[Teej]

I'm working on the tripower on my '58 and maybe I'm just getting too much information. It started out with a few stumbles and just not quite 'right' and I recently hooked up an LM1 AFR meter to the exhaust and have been chasing things ever since.

It's the stock 1958 370, manual transmission. Originally a 2bbl. I'm using a '57 intake and the carbs are all the small diameter carbs. I had the carbs rebuilt and they run pretty decent all together, but I think there is some room for tweaking, thus the LM1.

After a number of adjustments and playing with things, I finally took the end carbs off and wanted to nail down the center carb first. I could adjust the idle set screws for a decent idle, at about 14.5 AFR but driving at 2500 rpm was getting me about 9.8. WOT was about what I'd say was target with 12.5. I tried smaller jets and that helped a little, but not much. I had #66 in there, tried 64, then all the way down to 57. Still was at 10.3 or so.

Just recently I swapped out the venturi cluster with another center carb I had, and that helped some, now running at about 11.3 during cruise. WOT is still about 12.5.

I don't have the specs on the venturi cluster, but will try to measure everything tomorrow. I'm just wondering where to look next. The 57 jets seem quite small from what I've been reading but its still running rich.

Pulled the plugs, and they look pretty good. Vacuum is great, 20+ at cruise. Timing is what I'd consider normal. 12 Initial. Think it's about 38 total. Vac advance connected to manifold vac. Using the original distributor that is triggering an MSD 6AL and a new coil. Cap and rotor are good. Plug wires are a year or two old. Plugs are R45S.

I'd appreciate any ideas/suggestions or things to look at or try.

Thanks

Thad

[Tom Vaught]

There is more work that you can do to the center carb cluster as far as sizing the air bleed holes. You can also restrict the "progression holes" to lean out the part throttle by using a small press-in restriction in the throttle base for each bore.

Tom Vaught

[Tom Vaught]

http://www.pontiacsafari.com/L1Early...hester%202.pdf

Read 6B-44 (Part Throttle System)

Tom V.

[carbking]

Just guessing, based on the A/F ratio being richer at cruise than WOT, is that the power valve is not functional, and is open ALL of the time.

No offense meant, but the power valve should be inserted into the bowl using a special tool with an inset for the plunger on the power valve. Failure to do so will occasionally cause the power valve to stick in the open position. Check the position of the plunger first. If normal, test by removing the power valve and plugging the opening. Then check A/F ratio at cruise only, not WOT.

Jon.

[dadsformula]

Tri Power running rich. I can tell you this, forget attempting to tune that Center
Carb until you are sure the end Carbs are sealed and a free from Vac leaks as possible.
The end Carbs have Thick beveled throttle plates, and these must seal when closed
or the result will be a vac leak that will make tuning impossible. Trick is to get the
throttle bases separated and loosen the Throttle plate Screws (very carefully) then
holding the base up to a light adjust the plates to produce the least amount or Air leak.
You want to see as little light as possible between the Throttle Body walls and the
throttle blades. Any light you see is a Vac leak. Nearly impossible to get them
perfect, but you do need to get them as best you can to have any chance at a proper
tune on the trips. Remember those end Carbs do nothing until they are opened,
so they need to remain closed (and as air tight as possible). This is the main reason
lots of guys run block off plates under the end Carbs, and by the way not a bad idea
to do this for tuning the Center as its takes the ends out of the equation. I have seen
so many frustrated Tri Power owners because of this, but once you take the time to
really get those end carbs sealed as well as possible, Trips are easy to tune and live
with.

[carbking]

In my previous post, I should have also mentioned the power valve actuating valve. If this valve is stuck in the down position; or, if no vacuum is getting to the valve (wrong gasket, mismatched, housings, etc.) this valve could be keeping the power valve open.

From your post, I am assuming you have original tripower carbs.

The center from 1957 used a 62 jet, and that was downsized in 1958 to a 61 jet.

Jon.

[Teej]

Thanks for the advice, I'll look into those things and report back.. I am just running on the center carb, I figured once I get that dialed in I'll worry about the end ones.

And yes, they are original tri-power carbs.

It definitely sounds like the power valve may be a contributing factor.

And thanks for the link Tom.. I'd forgotten about all the stuff on that site.

[Tom Vaught]

On a Holley carb the fuel for the Power Valve comes from the same main well that the main jets feed. So if the power valve is open then it is impossible for the PV to affect a idle condition.

On a Rochester 2 BBl carb the Power valve is fed directly by the fuel bowl and then the fuel goes past the PVCR drillings and on to the mail wells.

As John mentioned, if the PV is stuck open then the fuel curve anytime the carb cluster booster nozzles are functioning will have an effect on the calibration.

Tom V.

[Teej]

I was thinking about this on the drive in... Can I simply put a 'cap' over the PV to test this theory. Something that would seal around the bottom and prevent fuel from coming in and the spring from pressing on it? John said there was a special tool for installing them and I know I don't have one. Can I make one?

Mine are the Rochester carbs.

[Tom Vaught]

The Power Valve is located in the bottom of the bowl. It has a little pin that sticks out and contacts the Power Valve actuator stem.

When there is less vacuum the spring forces the PV piston (in the lid of the carb) towards the Power Valve stem located in the bowl.

First test would be to see if you can move the PV Piston upward in the lid of the carb.
(Push the nail head part against the spring force and toward the lid).

If the nail head part is stuck, (Piston stuck inside the lid cavity), many times by tapping on the nail head part with a wooded dowel you can free the piston up.

If it does not move then that is the issue: The nail head stem is stuck downward and when you put the lid on the carb you open the power Valve in the bottom of the bowl and it stays open all the time.

Check that first.

Tom Vaught

[Teej]

OK, I'll try to check that tonight. Last time I had the carb open (Sunday) I checked the power valve by pushing the pin (it moved and sprang back into place), but didn't check the piston/spring.

And if the piston were stuck, would I still be able to get a good AFR at idle? I can get it to 14.5 using the idle set screws.

If the piston is moving easily, since the carb will be open, could I put something over the PV to prevent fuel from flowing into it? Not exactly sure what, but I'm picturing something shaped like a bottle cap. That would tell me if the PV was leaking if my AFR went way up.

[carbking]

The special tool can easily be made.

(1) Obtain a flat screwdriver with the proper thickness blade to fit the slot of the power valve.
(2) Put the screwdriver in your vice with the blade up.
(3) Use a good hacksaw or knife file to make a slot in the center of the blade that is slightly deeper and slightly wider than the pin in the power valve.

Or, if you have access to a mill and slitting saw, the task is much less labor intensive.

If you wish to be fancier, the original was a solid piece of steel with a hole machined in the center, and the screwdriver prongs milled on the bottom. Scroll down to T109-61 in this link: http://www.thecarburetorshop.com/Carter_tools.htm (and yes, the picture is of a Carter tool, not a Rochester, for removing/installing power valves). This one has a socket head on the top.

Tom's description of the test is spot-on.

Jon.

[Willshire]

i had the same problem after i tried to adjust the spring tension on the power piston. the spring was a quarter inch from the nail head when fully extended so, i opened the spring and clearly opened it too far and it opened the power valve when it should not have

[Tom Vaught]

Originally Posted by Teej

OK, I'll try to check that tonight. Last time I had the carb open (Sunday) I checked the power valve by pushing the pin (it moved and sprang back into place), but didn't check the piston/spring.

And if the piston were stuck, would I still be able to get a good AFR at idle? I can get it to 14.5 using the idle set screws.

If the piston is moving easily, since the carb will be open, could I put something over the PV to prevent fuel from flowing into it? Not exactly sure what, but I'm picturing something shaped like a bottle cap. That would tell me if the PV was leaking if my AFR went way up.

You have asked about covering the Power Valve twice now. The cup will not work because the carb float is right next to the Power Piston Spring and the Power Valve. If the "cup"
interferes with the float dropping then you will have other carb issues.

A quick test is to take a piece of thread (and using a needle) fish the thread up next to the stem of the Power Piston and temporarily make the spring shorter by using the thread to pull the coils closer together. This will take the spring tension off the piston. Push the piston upward and it will stay there. The Power valve will seal as you already posted that the stem moves easily upward when you push on it and let go.

The Power Valve will be OFF when the stem is up. And with the spring temporarily compressed it cannot open the valve during your test.

Do your A/F test and if the drive is good, then the power valve is sealing properly. Cut the thread in the power piston stem spring and work on getting the power piston to move
freely. DO NOT DO ANY HARD THROTTLE STUFF ON THE CENTER CARB AS THE POWER VALVE ADDS FUEL NORMALLY UNDER ACCELERATION AND THE POWER VALVE WILL BE CLOSED FOR THE TEST.

Tom V.

[KEN CROCIE]

Always polish the power piston with very fine paper or crocus cloth.

[Tom Vaught]

One other comment, the power piston retention ring is peaned over slightly by the air horn lid casting (holding it in place).
So if you get the piston to move freely, you can push it in to the casting and then let the nail head stem go and the piston will snap back against the retaining ring.
Do this quite a few times and the peaning will slowing go away and one time the piston (with the stem will fly out of the lid.
Point the nail head away from you when doing the last step so you do not get hit in the eye.

Once the piston has flown out of the carb lid then you can do the polish stem routine (Mr Crocie mentions) in the above step.
All nicely polished and moving freely and you can lightly repean the casting back over the retaining ring again. Carb will work like a champ on that circuit.

Tom V.

[Teej]

I pulled the top and checked the piston. It moves freely and the spring pushes it back out consistently. So i made a tool and pulled the power valve. It too seemed to work as it should. I could blow air against the top, with no air going through until I pressed the pin. Then it would flow easily. Release the pin, no more flow.

One thing I did notice, was that the pin in the PV was shorter than the pin in a PV from another center carb I had. I snapped a picture and Ill try to upload it. But in reality, the shorter pin would not have caused the running rich, but the longer one might have. I reassembled the carb as it was. There was a gasket under the PV on my car, but not on the spare with the longer pin I pulled from the other carb. Is the longer pin to make up for the gasket?

I'll try putting a string on the piston spring tomorrow.

Are there different length pistons? I'll try the thread trick tomorow.

[carbking]

Rochester made many different power valves, and yes, they did come with at least 2 different pin heights.

All of the power valves used a fiber washer under the valve.

Interchanging parts, unless from identical tag-numbered carbs, or known exact interchanges from two different tagged carbs to which you have the bill of material; is always a bad idea. It MIGHT work, but chances are better that it will not work.

Jon.

[Teej]

Car is my daily driver, so I got to test it this morning. No change. I was hoping the PV wasn't seated on the washer correctly or something. Still runs very rich at cruise (~10) and will be great at idle (14.5 @ ~750rpm) One thing I have noticed is that when I stop, the idle sits a little high, about 1000 -1100 rpm and the AFR is lean (17) until I 'blip' the throttle and get it to fall back to 750-800, then its back to ~14.

May try the spring/string thing this afternoon.

[Teej]

PV picture