Rochester 4bbl dead spot - Impala Tech
Troubleshooting Diagnosing problems

 
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post #1 of 14 (permalink) Old 10-01-2015, 11:05 AM Thread Starter
 
 
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Unhappy Rochester 4bbl dead spot

1. Tore apart the carb, dunked it in cleaner.
2. Rebuilt carb.

Figured the new accelerator pump would stop the problem, but nooooooo.

Still has a big dead spot. Yes, there is a squirt when you look down into it and gas it.

3. New plugs and wires.
4. New points.
5. I can see the vacuum advance moves when I open the little window on the distributor and give it gas.
6. Checked the suction bulb by sucking on the vacuum line from the carb to the distributor and putting my tongue against the line to see if it held suction. It did.
What am I missing? I know that pulling my hair out or strangling someone won't help, so any advice I can get here is very much appreciated!
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post #2 of 14 (permalink) Old 10-01-2015, 12:50 PM
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The accelerator pump is adjusted by selecting a different hole in the plunger arm and by bending the linkage rods to obtain a custom accelerator pump discharge curve.

If this where a Holley you have the option of changing the accelerator pump cam to affect when in relation to the throttle blades the accelerator pump activates, changing the accelerator pump size, to increase the volume of the discharge, or the squirter's orifice size to speed up or slow down the rate of discharge. If you had a Webber carburetor everything associated with the carb is adjustable. So you can modify the entire carb on the fly if you want to pay that much to have that amount of flexibility for racing.

All carbs work the same way and all will make the exact same power level for a given cfm rating when properly tuned. What separates one brand from another is the ability and flexibility to tune the carb. This is why half the population swear by Holleys while the other half swear at them. "With flexibility comes great responsibility" to tune it properly; to mis-quote Spiderman's Uncle Ben.

Rochester took the approach of tuning it once at the factory and replacing it with another if it ever died or wore out. They can be tuned but it is difficult and requires knowledge and little luck (I have broken more than few carbs trying to bend the rods without actually breaking things off the carb). Carter is a little more flexible in their tuning but most of it is preprogramed to respond to engine vacuum that is beyond your control.

Could be worse you could have to play with a Solex or Zenith carb on an English or German sports car.

Big Dave
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post #3 of 14 (permalink) Old 10-01-2015, 03:51 PM Thread Starter
 
 
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Big Dave
Thanks for the response!
So, if I'm catching you right, you're saying to adjust (carefully bend) the linkage to the acc pump so it begins pumping earlier on the downward stroke? That makes sense. I'll try that first.
The linkage is currently hooked into the hole of the plunger arm that will give it the longest stroke, so changing that is out. Looks like bending the linkage is the only option from here outside of replacement.

English or German sports car? Nah- I'll have none of that. Haha

Thanks again!
Brad
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post #4 of 14 (permalink) Old 10-01-2015, 05:57 PM
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Quote:
Originally Posted by Bradsss View Post
Big Dave
Thanks for the response!
So, if I'm catching you right, you're saying to adjust (carefully bend) the linkage to the acc pump so it begins pumping earlier on the downward stroke? That makes sense. I'll try that first.
The linkage is currently hooked into the hole of the plunger arm that will give it the longest stroke, so changing that is out. Looks like bending the linkage is the only option from here outside of replacement.

English or German sports car? Nah- I'll have none of that. Haha

Thanks again!
Brad

Me neither but I had a garage that specialized in hot rods and people would drag in a Jag, Triumph Stag or an M3 Beamer and expect me to save it from the idiots that had worked on them before I ever saw them. I have seen some strange things (such as corks stuffed into the throats of two out of three carbs to get the car to run, though poorly on just the center one).

I often wondered what kind of training these mechanics at the foreign car dealerships received as they hadn't a clue how a carb worked. And multiple carbs with progressive linkage baffled them as much as dark mater bothers me.

Big Dave
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post #5 of 14 (permalink) Old 10-01-2015, 08:23 PM
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Have you verified timing, dwell, idle speed, and set mixture correctly?

Two doors, four doors, wagons, and ragtops.
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post #6 of 14 (permalink) Old 10-03-2015, 01:14 AM Thread Starter
 
 
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so please forgive my ignorance and try not to laugh at me too much.
I grew up under an old school Dad who seemed to be able to listen to it and know what to do to make it run at it's best. And they always seemed to. I, however, haven't learned that skill. SO here I am...

Timing- Maybe I'm terribly off, but I was taught to advance the timing to where it barely pings on acceleration, then back it off slightly. (I told you not to laugh)

We put new points in and gapped it to .030 (I believe, and as I've been checking online, seems to be to way too high)

As far as idle speed, I just adjust it to where it sounds good.

Mixture- I have no idea how to properly set it.

Yeah, I know... sounds like a train wreck. I have a lot of learning to do. Thanks for any advice!
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post #7 of 14 (permalink) Old 10-03-2015, 01:18 AM Thread Starter
 
 
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Quote:
Originally Posted by Big Dave View Post
Me neither but I had a garage that specialized in hot rods and people would drag in a Jag, Triumph Stag or an M3 Beamer and expect me to save it from the idiots that had worked on them before I ever saw them. I have seen some strange things (such as corks stuffed into the throats of two out of three carbs to get the car to run, though poorly on just the center one).

I often wondered what kind of training these mechanics at the foreign car dealerships received as they hadn't a clue how a carb worked. And multiple carbs with progressive linkage baffled them as much as dark mater bothers me.

Big Dave
ok so I just spewed some of my ignorance to jayoldschool, but corks in carbs? That's a new one to me. I guess that's one way to choke it. haha
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post #8 of 14 (permalink) Old 10-03-2015, 09:54 AM
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You will need to buy a dwell meter if you are going to retain your points.

http://www.amazon.com/Actron-CP7605-Tachometer-Voltmeter-Analyzer/dp/B00062YUUS/ref=sr_1_1?ie=UTF8&qid=1443876997&sr=8-1&keywords=tach+and+dwell+meter .

Or you can convert your existing (and probably well worn but numbers matching factory points distributor) over to an HEI with a module from Pertronix. Here is the instruction sheet to convert it over to let you know what you are getting into:

http://www.classicchevy.com/assets/p...STALLATION.pdf

The advantages of HEI are no more points bounce (which is why high performance cars had dual point distributors) due to the fact that there are no points involved. there is also no more constantly adjusting your dwell all the time as the points burned under normal use. And the thing most cite as the best result, easier starts as the ignition is internally regulated to four volts so there isn't a voltage drop to worry about like there is with points in your ignition.

Even with an HEI ignition you will still need a tachometer that you can read under the hood and a BIG vacuum gauge that you can also read. A bigger gauge moves the analog markings further out from the central pivot so it is easier to read the actual amount of vacuum.



Here is a vacuum gauge made to adjust mechanical fuel injection with individual runners or as pictured for tuning Webber carbs. It allows you to tune each stack to get the same vacuum reading so you don't have to put corks in a nice doctor's Jag because you don't know how to tune multiple carburetors (something I grew up with back before big four barrels where invented).

And finally a timing light. Here you may want to think ahead because a awful lot of timing lights with a dial back feature do not work with a capacitive discharge ignition box. You can buy the expensive timing light from MSD that works with their boxes or a cheap one from Sears that also works where few others do.



Which brings up two more questions. First why bother with a dial back timing light when you can put on a piece of timing tape? That is because the factory timing marks are limited to twelve to fourteen degrees of advance when your motor is actually going to want about thirty four to thirty six degrees of advance total (vacuum and mechanical combined). With a dial back you turn the dial to what you want the total to be and it will flash a the TDC mark. With a standard timing light and tape it will work just as well until the tape falls off.

Second why even consider a capacitive discharge ignition system over the factory inductive ignition? If you thought it was easier to crank up with an HEI you will love a CD ignition system.

MSD stands for Multiple Spark Discharge. That is the spark plug fires up to four times when cranking to light off a lean mixture (MSD was introduced in the seventies when the factory was going overboard trying to meet emission laws that hadn't even been written yet). As the RPM increases the number of pulses drops off so that by 3,000 to 3,200 RPM it is only firing once. It is these multiple spark firings that confuses a digital dial back timing light.

One other feature that CD has over HEI is that the HEI is transistorized. So like all transistorized devices it is susceptible to heat. As your RPM increases the HEI module gets hotter and hotter. Generally above 5,600 RPM a factory HEI is missing about as frequently as it is firing. In later model computer controlled HEI distributors they used this "feature" as a rev limiter in BBC applications. MSD and many aftermarket HEI distributors have special modules with a built in heat sink that works with the billet aluminum body of the distributor to shed heat more quickly. The MSD billet Street distributor (includes a vacuum module) is good to 6,800 RPM before heat becomes a problem.

With a CD the ignition isn't dependent upon an HEI module to amplify the weak Hall Effect induced current. The magnetic pulse is just used to discharge the fully charged capacitors (that are charged from the battery, not from the magnetic field in the coil collapsing). The coil is now used only as a transformer, instead of a magnetic flux capacitor. So a MSD 6AL ignition box makes for a smooth start, and delivers a high energy spark to your ignition.

Big Dave
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post #9 of 14 (permalink) Old 10-03-2015, 01:03 PM
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Quote:
Originally Posted by Bradsss View Post
so please forgive my ignorance and try not to laugh at me too much.
I grew up under an old school Dad who seemed to be able to listen to it and know what to do to make it run at it's best. And they always seemed to. I, however, haven't learned that skill. SO here I am...

Timing- Maybe I'm terribly off, but I was taught to advance the timing to where it barely pings on acceleration, then back it off slightly. (I told you not to laugh)

We put new points in and gapped it to .030 (I believe, and as I've been checking online, seems to be to way too high)

As far as idle speed, I just adjust it to where it sounds good.

Mixture- I have no idea how to properly set it.

Yeah, I know... sounds like a train wreck. I have a lot of learning to do. Thanks for any advice!
I don't know if this is causing your problem but:

IAW the 1963 shop manual supplement (don't know what year your car is but this info should be good regardless), for the v8 engines the 'gap' on the points is .019" new, and .016" used. The dwell angle is listed as a range between 28-32 degrees. .030" gap is too high. If the points' setting is changed, the timing needs to be re set (8 degrees BTDC nominal is factory setting for the small block).

The factory settings are for overall performance, not maximizing 'power'.
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post #10 of 14 (permalink) Old 10-06-2015, 09:20 AM Thread Starter
 
 
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Quote:
Originally Posted by Big Dave View Post
You will need to buy a dwell meter if you are going to retain your points.

Amazon.com: Actron CP7605 Dwell/Tachometer/Voltmeter Analyzer: Automotive .

Or you can convert your existing (and probably well worn but numbers matching factory points distributor) over to an HEI with a module from Pertronix. Here is the instruction sheet to convert it over to let you know what you are getting into:

http://www.classicchevy.com/assets/p...STALLATION.pdf

The advantages of HEI are no more points bounce (which is why high performance cars had dual point distributors) due to the fact that there are no points involved. there is also no more constantly adjusting your dwell all the time as the points burned under normal use. And the thing most cite as the best result, easier starts as the ignition is internally regulated to four volts so there isn't a voltage drop to worry about like there is with points in your ignition.

Even with an HEI ignition you will still need a tachometer that you can read under the hood and a BIG vacuum gauge that you can also read. A bigger gauge moves the analog markings further out from the central pivot so it is easier to read the actual amount of vacuum.



Here is a vacuum gauge made to adjust mechanical fuel injection with individual runners or as pictured for tuning Webber carbs. It allows you to tune each stack to get the same vacuum reading so you don't have to put corks in a nice doctor's Jag because you don't know how to tune multiple carburetors (something I grew up with back before big four barrels where invented).

And finally a timing light. Here you may want to think ahead because a awful lot of timing lights with a dial back feature do not work with a capacitive discharge ignition box. You can buy the expensive timing light from MSD that works with their boxes or a cheap one from Sears that also works where few others do.



Which brings up two more questions. First why bother with a dial back timing light when you can put on a piece of timing tape? That is because the factory timing marks are limited to twelve to fourteen degrees of advance when your motor is actually going to want about thirty four to thirty six degrees of advance total (vacuum and mechanical combined). With a dial back you turn the dial to what you want the total to be and it will flash a the TDC mark. With a standard timing light and tape it will work just as well until the tape falls off.

Second why even consider a capacitive discharge ignition system over the factory inductive ignition? If you thought it was easier to crank up with an HEI you will love a CD ignition system.

MSD stands for Multiple Spark Discharge. That is the spark plug fires up to four times when cranking to light off a lean mixture (MSD was introduced in the seventies when the factory was going overboard trying to meet emission laws that hadn't even been written yet). As the RPM increases the number of pulses drops off so that by 3,000 to 3,200 RPM it is only firing once. It is these multiple spark firings that confuses a digital dial back timing light.

One other feature that CD has over HEI is that the HEI is transistorized. So like all transistorized devices it is susceptible to heat. As your RPM increases the HEI module gets hotter and hotter. Generally above 5,600 RPM a factory HEI is missing about as frequently as it is firing. In later model computer controlled HEI distributors they used this "feature" as a rev limiter in BBC applications. MSD and many aftermarket HEI distributors have special modules with a built in heat sink that works with the billet aluminum body of the distributor to shed heat more quickly. The MSD billet Street distributor (includes a vacuum module) is good to 6,800 RPM before heat becomes a problem.

With a CD the ignition isn't dependent upon an HEI module to amplify the weak Hall Effect induced current. The magnetic pulse is just used to discharge the fully charged capacitors (that are charged from the battery, not from the magnetic field in the coil collapsing). The coil is now used only as a transformer, instead of a magnetic flux capacitor. So a MSD 6AL ignition box makes for a smooth start, and delivers a high energy spark to your ignition.

Big Dave
Wow thanks for the detailed reply here Big Dave! Looks like I have a ton of "work" to do. Great information and love the explanations.
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post #11 of 14 (permalink) Old 10-06-2015, 09:22 AM Thread Starter
 
 
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Quote:
Originally Posted by japete92 View Post
I don't know if this is causing your problem but:

IAW the 1963 shop manual supplement (don't know what year your car is but this info should be good regardless), for the v8 engines the 'gap' on the points is .019" new, and .016" used. The dwell angle is listed as a range between 28-32 degrees. .030" gap is too high. If the points' setting is changed, the timing needs to be re set (8 degrees BTDC nominal is factory setting for the small block).

The factory settings are for overall performance, not maximizing 'power'.
My car is actually a 63 impala ss. Thanks for the information!!
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post #12 of 14 (permalink) Old 10-13-2015, 12:20 PM Thread Starter
 
 
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Videos to watch

Ok so here's an update.
-I set the dwell to 30 degrees
-Points gap is at .019
-Re-calibrated the floats in both primary and secondary
-Established smooth idle with front mixture screws.
-Bent linkage as needed to make accelerator pump shaft correct distance from top of carb. body

Still has a dead spot. I uploaded two youtube videos of what it looks like in the carb when I work the linkage.
Here's a video showing what it looks like to work the throttle with the motor shut off:
https://youtu.be/WIhlgpChnms

Here's what it looks like when it's running:
https://youtu.be/DwxcWOrOwMk

Thanks for watching and all your help!!
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post #13 of 14 (permalink) Old 10-13-2015, 12:57 PM
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Quote:
Originally Posted by Bradsss View Post
Ok so here's an update.
-I set the dwell to 30 degrees
-Points gap is at .019
-Re-calibrated the floats in both primary and secondary
-Established smooth idle with front mixture screws.
-Bent linkage as needed to make accelerator pump shaft correct distance from top of carb. body

Still has a dead spot. I uploaded two youtube videos of what it looks like in the carb when I work the linkage.
Here's a video showing what it looks like to work the throttle with the motor shut off:
https://youtu.be/WIhlgpChnms

Here's what it looks like when it's running:
https://youtu.be/DwxcWOrOwMk

Thanks for watching and all your help!!
You didn't mention re-setting the timing? Did you? Not saying that is your issue but it MAY be. If the timing was/is correct for the 0.030" point gap, it is 'wrong/off' for the 0.019" point gap.

I didn't think to mention it before, but are your plugs gapped properly? The GM factory spark plug gap is 0.035". If you check the plug gap, check them all.

This is all basic tune up stuff which needs to be correct, but may not be your problem. You won't know until you get it right.
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post #14 of 14 (permalink) Old 02-21-2016, 07:20 PM Thread Starter
 
 
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Quote:
Originally Posted by japete92 View Post
You didn't mention re-setting the timing? Did you? Not saying that is your issue but it MAY be. If the timing was/is correct for the 0.030" point gap, it is 'wrong/off' for the 0.019" point gap.

I didn't think to mention it before, but are your plugs gapped properly? The GM factory spark plug gap is 0.035". If you check the plug gap, check them all.

This is all basic tune up stuff which needs to be correct, but may not be your problem. You won't know until you get it right.
Ok so it's been awhile since I've replied, and for that I apologize!
So yes, I reset the timing, checked and rechecked the gaps, checked plug gap, timing light used, used dwell meter, and as a last resort, turned my head a little sideways and halfway closed one eye. Still dead spot.
I have a buddy that had an extra Edelbrock hanging around, so I swapped it out and, wouldn't you know it... problem solved.
Considering getting a reman one from Rock Auto and sending mine away. Anyone have any feedback from that?

Thanks again
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