The Ford Barn

Johngtate · 11-22-2019, 11:55 AM

I have a stock '40 that runs well but tends to have a rather large temporary RPM "dip" when returning to idle to the point it feels like it's going to stall out.

After a second or so it returns to a normal speed but I tend to pull the throttle knob to get it past this point for fear of having to restart in front of traffic.

Does anyone have any ideas as to what this could be? The float bowl screws, baseplate nuts, etc. all seem tight. Any advice is appreciated-thanks.

Mart · 11-22-2019, 12:04 PM

It may be running slightly lean, or the float level might be a little low. As you stop, the fuel surges forward in the float bowl, away from the metering area. This sudden leaning out gives you the dip you are describing.

That's my guess.

Mart.

supereal · 11-22-2019, 01:12 PM

Your engine idle speed may be too low. Increase it and see if it helps. Also be sure your clutch pedal free play is correct so the clutch disc is completely released. As the clutch ages, the free play decreases, and any drag can affect the engine idle.

philipswanson · 11-24-2019, 05:49 PM

Check the throttle return spring. It may be on the bellcrank.

Johngtate · 04-15-2020, 02:09 PM

Thanks for the great tips, to close this thread I simply turned the curb idle screw up about a 1/4 turn. Believe it or not, 1/2 turn was too much and resulted in a high idle. Now that the weather has warmed up to the 70s it's back to it's old tricks again with the idle dip, almost to the extent that it seems to droop way low and/or shudder a bit so not sure what that means; maybe the float level is a bit low.

GB SISSON · 04-15-2020, 10:37 PM

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Wife and I went for our first spring drive yesterday and exact same situation, except we actually stalled 4 times. Last years gas? I'll be watching this thread.

ford38v8 · 04-16-2020, 12:43 AM

Just a wild guess... Distributor advance plate sticking, unable to normalize immediately on idle down.

Tinker · 04-16-2020, 12:58 AM

I'm in the same boat of increasing the cold idle speed. If you have good compression. Lowering the idle speed to get that lump sound is interesting and sounds great.

Pulling the choke usually means a lean condition. Throttle rod might be linkage.

.

11-22-2019, 11:55 AM	#1
Johngtate Member Join Date: Dec 2012 Posts: 33	1940 stock flathead idle "dip" when stopping at a traffic light I have a stock '40 that runs well but tends to have a rather large temporary RPM "dip" when returning to idle to the point it feels like it's going to stall out. After a second or so it returns to a normal speed but I tend to pull the throttle knob to get it past this point for fear of having to restart in front of traffic. Does anyone have any ideas as to what this could be? The float bowl screws, baseplate nuts, etc. all seem tight. Any advice is appreciated-thanks.

11-22-2019, 12:04 PM	#2
Mart Senior Member Join Date: May 2010 Location: Solihull, England. Posts: 8,745	Re: 1940 stock flathead idle "dip" when stopping at a traffic light It may be running slightly lean, or the float level might be a little low. As you stop, the fuel surges forward in the float bowl, away from the metering area. This sudden leaning out gives you the dip you are describing. That's my guess. Mart.

11-22-2019, 01:12 PM	#3
supereal Senior Member Join Date: Jul 2014 Location: Iowa Posts: 1,634	Re: 1940 stock flathead idle "dip" when stopping at a traffic light Your engine idle speed may be too low. Increase it and see if it helps. Also be sure your clutch pedal free play is correct so the clutch disc is completely released. As the clutch ages, the free play decreases, and any drag can affect the engine idle.

11-24-2019, 05:49 PM	#4
philipswanson Senior Member Join Date: Oct 2015 Location: Bonita, CA Posts: 1,374	Re: 1940 stock flathead idle "dip" when stopping at a traffic light Check the throttle return spring. It may be on the bellcrank.

04-15-2020, 02:09 PM	#5
Johngtate Member Join Date: Dec 2012 Posts: 33	Re: 1940 stock flathead idle "dip" when stopping at a traffic light Thanks for the great tips, to close this thread I simply turned the curb idle screw up about a 1/4 turn. Believe it or not, 1/2 turn was too much and resulted in a high idle. Now that the weather has warmed up to the 70s it's back to it's old tricks again with the idle dip, almost to the extent that it seems to droop way low and/or shudder a bit so not sure what that means; maybe the float level is a bit low.

04-15-2020, 10:37 PM	#6
GB SISSON Senior Member Join Date: Jan 2014 Location: Orcas Island Washington Posts: 4,917	Re: 1940 stock flathead idle "dip" when stopping at a traffic light Sponsored Links (Register now to hide all advertisements) Wife and I went for our first spring drive yesterday and exact same situation, except we actually stalled 4 times. Last years gas? I'll be watching this thread. __________________ Owner/Operator of 'Jailbar Ranch' on the side of Mt. Pickett. Current stable consists of 1946 1/2 ton pickup turned woodie wagon with FH V8, 1947 Tonner Pickup (red) mostly stock with exception of a cummins 6at turbo diesel, 1946 Tonner Pickup (green) with 226 cu in 6 cyl flathead, 1979 Toyota landcruiser wagon, completely encased in 1947 Ford Jailbar sheet metal. Ok, cornbinder rear fenders..... 'Rusty ol' floorboards, hot on their feet' (Alan Jackson)

04-16-2020, 12:43 AM	#7
ford38v8 Senior Member Join Date: Jul 2011 Location: SF Bay Area Posts: 6,635	Re: 1940 stock flathead idle "dip" when stopping at a traffic light Just a wild guess... Distributor advance plate sticking, unable to normalize immediately on idle down. __________________ Alan

04-16-2020, 12:58 AM	#8
Tinker Senior Member Join Date: May 2010 Location: MN Posts: 7,053	Re: 1940 stock flathead idle "dip" when stopping at a traffic light I'm in the same boat of increasing the cold idle speed. If you have good compression. Lowering the idle speed to get that lump sound is interesting and sounds great. Pulling the choke usually means a lean condition. Throttle rod might be linkage. . Last edited by Tinker; 04-16-2020 at 01:10 AM.