[olde-goat]

Can someone please explain the function of the power valve located in a Tripower center carb. Thanks

[Dick Boneske]

The power valve is controlled by manifold vacuum. When under part of full throttle (engine developing power), vacuum level drops and a richer mixture is needed to properly fuel the engine. At idle or steady speeds, vacuum is fairly high and all the gas for the engine is supplied by the idle circuit and/or the main metering jets.

The power valve rod is held upward by engine vacuum against the spring around the rod. When vacuum drops, the spring pushed the rod downward and pushes the power valve open. This allows additional gas from the float bowl to flow into the venturi using the same path as the gas that flows through the main metering jets.

The strength of the spring is what determines when the power valve opens--a stiffer spring results in actuating the power valve with higher vacuum levels than a weaker spring. So with a hotter cam, there is less vacuum and we typically weaken the spring so the power valve does not actuate until vacuum is really low.

I hope this helps you understand the function of the power valve.

[Tom Vaught]

Good Explanation Dick, as usual.

Tom V.

[olde-goat]

Thank you for the replies.The power valve was always a mystery to me. I have restored a few Tripowers over the years for my vehicles and haven't had any major problems to speak of. I have one more 65 Tripower left to restore. Is the power valve a serviceable part? Can the power valve condition be evaluated during the restoration process or is the final test on the running engine? I appreciate the input.

[Dick Boneske]

Until the engine in run, driven, there is no measure of power valve performance. Most stock springs are about 150 grams. I cut 8 turns off the stock spring if the engine has a cam like the RAIV or more radical. That makes the spring about 100 grams. I've never had a problem with that method--but, I always measure the spring pressure as they vary from carb to carb.

[Jack Gifford]

I always disliked the trial-and-error method of pulling the top off the carb and guessing at a power valve spring alteration. So for my GMC engine ('58 382 c.i., '61 SD Tri-Power) I modified the valve to be externally adjustable, with a range of about 5 - 12 in.Hg. The hex on the adjuster is accessible without even removing the air cleaner. I "calibrated" it with a vacuum guage, so I know how many turns gives a 1 in.Hg. change (need to look at build log, though, to remember).

As luck would have it, my initial "guess" setting of 8 in.Hg. was perfect- have not had a reason to change the setting!

[olde-goat]

Great information. Once again thanks for the replies.

[carbking]

A bit more information:

The "power valve" is not a fixed calibration.

As Dick mentioned, one can trial and error cutting coils from the power valve actuating valve spring (or spring kits of varying tension are also available).

As to the power valve itself, there are more than a dozen different. These vary in orifice size (more or less fuel) and length of the plunger. Generally, the longer plungers were used with automatic transmissions (additional fuel required quicker due to fluid coupling losses), and the shorter plungers were used with manual transmissions.

However, the enthusiast may obviously change power valves for reasons he/she determines. It is another tuning tool.

Jon.

[Tom Vaught]

Some nice work, Jack, on that PV tuning design.

Tom V.