#1  
Old 09-12-2021, 05:38 PM
MidnightAuto's Avatar
MidnightAuto MidnightAuto is offline
Senior Chief
 
Join Date: Jul 2018
Location: NY
Posts: 367
Default Center jets

I just rebuilt my top end with kre heads and 60919 cam, on a 461. Mike at Pontiac tripower has been helping me with some bugs that I have mostly sorted out.
As well as recommending My jet set up is as follows, 75-64-75. This is 66 tripower. I am Using an af meter to dial in, plus reading plugs (that were pretty black). So itís a bit rich. Idle is about sorted now, fluctuates around 12, but light driving had the meter dropping to 10ish. Have not been able to see full wot. Anyway, I am sure I will figure it out, but my question is another topic.

Iíve studied the test omt did with his tripower and it was clear that the key was the center carb. That is where most of the power comes from. In that spirit, I have been hunting for the 1 3/8 Venturi 2 bbl, but they are a bit of a unicorn, never oem on a Pontiac, and infrequently on sbc 400 truck applications. So I gave up and bought this: https://www.summitracing.com/parts/jet-37001#overview, which has the 1 3/8 Venturi plus a couple other tricks to get it to 500 cfm purportedly.

My question is two fold as it relates to jetting. this carb uses Holley jets and came with 74.

(1) why are jets in the center carb always smaller then the end carbs in Pontiac applications?

(2) based on these 74 jets, would you start with those, or get Holley 64ís, similar to what I have now in the carb currently on the motor?

Thoughts welcome. Thanks guys

  #2  
Old 09-12-2021, 08:11 PM
carbking's Avatar
carbking carbking is online now
Ultimate Warrior
 
Join Date: May 2000
Location: Eldon, Missouri 65026
Posts: 3,424
Default

(1) The center carb has a power valve, the end carbs do not.

(2) No comment.

Jon.

__________________
"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).
  #3  
Old 09-13-2021, 12:59 AM
Goatracer1 Goatracer1 is offline
Ultimate Warrior
 
Join Date: Oct 2003
Location: NH
Posts: 3,132
Default

Your jets should not be too big. Maybe your power valve is open too soon.

  #4  
Old 09-14-2021, 09:43 PM
Dick Boneske's Avatar
Dick Boneske Dick Boneske is offline
Ultimate Warrior
 
Join Date: Mar 2004
Location: Winneconne, Wisconsin
Posts: 5,211
Default

With the cam you have, mods have to be made to the venturi cluster and power valve spring. Until this is done, idle will be too rich.

I have no idea what hole size a Holley #74 jet has, but with a carb that has 1 3/8" venturi''s vs. the standard '66 canter carb with 1.3/16" will need larger jets. I would start with .065".

Are you aware there are many Rochester 2 bbl. carbs larger than 1 3/16"? Why not find one of those and have someone tweak the venturi cluster and power valve to give you decent idle without the rich condition?

Here is an end carb I modified with 1.415" venturi's, larger than the stock 1.310' for the end carbs. This is as large as can be done with the end carbs. You can do the same for a center carb.
Attached Thumbnails
Click image for larger version

Name:	IMG_0983.jpg
Views:	32
Size:	55.8 KB
ID:	573051  

__________________
BONESTOCK GOATS

'64 GTO Tripower Hardtop (Wife's Car)
'64 GTO Tripower Post Coupe (My Car)
'08 Grand Prix GXP LS4 V8 (Wife's Car)
'09 Pontiac G8 GT 6.0 V8
  #5  
Old 09-15-2021, 05:44 AM
Kenth's Avatar
Kenth Kenth is offline
Ultimate Warrior
 
Join Date: Feb 2003
Location: The Kingdom of Sweden
Posts: 4,765
Default

It really is impossible to advice jetting on an unknown carb.
ALL restrictions, idle tubes, idle airbleeds, idle downchannel restrictions, main airbleeds, acc-pump jets needs to be measured and then modified/adjusted properly for the application.

For an original 1966 Pontiac Tripower on a 455 75-64-75 does not seem too far off.
BUT the idle tubes and downchannel restrictions needs to be opened some.
.036"-.038" idle tubes and .050"-.052" idle channel restrictions to begin with.

And, Holley jets are for Holley carbs, they are NOT equal to Rochester jets in any way.

FWIW

__________________
1966 GTO TriPower
1970 GTO TheJudge
http://www.poci.org/
http://gtoaa.org/
  #6  
Old 09-15-2021, 09:23 AM
MidnightAuto's Avatar
MidnightAuto MidnightAuto is offline
Senior Chief
 
Join Date: Jul 2018
Location: NY
Posts: 367
Default

thanks for all the feedback guys. I've now pretty much got my idle mixture to a good spot with adjusting the adjustment screws. the issue is when I am cruising my a/f drops to 10. So that is one issue is solving for cruise. I did cut the spring on the powervalve a few coils and checked the float height so believe I am all good there. Not sure what took at next.


Dick - as for using the bigger carb with that venturi (but with holley jets) - i had no luck finding a rochester 1 3/8 or bigger, and I thought if I opened up the venturi myself I would somehow screw up the velocity. Your carb looks great, thats exactly the kind of thing I was aiming for...

  #7  
Old 09-15-2021, 10:19 AM
Dick Boneske's Avatar
Dick Boneske Dick Boneske is offline
Ultimate Warrior
 
Join Date: Mar 2004
Location: Winneconne, Wisconsin
Posts: 5,211
Default

As Kenth pointed out, the venturi cluster needs to be tweaked to work with the mods you've made to your engine. Adjusting the idle screws is the last step in the process of setting up for decent idle. As Kenth said, the idle tubes need to be .038" and the idle bypass about .052". These sizes have no relation to the venturi size, as they are in play at idle and small throttle openings.

__________________
BONESTOCK GOATS

'64 GTO Tripower Hardtop (Wife's Car)
'64 GTO Tripower Post Coupe (My Car)
'08 Grand Prix GXP LS4 V8 (Wife's Car)
'09 Pontiac G8 GT 6.0 V8
  #8  
Old Yesterday, 12:26 PM
MidnightAuto's Avatar
MidnightAuto MidnightAuto is offline
Senior Chief
 
Join Date: Jul 2018
Location: NY
Posts: 367
Default

Well here is a long winded update, that has left me puzzled.

I dropped on that bigger Venturi carb with the Holley jets and the car would not start. Once in awhile I could get it to start for 3 or 4 seconds, but then it would just die. I tried all kinds of throttle positions pedal to floor, pedal half way, etc. it’s like it runs out of fuel (or air I suppose). Accelerate pump is spraying fine, carb bowl had plenty of gas when I took top off. Have no idea what I should trouble shoot with that symptom so I took it off and went back to my original Pontiac center carb.

Motor started right up, but the rich symptom is still their, at idle (12.5- 13.5afr) and even worse at cruise (9-10 afr). While I did not think my 64 center jets were too big as others have suggested, I went ahead and dropped to 63. Had no impact on readings. To list what I have done- cut 5 coils off the power valve spring, opened up idle tubes to .38, opened up idle mixture holes where adjustment screws are .73. Car will not hold steady idle with screws all the way in. Screws are about 1.5 turns out and idles fine. No dripping or flooding visible of any kind, but gas stink out exhaust and black plugs.

Here are some readings - with vacuum advance disconnected, 22 timing @ 900 rpm, 12.5-13.5 afr, vacuum at 10. With advance connected, 32 timing at 970 rpm, 12.5-13.5 afr, vacuum at 13.

I was thinking of retarding the timing to see what that does.

My bigger problem of course is cruise, not sure what to do at this point. Mike thought powervalve might be bad but it was fine on smaller cam/ heads two months ago on this same motor. So not sure that is smoking gun. Any thoughts welcome.

  #9  
Old Yesterday, 12:55 PM
Dick Boneske's Avatar
Dick Boneske Dick Boneske is offline
Ultimate Warrior
 
Join Date: Mar 2004
Location: Winneconne, Wisconsin
Posts: 5,211
Default

Need to know what vacuum reading you have. It needs to be sufficient to pull the power valve rod off the power valve at idle.

For your application. You need to open the idle bypass restrictors to at least .052Ē. They are now likely .043Ē or.046Ē.

You may also have to drill holes in the throttle platesóabout 1/8Ē on the center carb only. This is to prevent nozzle drip when the engine requires the throttle plates are open beyond the idle slots in the throttle body.

Call or text me at 920-450-1040 to discuss if needed.

__________________
BONESTOCK GOATS

'64 GTO Tripower Hardtop (Wife's Car)
'64 GTO Tripower Post Coupe (My Car)
'08 Grand Prix GXP LS4 V8 (Wife's Car)
'09 Pontiac G8 GT 6.0 V8
  #10  
Old Yesterday, 04:44 PM
Kenth's Avatar
Kenth Kenth is offline
Ultimate Warrior
 
Join Date: Feb 2003
Location: The Kingdom of Sweden
Posts: 4,765
Default

What Dick said.
Also, check float settings and inlet valve binding due to needle clip in wrong location. In doubt remove the clip and run without the clip.
Too high fuel pressure will cause high fuel level in bowl starting the fuel delivery circuits too early making mixture too rich. Look for no more than 5 psi.

__________________
1966 GTO TriPower
1970 GTO TheJudge
http://www.poci.org/
http://gtoaa.org/
  #11  
Old Yesterday, 06:34 PM
MidnightAuto's Avatar
MidnightAuto MidnightAuto is offline
Senior Chief
 
Join Date: Jul 2018
Location: NY
Posts: 367
Default

Thanks again guys. I connected with dick and have the drill bits coming for me to open up bypass as a first step. Let see how that goes.

Thanks for feedback as well Kenth, my fuel pressure regulator is set to 3.5, but I donít actually have a gauge so that will be something to check if I am still running into issues. On the inlet, I think that is where the seat goes? I donít use those needles with the clips if that is what u mean, but let me know otherwise and Iíll check out.

  #12  
Old Today, 03:51 AM
Kenth's Avatar
Kenth Kenth is offline
Ultimate Warrior
 
Join Date: Feb 2003
Location: The Kingdom of Sweden
Posts: 4,765
Default

I would NOT use any other inlet valve style but the original style and size.
This is IMPORTANT to avoid all sorts of fuel delivery issues as flooding, reduced flow etc.
Most troublesome are the integrated "disc" type inlet valve, i have since long lost count of all the folks i have helped with their carbs where this inlet valve was used.

A 1966 Pontiac Tripower center carb uses a .120" inlet valve seat with a clip on the needle and the outer carbs uses .086" valve seat, no clip.

FWIW

__________________
1966 GTO TriPower
1970 GTO TheJudge
http://www.poci.org/
http://gtoaa.org/
  #13  
Old Today, 10:25 AM
carbking's Avatar
carbking carbking is online now
Ultimate Warrior
 
Join Date: May 2000
Location: Eldon, Missouri 65026
Posts: 3,424
Default

Quote:
Originally Posted by Kenth View Post
I would NOT use any other inlet valve style but the original style and size.
This is IMPORTANT to avoid all sorts of fuel delivery issues as flooding, reduced flow etc.
Most troublesome are the integrated "disc" type inlet valve, i have since long lost count of all the folks i have helped with their carbs where this inlet valve was used.

A 1966 Pontiac Tripower center carb uses a .120" inlet valve seat with a clip on the needle and the outer carbs uses .086" valve seat, no clip.

FWIW
The valve Kenth mentions has a wafer of neoprene, encapsulated in an aluminum frame, and is located between the seat and the plunger. If float drop is not perfect, it is possible for the wafer to jam, and full fuel flow at all times. Not a good thing Have not seen a lot of these, thankfully.

Had far more issues with the two ball valve, mainly because we have seen more of them. Sure glad someone sells them, because folks that have tried them purchase kits with the original type valve from us. Really help our business

Jon

__________________
"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).
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 11:14 AM.

 

About Us

The PY Online Forums is the largest online gathering of Pontiac enthusiasts anywhere in the world. Founded in 1991, it was also the first online forum for people to gather and talk about their Pontiacs. Since then, it has become the mecca of Pontiac technical data and knowledge that no other place can surpass.

 

Cold Case Radiators

Cold Case Radiators is a Max Performance company that produces high end aluminum performance radiators at a price point that won't break the bank. Polished and stamped tanks. 2 rows of 1" tubes. Lifetime Warranty.

GTOG8TA.com

GTOG8TA.com is a Max Performance company that manufacturers parts for the 04-06 GTO, 08-09 G8, and 93-02 Firebird/Trans Am platforms.

Copyright © 2017