[BruceWilkie]

About to prep a 2jet but for now just afew mock up pics of the slant6 its going on. (in a 34 dodge coupe, originally straight 8, these cars have a gangster look...looong narrow hood, very low roof, suicide doors) The slant six is actually a tight fit due to narrow hood and frame but lots of height available is why the turbo and piping is routed the way it is. I wish this was a 30's Pontiac coupe with a Sprint 6 but the Pontiacs are just as hard to find!

I'll post blow-thru mods as I do them. For now just a parts carb mounted.

[Tom Vaught]

Ken Crocie did a LOT of testing on his Q-Jet (Dyno Time) to get it to have enough fuel under boost to make the power he wanted. You are limited by the basic 2G Power Valve design. Very hard to add extra fueling circuits to that circuit like you can do with a Holley Metering Block. Normal Metering Block has two fuel passages to the main wells when the power valve opens. Some add 2 more drillings and get more fuel. Some like Charlie66 had 6 fueling circuits in his Power Valve systems on his 4 bbl carb.

So what power level are you trying to achieve Bruce?

Tom V.

[BruceWilkie]

Considering actual use of the street rod and the stock nature of the motor, if I can double its stock 90 hp at 4000 and up the tq considerably where the stock motor peaks(around 2500) I'll consider it good enough. The owner (my boss?) decided to buy this ebay kit vs the shopping list I suggested. Its a small turbo(49mm) and dont think I'll let it get past 10 psi. (My Rajay E flow is a much better match and have 3 different a/r housings for it)
Plan to jet abit on the rich side in cruise mode and open the pv holes(all 4 holes) till I get down to @ 11.5 afr under load. Its likely going to need off idle transition work. On the plus side the bowl vent is large and a bit funnel shaped and right in the air horn, and it has a solid float. The throttle shaft will be easy to route some boost to and help keep fuel from seeping out.
Un boosted its going to be a bit fat but MSD should help keep the plugs firing.

The carb supported my 290 hp 400 in my old 68 Catalina so I think it can pass enough fuel.
With more coaxing maybe I can convince him to go EFI...Then I'll build the early 225 in the shop and take advantage of a meatier block with a forged crank.

[charlie66]

Bruce, I hope you post some pictures of the internals of the carburetor as you make the modification's to it. Im interested in what needs to be done to make this work. I like what you're doing . Its out of the box of what is normally attempted an I like that! Good luck!

[charlie66]

The one thing I learned with my 4 barrel carburetor is you really don't need the second set of barrels like you would N/A. Its amazing what cramming air through a pair of small holes and venturies can accomplish ... I think if you wanted to , you could make a lot more then what you're looking for...

Good luck again!

[Tom Vaught]

Originally Posted by BruceWilkie

Plan to jet abit on the rich side in cruise mode and open the pv holes(all 4 holes) till I get down to @ 11.5 afr under load. Its likely going to need off idle transition work.

Casting has two passages to the main wells. I was referring to those passages vs the actual PV holes in the power valve. Holley Power valves have 2 and 4 window valves that are very large but the controlling point (restriction to flow) is the two drillings to the main wells.

Opening up the 4 holes in the power valve does little if the flow passages to the main wells are too small. You can remove the lead plugs and drill them larger though if you are careful and can seal the passages properly at the end of the day.

Tom V.

[BruceWilkie]

My parts carb has smaller venturis otherwise near identical to the bigger carb and uses all same gaskets etc so Im going to use that to get a stronger signal at the boosters. IIRC its off of my old 68 Firebird 350.

I'll be doing wiring on our hemi powered 33 Plymouth coupe tomorrow. So looks like Monday before jumping back on the Rochester.

Cant post a copy of the Rochester manual, but you can find it at http://www.carburetor-parts.com/assets/2%20Jet.pdf
(many other carb manuals at same site and they are free)

[BruceWilkie]

Originally Posted by Tom Vaught

Casting has two passages to the main wells. I was referring to those passages vs the actual PV holes in the power valve. Holley Power valves have 2 and 4 window valves that are very large but the controlling point (restriction to flow) is the two drillings to the main wells.

Opening up the 4 holes in the power valve does little if the flow passages to the main wells are too small. You can remove the lead plugs and drill them larger though if you are careful and can seal the passages properly at the end of the day.

Tom V.

Took a look at the carb again... I'm going to leave pv restrictions as is and add a second power enrichment system much like a Mikuni power jet carb.

http://thunderproducts.com/shop/thunder-powerjet/

A pair of these properly placed(above fuel level) will draw fuel(from bottom of bowl) at heavy load(beyond main jetting and power valve) and when boost pressurizes the fuel bowl additional fuel will be metered proportional to boost. They use a screw to adjust fuel metering.

I've used this company's Dial-a-Jet product on snowmobiles for years but they incorporate air bleeds and wouldnt work with boost. Mikuni Power jets could be made to work but would be harder to adapt plus I'd need a big collection of Mikuni jets.(I only have a small assortment left.)

[taff2]

I'm assuming you would be introducing the new fuel source above the carbs throttle plates, so wouldn't the boost pressure above the throttle blades be equal to or above the boost pressure in the fuel bowl and thus cancel out any fuel metering proportional to boost?

[BruceWilkie]

I'll a swer with a question. How do the nozzles draw fuel under boost?

[Tom Vaught]

If Bruce Pressurizes the Fuel Bowl, (with Boost), all he did is add more pressure vs the normal 14.7 psi that the bowl sees already.

Tom V.

[taff2]

Same as they do in a n/a application,by the venturis creating a pressure drop due to the velocity of the air. As velocity goes up ,pressure drops, a low pressure area is created at the nozzles and fuel is drawn from the float bowl into this low pressure area. An unmodified carb will start to fall short of fuel as the boost gets higher,as I understand it the mass flow increases but the velocity does not increase. As unmodified carbs work off the velocity principle,additional fuel won't be metered in to the airstream in line with the density increas.
So, an n/a carb has atmospheric pressure above throttle blades and atmospheric pressure in the float bowls. A blow through carb has boost pressure above throttle blades (say 10 psi) and boost pressure in the float bowls (say 10psi), how would it meter proportionately more fuel as the boost goes up if the pressure above the throttle blades (where the new nozzles are located) and the float bowl pressure is at the same pressure?
This is why old school blow through carbs used vent extensions-to add the velocity aspect of the incoming air charge to pressurise the float bowls to a little higher pressure than exists above the throttle blades, thereby forcing more fuel to exit the nozzles as the boost goes up.
I could understand the nozzles of the secondary power enrichment system adding more fuel over and above the existing carb nozzles-but not in proportion to boost.

[BruceWilkie]

the vent on this Rochester is large and funnel shaped. Room to add more vent if needed. The dial a jet kits I used on mikunis mounted quite a bit ahead of venturi yet still pulled additional fuel under load. The bowls were vented to air box. I'll post a pic of the rochester vent tomorrow.

[taff2]

Ah right,that sounds like it would work. I remember a thread a while back on the turbo forums where one of the guys put a flare on the ends of his vent extensions and they worked too good in his application. Sounds like the Rochester was built for boost!

[Tom Vaught]

Correct, before the guys got involved in the Many Mods to the PV circuit, they were all trying the "make the vent into a velocity tube" and air velocity would increase the pressure in the bowl and add extra fuel for boost.

Key there was the calibration vent(s) in the velocity tube that kept the bowl "circuit" from going extremely rich under higher boost conditions.

The C&S pick-up tube/air cleaner hold down is a similar type device except it turns on the Power Valve circuit and you calibrate the fuel based on PVCRs in the circuit.

Tom V.

[BruceWilkie]

Pics of the air horn showing bowl vent location and shape. It tapers down to @ 7/16" hole. There is room to enhance the funnel or add more vent. I think this will be adequate.

More pics as I get into things.

[charlie66]

That vent looks bigger then a vent in a 4150 carb. I think its more then big enough. You're not looking for big boost any way..

[Tom Vaught]

Originally Posted by BruceWilkie

Pics of the air horn showing bowl vent location and shape. It tapers down to @ 7/16" hole. There is room to enhance the funnel or add more vent. I think this will be adequate.

More pics as I get into things.

Learned something today Bruce, Thanks. Does it have any sort of a baffle to keep fuel from pouring into the venturi area under hard braking?

Tom V.

[BruceWilkie]

Originally Posted by Tom Vaught

Learned something today Bruce, Thanks. Does it have any sort of a baffle to keep fuel from pouring into the venturi area under hard braking?

Tom V.

Fuel bowl faces forward with jets nearly on bottom by venturi wall...hard decell might uncover jets but if braking that hard a brief loss of fuel shouldnt be an issue...the booster legs at top has what appears to be an anti-pullover for the pump nozzles and its width suggests perhaps the bleeds for the main wells... I'll look closer.
BTW the bowl wall seals to the airhorn behind the vent.

Accerator pump is internal to bowl so bowl pressure will keep it balanced.
The actuator arm extends thru airhorn casting so i'll have to bleed boost to it near shaft end to keep fuel from seeping out, same as i'll do for throttle shaft.(I'll source it from "hat"). Most likely deleting choke so shaft holes can be plugged. I could make seals easy enough should I decide to keep it.

Question...I saw this posted recently but not finding it... Holley jet tap size...is 1/4-32? (I have a 1/4-36 and it seems too fine) Seeing I have a bunch of Holley jets and few Rochester...converting seems my best option.

Source of the Rochester manual(posted above) sells assortments of aluminum plugs should I need some.

[Tom Vaught]

https://www.holley.com/products/fuel...tes/parts/26-1

1/4-32 UNF Thread

As you can see from the Holley Price, not a common tap or PRICE.

Worth converting Rochester carbs as Rochester jets are hard to find and they want 1/3 of the tap price for 4 Rochester jets.
Buy a whole Holley jet kit for less money to do a couple of Tri-Powers.

Tom V.