Slant Six Forum :: Big dead spot

It's a stumble/stall right off idle. When it's warm, it's a stumble right off idle. I'm not sure if we got the choke dialed in yet, we swapped manifolds two weekends ago (the old one was cracked into two pieces) and we're still tunning the choke. It did this before the manifold swap, back when the choke was set correctly though.

By the Way: Thanks for the links Dan. I still haven't got a chance to read them though because I've been burried alive with homework.

School

It's a stumble/stall right off idle. When it's warm, it's a stumble right off idle.

haha you should consider adapting a homework optional policy. After enacting such a policy, my ratings shot through the roof on self polls

Quote:

It's a stumble/stall right off idle. When it's warm, it's a stumble right off idle.

That'll be an accelerator pump problem. With the engine off and the air cleaner lid removed (and choke open), peer down into the carburetor as you operate the throttle lever. You should see a good strong shot of fuel squirting into the middle of the carb throat each time you operate the lever. If no shot (or a weak/piddly one) you've got accelerator pump problems. Maladjusted or missing linkage, missing pump drive spring, broken or worn pump operating cam, missing pump check ball in the main metering block of the carb (or pump check ball seat so badly corroded it no longer seals)...

So is my train of thought correct? Or is there still something I'm missing?

Author:	/6 Matt [ Tue Mar 23, 2010 8:04 pm ]
Post subject:	Big dead spot
About 1 1/2 - 2 years ago me and my Dad rebuilt the Holley 1920 Carb on my Dart, and ever since then it's had a huge dead spot. I never got around to asking the more knowlegable one's onpinions on the matter untill now, so here I am. Symptoms Idles fine WOT is fine At 15-20% there is a huge dead spot that causes the car to stall in gear, in park or neutral it will stumble when your rev the engine. Timing is where it should be The dead spot shrinks as the engine warms up and will almost disappear in an 80+ degree enviroment at normal opp. temp. The only way to avoid stalling (besides giving 7 mins. to warm-up before driving of course) is to "feather peddle it" - to let off really quick and hit the gas quickly. I don't remember the brand of the rebuild kit, but we got it from autozone which could be the problem. Immediatley after the rebuild, there was a vaccum leak that would cause a really annoying high-pitch whisling noise. That problem was soon fixed by spraying ether around the carb a listen for the engine to Idle up to locate the leak, then we re-used an old gasket (still in very good condition though) along with the newer gasket and that fixed the problem well. I can't remember the name of the gasket but it was located above the fuel bowl under a small cast iron plate held to the carb. body by three screws, it had a diaphram in between if I recall correctly. The re-used gasket could most likely also be the problem. Me and my Dad's theory was that a piece of trash got caught in the jet and it runs too lean when it's cold, when it warms up, it handles the lean mixture better. My Auto. Tech. teacher was thinking it was a vaccum leak because the metal would expand on the carb. to seal up the leak when it gets warm, but unfortunately the semester ran out before we had a chance to test that theory. But he did acknowledge me and my old man's theory as a valid one. Regardless, up 'till now the problem was merely an inconvience, but this morning on the way to school she stalled in an intersection and we almost had a wreck. I believe I covered all the bases, if not I'll be back on tommorrow morning before school. Good night all, and thanks for the words of wise.

Author:	Doctor Dodge [ Tue Mar 23, 2010 8:22 pm ]
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Does the accelerator pump shot work? Does the distributor vacuum advance work? What is "timing where it should be"... more advance may help. DD

Author:	SlantSixDan [ Tue Mar 23, 2010 8:52 pm ]
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It's all been done. Carburetor operation and repair manuals and links to 1920 repair/modification threads are posted here for free download.

Author:	/6 Matt [ Wed Mar 24, 2010 2:58 am ]
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Doc. The accelerator pump shot is working working like it should (I forgot to mention that part). I believe the vaccum advance is in working order, we need to check that though. We set the timing at TDC, then advanced it untill we heard a ping, then backed it off a little. If I was to guess, I would say 5-7 degrees BTDC. I kinda wish we had one of those timing lights that allows you to set the advance. Dan. I'll check out that link after school.

Author:	carlherrnstein [ Wed Mar 24, 2010 7:27 pm ]
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You say that idle and wide open throttle are fine but, in between is not quite right? till ti warms up? hows your choke is it adjusted properly? Thats how mine is till its warm, but it has no choke. good luck

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Big dead spot https://www.slantsix.org/forum/viewtopic.php?t=39537	Page 1 of 2

Author:	SlantSixDan [ Wed Mar 24, 2010 9:02 pm ]
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If it's not a stumble as soon as you tip into the throttle from idle, but is more a hesitation/sag/surge after the accelerator pump shot is used up (or on acceleration from one cruising speed to another) then it's usually in the step-up/power valve/"economizer" system. Extensive material on repairing and dialing-in this system is in the 1920 threads linked from the carb manuals thread posted above.

Author:	/6 Matt [ Thu Mar 25, 2010 4:28 am ]
Post subject:
It's a stumble/stall right off idle. When it's warm, it's a stumble right off idle. I'm not sure if we got the choke dialed in yet, we swapped manifolds two weekends ago (the old one was cracked into two pieces) and we're still tunning the choke. It did this before the manifold swap, back when the choke was set correctly though. By the Way: Thanks for the links Dan. I still haven't got a chance to read them though because I've been burried alive with homework. School

Author:	Karasik87 [ Thu Mar 25, 2010 6:43 am ]
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haha you should consider adopting a homework optional policy. After enacting such a policy, my ratings shot through the roof on self polls

Author:	rustyfords [ Thu Mar 25, 2010 7:10 am ]
Post subject:
Quote: It's a stumble/stall right off idle. When it's warm, it's a stumble right off idle. I'm not sure if we got the choke dialed in yet, we swapped manifolds two weekends ago (the old one was cracked into two pieces) and we're still tunning the choke. It did this before the manifold swap, back when the choke was set correctly though. By the Way: Thanks for the links Dan. I still haven't got a chance to read them though because I've been burried alive with homework. School I hear you. I'm 41 years old and am buried in homework for the master's degree that I'm working on, and it's been keeping me from my slant car and my 54 Ford project.

Author:	SlantSixDan [ Thu Mar 25, 2010 8:35 am ]
Post subject:
Quote: It's a stumble/stall right off idle. When it's warm, it's a stumble right off idle. That'll be an accelerator pump problem. With the engine off and the air cleaner lid removed (and choke open), peer down into the carburetor as you operate the throttle lever. You should see a good strong shot of fuel squirting into the middle of the carb throat each time you operate the lever. If no shot (or a weak/piddly one) you've got accelerator pump problems. Maladjusted or missing linkage, missing pump drive spring, broken or worn pump operating cam, missing pump check ball in the main metering block of the carb (or pump check ball seat so badly corroded it no longer seals)...

Author:	/6 Matt [ Thu Mar 25, 2010 2:38 pm ]
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Quote: haha you should consider adapting a homework optional policy. After enacting such a policy, my ratings shot through the roof on self polls Quit being a bad influence! lol

Author:	/6 Matt [ Thu Mar 25, 2010 2:39 pm ]
Post subject:
Quote: Quote: It's a stumble/stall right off idle. When it's warm, it's a stumble right off idle. That'll be an accelerator pump problem. With the engine off and the air cleaner lid removed (and choke open), peer down into the carburetor as you operate the throttle lever. You should see a good strong shot of fuel squirting into the middle of the carb throat each time you operate the lever. If no shot (or a weak/piddly one) you've got accelerator pump problems. Maladjusted or missing linkage, missing pump drive spring, broken or worn pump operating cam, missing pump check ball in the main metering block of the carb (or pump check ball seat so badly corroded it no longer seals)... Alright, I check to see how the shot o' fuel is operating.

Author:	/6 Matt [ Sat Apr 03, 2010 10:40 am ]
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I never did get back to you guys about the accellerator (it's too early in the morning to spell that right) pump shot. It's working fine, but in other news my dad was listening to the dead spot (revving the engine without the air cleaner on it) when he lost most of his hair (what little is left anyways ) to a jet of flame that came up through the carbeuretor. So our next stop was the distributer. Now last time we set the timing, we had the timing light and we set it to TDC like Haynes stated too and found a skip on idle. So dad andvanced the timing about 2-3 degrees and the skip went away so we left it there. So, skipping back forward to the present, we decided to just leave the timing light out of the mix and adjust it while reving the engine (with our faces away from the carb) and seeing if the dead spot might dissapear. By finding our "optimal" timing that way, we almost eliminated the dead spot. But we're back with the skip agian. By the way, our current timing is a little retarded from what it was before. So, here's the wierd part: the skip only persists on idle and in park. When in gear the skip is gone and only occurs slightly if you let off the gas while moving. So we came to this conclusion: the skip will only occur if there is little load applied to the transmission. Now Dad thinks that we might possibly have a burnt valve, and for a while I went along with that theory. But then a thought so simple occured to me that I almost dissmissed it: valve lash... Duh!!! . I still haven't mentioned it to dad, but the more i think about it, the more it makes sense. Improper valve lash would be giving us our crappy idle, it could have caused the intake valve to be open and allow the flamethrower effect to take place when we had the timing too advanced. And to further confirm my theory 1. the rocker arms are very noisey and 2. the last time they were adjusted was about 4 years ago or approximatley 25k miles (that part is a rough estimation). I'm still suspecting that the dead spot is a carb problem because it occured directly after we rebuilt it (meaning it was possibly our fault). I never mentioned the reason we rebuilt the carb, was because the car sat unregistered for almost a year and it was reregistered only to find a leaking fuel bowl. In conclusion, I've left with two things from all this: A. dad got lucky (cause he still has his face) and B. old, un-restored, and mediocrely maintaned vehicles usually have more wrong with them than one would first suspect. So is my train of thought correct? Or is there still something I'm missing?

Author:	Aggressive Ted [ Sat Apr 03, 2010 4:23 pm ]
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Quote: So is my train of thought correct? Or is there still something I'm missing? 1. Do the valve lash first .010 and .020. That will eliminate any valves hanging open too long and will clean up the idle. 2. If the accelerator pump cam is sticky or gritty from the shaft rusting, clean it well and clean the shaft with a red scotch-brite pad oil the shaft and (bend the linkage) so there is no slop, causing a delayed pump shot. 3. Next tell us what size jet your running. It should be at least a #58 and a #60 is even better. 4. Re-check the pump shot to see how quickly it engages and how much fuel comes out. If it is still weak you can clean it out with a monofilament fishing line and some spray cleaner. With the economizer block off push the fishing line through from the inside. Sometimes it can oxidize around the outlet and not allow good shot to come out. When you spray carb cleaner through the hole it should be a real healthy shot. 5. Set your timing at 10 degrees. You can set it at 6 which will be a big difference, but at 10 you will not feel the delay if you got a good healthy pump shot. Don't run it near TDC per the Peterson manual. You will only get about 18 mpg....... you should be in the low 20's. This will make a big difference in how the car feels. Ultimately you should do a distributor re-curve to get at least 50 degrees of advance. If you motor is in fair shape, you should get at least 23 to 24 mpg if not more.

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Author:	/6 Matt [ Sat Apr 03, 2010 7:27 pm ]
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Ted, The car gets me back and forth between school each day, so theres no telling how soon we might be able to work on that valve lash. But me and dad will address the valve lash and the timing first and if problems still persist well address the carb. As for recurving the distributor, we might have to find some one local as I cant comprehend the artical on here, my dad doesnt know how to do it, and my auto teacher thinks the factory setting is always right. Besides that, well hopefully be swapping to electronic ignition this summer and sometime within the next year a two barrel will be hangin off the side of the engine.