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Stumbling under load - it's fixed!
https://www.slantsix.org/forum/viewtopic.php?t=57550		 Page 1 of 4
Author:  sargentrs [ Mon Apr 20, 2015 3:59 pm ]
Post subject:  Stumbling under load - it's fixed!
New development in the past few days. She started stumbling under load. As long as I'm easing up to speed she's good and cruises fine. If I give her some gas too fast, she stumbles and stutters until she gets to speed. Same thing going uphill. I've checked the vacuum advance with a vacuum pump and it's working ok, increases rpm with applied vacuum up to 20 psi. Getting around 17 psi vacuum at idle. Timing hasn't been messed with and was running fine 3 days ago with good acceleration and no hesitation. Tuned to max vacuum with vacuum advance plugged and capped. Adjustment at distributor is maxed out to the adjusting bracket. I've adjusted the air/fuel screw in the Holley 1945 to max out vacuum at 1000 rpm. Rotor on distributor feels tight with no wobble. New PCV valve and breather cap. What should I check next? Any and all advice is greatly appreciated!
Author:  SlantSixDan [ Mon Apr 20, 2015 4:46 pm ]
Post subject:  Re: Stumbling under load
What is your actual timing setting? Shoot for 5° BTDC.

Where does 1000 rpm come from? That is way too high an idle speed; you should be shooting for 750.

Is your vacuum advance connected directly to the carb, or does it still run to the OSAC valve on the firewall?
Author:  sargentrs [ Mon Apr 20, 2015 5:11 pm ]
Post subject: 
Sorry, I mis-phrased that. She's always idled around 700 rpm although I didn't hook the tach up this time. When I pulled the vacuum advance, she revved to up to around 1000 and when I capped it she went back down. Hooked up the vacuum gauge to the manifold and it was around 16 psi. I turned the screw until it steadied out at a max of 17 psi. Hooked the vacuum advance back up and it's idling around 700. Wasn't really any appreciable change over what it was. More or less just verified the setting.

Vacuum advance is run straight to the base of the carb per the Holley 1945 manual.

Not sure what happened, it was running fine until halfway to work this morning.
Author:  sargentrs [ Mon Apr 20, 2015 5:18 pm ]
Post subject: 
Sorry, wifey yelled and had to cut that post short. Didn't hook up the timing light to verify what it's currently at. Since it happened so suddenly and runs fine while cruising, I was focusing on a possible carb problem. I'll hook up the timing light and check that tomorrow when I get home from work, if she makes it.
Author:  SlantSixDan [ Mon Apr 20, 2015 5:48 pm ]
Post subject: 
Quote:
When I pulled the vacuum advance, she revved to up to around 1000 and when I capped it she went back down.
That ain't good. Sounds like you've got too much initial and/or vacuum advance.
Quote:
Hooked up the vacuum gauge to the manifold and it was around 16 psi.
Not unless you have a very big turbocharger! Pressure is measured in PSI; vacuum is measured in inches of mercury ("in. Hg").
Author:  wjajr [ Tue Apr 21, 2015 4:23 am ]
Post subject: 
Quote:
Vacuum advance is run straight to the base of the carb per the Holley 1945 manual.
Base of carburetor vacuum tap would see intake manifold vacuum. Advance needs to be plugged into ported vacuum tap located above throttle plates so that it only sees vacuum off idle when throttle is open at steady speed, and sees drop off of vacuum when accelerating.
Author:  sargentrs [ Tue Apr 21, 2015 7:28 am ]
Post subject: 
So, if that is the case, which port should I be using and what do I do with the lines connected to the other ports?

Image
Author:  sargentrs [ Tue Apr 21, 2015 7:33 am ]
Post subject: 
Quote:

That ain't good. Sounds like you've got too much initial and/or vacuum advance.
I'll put a light on it this afternoon and verify.
Quote:
Not unless you have a very big turbocharger! Pressure is measured in PSI; vacuum is measured in inches of mercury ("in. Hg").
Haha! PSI is just easier to type than in./Hg. But wouldn't that be something!

Thanks, guys!
Author:  SlantSixDan [ Tue Apr 21, 2015 10:05 am ]
Post subject: 
Quote:
So, if that is the case, which port should I be using
The one labelled "To distributor spark advance control", which is probably where you have it hooked up. It is at the base of the carburetor, as was the correct port on this carb's predecessor (the 1920). The physical location of the vacuum nipple doesn't mean anything, so don't get sidetracked or confused that way.
Quote:
and what do I do with the lines connected to the other ports?
See to it they're routed correctly.
Author:  sargentrs [ Tue Apr 21, 2015 1:13 pm ]
Post subject: 
I have everything connected as shown. My OSAC was gone, and all but the charcoal canister, emissions controls were stripped when I bought her. I've connected that OSAC line, bottom of the carb, to the vacuum advance on the distributor. Thanks!
Author:  sargentrs [ Tue Apr 21, 2015 1:16 pm ]
Post subject: 
Ok, this just can't be right. Vacuum advance capped and plugged. She:s idling at 650 rpm and tming reads 5* ATDC! No wonder.
Author:  Reed [ Tue Apr 21, 2015 2:02 pm ]
Post subject: 
You need to verify that the timing mark on the vibration damper/crank pulley has not slipped. Over time the rubber bonding the outer ring to the inner ring can degrade and allow the outer ring to slip. You need to get a piston stop tool and verify that the TDC on the damper is still true TDC for the engine.
Author:  sargentrs [ Tue Apr 21, 2015 4:56 pm ]
Post subject: 
Running great now! Verified TDC on damper like Reed suggested and it hasn't floated, dead on zero. Set timing to 5* BTDC, vacuum advance capped and plugged, and set idle to 750 rpm per Dan's direction. Took her for a test drive, WOT from stop and up a 30% grade at increasing rpm and no stutter at all. It's amazing how well a car runs when the timing's set right, ain't it? Thanks to all who posted help and advice!
Author:  sargentrs [ Wed Apr 22, 2015 3:14 pm ]
Post subject: 
And she was doing so well! Drove it around yesterday, to work this morning, and then about 1/2 way back it started again. Babied it home, checked the timing and it was still at 5* BTDC. However, it's not a rock steady 5*. Seems to float between 5*-10*, stable at 5* for a couple of seconds, erratic for a few seconds, stable for a few seconds, up to 7*, back down to 5*, randomly. Increased timing to 7*-8* and took her for a drive. It was better at 1st-2nd but when the tranny shifts into 3rd it starts stumbling when you give it too much throttle. Took it up to 10* and no noticeable change. Took it back down to 7* and left it there. During all of this, I plugged and unplugged the vacuum advance each time. On the next to last trip, I left it unplugged and it stumbled through 1st-2nd too. Plugged it back and up and smooth through 1st-2nd but stumbles in 3rd with heavy throttle. Not as bad easing into to it but won't take heavy acceleration without stuttering. With the erratic timing readings and stuttering under acceleration, would this indicate the distributor as a problem? What further testing can I do? Thanks for any and all help and advice!
Author:  wjajr [ Wed Apr 22, 2015 3:55 pm ]
Post subject: 
The governor weights may be sticking causing slow reaction to rpm change, and or there is a spring problem where it is broken, and or too weak to retract weights. Time to investigate distributor's inner workings.

One way to see what is going on with timing is to start at idle rpm with vacuum advance disconnected and record mechanical advance in degrees every 50 or 100 rpm to 3000 rpm. Second task would be to connect vacuum advance go through 50 or 100 rpm increments until the additional advance contributed by vacuum advance. This information will show your advance curve, and help us to guide you to a solution.
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