Off idle hesitation - solved

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hotrod swinger

hotrod swinger

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1966 Signet
170 slant six
Stromberg WW3 carb
Vacuum is a steady 17
Initial timing 7 degrees
Idling at 750 in neutral
No vacuum leaks I can see
Idle mixture screws adjusted properly
Accelerator pump functioning properly

I’ve tried advancing the timing, I’ve tried adjusting the idle mixture screws, I’ve disconnected the vacuum advance as a test, can’t seem to get rid of this hesitation off idle.

Any ideas? Questions?

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Low speed idle enrichment screw.

If turning it in and out with no change, the the idle enrichment circuit is plugged.

Take the screw out, and go after the enrichment port with carb cleaner and compressed air > multiple times.

Then readjust again.


☆☆☆☆☆
 
Make sure Transfer Slots in the carburetor are not over exposed. If they are then that has to be corrected.
 
George Jets said:
Low speed idle enrichment screw.

If turning it in and out with no change, the the idle enrichment circuit is plugged.

Take the screw out, and go after the enrichment port with carb cleaner and compressed air > multiple times.

Then readjust again.


☆☆☆☆☆
Click to expand...
I’ve ensured these passages are clean, the idle air screws are working properly, vacuum drops off at around 2 turns.
 
If you feather the throttle just before and during the time it would hesitate does it still hesitate?
 
Dana67Dart said:
If you feather the throttle just before and during the time it would hesitate does it still hesitate?
Click to expand...
Sometimes it still hesitates even if I feather the throttle. It’s inconsistent. There is also a hesitation at light throttle from time to time.
 
340sFastback said:
Make sure Transfer Slots in the carburetor are not over exposed. If they are then that has to be corrected.
Click to expand...
Is there a way to check this without removing the carb? I don’t think I can see the slots but I’ll try again.

For what it’s worth my throttle blades are closed almost as far as they can be. My idle speed screw is only tightened a half turn.
 
I’ve been running it with the screws 2.5 turns out. Turning them out further doesn’t change the idle or vacuum level.
 
hotrod swinger said:
1966 Signet
170 slant six
Stromberg WW3 carb
Vacuum is a steady 17
Initial timing 7 degrees
Idling at 750 in neutral
Click to expand...
Seems a litle high relative to factory. Lemme look in the FSM. Don't think that alone should be a cause though.

hotrod swinger said:
I’ve disconnected the vacuum advance as a test,
Click to expand...
And forgive me if this sounds too basic,; Was the timing and idle set with the vacuum advance disconnected?
hotrod swinger said:
Sometimes it still hesitates even if I feather the throttle. It’s inconsistent. There is also a hesitation at light throttle from time to time.
Click to expand...
That's a good clue its not the accel pump but the something more basic. Fuel level is the only one I don't think has been brought up yet.
hotrod swinger said:
Is there a way to check this without removing the carb? I don’t think I can see the slots but I’ll try again.
Click to expand...
Sadly. No. And I can't even give you an approximate what to look for on the Stromberg. However I agree that knowing that the idle stop screw is only 1/2 turn does suggest that this is not the issue. Putting this together with the timing and idle speed it may be that the timing needs to be reduced and the throttle position opened. I mean a little. Let me look up the factory spec before I speculate more on this.
 
Mattax said:
Seems a litle high relative to factory. Lemme look in the FSM. Don't think that alone should be a cause though.


And forgive me if this sounds too basic,; Was the timing and idle set with the vacuum advance disconnected?

That's a good clue its not the accel pump but the something more basic. Fuel level is the only one I don't think has been brought up yet.

Sadly. No. And I can't even give you an approximate what to look for on the Stromberg. However I agree that knowing that the idle stop screw is only 1/2 turn does suggest that this is not the issue. Putting this together with the timing and idle speed it may be that the timing needs to be reduced and the throttle position opened. I mean a little. Let me look up the factory spec before I speculate more on this.
Click to expand...

I think 750 is correct idle as per FSM.

Timing and idle were set with vacuum advance disconnected.

I haven’t confirmed the float level is correct. I have to remove the carb for that, which I haven’t done yet.

I tried reducing the timing to 2.5 and opening the throttle position but the hesitation was still there off idle.
 
hotrod swinger said:
I’ve been running it with the screws 2.5 turns out. Turning them out further doesn’t change the idle or vacuum level.
Click to expand...
OK. I think I follow you.
I'll explain why I asked and maybe you've already got this but if not then we've put it to rest.
Making it richer the idle and vacuum fall off more slowly than when turning it leaner.
Turning it in (leaner) the drop from right on the money to dying is pretty quick. So normally that's considered the best approach to adjusting the mixtures; turn in until rpm or vac just starts to drop and then turn it back an 1/8 to a 1/4 turn. Its better to go back a little more because when the engine is put in gear it needs to be a bit richer to have power to turn the transmission.

I hope that made sense.

Unfortunately I haven't worked with that carb so don't know if 2 to 2.5 is typical. I expect someone here will know
 
Mattax said:
Turning it in (leaner) the drop from right on the money to dying is pretty quick. So normally that's considered the best approach to adjusting the mixtures; turn in until rpm or vac just starts to drop and then turn it back an 1/8 to a 1/4 turn. It’s better to go back a little more because when the engine is put in gear it needs to be a bit richer to have power to turn the transmission.
Click to expand...
I turned them in until vacuum dropped and then back out a half turn, which is about 2.5 out overall.
 
According to the FSM its right in between your two experiments. 5* BTDC at 550 for a 49 state car.
(If its a CAP car and has the original distributor, then it should be set retarded because advance is fast and long.)
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Regardless of your accelerator pump operation description, that's what I'm going with. The pump shot needs to be STRONG and LONG each time and it should start with IMMEDIATE moving of the throttle lever. No delay whatsoever. Can we get a picture of this beast?
 
I basically agree RRR.
But on a very very gentle acceleration the pump shouldn't do much and the problem should be noticibly less.
I'd be curious how it acted in neutral. I think what I'd do is check the spark plugs and in the cap (because they alsways have potential to reveal a good clue). Then take the aircleaner off and observe the carb and engine while manually opening the throttle. If it happens under no load, then it will certainly happen under load.

@hotrod swinger
With two idle circuits, each one contributes the same a single would have. That's a long way of saying 1/2 turn richer on each mix screw is probably double what it needs.
 
RustyRatRod said:
Regardless of your accelerator pump operation description, that's what I'm going with. The pump shot needs to be STRONG and LONG each time and it should start with IMMEDIATE moving of the throttle lever. No delay whatsoever. Can we get a picture of this beast?
Click to expand...
I’m getting an instant strong and long shot from the accelerator pump. I removed the air horn and oiled and flared the plunger and tested the check balls, it’s all operational.

A pic of the car or the carb?
 
Mattax said:
I basically agree RRR.
But on a very very gentle acceleration the pump shouldn't do much and the problem should be noticibly less.
I'd be curious how it acted in neutral. I think what I'd do is check the spark plugs and in the cap (because they alsways have potential to reveal a good clue). Then take the aircleaner off and observe the carb and engine while manually opening the throttle. If it happens under no load, then it will certainly happen under load.

@hotrod swinger
With two idle circuits, each one contributes the same a single would have. That's a long way of saying 1/2 turn richer on each mix screw is probably double what it needs.
Click to expand...
I have taken the air cleaner off and manually operated the throttle to observe the pump shot in operation. Sometimes there is a hesitation, most times not.
 
hotrod swinger said:
I’m getting an instant strong and long shot from the accelerator pump. I removed the air horn and oiled and flared the plunger and tested the check balls, it’s all operational.

A pic of the car or the carb?
Click to expand...
The carburetor.
hotrod swinger said:
I have taken the air cleaner off and manually operated the throttle to observe the pump shot in operation. Sometimes there is a hesitation, most times not.
Click to expand...
Yeah, but you're asking for our opinion, not yours. lol Since we have no pictures and preferably a video to see anything, I'm sticking with classic problems to classic symptoms. lol
 
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