When you Port a head and pick up flow alot does it usually require more jetting?

[Geoff2]

Yes, vacuum advance connected to manifold vacuum is greatly misunderstood...& under utilised. My engine likes 48* at idle.

Although 45-55* at cruise is often touted as 'ideal', there are combos where 60* is needed. The engine will tell you what it likes.....

[NormS]

If ,at idle, the manifold vacuum is fluctuating enough,above and below the power valve rated vacuum, then the power valve diaphragm can flutter and act like a pump, making the idle mixture rich. In that case, the power valve should be chosen so that its rating is just below the lower end of the manifold vacuum fluctuation.

[cjperformance]

If ,at idle, the manifold vacuum is fluctuating enough,above and below the power valve rated vacuum, then the power valve diaphragm can flutter and act like a pump, making the idle mixture rich. In that case, the power valve should be chosen so that its rating is just below the lower end of the manifold vacuum fluctuation.

How much fluctuation and speed of fluctuation is required for this to take place ?

[NormS]

If you can see the fluctuation of the manifold vacuum, on a vacuum gauge connected to the vacuum port on the front of the baseplate, that's the rate of fluctuation that the PV diaphragm will have. It can be quite fast and violent. It's usually way faster than you can count... the needle in the gauge may look like a blur, it's moving so fast, even at idle.

[cjperformance]

If you can see the fluctuation of the manifold vacuum, on a vacuum gauge connected to the vacuum port on the front of the baseplate, that's the rate of fluctuation that the PV diaphragm will have. It can be quite fast and violent. It's usually way faster than you can count... the needle in the gauge may look like a blur, it's moving so fast, even at idle.

Under this condition, have you removed and plugged the PV and reassesed with either the same jet size OR a jet size that compensates for pvcr?

[NormS]

In some cases , yes. More often in drag racing applications, than in others. In street and other applications where part throttle/ light load/ low rpm is an important part of the vehicle's use, I try to retain the PV. There are some things that can be done to dampen the effect of the vacuum fluctuations as far as the power valve is concerned.

[cjperformance]

In some cases , yes. More often in drag racing applications, than in others. In street and other applications where part throttle/ light load/ low rpm is an important part of the vehicle's use, I try to retain the PV. There are some things that can be done to dampen the effect of the vacuum fluctuations as far as the power valve is concerned.

I too always use a PV in almost all applications and would typically only not use a PV simply for the purpose of testing something in some situations.
Are you seeing these examples go rich or lean due to PV pumping?
What effect are you seeing on same examples when the PV is plugged to prove that it is the PV causing this ?

[NXBOY]

If ,at idle, the manifold vacuum is fluctuating enough,above and below the power valve rated vacuum, then the power valve diaphragm can flutter and act like a pump, making the idle mixture rich. In that case, the power valve should be chosen so that its rating is just below the lower end of the manifold vacuum fluctuation.

What I remember is exactly when then 2.5 to 3 lbs of vacuum sweep and it dips below the number of power valve while stareing at vacuum gauge. Usually after someone done got the plugs closed to fouled and ask me to come help. I could raise the Idle a tiny bit and raise the dip a hair and it sound better with no flutter. Alot of my friend back when I played alot with cars and carbs they could only afford a double pumper and usually single circuit idle and like a 292 cam. I wonder if that helped in the transfer slot being open more. I didn't care for a street /strip car to drill butterflys as it seem to require bigger squirter then and with street driving didn't like that alot. Small converters and big cams usually go lean at track and makes the squirter size need to be bigger to crutch. I'll have the friend read this and let him decide what he wants to do. Hope it don't see 230 degrees anymore. Big valve ported dove heads can crack if over heated too much.

[Geoff2]

If the throttle blade position is correct relative to the transfer slot then a PV that might be open has no effect on idle mixture. This is an old wives tale that crops up regularly. The PV is part of the main system, which means not only would fuel be pulled by the PV [ when it is open ], but fuel would also be puled through the main jets. There is not enough airflow at idle speed to activate the main system.

[cjperformance]

If the throttle blade position is correct relative to the transfer slot then a PV that might be open has no effect on idle mixture. This is an old wives tale that crops up regularly. The PV is part of the main system, which means not only would fuel be pulled by the PV [ when it is open ], but fuel would also be puled through the main jets. There is not enough airflow at idle speed to activate the main system.

I cant believe how many times this can be explained and still people just dont get it. Almost debated as much as the moon landing or bigfoot
The PV's i use certainly never affect the idle, must just be those other ones , i might try asking my parts supplier for rich or lean idle PV's on next order

[NormS]

When a power valve is fluttering open and closed with rapidly fluctuating manifold vacuum, that causes a fluctuation of fuel pressure in the main metering wells in the metering block.Being that the power valve channel restrictions are nearly the same level as the passages that feed the low speed circuits from the main wells, the pumping action of the fluttering PV causes a rapid pressure fluctuation in the low speed circuits, making them literally pump more fuel out of the idle circuits, and out of however much of the transition slots that are showing below the butterflies. Pumping of fuel out of the boosters is much more rare, but I have seen that a couple of times, mainly due to the float level having been set too high.

[groberts101]

When a power valve is fluttering open and closed with rapidly fluctuating manifold vacuum, that causes a fluctuation of fuel pressure in the main metering wells in the metering block.Being that the power valve channel restrictions are nearly the same level as the passages that feed the low speed circuits from the main wells, the pumping action of the fluttering PV causes a rapid pressure fluctuation in the low speed circuits, making them literally pump more fuel out of the idle circuits, and out of however much of the transition slots that are showing below the butterflies. Pumping of fuel out of the boosters is much more rare, but I have seen that a couple of times, mainly due to the float level having been set too high.

I've seen that same thing happen with straight manifold vac assisted distributor advance pots fluctuation as well. If the manifolds vac is right near where the vac pot adds its assist(typically around 6-8") it can cause an exaggerated needle movement as the vac pots diaphragm moves in and out of its advance assist mode. Motor goes lean then back to rich. Causes small erratic drips from the boosters and wreaks even more havoc on the entire tuning process. Lowering fuel bowl levels helps to bandaid the situation but the best cure I typically see/utilize is through adding more base timing/restricting mechanical sweep, and using a weaker vac pot diaphragm with mod'd travel length.

[NXBOY]

I don't know for sure, except some factor is causing it or duplicating it and I never run or mess much with vacuum advance. Just talked to my old Mentor mechanic/ race car tuner/ shop owner and he was more sure then me about it made a difference in Idle. I'm smart enough to know I don't know everything, unlike my OLD Buddy lol. Wish I realised this was a debate thing, I would of never mentioned it. I am willing to bet its not on all carbs and tunes atleast like things mentioned, hope nobody got too upset, This post is the first time I've heard of this.

[NXBOY]

But back to my original post its not the same shortlblock now. Its .030 with Forged Wisco piston with a hair more compression. Would think the fresh /tight motor would be a hair more friction over a 100,000 rering that wore out again. So running cool before heads ported I don't think means alot. It took, he said several miles on a incline for temp to climb. Its fine on normal highway. It was in the 90s that day. I first just wondered about head flow with he exhaust just 60 percent of Intake port flow that some reversion was making it leaner. Cam only has a 10 split as anything bigger then 260@ .050 on the Exhaust lobe with a Hydraulic, I didn't know how reliable on the street that was and thought more dur. meant more over lap and could effect drivabilty and Idle some.

[Geoff2]

There is no pumping action in the main wells at idle. The fuel level in the main well [ at idle ] is at the same level as the fuel in the fuel bowl. If the PV is open at idle, the fuel simply exhausts through the PV supply opening, back into the fuel bowl.