The Ford Barn

SoCalCoupe · 06-14-2023, 01:48 PM

Not the worst thing in the world, but my '41 flathead sometime stalls when coming off speed to idle. Much worse when cold, happens about a third of the time when up to temperature.

If I'm cruising, take my foot off the gas letting the engine brake the car, then push in the clutch, that's when it usually happens. The engine will stumble at a below idle speed then stall. Sometimes it catches itself and comes back to idle speed. If I blip the throttle that always stops it. It never happens just a straight idle except when cold.

Car has an Eddie Meyer hi-rise dual carb manifold with two Stromberg 81's. The carbs have been extensively tuned, first with a "whatever it's called" to synchronize the throttles then each of the four idle mixture screws to maximize vacuum at about 14".

Have a new Skip Haney coil and a new Tubman condensor. Normally, car runs great under all conditions when warmed up. Very occasionally under strong acceleration, the car will hiccup, not even stumble, just a near instantaneous pause.

Checked for vacuum leaks everywhere multiple times and found no trace. Plugs are very clean indicating car isn't running rich either at idle or at speed.

I'm leaning towards disassembling the carbs and trying to blow out all the passages even though I don't have an air compressor.

Thoughts?

Mart · 06-14-2023, 01:58 PM

I had a similar problem in my truck. ran along ok but would not idle once warm. Power valve was leaking. I know mine is a 94 and yours are strombergs but once I fixed it the idle is nice and steady regardless of when I lift off the gas or brake. Look for something that would give you an unexpected over rich condition.

Mart.

drolston · 06-14-2023, 03:33 PM

I had the same problem on my '41 flathead with Offy tri-power manifold with 97 carbs (similar to 81's) and progressive linkage. The problem happened when slowing down to a stop. That is, when I let off of the gas pedal gradually it would sometimes stall when I put the clutch in.

The problem was sticking linkage to the secondaries. Thinking that the throttle shaft return springs were enough, I disconnected the accelerator pump return springs on the end 97 carbs to reduce the pedal pressure needed to open the secondaries. Turns out that even with the expensive version of progressive linkage, there is enough stiction in the linkage that sometimes the throttle plates on the secondaries did not go all the way closed, leaving a big vacuum leak at idle. That resulted in a lean condition that stalled the engine. It would always start right back up, but that problem is more than an aggravation; it can be a safety issue. I put the accelerator pump return springs back on and added a separate spring acting to return the linkage to the secondaries. Problem permanently solved!

Sounds like you may have the same problem. Step one on your dual carb manifold is to be double damned sure that when your foot is off of the gas and both idle stop screws (not the jets) are completely backed off, both throttle butterfly valves go completely, precisely and simultaneously closed.

It is hard to tell looking down through the carbs, so remove both carb bodies from the cast iron bases and adjust the linkage that joins the carbs until you have precise, simultaneous closure. Be sure there is no free play or slop in the linkage joining the two carbs. When setting the idle speed, use only one of the idle stop screws; if the linkage is tight, it should hold both throttles open equally. Check that before you put the carb tops back on. Both carbs will need the throttle shaft return spring and the accelerator pump return spring.
Then you can sync at idle using the air thingy and idle mixture adjustment screws.

drolston · 06-14-2023, 03:35 PM

Quote:

Originally Posted by Mart

I had a similar problem in my truck. ran along ok but would not idle once warm. Power valve was leaking. I know mine is a 94 and yours are strombergs but once I fixed it the idle is nice and steady regardless of when I lift off the gas or brake. Look for something that would give you an unexpected over rich condition.

Mart.

Or, over lean!

SoCalCoupe · 06-14-2023, 11:18 PM

Quote:

Originally Posted by drolston

Sounds like you may have the same problem. Step one on your dual carb manifold is to be double damned sure that when your foot is off of the gas and both idle stop screws (not the jets) are completely backed off, both throttle butterfly valves go completely, precisely and simultaneously closed.

It is hard to tell looking down through the carbs, so remove both carb bodies from the cast iron bases and adjust the linkage that joins the carbs until you have precise, simultaneous closure. Be sure there is no free play or slop in the linkage joining the two carbs. When setting the idle speed, use only one of the idle stop screws; if the linkage is tight, it should hold both throttles open equally. Check that before you put the carb tops back on. Both carbs will need the throttle shaft return spring and the accelerator pump return spring.

Checking this with the carb bodies off sounds like the way to go. I strongly suspect it's something with the throttle plates but like you said, you can't tell anything by looking down the barrels with the bodies on.

Great tip!

Brad

Flathead Fever · 06-15-2023, 05:08 PM

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I would start with adjusting the carbs again. If I didn't setup them up I would want to make sure they were adjusted correctly.

What happens if you slowly close the throttle coming to a stop. If the plates slam shut too fast when the engine is still not down to idle rpm it creates a very strong vacuum signal that can draw out extra fuel and momentarily flood the engine. Float level could also be too high and sloshing extra fuel out the vents when you come to a sudden stop. Air fuel mixture not correctly adjusted. Throttle plates not adjusted evenly. You need to figure out if it is getting too lean or too rich, could be either of them. Try pulling the choke closed a little and creating a rich mixture as it switches back to the idle circuit and see if it likes. that rich mixture.

SoCalCoupe · 06-15-2023, 09:46 PM

Quote:

Originally Posted by Flathead Fever

Try pulling the choke closed a little and creating a rich mixture as it switches back to the idle circuit and see if it likes. that rich mixture.

Did that. Car ALWAYS like the choke out just a tad. That points to the idle being a little lean, as do the plugs. Problem is, when I enrich the idle, it idles too fast and I can't can't slow it down even by backing all the way off the idle screws, throttles completely closed. Air's still getting in somewhere. Either throttle plates, vacuum leak or the via the air bleeds and emulsion tubes.

Like I said, checked everywhere twice for vacuum leaks and couldn't find any. I'll check again though. Been doing it by squirting carb cleaner at all the manifold joints and vacuum connections.

That's why I strongly suspect throttle plates aren't closing all the way even with the idle speed screws backed all the way out. This weekend I'm going pull off the carb bodies and see what's really going on with the throttle plates and with the dual linkage setup.

BTW, before I installed the dual carb setup this was not a problem. For that reason and others, I really don't suspect ignition issues.

I know, I don't need dual carbs, but it's so cool and so much fun. At least I THINK it is.

SoCalCoupe · 06-17-2023, 02:13 PM

Quote:

Originally Posted by drolston

Step one on your dual carb manifold is to be double damned sure that when your foot is off of the gas and both idle stop screws (not the jets) are completely backed off, both throttle butterfly valves go completely, precisely and simultaneously closed.

It is hard to tell looking down through the carbs, so remove both carb bodies from the cast iron bases and adjust the linkage that joins the carbs until you have precise, simultaneous closure.

Did this this morning. All throttle butterfly valves go completely, precisely and simultaneously closed. However, it looks like one of the four throttle plates has been clipped and is out of round. Going to order some new plates.

RKS.PA · 06-17-2023, 06:21 PM

Brad, yes you DO need dual carbs!! Betting it gives you great pleasure looking at them each time you raise the hood!! See that slice in the throttle plate. Hope that's your problem.

SoCalCoupe · 10-29-2023, 01:07 PM

Fixed it! Replaced both of the throttle shafts with oversize shafts. Idles smother, slower and steadier. Unlike before, no hunting at idle; rock solid idle speed whether sitting at a light or braking to a stop. So it was a vacuum leak through the throttle shafts.

Bought a reaming tool to finish the throttle shaft bores but could have done the job without it. Might have knocked off a few high spots with the tool but that's all. I don't have the story on these old Stromberg 81 carbs. Maybe someone reamed them out while rebuilding then didn't install oversize shafts

Also, fixed the "hiccup, not even stumble, just a near instantaneous pause...under strong acceleration." Separate problem. The points weren't adjusted correctly. There was almost no gap at all. Could actually hear spark jumping the gap inside the helmet distributor. Reset them with .015" gap. Hasn't happened again. Checked it later with my new timing fixture.

Terry,OH · 10-31-2023, 06:56 AM

From your description of your Carb. adjustments, they have not been done properly. Adj. both Carb. idle mixture, first, for the highest and smoothest idle RPM. If necessary back off the idle RPM adjustment to lower RPM don't adj the mixture adj for a lower idle RPM. AFTER idle mixture then use synchronizer adjusting idle RPM screws so each Carb. contributes equally to idle. Do adjustments with the linkage between the two carbs disconnected reconnect linkage after adjustment.

06-14-2023, 01:48 PM	#1
SoCalCoupe Senior Member Join Date: Sep 2021 Location: Point Loma, San Diego, CA Posts: 424	Engine sometime stalls slowing down to idle Not the worst thing in the world, but my '41 flathead sometime stalls when coming off speed to idle. Much worse when cold, happens about a third of the time when up to temperature. If I'm cruising, take my foot off the gas letting the engine brake the car, then push in the clutch, that's when it usually happens. The engine will stumble at a below idle speed then stall. Sometimes it catches itself and comes back to idle speed. If I blip the throttle that always stops it. It never happens just a straight idle except when cold. Car has an Eddie Meyer hi-rise dual carb manifold with two Stromberg 81's. The carbs have been extensively tuned, first with a "whatever it's called" to synchronize the throttles then each of the four idle mixture screws to maximize vacuum at about 14". Have a new Skip Haney coil and a new Tubman condensor. Normally, car runs great under all conditions when warmed up. Very occasionally under strong acceleration, the car will hiccup, not even stumble, just a near instantaneous pause. Checked for vacuum leaks everywhere multiple times and found no trace. Plugs are very clean indicating car isn't running rich either at idle or at speed. I'm leaning towards disassembling the carbs and trying to blow out all the passages even though I don't have an air compressor. Thoughts? Last edited by SoCalCoupe; 06-14-2023 at 11:35 PM.

06-14-2023, 01:58 PM	#2
Mart Senior Member Join Date: May 2010 Location: Solihull, England. Posts: 8,755	Re: Engine sometime stalls slowing down to idle I had a similar problem in my truck. ran along ok but would not idle once warm. Power valve was leaking. I know mine is a 94 and yours are strombergs but once I fixed it the idle is nice and steady regardless of when I lift off the gas or brake. Look for something that would give you an unexpected over rich condition. Mart.

06-14-2023, 03:33 PM	#3
drolston Senior Member Join Date: Jun 2014 Location: Williamsburg, VA Posts: 1,627	Re: Engine sometime stalls slowing down to idle I had the same problem on my '41 flathead with Offy tri-power manifold with 97 carbs (similar to 81's) and progressive linkage. The problem happened when slowing down to a stop. That is, when I let off of the gas pedal gradually it would sometimes stall when I put the clutch in. The problem was sticking linkage to the secondaries. Thinking that the throttle shaft return springs were enough, I disconnected the accelerator pump return springs on the end 97 carbs to reduce the pedal pressure needed to open the secondaries. Turns out that even with the expensive version of progressive linkage, there is enough stiction in the linkage that sometimes the throttle plates on the secondaries did not go all the way closed, leaving a big vacuum leak at idle. That resulted in a lean condition that stalled the engine. It would always start right back up, but that problem is more than an aggravation; it can be a safety issue. I put the accelerator pump return springs back on and added a separate spring acting to return the linkage to the secondaries. Problem permanently solved! Sounds like you may have the same problem. Step one on your dual carb manifold is to be double damned sure that when your foot is off of the gas and both idle stop screws (not the jets) are completely backed off, both throttle butterfly valves go completely, precisely and simultaneously closed. It is hard to tell looking down through the carbs, so remove both carb bodies from the cast iron bases and adjust the linkage that joins the carbs until you have precise, simultaneous closure. Be sure there is no free play or slop in the linkage joining the two carbs. When setting the idle speed, use only one of the idle stop screws; if the linkage is tight, it should hold both throttles open equally. Check that before you put the carb tops back on. Both carbs will need the throttle shaft return spring and the accelerator pump return spring. Then you can sync at idle using the air thingy and idle mixture adjustment screws.

06-15-2023, 05:08 PM	#6
Flathead Fever Senior Member Join Date: May 2010 Location: Yucaipa, CA Posts: 1,095	Re: Engine sometime stalls slowing down to idle Sponsored Links (Register now to hide all advertisements) I would start with adjusting the carbs again. If I didn't setup them up I would want to make sure they were adjusted correctly. What happens if you slowly close the throttle coming to a stop. If the plates slam shut too fast when the engine is still not down to idle rpm it creates a very strong vacuum signal that can draw out extra fuel and momentarily flood the engine. Float level could also be too high and sloshing extra fuel out the vents when you come to a sudden stop. Air fuel mixture not correctly adjusted. Throttle plates not adjusted evenly. You need to figure out if it is getting too lean or too rich, could be either of them. Try pulling the choke closed a little and creating a rich mixture as it switches back to the idle circuit and see if it likes. that rich mixture.

06-17-2023, 06:21 PM	#9
RKS.PA Senior Member Join Date: Jun 2010 Location: Marana, AZ Posts: 1,194	Re: Engine sometime stalls slowing down to idle Brad, yes you DO need dual carbs!! Betting it gives you great pleasure looking at them each time you raise the hood!! See that slice in the throttle plate. Hope that's your problem.

10-29-2023, 01:07 PM	#10
SoCalCoupe Senior Member Join Date: Sep 2021 Location: Point Loma, San Diego, CA Posts: 424	Re: Engine sometime stalls slowing down to idle Fixed it! Replaced both of the throttle shafts with oversize shafts. Idles smother, slower and steadier. Unlike before, no hunting at idle; rock solid idle speed whether sitting at a light or braking to a stop. So it was a vacuum leak through the throttle shafts. Bought a reaming tool to finish the throttle shaft bores but could have done the job without it. Might have knocked off a few high spots with the tool but that's all. I don't have the story on these old Stromberg 81 carbs. Maybe someone reamed them out while rebuilding then didn't install oversize shafts Also, fixed the "hiccup, not even stumble, just a near instantaneous pause...under strong acceleration." Separate problem. The points weren't adjusted correctly. There was almost no gap at all. Could actually hear spark jumping the gap inside the helmet distributor. Reset them with .015" gap. Hasn't happened again. Checked it later with my new timing fixture. Last edited by SoCalCoupe; 10-29-2023 at 01:17 PM.

10-31-2023, 06:56 AM	#11
Terry,OH Senior Member Join Date: May 2010 Posts: 4,751	Re: Engine sometime stalls slowing down to idle From your description of your Carb. adjustments, they have not been done properly. Adj. both Carb. idle mixture, first, for the highest and smoothest idle RPM. If necessary back off the idle RPM adjustment to lower RPM don't adj the mixture adj for a lower idle RPM. AFTER idle mixture then use synchronizer adjusting idle RPM screws so each Carb. contributes equally to idle. Do adjustments with the linkage between the two carbs disconnected reconnect linkage after adjustment.