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Old 10-18-2020, 09:41 PM   #1
drolston
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Default Chapter 2. Missing at part throttle.

I am going to get this old flathead purring like a kitten if it takes me a year! Which I am well into.

On my recent "Missing at full throttle" post, you helped me fix that problem. Turns out two ignition wires had gone open circuit. They would jump the gap and run fine at low rpm but give out at high rpm. Now it revs right up to 5,000 rpm without missing a beat. Until it starts starving for fuel and losing power. I turned the fuel pressure up from 2.5 to 3.0 psi and that helped a little but not completely. Pulled the top off of the center carb (97) and found that the float was set way low; like almost 3/4 inch. I reset to a shade under 1/2 inch. Will road test that fix soon.

But another problem has shown up. The engine idles perfectly but runs rough as I slightly open the throttle from idle to about 1,500 rpm. Above which it smooths out. Smooth at idle and high rpm pretty much rules out ignition:

- Except that in trouble shooting the full throttle problem I replaced the Pertronix distributor with a rebuilt points distributor. For whatever reason, the Pertronix distributor had a lot (like five turns) of vacuum brake; the points distributor is set with the brake having a very light drag. Could too much advance too early cause the low rpm stumble. I have the advance setting advanced 3 degrees, which is what ran fine on the Pertronix.

Any suggestions?
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Old 10-18-2020, 10:34 PM   #2
WABOOM
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Default Re: Chapter 2. Missing at part throttle.

I can't help but I'm always interested to learn more. I have been trying to figure out a low rpm partial throttle stumble that I am getting on my car.
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Old 10-19-2020, 09:40 AM   #3
51woodie
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Default Re: Chapter 2. Missing at part throttle.

The tag that Bubba attached to the rebuild he did on my '46 distributor, showed the vacuum brake set at 3 turns in. and the drivers side points set to 22 1/2 deg., passenger side =36 deg., and 22 deg. advance @2500 rpm.
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Old 10-19-2020, 12:26 PM   #4
drolston
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Default Re: Chapter 2. Missing at part throttle.

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Originally Posted by 51woodie View Post
The tag that Bubba attached to the rebuild he did on my '46 distributor, showed the vacuum brake set at 3 turns in. and the drivers side points set to 22 1/2 deg., passenger side =36 deg., and 22 deg. advance @2500 rpm.
My rebuilt distributor came with the same calibration and vacuum brake adjustment, which I backed off to where the brake has a light drag with no vacuum.

So again, is spark advance a suspect in an engine that stumbles at low RPM?

Due to white plugs instead of brown, I re-jetted from .045 to .047 in the Stromberg 97 carb. Could that be the problem? Or maybe I was too timid in seating the replacement jets. If a main jet vibrated loose, could that cause the low RPM stumble?
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Old 10-19-2020, 05:51 PM   #5
Phil Gillespie
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Default Re: Chapter 2. Missing at part throttle.

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Originally Posted by drolston View Post
My rebuilt distributor came with the same calibration and vacuum brake adjustment, which I backed off to where the brake has a light drag with no vacuum.

So again, is spark advance a suspect in an engine that stumbles at low RPM?

Due to white plugs instead of brown, I re-jetted from .045 to .047 in the Stromberg 97 carb. Could that be the problem? Or maybe I was too timid in seating the replacement jets. If a main jet vibrated loose, could that cause the low RPM stumble?
running single stromberg 97 jetted 56 with 65 pv. 3 5/16 bore.
I have a very similar issue with a stumble at low rpm & speed 20 to 30 mph.
changed out a carb still seemed to be there. checked timing was ok.
thinking its in the idle circuit and at c/o to main circuit its ok. was also considering vacuum brake adjustment.
as on open road 50 to 60 mph runs fine.
i watch this with interest.
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Old 10-19-2020, 06:01 PM   #6
Bill OH
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Default Re: Chapter 2. Missing at part throttle.

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Some ideas!! First, do you have room behind the 3rd carb to install a mechanical fuel pump and use the electric for priming? I have that arrangement - mech pump primed by electric.
I would back the vacuum brake full out. I had a 221 - 1938 that the vacuum brake was tightened down and max speed was 74 mph. I backed the brake off and the speed went to 84 mph under the same conditions. At one point I saw 92 mph, but I could always count on 86 - 88 mph.

Let’s give some attention to the center carb - pull it off, open it up and give it alternate doses of Gumout and compressed air in each of the passages (idle, accelerator pump discharge and main circuit) and examine the condition of the accelerator pump piston and the two check valves. You may want to put a new kit in the carb - your call. I have a MOTORS MANUAL and the float setting for your year Stromberg is 15/32 and I do not think 1/32 is going to make that much of a difference, your float level now being 16/32.

With the engine warm, you might try opening the idle mixture screws to the rich side of a smooth idle - a judgement call. A quarter of a turn out just might solve your problem.

Do you have a timing mark establishing TDC by the dead stop method? If so I would set the timing 4 degs advance - motor will be more responsive.

The spiral ignition wires - you have two of them gone South. I have mentioned in the past the value of using Packard 440 with ends stripped back about 1/2 inch and folded over to make good contact with the terminals, now that you are using points and condenser.

Is the heat riser blocked with cupped core plugs - mine is.

While I think your problem is fuel, the other matters I have mentioned will make a tiger out of a kitty cat. I am open to further discussion.
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Old 10-19-2020, 08:08 PM   #7
drolston
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Default Re: Chapter 2. Missing at part throttle.

I run an electric fuel pump through the mechanical pump. I have the cheapo fuel pressure regulator which has worked very well so far. The pressure is set a shade under 3 psi.

First, I am going to check all of the plugs to see what that reveals.
I will set the idle mixture a little richer and see if that makes a difference.
I will back off the vacuum brake and check again.
I will take out the advance I have on the side of the distributor and see if that makes a difference.
If still not smooth, I will try the good 94 carb I have under the bench.
If all of that fails, I will switch to copper core ignition wires.
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Old 10-20-2020, 03:33 PM   #8
B-O-B
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Default Re: Chapter 2. Missing at part throttle.

drolston hope you are doing a fix at a time.
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Old 10-20-2020, 05:26 PM   #9
drolston
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Default Re: Chapter 2. Missing at part throttle.

Did two fixes on separate (I think) issues. Fixed the primary carb float level from almost 3/4" to very close to 1/2 inch. Road tested to full throttle in low and second. Wound right up to 5,000 rpm with no miss and no loss of power, starving for fuel.

Second fix was to verify continuity in the plug wire and replace the plug on the one cylinder that showed a sooty, oily plug. After running a bit, that seems to have fixed the missing at low rpm.

Next I will adjust the idle circuits to be sure they are not too lean.
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