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#21
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Similar problem on a 9.5:1 455 Tripower engine in a '64 GTO. Cam is a little more radical than the RAIV--110 degree overlap. Fresh rings, valve grind, etc. Tripower is a stock '65 setup with .064 jets in center carb, .070 jets in end carbs. Idle mixture screws give max. idle rpm at 1 3/4 turns out. Idle speed about 800 rpm. Very good throttle response and power.
But, at a steady 1500-2000 rpm in 3rd or 4th gear, it bucks fairly badly. This makes city driving uncomfortable. Today, we lowered the spring pressure on the power valve rod by removing 13 coils off a 33 coil spring. Bucking problem is better, but still there a little as described.
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BONESTOCK GOATS '64 GTO Tripower Hardtop (Wife's Car) '64 GTO Tripower Post Coupe (My Car) '99 Bonneville SE Sedan |
#22
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I seem to remember that one of the modifications Royal Pontiac made on cars with their "Bobcat" treatment was to remove the factory's split jetting and put 69 jets across all 3 carbs. Over the factory's setting, this resulted in going up in jet size on the center carb and down on the ends.
I suppose Royal's "even" jetting resulted in better times that were proven at the strip, otherwise why would they have done it? |
#23
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re
Mike Wasson just sent me 63s and 73s for my '65. I had been running 65s and 74s and seemd to be running rich. I asked for stock jets and this is what I was sent.
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#24
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Found the Car & Driver test article. It says normal jetting for the car was 73/66/73 which would have been factory stock 421 jetting. When Royal got their hands on the car, the article says they put in 69/69/69. To me, that would seem lean on the outboard carbs but that car really ran the numbers so the jetting must have been working for them, right? According to the article, that car ran 115mph in the quarter.
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#25
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RE
I was given a jet chart and its says 65 421s were 60 and 61 for auto and manual trans. Mike wasson gave me 63 for the center due to Ethanol laces fuels. He confirmed 60/61 and always bumps up the size a notch or two because of the gas of today.
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#26
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Bringing this one back up. I noticed when cold I have very little torque at low RPM. So much so, that it is often difficult to get rolling without stalling when you let out the clutch. That requires more RPM which I do not like to give it when cold. Is that a sign of a too lean idle condition?
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#27
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You either have too lean a mixture from the center carb or, more likely, not enough timing advance at low rpm's. Our old Pontiacs need about 12 degrees initial and near 30 degrees total by 2000-2500rpm. What do you have?
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BONESTOCK GOATS '64 GTO Tripower Hardtop (Wife's Car) '64 GTO Tripower Post Coupe (My Car) '99 Bonneville SE Sedan |
#28
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I think you hit the nail on the head. Not enough initial advance. Some time ago, we put 13 degrees advance in the distributor and limited the vacuum advance to 6. This engine (421) likes about 34 degrees total at 3000-3500. We slowed up the curve to help with pinging. So 34 minus 26 is only 8 degrees initial.
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#29
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Bringing this one back up. Advancing the initial timing helps with the low RPM stumble so not enough initial timing was the problem. I had forgotten that I backed it off to 28-30 total degrees for pump gas. The 13 degree curve in the distributor was putting the initial timing way too low..... like only 2-4 degrees initial.
It's ping city after 30 total timing, so I probably will shorten up the advance in the distributor. Currently set at 8 degrees initial and 32 total which is still ping city but helps with the bottom end. 28 total seemed good with pump gas. I think the best solution would be to have one distributor done up for pump gas, and another one done for when running race gas. What distributor curve and timing are you guys running in your 421s? |
#30
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15 Initial, 9 in the distributor (adds 18 to the initial so 33 total.
The vacuum advance is not hooked up as you are not going to improve the F.E. that much with a 255/260 duration camshaft at .050" checking points. Tom V.
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"Engineers do stuff for reasons" Tom Vaught Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward. |
#31
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We are going to go from 13 degrees in the distributor down to 10 or 11 degrees and see if that helps with drivability/pinging.
Going to scribe two lines on the distributor and hold down clamp - one position/setting for when running race gas, and one position for pump gas. |
#32
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We have another carb problem showing up. With the higher initial timing setting of 8-10 degrees, we cannot get the idle down below 1100rpm. Center carb throttle stop screw is already backed out all the way.
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#33
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What does the vacuum gauge say? Haven't seen much discussion about that so far - would be very instructive to know what the readings are when you start to feel the stumble.
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#34
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Quote:
I have my timing at 14 initial with 20 centrifugal and 10 degrees adjusted at the can (manifold) I can back the idle screw all of the way out if I have the idle screws turned out more than 2.5 turns so Im at 2 turns and it Im idling at 750 rpms So you might need to turn in your idle mixture screws
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1966 GTO 1966 421-9.3-1 comp-Race Tec 23cc Pistons 1966 Tri-Power 1967 670 Heads Pontiac "Highlift" TriPower Cam by SpeedPro DUR 214 Int 224 Exh @.050 - 107 ICL LIFT .445" Int .465" Exh Tri-Y-Headers by Tribal Tubes w/ Goerlich Mufflers 1966 Muncie Wide Ratio 4-Speed 1968-72 Chevy 12 Bolt Rear End w/ 3.73 rear gears |
#35
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When discussing idle mixture control screw settings, remember that the taper on the screw changed in the 1968~1970 era because of smog emission requirements. The earlier screws have the short taper, so a good range is 1 turn to 1 3/4 turn; whereas the later carbs with the long taper would have a range of 1 1/2 to 3 1/2 turns.
The long taper would give a much more precise calibration, and thus was implemented to help with smog emission adjustment. 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. |
#36
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Modified tri-power center carb stumble when accelerating
Good to know Jon. Thank you
Sent from my iPhone using Tapatalk
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1966 GTO 1966 421-9.3-1 comp-Race Tec 23cc Pistons 1966 Tri-Power 1967 670 Heads Pontiac "Highlift" TriPower Cam by SpeedPro DUR 214 Int 224 Exh @.050 - 107 ICL LIFT .445" Int .465" Exh Tri-Y-Headers by Tribal Tubes w/ Goerlich Mufflers 1966 Muncie Wide Ratio 4-Speed 1968-72 Chevy 12 Bolt Rear End w/ 3.73 rear gears |
#37
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At 10 degrees initial timing, my idle is still too fast even with the throttle stop screw out all the way. Center carb is still a 455 2bbl.
Idles much better (lower) at around 6-7 initial. What could cause this high idle at only 10 degrees initial timing? |
#38
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ALL 1966 to 1970 Pontiac 2GV and 2GC, incl. Tripower, uses the same part-# on the idle mixture screws.
The 1966 Pontiac Shop manual says to adjust for smoothest idle. My 1966 Tripower w/068 cam idles best/smoothest in gear (A/T) with the idle screws 2,5 turns out, no way it will run with less than 1,5 turns. Also, from what i´ve seen the earlier 2-Jets and most 4-jets uses idle screws with longer and overall thinner tapers. |
#39
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Just for the record, when I still had my 1964 GTO w/tripower:
Fresh engine (zero miles), erratic idle in neutral at 900 - screws at 1.75 turns. After 500 miles, decent idle in drive at 750 - screws at 1.25 turns After 1000 miles, excellent idle at 550 in drive - screws at 1 turn each. Zero changes in calibration from zero miles to 1000 miles except reducing the number of turns on the idle mixture screws, and reducing the setting of the curb idle screw. Also, no DAG-213 or other sealer on end carbs. If the plates are centered properly, it isn't needed, as the above idle readings show. The biggest problem I had with the tripower was the two-speed power-slide transmission! 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. |
#40
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I have found 1964-65 Pontiac Tripower´s uses idle mixture screws has longer taper and smaller diameter were the taper starts, thus letting more fuel/air mixture thru.
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