle linkage spring is good and the linkage is free moving. With a Carter carburetor, it won't take much resistance to mess things a bit and it sounds like something is keeping your carburetor from settling correctly on the throttle stop. It is worth mentioning the Rochester had a strong spring inside which pushed the throttle linkage back as far as it could go and the Carter is built differently. For one comparison, the Carter accelerator diaphragm works "backwards" from the old Rochester pump type and for another comparison the internal spring isn't as stiff. With the Carter, there should be one spring on the diaphragm rod which connects between the bottom casting piece and a sort of C clip about 3/4 of the way up the rod. The pump lifter sits on top of that C clip (remember it pulls up on the diaphragm to squirt gas). Straight on top of this sits the metering rod arm, spring and metering rod and on top of that you'll have one or two springs (depending on the age of the carburetor and how it was designed) and another C type clip at the top. Image of this is attached for you. In the Rochester a strong spring beneath the accelerator pump sits and it will push the pump up smartly. Good luck.

Thanks Jon, I will open it up and make sure I put it back together right. I do remember the double springs and the two c clips and that I put it together wrong the first time. The accelerator pump is working, so part of it at least is correct. The return spring on the throttle is pretty strong too, though I have nothing to compare it to. I removed the complete accelerator linkage and cleaned and lubricated everything. It all looked okay with no over worn parts that I could see. Thanks for the picture.
Lee.

There is also an adjustment for the metering rod which ought to be done any time the carburetor is upset. Did you get that from the rebuild data you had? (won't affect idle speed but if wrong will affect mixture timing, etc)

I do have the information for adjusting the metering rod, but to me it is not real clear. I cannot tell how to determine if the rod is all the way down in the hole.

Lee.

This sounds like a mild case of carb icing . Is your manifold heat flap working properly?

The heat riser is stuck in the open position; I tried for several days the spray and hammer tap procedure with no luck.

First close the throttle valve all the way. Now press down on the diaphragm shaft until it bottoms out and hold it there. Lift up slightly on the metering rod and set it down again if you want to check. You can feel it hit the bottom of the carb body and watch carefully to see that it doesn't hit bottom too soon. If you don't feel it touch, press gently on it to see how much it is off and then gently bend the lip of the metering rod arm. Ideally the metering rod arm and the lifter arm should sit straight on top of one another and ideally the metering rod bottoms when the diaphragm is all the way down.

You may have to remove the exhaust manifold to get the heat flap unstuck. Some of those are stuck awfully badly (as you've learned). Where in Texas are you?

I guess I have to remove the end of the metering rod from its attach point then to see if it is bottoming out st the correct point. I’ll check that next. I dropped the exhaust pipe from the manifold, and that is how I know it is stuck open. I didn’t try it, but I might be able to spray both sides of the shaft with the pipe out of the way and be able to free it.
I am in San Angelo.

Be patient with the flapper valve. The good thing about it being stuck is, at least the holes in the manifold aren't wallered out. (Yes, wallered is a word, just don't try to look it up.)
Spray Kroil, hammer ends of shaft back and forth. Spray more Kroil, hammer, wait, try to move the weight. Repeat. It will come loose.
Don't bother with the carburetor until you can free up the flapper. The carburetor may be fine.
There is an old adage, "Save the carburetor for last. It is never the carburetor." Even if it does end up being the carburetor, you cannot make it work right if there is something else amiss.
Carl

Please don't remove the end of the metering rod. Just lift up carefully and let it down and you can feel it bottom out or you can touch it on top to see if there's room left to move down. Once you get the feel for this procedure it is like riding a bike. San Angelo is a nice place. Could use more rain but most of Texas could. I spent a decent amount of my youth out that way. Like Carl says, be patient with the flapper. You'll get it to free up in time. I don't believe that's the answer to the idle rpm question, but it will keep the engine from running as it should.

I checked the metering rod as you mentioned and it appears to be set correctly, so I won’t be messing with it. I went through all the linkages one more time to see if something is binding, and nothing appears to be. I’ll keep driving it and see if it recurs and maybe I can figure it out. It is the drop in RPM that has me stumped; I can think of several things that would cause a rise in RPM, but not a drop.

As far as the heat valve goes, I’ll get back to working on it and see if I can free it. Like I have read here in several posts, it just takes patience.

Thanks, Lee.

Hi. I can tell you that on my old carb - 62 Rochester one barrel - the throttle shaft was worn and sloppy and the carb would suck air through there. Set the idle in the driveway, running great, stop at a light a little later and it dies. The throttle would be closing in a different place. Reset the idle, runs fine a bit, stop at a light and now it idles too high. Throttle is now closing back where it was before. Doesn’t take much variation to throw off the idle. On careful inspection, I could see what was going on. I solved this when I located a NOS carb on eBay. I think you have to watch out for the rebuilt ones as sometimes they aren’t any better. Possibly you could drill out the bores and add bushings, or make some kind of seal to reduce the air leak.

I always found that my Carter YFs were more sensitive to the throttle blade position then a Rochester and I usually had to add a second spring to make sure the throttle closed completely against the idle speed screw stop. You cant check this by snapping the linkage. You want to press the accelerator down half way or so (engine off) and gently let it up and then look to see if the throttle is closed against the idle speed screw stop. if not, you may have to add a extra spring to make sure that it does.

Thanks to all for your suggestions; I could not find anything specific, and even put the Rochester back on to see if it was not related to the carb and maybe a timing issue (ran fine). So I took apart the Carter again and cleaned it out real well, blew out all the passages with air, put it back together, and so so far it is much improved (except the time I forgot to reattach the wiper hose!).

Lee.