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Old March 5th, 2008, 03:11 PM   #1
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L69 Tri-Carb vs. Pontiac Tripower

The '66 Olds L69 setup used on 442's used the same Rochester carbs as the Pontiac in '66 only. All three carbs had the same casting numbers between Olds and Pontiac. Olds ran a slotted rod linkage piece to the front carb to actuate the secondary throttles, Pontiac also used a slotted rod, but to the rear carb.

The question I have relates to the center carb. The photos I've seen of the Olds Tri-Carb setups have ported vacuum off the center carb routed to the vacuum advance on the distributor. The vacuum fitting is located on the center carb passenger side toward the front. All the '66 Pontiac Tripowers I've seen and owned use manifold vacuum for the distributor advance taken off the base of the front carb. On Pontiac Tripowers prior to '66, ported vacuum was used for automatic cars and manifold vacuum was used for stickshift cars.

Does anyone know about this?
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Old March 5th, 2008, 06:01 PM   #2
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I'm no expert on Pontiac tri-powers, but I am surprised to hear that they would use a vacuum source from the front carb. My understanding is that on the Olds setup, the vacuum source for the dist. advance is above the throttle butterflies (i.e. "ported"vacuum) so that unlike manifold vacuum, it is lowest at idle and then when the throttle plates are opened acts more like manifold vacuum. So if using a ported vacuum source, I would expect it to be from the center carb. On the Olds setup, the outer carbs don't even kick in for a significant portion (something like 60%?) of accelerator travel / throttle movement on the center carb, and I'd expect the same to be true for the Poncho. So I would wonder how vacuum from an outer carb (front OR back) would help regulate dist. advance.

Are you sure its not the power brakes and not dist. advance. that draw vacuum from the front carb on the Pontiac setup?

Last edited by aliensatemybuick; March 5th, 2008 at 06:39 PM.
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Old March 6th, 2008, 07:15 AM   #3
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Yes, the vacuum source on Pontiac Tripowers for the distributor was off the center carb, not the front one. It was, however, manifold vacuum from the center carb base on stickshift cars and ported vacuum from the center carb on automatic cars. Power brakes and vacuum linkage for the carbs used the front carb base as a manifold vacuum source. This same port (since the front carb on Pontiac setups is placed in the rear on '66 Olds setups) is used by Olds for manifold vacuum operated heater controls. None of these setups used ported vacuum from end carbs.

My question was related to the '66 Olds setup. Do all the '66 Olds Tri-Carb setups use ported vacuum for distributor advance---or just the automatic ones?
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Old March 6th, 2008, 09:36 AM   #4
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Sorry if I misunderstood your question. I can say that there were NO factory 1966 tri-carb setups on automatic equipped cars. The set-up was only available from the factory on 3 speed or 4 speed cars. As for dealer and owner installed setups on automatic cars, my guess is the proper way to install them would be to do as the factory did, and use the ported vacuum port from the center carb.
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Old March 6th, 2008, 06:49 PM   #5
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Originally Posted by aliensatemybuick View Post
........ my guess is the proper way to install them would be to do as the factory did ........
Your guess is accurate.

Changing the source would require a different distributor curve, to compensate for the difference in advance at idle.

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Old March 29th, 2008, 10:00 AM   #6
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The Olds linkage can be set for 30 or 40 deg. opening position for the secondaries. As noted above, all Tri Carbs were stick cars and all used ported vacuum from the fuel bowl of the center carb. I've been using Tri Carbs for 35 years. In the mid-70s I compared properly jetted Tri Carbs against the 800 Holley and Edelbrock Torker. The ancient, cast iron lump from Olds with those sorry little 2 jets ran 1-2 tenths better in the quarter
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Old March 29th, 2008, 10:00 AM
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