[SD421]

Hey guys,

Thought I’d post this in here cause I run a tri-power.

I changed my cam from a 224/234 duration to a 214/224 duration cam (changed my rearend from a 3.73 to a 2.56).
Question:
Is there a rule of thumb regarding the power piston spring ?

I took some coils off to compensate for the lower vacuum (12 in) of the 224/234 cam, Iso should put a stock spring back in now that my vacuum is back up around 16-18 in?

Thanks,
Don

[Tom Vaught]

Yep, that would be a good idea if the power piston you have is in good shape. Putting the carb back to a milder camshaft/stock condition vs Carb Calibration is always a good idea.
You want the engine vacuum and the Power Piston syncronized for the camshaft profile.

Tom V.

[Kenth]

You may gain some mpg keeping the spring as is, but if light throttle surging/hesitation occurs you´ll have to replace the spring with an original.

[Tom Vaught]

Originally Posted by Kenth

You may gain some mpg keeping the spring as is, but if light throttle surging/hesitation occurs you´ll have to replace the spring with an original.

X2 exactly what would happen.

Stock spring and stock camshaft, the power valve
movement is controlled by the balance of the vacuum applied to the power valve piston and the spring pressure.

So with the hotter camshaft you have less vacuum and with the stock spring the PV will be opened earlier. So a bit more fuel earlier in the transition. When the piston stem drops it opens the Power Valve in the bottom of the fuel bowl.

The Rochester Engineers always put a lot of effort into their calibrations so I typically go by their efforts and if the calibration is back (close to stock) use their knowledge and selection of parts and spring rates.

Tom V.

[SD421]

Ok, thanks for the responses guys!!
I will get a teplacement spring.
Thanks!
Don

[Dick Boneske]

I have experimented with the power valve spring on quite a few engines. Cutting 8 turns off the spring with a higher duration cam has worked fairly decently. Most stock springs require 180-200 grams of force to compress them a small amount. With 8 turns off the spring, the force is reduced to about 120-140 grams.

As some have said, a cam that runs with less vacuum (more duration) results in too rich a mixture as the stock spring pushes the power valve rod downward opening the power valve. The wrong combination of power valve spring, cam duration, etc., will result in surging at low rpm's in higher gears under light throttle. A spring that is too weak will keep the power valve closed under light throttle, resulting in a lean mixture and surging. This is really annoying with a manual trans when driving around 15-35 mph.

[Tom Vaught]

Most engines can live with a bit rich calibration, Lean surge with a 2.56 axle ratio is a issue just waiting to hurt the engine.

You really want that stock spring to open that power valve properly vs be lean with a tall gear ratio.

Tom V.

[dlloyd]

I am confused. I need a little explanation of how the power valve and power piston and spring works with the vacuum.
The power valve is in the bottom of carb, right?
It has a spring that keeps it up and closed, right?
When it is pushed down it is open, correct?
The power piston spring pushes down on power valve until there is enough vacuum to pull it up and off power valve, correct?
At idle the power valve would be open because of lower vacuum and closes at higher vacuum?
This all sounds backwards to me. I know I have something wrong here but I need someone to explain it to me.
Thanks in advance.

[Jack Gifford]

If you trace the vacuum passage in the carb's body, from the power valve piston- you'll find that it opens into the base below the throttle valves. Thus it sees manifold vacuum, which is high at idle and under light load conditions.

[Dick Boneske]

Originally Posted by dlloyd

I am confused. I need a little explanation of how the power valve and power piston and spring works with the vacuum.
The power valve is in the bottom of carb, right?
It has a spring that keeps it up and closed, right?
When it is pushed down it is open, correct?
The power piston spring pushes down on power valve until there is enough vacuum to pull it up and off power valve, correct?
At idle the power valve would be open because of lower vacuum and closes at higher vacuum?
This all sounds backwards to me. I know I have something wrong here but I need someone to explain it to me.
Thanks in advance.

The spring is there to OPEN the power valve. Under high vacuum conditions, such as idle, closed throttle, light throttle, the piston keeps the power valve rod up off the power valve. Under low vacuum conditions, such as wide open throttle, brisk acceleration, the power valve spring pushes the rod downward to open the power valve and provide a richer fuel/air mixture to the engine.

You had it all right except that idle does produce fairly high vacuum, not lower as you thought.

[Tom Vaught]

Milder cam vs modified cam means more vacuum in the intake manifold and a higher depression across the carb so less cam might be 18" of vacuum vs 14" of vacuum with the bigger cam at idle.

Those additional 4" of vacuum create a stronger pull on the power piston and keeps it up (away from the Power Valve stem) so the PV is closed even with the stock spring installed.

If you have less vacuum you need to reduce the spring force (cut coils) so that the vacuum can still keep the piston up with less idle vacuum on the power piston.

Tom V.

[dlloyd]

Thanks everyone for the explanation.
I thought that the vacuum was low at idle like the vacuum on the power brakes.
I guess now I need to know why is the vacuum different at the power brakes?
I hate to be so dumb about all this but really like to understand.

[Dick Boneske]

The vacuum that the power valve piston sees is the same as what the power brakes see--it's manifold vacuum. That vacuum varies from about zero at wide open throttle to more than 19" of vacuum at idle or while engine braking with the throttle closed.

[dlloyd]

When I had my motor on the run stand, I had my vacuum gage connected to the power brake line. At idle, it registered about 12” of mercury. When I reved the motor the vacuum went much higher around 20” or more.

[Greg Reid]

Where do you have your power brake booster tapped in? Sounds like you may be tapped into the ported vacuum above the throttle plates.
To give you an analogy, it's kind of like sucking a milk shake through a straw. You have 100% of the vacuum being produced at the bottom of that straw. If you cut a hole in the side of that straw, you're still producing the same vacuum but the amount felt at the bottom of the glass drops sharply and nearly 100% of it is now felt at the hole in the side.
By opening the throttle plates in the carb it's like opening a hole in the straw.
Manifold vacuum goes to near zero. The same vacuum is being produced but all the vacuum is felt at the carb opening as it bypasses the manifold fittings.

[dlloyd]

Originally Posted by Greg Reid

Where do you have your power brake booster tapped in? Sounds like you may be tapped into the ported vacuum above the throttle plates.
To give you an analogy, it's kind of like sucking a milk shake through a straw. You have 100% of the vacuum being produced at the bottom of that straw. If you cut a hole in the side of that straw, you're still producing the same vacuum but the amount felt at the bottom of the glass drops sharply and nearly 100% of it is now felt at the hole in the side.
By opening the throttle plates in the carb it's like opening a hole in the straw.
Manifold vacuum goes to near zero. The same vacuum is being produced but all the vacuum is felt at the carb opening as it bypasses the manifold fittings.

Thanks Greg for the straw analogy that makes sense now. So when at idle the power brakes has 12” of vacuum while manifold vacuum is over 20”. While accelerating the power brake vacuum goes up while manifold vacuum goes down.
BTW, my power brake vacuum comes off the back of the front carb like all 65 tripower. I assume that would be above the throttle valve.

[Tom Vaught]

Power Brake line going to the front carb base plate sees vacuum BELOW the front carb throttle blades dlloyd. Same vacuum that the other parts of the intake see just in a more convenient spot. If you have reduced vacuum there then either the two springs holding the throttle plates on the front and rear carb shut are too weak or you have a misalignment in the blades to the bores in the bases. Sometimes the blades don't seal well with perfect alignment and you need the DAG treatment to get them to seal better.

Vacuum should be the same in the entire intake unless you have leaks dropping the number at given spots.

Tom V.

[dlloyd]

Originally Posted by Tom Vaught

Power Brake line going to the front carb base plate sees vacuum BELOW the front carb throttle blades dlloyd. Same vacuum that the other parts of the intake see just in a more convenient spot. If you have reduced vacuum there then either the two springs holding the throttle plates on the front and rear carb shut are too weak or you have a misalignment in the blades to the bores in the bases. Sometimes the blades don't seal well with perfect alignment and you need the DAG treatment to get them to seal better.

Vacuum should be the same in the entire intake unless you have leaks dropping the number at given spots.

Tom V.

When I had the motor on the run stand and had my vacuum gage on the power brake hose, I had 12” of vacuum. So when I reved up the motor it should have went to zero?
I am totally confused again.
I saw it go over 20” while reving.
I hear what you guys are saying and I am not doubting what you say. Because I know what I saw on my run stand it is making me confused.
I guess I should disconnect the power brake line and put a vacuum gage in it and see what I get now.

[Greg Reid]

Yeah, try that again because something's not right with that.

[Dick Boneske]

With number like that, idle must be horrible.