Off idle stumble.

[Nate S]

Well lets see....
I dont race.
I drive on the street and I have a large slightly modified internal combustion engine that should be quite capable of accelerating without an off idle bog.
Maybe I am screwing up my car, but if so I am doing it while requesting help from folks more knowlegeable than me.
I changed the "squirters" (accelerator pump jets) because I read somewhere that an off idle stumble could be a lack of fuel provided by the accelerator pump and larger "accelerator pump jets" (geez I hope that is the correct term) would help. I did it over 2 years ago and it's the only time in my life that I have ever done it and I recall that they were designated with a number but, silly old me, I don't remember the number and didn't feel like yanking them back out of the carb unless it is required to help my problem.
Thanks.

Absolutely. If someone can’t make a car respond appropriately when stuffing the pedal to the floor hard at any and all speeds -and- have it perfectly driveable there’s something wrong. They did it from the factory, not much of a stretch to expect it when altered a bit.

Most of the comments are in about the right direction. Initial timing could be a bit higher, maybe 16-18 (but keeping total at 36-38). Should be able to fit a thin (.002) feeler gauge between the cam and pump lever at idle (no preload). Squirters can go bigger and accel pump can go to a 50cc but that really shouldn’t be required. If secondaries open too quick it can cause this bog. (No ball and white spring for example).

Other less common stuff is starting lean because carb shafts are worn/leaky. Another is the pump check not working well. Old ones had a ball in there newer have a rubber diaphragm. The diaphragm works a little better.

 

[MWFan]

Ok heres the latest. Checked with vac guage and timing light.
Disconnected vac adv, plugged
800rpm, 12° BTDC, vac 18lbs.
Full adv @2500, 34°.

Reset initial to 14°, vac 18lbs
Full adv @2500, 37°

Reconnect vac adv, idle set to 800, full total advance now 59°.

Made SURE the accelerator pump moves INSTANTLY with throttle, checked secondaries on initial acceleration in neutral and they never move.

Now if the fricken rain would stop I'd road test it...

 

[pedal2themetal]

HI You can't set total timing with out everything hooked up including vacuum advance.
To set total timing everything needs to be hooked up including Vacuum advance, then set it at 36* @ 2,500 RPM's
and you should be good to go..

good luck
tim

 

[MWFan]

HI You can't set total timing with out everything hooked up including vacuum advance.
To set total timing everything needs to be hooked up including Vacuum advance, then set it at 36* @ 2,500 RPM's
and you should be good to go..

good luck
tim

Thanks.

 

[Nate S]

HI You can't set total timing with out everything hooked up including vacuum advance.
To set total timing everything needs to be hooked up including Vacuum advance, then set it at 36* @ 2,500 RPM's
and you should be good to go..

good luck
tim

This is incorrect. Set to 36 with vac advance off. (Unless you’re nuts and running it full load on a dyno). With vac advance on this would be roughly 50 some-odd degrees. When under full throttle there’s no vac and therefore no vac advance.

 

[MWFan]

Got a test drive in between raindrops. Honestly it is somewhat better, not completely. I think I'm going to let this shortblock get broke in, listen carefully for detonation and see how it runs in a couple hundred miles.
Other than the slight stumble left upon dropping the hammer, this car runs beautifully.
With this much total timing, I obviously am concerned about detonation , but the all in timing is only a couple of degrees more than suggested in the Hot Rod Magazine article provided by Leo. It might be within tolerance of my timing light. I do know how to adjust the vacuum advance cannister on the distributor so if it pings, I'll retard it a couple.
Thanks all.

 

[RemCharger]

Wouldn't hurt to just leave the vac adv plugged until you get some more miles on it.
I personally wouldn't use it at all.

 

[MWFan]

This is the HotRod article MoparLeo provided.

 

[oldbee]

HI You can't set total timing with out everything hooked up including vacuum advance.
To set total timing everything needs to be hooked up including Vacuum advance, then set it at 36* @ 2,500 RPM's
and you should be good to go..

good luck
tim

Pedal— ur wrong there. See post#26.

 

[Nate S]

Wouldn't hurt to just leave the vac adv plugged until you get some more miles on it.
I personally wouldn't use it at all.

This works except for adding heat and giving up 2-3 mpg.

 

[RemCharger]

To each their own.

This works except for adding heat and giving up 2-3 mpg.

 

[MWFan]

Pedal— ur wrong there. See post#26.

And #28

 

[pedal2themetal]

HI MWFan... If you look at Part F of the photo in post #28 It say's
"Total advance , Mechanical Plus vacuum advance"
How can it be all in when its NOT all in without the vacuum advance?
Good luck
tim

 

[Crackedback]

Vacuum advance is in additional to any advance from initial and mechanical. You set TOTAL for what WIDE OPEN THROTTLE would represent, which should NEVER have vacuum advance involved in real world applications.

You also can't pick a rpm number out of thin air and say set timing to 36* at 2500rpm. What happens if the distributor has 10 more degrees in it at 3000? Now you have 46* total... which is not going to be good for most any engine with todays pump swill. You MUST find the point where the distributor is no longer advancing. That is the RPM where you define and set total timing with vacuum advance disconnected. Seen plenty of distributors not stop advancing until over 4K including MSD's OOB with the garage door springs on them causing issues.

After you get the initial and mechanical sorted out, you tune the vacuum advance for your application. Some cars can handle vacuum, some can't. Depends on the build and if you can actually get the fuel system to get lean enough to benefit from the additional timing. Lean mixtures require more timing.

I disagree with plenty in that article on setting timing. Total timing method is a horrible way to set up a street car. Think building a house. Do you build the walls or roof first, nope. The foundation of the timing profile starts with initial/idle timing. You build from there. The cars were initial timing crutched even in the late 60's with emissions requirements. Stock 318's like 10-14 idle timing. How do you think an engine with a bigger cam would like 5* as that article suggest as a starting point. It does have good explanations, not so good on real world settings. Bottom line, if you have the idle set and you give engine more timing and it increases RPM, it likes the timing.

Ever hear mopars grind the starter for hours trying to start or having to mash the pedal to get them to start? That's usually a symptom of insufficient initial timing.

 

[MWFan]

Vacuum advance is in additional to any advance from initial and mechanical. You set TOTAL for what WIDE OPEN THROTTLE would represent, which should NEVER have vacuum advance involved in real world applications.

You also can't pick a rpm number out of thin air and say set timing to 36* at 2500rpm. What happens if the distributor has 10 more degrees in it at 3000? Now you have 46* total... which is not going to be good for most any engine with todays pump swill. You MUST find the point where the distributor is no longer advancing. That is the RPM where you define and set total timing with vacuum advance disconnected.

After you get the initial and mechanical sorted out, you tune the vacuum advance for your application. Some cars can handle vacuum, some can't

Vacuum advance is in additional to any advance from initial and mechanical. You set TOTAL for what WIDE OPEN THROTTLE would represent, which should NEVER have vacuum advance involved in real world applications.

You also can't pick a rpm number out of thin air and say set timing to 36* at 2500rpm. What happens if the distributor has 10 more degrees in it at 3000? Now you have 46* total... which is not going to be good for most any engine with todays pump swill. You MUST find the point where the distributor is no longer advancing. That is the RPM where you define and set total timing with vacuum advance disconnected.

After you get the initial and mechanical sorted out, you tune the vacuum advance for your application. Some cars can handle vacuum, some can't. Depends on the build and if you can actually get the fuel system to get lean enough to benefit from the additional timing. Lean mixtures require more timing.

Thanks. The total mechanical on THIS distributor is all in at about 2500.

 

[Dan64]

HI MWFan... If you look at Part F of the photo in post #28 It say's
"Total advance , Mechanical Plus vacuum advance"
How can it be all in when its NOT all in without the vacuum advance?
Good luck
tim

Total advance is initial timing plus Mechanical advance in the dist. only. Vacuum advance isn't available under wide open conditions because there isn't any vacuum to operate the canister. Vacuum advance is only for part throttle cruising when the engine is producing vacuum and needs more timing is complete the burn.It needs more time ( advance) because the cylinders mixture is not tightly packed, it takes more time for the gas and air to find each other and party. Under wide open conditions the cylinders are packed full of gas and air, the flame front travels fast meaning there is not a need or desire for additional timing. All timing procedures should be done with the vacuum advanced off and plugged. I would also think you wouldn't want to see more than 48-50 degrees with the vacuum advance kicked in , remember truth is you really don't even need it so why allow the vacuum advance to run it up too high and possibly hurt the motor. If the arm on your vacuum canister has a 8 stamped in it that means that it increases your part throttle timing by 16* ( crank degrees vs. dist. degrees) hope this helps.

 

[Nate S]

If the arm on your vacuum canister has a 8 stamped in it that means that it increases your part throttle timing by 16* ( crank degrees vs. dist. degrees) hope this helps.

Good point here ^^, and rarely mentioned. Both the vac can and the the mechanical limiter have these numbers. They are always in distributor degrees, just like the distributor specs in the manuals. These vary a huge amount between different distributors. I’ve always used junk distributor parts to mix-n-match to get the mech and vac advance I want.

Amazing how all over the place the factory settings were. Examples:
‘64 426W, init=10, all in = 26 (vac=18)
‘64 383-4, init=10, all in = 30 (vac =18)
‘64 361-2, init=10, all in = 33 (vac = 26)
‘70 440-4, init=tdc, all in = 22! (Vac = 22)
‘70 440-6, init=5, all in = 31 (vac = 26)
‘70 426, init=tdc, all in = 30 (vac = 16)

 

[MWFan]

Total advance is initial timing plus Mechanical advance in the dist. only. Vacuum advance isn't available under wide open conditions because there isn't any vacuum to operate the canister. Vacuum advance is only for part throttle cruising when the engine is producing vacuum and needs more timing is complete the burn.It needs more time ( advance) because the cylinders mixture is not tightly packed, it takes more time for the gas and air to find each other and party. Under wide open conditions the cylinders are packed full of gas and air, the flame front travels fast meaning there is not a need or desire for additional timing. All timing procedures should be done with the vacuum advanced off and plugged. I would also think you wouldn't want to see more than 48-50 degrees with the vacuum advance kicked in , remember truth is you really don't even need it so why allow the vacuum advance to run it up too high and possibly hurt the motor. If the arm on your vacuum canister has a 8 stamped in it that means that it increases your part throttle timing by 16* ( crank degrees vs. dist. degrees) hope this helps.

Thanks. To my knowledge, the harm that too much advance will cause comes in the form of detonation or "pinging".
With this setting on my car that is what I am concerned with. The car always gets 93 octane.
The car runs very well. It starts cold or hot nearly instantly and has amazing throttle response, UNLESS it it running at something below 1200 rpm and you stomp the throttle. Thats what this thread was stared for, not necessarily a how to on acheiving the best advance curve, although I do understand that you can't get one without the other. TO A POINT. The main reason I wanted to hook up the vacuum advance after this rebuild is that I think I may do more highway travelling than I did before. Maybe go to some rally's and shows out of town. But I am convinced that I should be able to hammer this throttle at idle and not have a stumble. Since that condition means virtually no vacuum, what the total highway vacuum specs are shouldn't be the major factor in the bog unless I totally misunderstand spark timing.
Now that I have it working fairly well with vacuum advance functioning, I think it is time to rule out timing as the source of the stumble and go back to the likeliest culprit, carburetor. This is a Holley 750 form the factory with one modification, the accelerator pump squirter is the next size larger than what was in it from the factory.
I plan to do the pump adjustment procedure provided by another member today and see if it has effect.

Thanks everyone.

 

[dadeo]

Well now we know it's not off idle stuff but secondaries kicking in too fast. Now most likely a spring change is needed in your vacuum secondaries.
Andy
(Happy New Year)

 

[RJS]

Your last entry talks about stomping the throttle when you get the stumble. If that has beed the case the whole time yes the stumble can very well be rear of carb. and req. a spring change in the sec. canister.

When you are doing your idle adjustments with a vacuum gauge proper is to check vacuum idling IN gear with emergency brake set unless your car is a 4 speed.
Ron