#41  
Old 05-05-2016, 11:00 AM
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The stumbling is really only when the motor is cold. When fully warmed up, it goes away and has way more torque down low. I will put a vacuum gauge on it today.

It has an aluminum flywheel so it takes some feathering of the clutch in first gear when cold.

  #42  
Old 05-06-2016, 02:34 AM
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Quote:
Originally Posted by elefantrider View Post
... The stumbling is really only when the motor is cold. When fully warmed up, it goes away and has way more torque down low...
Sounds like you need to address choke function.

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Old 05-06-2016, 12:10 PM
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Appears that way. I guess I not think a choke would matter because it is 70 degrees outside and it is already seems rich. The lightweight flywheel seems to accentuate these problems.

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Old 05-10-2016, 09:13 AM
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Vacuum is about 10 PSI at idle and around 25+ at cruise.

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Old 05-10-2016, 01:16 PM
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Engines still need choke at startup and warmup, even at 70 degrees. 10" of vacuum is pretty low, indicating a non-stock cam or problem with carbs, timing, or ?

You'll be surprised at how the choke will eliminate stumbling on warmup.

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Old 05-10-2016, 01:27 PM
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It's a Comp hydraulic roller. 230-236 @ .050, with .510 and .520 lift on a 110.
Just enough vacuum for the brake booster.

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Old 05-22-2016, 04:31 AM
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Even if carb is already set rich, a choke will make a difference? Maybe my carb mixture at idle is not as rich as I thought.

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Old 05-22-2016, 09:55 AM
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Chris,something is wrong.That little cam in that size motor should have tons of vac!My 245-252 in a 455 would work fine with power brakes,not for a few panic stops but never a issue with one.Tom

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Old 01-16-2020, 11:17 AM
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Back to sorting out this project. The 455 center carb does not want to idle down. The idle adjuster is backed all the way out and I'm still at 1100 rpm when hot. What could that be?

I'm about to swap back to my '65 manifold so I can run the small original center carb.

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Old 01-16-2020, 11:52 AM
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If the carb Hot Idle Compensator is frozen then you will get a much higher idle vs normal.

It is the springy thing between the two venturis and attached with two small screws to the carb inside.

Be careful if you want to check it out and remove the carb and then look at the HIC.

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Old 01-16-2020, 06:56 PM
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That metal strip that doesn't look like it would fail on a new NOS carb. Is that a common failure?
The is the carb which has the throttle blades drilled stock. Maybe that is just letting too much air through.

Carb number is 7041064 and it was NOS in the original box. We just put in new gaskets and an accel pump in it.

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Old 01-17-2020, 04:13 PM
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Many of the large 2G carbs have holes in the throttle plates as the factory did on all automatic transmission cars. If yours does have those holes, that explains the high idle. Also can work with you on idle circuit parameters.

PM me if you need throttle plates without holes--I have plenty. Will cost you shipping only.

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Old 01-17-2020, 06:15 PM
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That makes sense, thanks.
Do you think I can JB Weld shut the holes to try? Brazing is low heat and sandable, and could be an option also.
I'd hate to undo a throttle shaft screws on a NOS carb.
Also, I think the main jets are 61 but it does not seem to run too lean.
I'll remove the carb this weekend to check everything, and see if I can seal up the holes with a little JB weld.

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Old 01-17-2020, 06:39 PM
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Quote:
Originally Posted by Dick Boneske View Post
Many of the large 2G carbs have holes in the throttle plates as the factory did on all automatic transmission cars. If yours does have those holes, that explains the high idle. Also can work with you on idle circuit parameters.

PM me if you need throttle plates without holes--I have plenty. Will cost you shipping only.
Dick, what about the End Carbs?
Should they have holes in the throttle plates too?
Thanks,
Don

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Old 01-17-2020, 06:42 PM
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End carbs don't have the holes.

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Old 01-17-2020, 07:03 PM
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I would not hesitate to change the throttle plates. Closing those holes or making them smaller will resolve your idle rpm problem. I could also send you a different throttle base if you don’t want to alter yours.

But, after reading the earlier posts more carefully, here is what I think.

Being that it stumbles only when cold points to a definite lean condition. You said the carb is a stock 2 bbl. off a 455. If so, the idle tubes & idle bypass restrictors are too small for your cam. I would open the idle tubes to .038” and the idle bypass to .050”. You will find they are now at about .033” & .046” respectively. I did that on our two 434 engines and they behave really well when cold.

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  #57  
Old 01-17-2020, 10:13 PM
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Quote:
Originally Posted by elefantrider View Post
The stumbling is really only when the motor is cold. When fully warmed up, it goes away and has way more torque down low. I will put a vacuum gauge on it today.

It has an aluminum flywheel so it takes some feathering of the clutch in first gear when cold.
After reading the entire thread, I found this post.

I also have an aluminum flywheel. I love the performance (hot), but I finally had to install a carburetor with manual choke to be able to drive it in town for the first 20 or so minutes even in the summer. After 20 minutes or so, the choke is not necessary. With a RAIV cam, exhaust crossover blocked (huge mistake ), the aluminum wheel, and 48 heads on a 350, I can get a really good idle at 900 once hot without choke. Carb is an 850 spread-bore. With an 800 carb and automatic choke, the car was not driveable in town for 20 minutes. Stalled at every stop sign. Granted, my "heel and toe" is not as good as it once was (like too many other things ).

Just as a test, adjust your choke richer.

Jon.

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  #58  
Old 01-18-2020, 08:52 AM
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I agree the stumble is a combination of the aluminum flywheel and the lean condition.

I would have thought a '71 455 would draw need more fuel at idle than my 433ci.
61 main jets are certainly much smaller on this big carb than was stock for tripowers center carbs. Probably due to increasing emissions regulations in the '70s.
I believe they are not in use when the throttle is less than 1/8 to 1/4 open, so probably need to open up the idle tubes. I will look for the equivalent size drill bits.

There is no choke currently hooked up.

I will post a picture to see how a choke could be made to work. I'd need a '66 choke stove assembly , and could somehow rig up the carb to that.


Last edited by elefantrider; 01-18-2020 at 09:05 AM.
  #59  
Old 01-18-2020, 10:25 AM
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Enlarging the idle tubes may help or it may not; consider the "Law of Unintended Consequences" first.

If the idle now is fine, and the hesitation goes away when the engine is at normal temperature; then the idle may be too rich at normal operating temperature with more idle fuel.

By hooking up a choke, you can maintain existing calibration when hot, AND add additional fuel when cold.

In a different lifetime, when I actually had time to build carbs, built a number of the 455 2-barrel carbs for the dirt-track racers. NEVER had to open the idle tubes, but NEVER removed the choke. Because of having the choke butterfly to straighten eddy currents when the driver was off the throttle in the corners, and the fact we kept a functioning power system; the carbs we built had no hesitation when coming out of the corners, and drivers loved them.

The aluminum flywheel acts as a double edged sword, and not a lot of Pontiac folks use one; as Pontiac engines, mostly, are "torquer" engines. The mass of the cast flywheel helps the engine at lower RPM when the engine is cold.

As my 350 is built for RPM and NOT torque, and because of previous experience with aluminum flywheels on smaller displacement high revving engines; I installed one . The speed of change of RPM with the aluminum wheel is stunning. But as I posted earlier, it requires a choke for city driving, even when the ambient is 80 degrees F. until the engine is completely warmed up.

The 800 CFM I tried first, with electric choke, works well on Pontiac 400 and mild to medium 455's; but the choke went off too soon, and still had issues. With the manual choke, I can pull the choke out a quarter of the way at each stop sign for the first 20 minutes, then push it in once the car is moving again. Once the engine is warm, the choke is no longer needed. Oh, and I have a very wide ratio transmission, so the overall gear ratio in low gear is over 9.

And just for the record, while I know very few enthusiast have any interest in fuel economy; my OVERALL, with maybe 40 percent city driving, is better than 20 MPG. I never purchased stock in Shell Oil.

In short, can you modify that carb so no choke is necessary on the street? Probably. But the overall result may be less than satisfactory.

Any possibility you have a unmodified 2-barrel that you could throw on for a test? If so, wire the choke so that the butterfly is at about 75 degrees instead of 90. If the hesitation goes away when cold, you will then know what to do.

Lots of theory on this thread, but a single test might give us an answer, without doing any modifications to your existing carb.

Jon.

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  #60  
Old 01-18-2020, 10:40 AM
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I'll wire the butterfly to 75 degrees today and report back. This carb still has the choke butterfly. It is just not hooked up to the heat riser on the manifold.

My carb's choke looks like this.
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