#61  
Old 01-18-2020, 11:27 AM
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With a 230-236 HR Extreme energy on 110 LS cam and 1.6 rockers in a 389 using a stock 1971 2GC, you will have to adjust the idle circuit due to the lower than stock intake manifold vacuum signal from that cam, there is NO way around this.
Before this is done you´ll just chasing your tail.
Reduced pressure difference between atmosphere and intake manifold means less fuel into the combustion chamber and the already lean factory calibration in your 1971 carb needs help.
At minimum .038" idle tubes and .052" idle channel restrictions is needed.
Some, .057-.096" bypass air thru the center carb throttle blades is helping the blades in correct position avoiding nozzle drip, due to the hotter than std cam.
And regarding the choke, all engines not reached the normal driving temperature are cold, and cold engines needs additional fuel from the choke to run properly until it gets hot. The engineers knew this and put a choke on every carb from factory. This has nothing to do with the outside temperature, it´s the engines temperature that counts.
And, at least all 1964-66 Pontiac tripower end carbs have .030" holes in the throttle plates.

Hope this helps some.

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  #62  
Old 01-18-2020, 11:40 AM
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Anyone have a good, big transparent drawing of a 2G and can recommend from what side I should drill?

I have 2 of these carbs, both NOS so don't mind modifying one.
I looked forever and got them when I was convinced I needed bigger flow on the center than the stock small '65 center carb could offer.
It works nice in that respect. More power when running on the center carb only.

Engine is a high compression 433 cI which needs about 50% race gas.


Last edited by elefantrider; 01-18-2020 at 11:50 AM.
  #63  
Old 01-18-2020, 12:12 PM
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Kenth - I do not necessarily disagree with your post on enlarging the idle circuit; I simply do not have ALL the calibrations on the smog Rochester carburetors to make this recommendation.

I do have these calibrations on both Carter and Stromberg carburetors. The engineers knew that enthusiasts had been drilling jets since at least the 1930's, so their solution to minimize "tampering" was to install LARGER gasoline jets on the smog carburetors; but also MUCH LARGER air bleeds. Thus the key to enrichening the mixture on these units is to pull the air bleeds, and fabricate new ones of reduced size. They figured the average enthusiast did not have the necessary tooling to fabricate smaller air bleeds. A lot changed on carburetors from 1967 to 1968, and more so each succeeding year.

The fact that the OP has stated that the hesitation goes away once the engine warms suggests to me that the missing choke may be the culprit.

And, even if making the choke operational fixes the hesitation; drilling the idle circuit MAY still be of benefit. Some A/F readings would be helpful in making that determination. But I would suggest working with the choke first.

Jon.

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  #64  
Old 01-18-2020, 02:27 PM
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I safety wired the flap closed for first cold start. It started great and better low end to pull it out of the garage. 30 seconds later it was sputtering so i opened the flap more. Works better. It still has a low rpm stumble when clutch is out and giving it throttle.

Here is a couple pics. How can i get this choke to work with a stove?

The throttle screw is still backed out. I will try plugging the butterflies with jb weld tomorrow.
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  #65  
Old 01-18-2020, 02:32 PM
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can you put a electric choke on the carb?Tom

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Old 01-18-2020, 02:40 PM
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Hopefully pics straight this time.
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  #67  
Old 01-18-2020, 02:47 PM
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Can someone remind me the purpose of the black capped pipe on the airhorn of the front carb, as seen from the drivers side?

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Old 01-18-2020, 04:14 PM
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Engine likes idle mixture screws 2.5 turns out. Was previously at 2.0. Screwing out a half a turn improved the stumble a little.
3.0 turns was a little too fat.

Idles still at 1250 RPM hot, minimum.

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Old 01-18-2020, 04:25 PM
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Quote:
Originally Posted by elefantrider View Post
I safety wired the flap closed for first cold start. It started great and better low end to pull it out of the garage. 30 seconds later it was sputtering so i opened the flap more. Works better. It still has a low rpm stumble when clutch is out and giving it throttle.

Here is a couple pics. How can i get this choke to work with a stove?

The throttle screw is still backed out. I will try plugging the butterflies with jb weld tomorrow.
This will get your choke working:
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Old 01-18-2020, 04:28 PM
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Quote:
Originally Posted by elefantrider View Post
Can someone remind me the purpose of the black capped pipe on the airhorn of the front carb, as seen from the drivers side?
The pipe is used on A/T cars for the vacuum switch, and is capped with M/T.

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Old 01-18-2020, 04:42 PM
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Quote:
Originally Posted by elefantrider View Post
Engine likes idle mixture screws 2.5 turns out. Was previously at 2.0. Screwing out a half a turn improved the stumble a little.
3.0 turns was a little too fat.

Idles still at 1250 RPM hot, minimum.
Take a flash-light and look into the venturies for fuel dripping from the main nozzles at that idle speed.
If it drips your engine is running partly on the main circuit at idle and the idle circuit is partly out of play.
Then the cure is to enrich the idle circuit so that the engine will run only on the idle circuit at idle speed.

Also, i would prefer a timed source for the vacuum advance since the intake vacuum suffers from the camshaft used and wont fully activate the vacuum advance at normal idle speeds.
Using constant vacuum for the ignition vacuum advance with this cam and the timing will drop like a stone under certain rpm´s, usually where we want the idle speed, and take the rpm´s further down due the lack of ignition advance..

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  #72  
Old 01-18-2020, 05:21 PM
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Jon, i have found measuring the total calibration recipe of about all Quadrajet applications made, that Rochester only went to the huge main and idle air-bleeds for 1970 and later Chevrolet passenger car applications, the other divisions Rochester did not change the air-bleeds at all or very little for certain applications.

I agree fixing the choke first would be a step in the right direction.

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  #73  
Old 01-18-2020, 06:03 PM
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Venturies not leaking at idle, but actually it hard to see even at RPM due to atomization.
I can't seem to get the vacuum can to work when sucking the hose, so it is probably stuck.

Timing is currently about 8 initial and 36 total, according to the light.
We slowed up the curve to help reduce detonation, so it all comes in pretty late.


The choke, I will fix with a stove.
The high idle when hot, I will try filling in the butterfly holes with JB weld.

Idle RPM should be much lower with 8 degrees initial timing.

  #74  
Old 01-18-2020, 06:05 PM
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Reading the first postings on this didn't tell us the choke is inoperable. That will ALWAYS result in stumble with a cold engine, regardless of ambient temp, which some have pointed out.

The owner cannot use the '66 choke stove and rod with no manifold heat. You can see from the photos the heat risesr is blocked. I recommend putting an early '60's big Pontiac base on the carb so he can run an electric choke. I've done this on several '66 Tripowers and it works really well. With that, you retain the internal choke pull-off piston AND have an adjustable electric choke.

elephantrider, contact me via PM if you want one of those early bases with the needed choke linkage, electric dial, etc.

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  #75  
Old 03-11-2020, 01:22 PM
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Do we know for sure that plugging the 2 throttle blade holes will lower idle?

I think the original purpose of these holes is to prevent fuel pooling. Not to let more air through.
But I could be wrong.

  #76  
Old 03-11-2020, 05:58 PM
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The purpose of the throttle blade holes is to allow the same air flow at a smaller opening of the throttle plates.

Fuel through the idle circuit is delivered because of the air velocity of air passing the slightly open throttle plates. If the plates are too far open, the air velocity decreases, thereby allowing less fuel flow from the idle and off-idle circuit. Having the holes will allow some air to pass through the holes, less air to pass the throttle plates, but at a higher velocity.

(Opinion) plugging the holes will significantly hurt your idle quality!

Jon.

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Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
  #77  
Old 03-11-2020, 06:05 PM
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Thanks Jon.
So plugging the holes will not reduce idle RPM on this high compression engine? The throttle stop screw is all the way backed out and idle still too high (1300 RPM) when hot.

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Old 03-11-2020, 07:14 PM
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If you have not done so, just for grins, disconnect the throttle linkage from the footfeed at the center carburetor, and see if this has any effect on the idle RPM.

Jon.

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"Good carburetion is fuelish hot air".

"The most expensive carburetor is the wrong one given to you by your neighbor".

If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!

Owner of The Carburetor Shop, LLC (of Missouri).

Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
  #79  
Old 03-11-2020, 07:34 PM
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Cable was slack so i don’t think that will change anything. I will temporarily plug the holes with tape or jb weld instead of tig weld, to see if it drops iidle RPM.

  #80  
Old 03-28-2020, 12:41 PM
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Tripower linkage bar was too tight, preventing the carb throttle arm from going down completely. Duh.
What worked for the small 2GC, doesn't for the big 455 2GC. I've lengthened the arm a little and expect the idle to be fine now.
Need to check that this still allows the end carb throttle blades to be fully open.. No need to plug the 455 2GC blade air holes.

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