The Stovebolt Site Forums The Shop Area Engine Shop Throttle hesitation on 235

I recently got my '53 3100 running. It has a 235 with a Rochester 1bbl on it. When I apply throttle it has a big dead spot. I have to gradually apply throttle so it doesn't sputter or even die. The carb was completely gone thru and I installed a new carb kit. When I assembled it tho, I noticed that the accelerator pump diaphragm was smaller than the bore it went in, so I soaked it in marvel mystery oil for a few hours. It did swell the leather diaphragm enough to apparently get a slight squeeze fit in the bore. Could this be the reason there is a dead spot? I'm thinking that maybe after assembly that it could have shrunk again. Now that it is saturated with gas inside the carb will it tend to swell or shrink? Should I put another new accel pump in or what should I do? Thanks for any help.

What method did you use to adjust the carburetor mixture and "idle speed"?

What method did you use to adjust timing?

In what order-of-operation did you do those adjustments?

I set timing with vac advance shut off, I put a valve inline just for setting timing, and at approx 600-650 rpm. I used a timing light and put the timing ball right on the pointer. I put idle screw back to where it was originally, then in 1/4 turn increments I adjusted it a few turns in and out from starting point and it didn't help the hesitation. I'm assuming at some point in the adjustment it would have been "in the ballpark". What method should I use to adjust idle screw, vac or rpm? Adjust screw to obtain max vac/rpm?

I prefer the "maximum vacuum at idle" technique for adjusting the carburetor (and dual/multiple carburetors) and the timing. I do not disconnect the vacuum advance, unless it is an engine/carburetor that requires that. Connect the vacuum gauge to the wiper port on the intake manifold.

Some people adjust as I describe and then "fine-tune" the timing using the "ping" method. Most likely someone will post that procedure.

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0620.htm

I use the 5 step idling adjustment from the manual. ...example...(CARTER)
only difference Ill advance my distributor a bit more due to Petronix ignition


did the pump look different from the one you removed?

If by pump, you mean accel pump, yes it looked the same. Although the old one was dried up and cracked. It was a pinkish "cup" shaped little fella. It just seems to me that it's acting like the accel pump isn't working. Which is funny because it's a new one, but like I mentioned, I'm wondering if the new one is B.O.

Crawl up in the engine bay until you can see down the carb, then manually move the throttle lever and look for a squirt of gas. That is the accelerator pump showing you that it is working.
Carl

Like Carl suggested, test the pump.

Start the engine so you know there is fuel in the bowl (it could have evaporated out overnight in a heated garage).

Stop the engine.

Remove the air cleaner.

With one hand open the choke valve and hold it open if automatic choke.

Look down beneath the choke plate.

Move the throttle from closed to open.

If you see a spurt of gas, pump is OK.

Jon.

Ok, I should have thought of doing that already. I will check it out tomorrow hopefully and give a report. Thanks again guys!

Ok, I fired the old girl up tonight. After shuttin the engine off I visually checked the accel pump squirt, and it seemed good. I originally set the point gap to .019" like the book says to do for new points, but at that gap the dwell was 42*. So I had to set the gap at about .027" to obtain 32* of dwell. I then re-set the ignition timing to right on the ball at approx 550 rpm. Vac advance at that rpm didn't seem to make a difference if it was on or off. When I revved the engine up the dwell stayed right at 32*, hardly any dwell wandering. The hesitation upon throttle application got noticeably better, but still a bit of a dead spot. I didn't monkey with the idle jet adjustment because I didn't have a vacuum gauge. When I locate a vac gauge I will adjust idle jet, hopefully that'll clear up the hesitation. Thoughts???

Vacuum is "ported" so no vacuum at idle to the advance. My truck had a sticky power valve and after I fixed that it worked much better. The vacuum advance will move the whole distributor when you blip the throttle. But the port for the vacuum is not open until the throttle is opened. The dwell is a bit weird as reported. Be certain that you are on the right scale. And the correct voltage. Also check that the coil is wired correctly. Ignition to + and - to the points. Points at 0.027 is rather large. Idle mixture can be set with out a vacuum gauge. Not perfect but quite close. When the timing is set. And the idle speed is set. Turn the mixture screw in until the engine starts to stumble. Just as it starts to stumble. You can feel it, then turn back out 1/2 to 3/4 turn. If it still has a bad stumble then you might need to clean and rebuild the Carb. Get the points and dwell sorted first. Others will have some thoughts to help.

I was on the right scale, and 12v. truck is converted to 12v. Coil is in proper - to distributor. It is a coil with internal resistor. I really don't know of anything else to sort out regarding this situation. It is a bit puzzling.

Have you tried to advance the distributor?

What happens if you set the idle to 450-500rpm, does it stumble?

32 degrees of dwell is right for a V8, which has a total available dwell of 45 degrees. You're looking for a 66% "points closed" number, which would be 40 degrees on a 6 cylinder engine, (60 degrees available) or something close to that number. I'd shoot for 38-40 degrees for dwell on a stovebolt distributor, which should get the point gap somewhere close to the specification.
Jerry

DADS50; I have not advanced the timing in effort to eliminate the stumble. I've had the idle as low as 500 rpm and it still idles smooth but stumbles on rapid throttle input.
HOTROD; you're saying 38-40 for dwell, I had 42 dwell @ .019 gap. My research told me 26-33 dwell, so I adjusted the gap to .027 to obtain 32 dwell. So which is it, 33 or 40 max dwell? I can obtain the proper dwell, I just don't know whose "proper" dwell spec I should set it at. Perhaps I should just shoot for a happy medium and set the gap at .022 and call it a day.

The term "dwell" literally means "the time the points 'dwell' together"(points closed). On the Chevy sixes, the dwell specifications are all over the map, anywhere from 32-33 degrees up to 39-42, depending on the year and distributor part number. I'd need to know your part number to find an accurate specification. Obviously, the design of the distributor cam has a lot to do with the necessary settings. The 66% rule of thumb is pretty much industry standard for the maximum coil magnetic saturation that still gives the condenser time to discharge properly between "points open" and "points close" cycles. If the condenser does not discharge properly, the arc across the points as they close leads to rapid metal transfer and point failure. There's a lot of physics and electronics involved in proper distributor design and function, and a lot fewer of us around who were educated on that subject since electronic ignition has been standard since the mid-1970's!
Jerry

Ok Hotrod, I will let you know my distributor part/model number later today. If you can find some specs for me that would be great! Thank you very much.
Edited 5 min later; I found a photo of the dist on my phone, so I don't have to wait til I get home from work. It is a Declo-Remy model # 1112363 serial # 1B21

Found it! 1112363 is a for a 1951 passenger car with a Powerglide transmission, which means it was original equipment on a splash-oiler 235. Dwell angle is supposed to be 39 degrees, or .015"-.022" point gap. Dwell and point gap are an either/or situation as changing the point gap changes the dwell angle. Changing 1 degree of dwell also makes a 2-degree change in ignition timing, so it's necessary to reset the timing anytime a dwell or point gap change is made. The centrifugal advance at 1850 engine RPM should be 16.5 crankshaft degrees, and vacuum advance will be 9-11 degrees at 16 1/2 inches of vacuum on the distributor vacuum line. My ballpark guess of 40 degrees of dwell was a whole 1 degree off! My Bad!

Whether or not those figures will be right for your engine- - - -who knows? Just try them for original settings and tinker with the initial timing with the octane selector if necessary.
Jerry

Jerry/hotrod, thank you very much. Invaluable information right there. Much appreciated. I guess I'll be heading back out in the garage for some more tuning. Hopefully I can get the dead spot out of it now.

Well, I set the points at approx .020" (my 20 may be different than your 20, everyone has there own "feel") that put the dwell at 37 1/2 - 38 degrees. I'll stick with that. Re-set the timing right on the ball, adjusted the idle jet out 5/8 of a turn after it stumbled while turning it in. The engine purrs like a kitten, but still has that hesitation if I give the throttle a little snap from idle. If I get in the throttle easy it revs up nicely. I might have to live with it, after all I realize it's not a top fuel dragster! Thanks to everyone for the help. This forum is simply awesome, it's great to be part of the bolt!

I am curious why you are not setting the dwell at 39 degrees. Are the points brand new? If not I would start with new points and condensor and set the dwell at 39 degrees.
If the stumble continues, you might have an internal leak in the air horn due to slight warpage (some Rochester B carbs are notorious for that).
If you look down the throat of the carb while the engine is running and open the throttle a bit you can sometimes see fuel making a wet spot on the edge of the gasket between the air horn and the main case below. It will get wet when you open the throttle, then dry right up when the throttle is closed.
The stumble comes from fuel going where it does not belong.
Carl

The stumble is probably lean, and/or late timing. A degree or two of dwell change isn't likely to make much difference, but if you'll time it a ball's width or two before the timing pointer you'll see a much better off-idle response. Adjust the idle mixture to the best manifold vacuum (not distributor vacuum) plus 1/8 turn rich and set the idle speed to 450-500 RPM after adjusting the mixture.
Jerry

I guess I am comfortable at 38 dwell. I know how things can go for me, I could end up going back an forth all day trying to get exactly 39. Everything in the dist is new. Next time I run it ill check for that fuel wet spot. I'll try advancing the timing a bit and see what happens. Not to sound too green, but what direction on the idle mixture screw would rich be?

Rich is counterclockwise (left) Turning the idle needle full right shuts the flow off- - - -CCW makes it richer.
Jerry

You probably already check the vac advance

Make sure it advances and returns properly
Goose it and have someone look at the distributor movement.

Yes, the vac advance appears to be working properly. When adjusting the idle mixture screw, how important is it that the air filter be on? I do not have the air filter on, could this possibly be contributing to the dead spot on initial throttle application? Can I get rid of the oil bath filter for a more modern paper element?

Hi Lemmm117 ,I gave up on the carb that was stock on my 1948 2 ton with a 216 . I had 1985 Dodge truck with a 225 slant six which I junked but kept the engine .So I put this carb on my 216 with adaptor plate I made .Wow my engine runs great . I have over 6000 to 7000 miles on this set up . I even have the choke disabled (I'm lazy didn't hook it up) .Even at 16 degrees the engine starts with a few pumps of the gas peddle and settles very quickly into a nice idle .I do have a HEI dis. on it also .This is my daily driver not some babied trailer queen .
Hope this helps Paul