PY Online Forums - Bringing the Pontiac Hobby Together

PY Online Forums - Bringing the Pontiac Hobby Together (https://forums.maxperformanceinc.com/forums/index.php)
-   Tri-Power Tech (https://forums.maxperformanceinc.com/forums/forumdisplay.php?f=569)
-   -   Center jets (https://forums.maxperformanceinc.com/forums/showthread.php?t=853210)

MidnightAuto 09-12-2021 05:38 PM

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 it’s 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.

I’ve 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 64’s, similar to what I have now in the carb currently on the motor?

Thoughts welcome. Thanks guys

carbking 09-12-2021 08:11 PM

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

(2) No comment.

Jon.

Goatracer1 09-13-2021 12:59 AM

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

Dick Boneske 09-14-2021 09:43 PM

1 Attachment(s)
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.

Kenth 09-15-2021 05:44 AM

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

MidnightAuto 09-15-2021 09:23 AM

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...

Dick Boneske 09-15-2021 10:19 AM

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.

MidnightAuto 09-19-2021 12:26 PM

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.

Dick Boneske 09-19-2021 12:55 PM

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 .052”. They are now likely .043” or.046”.

You may also have to drill holes in the throttle plates—about 1/8” 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.

Kenth 09-19-2021 04:44 PM

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.

MidnightAuto 09-19-2021 06:34 PM

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 don’t 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 don’t use those needles with the clips if that is what u mean, but let me know otherwise and I’ll check out.

Kenth 09-20-2021 03:51 AM

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

carbking 09-20-2021 10:25 AM

Quote:

Originally Posted by Kenth (Post 6281404)
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

MidnightAuto 09-20-2021 08:48 PM

1 Attachment(s)
Drill bit here tmrw. Went ahead and took cluster off and checked check ball under spring and t bar to make sure seated. I mangled spring on way out. The kit replacement slightly bigger as noted in pic. Is that ok or should I trim spring to match? Or was original to short?

Dick Boneske 09-20-2021 09:34 PM

Spring in the kit will be fine. I use both lengths interchangeably.

MidnightAuto 09-20-2021 10:18 PM

Got it. Ty.

carbking 09-21-2021 08:34 AM

The springs are of slightly different tension; Rochester's attempt at "timing" the pump shot.

The pump discharge check "valve" (ball, spring, and retainer) were chosen such that negative pressure at this area during normal cruise (high engine vacuum) would be insufficient to pull the ball off the seat; yet of resistance sufficiently small such that fuel pressure when the pump was activated would push the ball off the seat and deliver the pump shot to the engine.

To my knowledge, Rochester used only the two different tension springs.

Carter did a LOT more. Carter did not use springs, but rather the mass of the valve itself. Carter used at least 7 different valves in the AFB series alone.

If the valve releases during normal cruise, fuel will siphon through the pump discharge valve, causing an overly rich mixture, and poor fuel economy.

If the valve release late, there may be a hesitation on acceleration.

For basically stock engines, I have found that the valve supplied by the carburetor manufacturer is best: QUOTE TOM V: "Engineers do things for a reason"!

For a modified engine, especially one with a cam with lots of overlap; the valve becomes another tool in the toolbox of the carburetor tuner.

Jon

"QUICK-SILVER" 09-21-2021 11:03 AM

Power Valve??
 
Quote:

Originally Posted by MidnightAuto (Post 6281213)

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.

Working then does not mean it isn't hanging open now.

Could be a piece of trash or just plain worn out. Worn out as in a rusty spring that wouldn't push the valve closed. Cutting the spring on the actuating rod doesn't help if the valve isn't sealing on its own.

I've seen them damaged from R&R when a notched screwdriver blade wasn't used.

I'd be tempted to change it before drilling holes that can't be un-drilled.

Clay

carbking 09-21-2021 12:17 PM

Quote:

Originally Posted by "QUICK-SILVER" (Post 6281665)
Working then does not mean it isn't hanging open now.

I've seen them damaged from R&R when a notched screwdriver blade wasn't used.

Clay

One of the tricks of Mr. Goodpliers.

For the "build an occasional carburetor" hobbyist, slotting a screwdriver will make a serviceable tool.

For the professional carburetor rebuilder, or a hobbyist that does lots of Rochesters, Carter made a really nice special screwdriver that works very well with Rochester power valves.

Jon.

MidnightAuto 09-21-2021 10:07 PM

small update. I got the drill bits and was able to drill the bypass out. I started with a .0462, and then finished with the .052. So it was smaller then .0462 to start. I figured while I was under the hood I would go ahead and get a new power valve that will be here tomorrow just to be safe the old one is not leaking.

With that said, It seems I can't get out of project without something else going wrong. In this case, I decided to go back to the 64 jets, removing the 63, and somehow it seems one of the jet hole threads stripped. unreal. It just so happens that I had the exact same carb bowl on the shelf, so i put the mod'd cluster on the alternate carb bowl, with my original base plate, and original air horn. Tomorrow power valve comes and will see how it runs.

Curious - what size tap is the rochester 2G jet thread?


All times are GMT -4. The time now is 06:20 PM.