Off idle hesitation - solved

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hotrod swinger said:
I believe that factory initial timing is 2.5 BTDC at 750 RPM.
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Notice even in '66 the CAP engines idle 100 to 150 rpm higher than the standard equipment.
HC and smog reduction methods required less initial advance (sometimes retarded) to put more heat into the cylinder walls, and the loss in power was made up for with high initial rpm. The retarded initial advance was made up for in the distributor with a really fast and longer advance curve. This can become a headache when distributors are swapped around.
 
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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.
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Do it slow with minimal pump shot. If it happens at all going slow, then its something more basic. Idle circuits, fuel level, or maybe not fuel related. If it happens when driving real slow, not accelerating, then its definately not the pump shot.
 
Mattax said:
HC and smog reduction methods required less initial advance (sometimes retarded) to put more heat into the cylinder walls, and the loss in power was made up for with high initial rpm. The retarded initial advance was made up for in the distributor with a really fast and longer advance curve. This can become a headache when distributors are swapped around.
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I should mention, I have no idea about the distributor. It came with the car and for all I know it’s in need of an overhaul.

I have purchased a new distributor from Ray and it’s en route from PA. Something ive been meaning to do for awhile now.
 
hotrod swinger said:
I should mention, I have no idea about the distributor. It came with the car and for all I know it’s in need of an overhaul.

I have purchased a new distributor from Ray and it’s en route from PA. Something ive been meaning to do for awhile now.
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Was it a stick or auto? and was it from California?
 
Mattax said:
Do it slow with minimal pump shot. If it happens at all going slow, then its something more basic. Idle circuits, fuel level, or maybe not fuel related. If it happens when driving real slow, not accelerating, then its definately not the pump shot.
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It doesn’t hesitate when I go slow manually with the air cleaner off observing.

Sometimes it hesitates when I’m driving slow at part throttle.
 
hotrod swinger said:
It’s auto from Canada.
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Aw geez. I don't have a Canadian book. They did a lot of stuff different in the 60s. Prob not the timing. lets assume its a typical 170 auto. If the distributor still has a tag, the number will match the distributor number in the book.
 
I followed all of the FSM service procedures for the accelerator pump and everything checks out.
 
hotrod swinger said:
I just don’t know what more I can do with it. The plunger is flared out, the linkages are set up properly, the check balls operate properly and it gives an immediate strong shot to both barrels.
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That's why I'm asking for a video. You want our opinion, right? It may be fine, but lets rule it out 100% since it is easy to do. Agree?
 
You should be able to see the timing mark advance with rpm.
With a dialback light or timing tape you can measure timing beyond the marks on the engine.

7* at 750 rpm falls within spec for the original distributor if its initial was set at 5*, 550 rpm.
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(All I did to create this graph was to mark 5* at 550 rpm. Then take the distributor degrees advance from the book, double them and add them to the 5* initial.)

I don't think this is timing related based on what you've described so far. Having driven it wih vac advance plugged and still having it happen, and its just in the off idle range, it just doesn't seem like it would be the cause.
 
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Does it have a stock cam? A larger than stock cam changes the idling & off idling dynamics.

Try this simple test: engine idling, slowly close the choke blade until engine sound changes. Wire/lock the blade in this position for a test drive. If stumble is improved/gone you have a carb problem: lean. Report back after the test.
 
I totally disagree that that is a strong pump shot. Especially on the right hand side.
 
I’m not sure what to try next. I followed the FSM and tested the plunger and the check balls, everything is operational during the tests.

I guess there must be some problem with the float or the linkage.
 
While being called a dork by @RustyRatRod did hurt a little bit, it motivated me to look more closely at the accelerator pump linkage. It was hanging up and the pivot screw was loose. Problem solved. Thanks for the assistance everyone.
 
hotrod swinger said:
While being called a dork by @RustyRatRod did hurt a little bit, it motivated me to look more closely at the accelerator pump linkage. It was hanging up and the pivot screw was loose. Problem solved. Thanks for the assistance everyone.
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Suck it up, buttercup. LMAO. Seriously, glad you got it fixed, man. Nice job!
 
hotrod swinger said:
I’m not sure what to try next. I followed the FSM and tested the plunger and the check balls, everything is operational during the tests.

I guess there must be some problem with the float or the linkage.
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You said balls.
 
Here’s a pic of the carb. It was the big pivot screw that was a bit loose causing the plunger linkage to bind up.

IMG_5300.jpeg
 
gotta ask, I aint familiar with Stromberg carbs either. But now that I see that its a 2 bbl and not a 1 bbl it should have 2 mixture screws. are they both even? By past posts in this thread it sounded like you were only messing with 1 idle mixture screw. Are both "peaked out" where the vac shows highest with each screw where its currently at? If they're off balance then that can cause issues too.
 
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