Test hit carb question
 

I finally got out over the weekend and did a test hit in my catalina.i video taped all my gauges during the run so i could dissect it later.wasn't totally sure what it was going to do.i had a 7lb spring in the wastegate.timing was conservative at 26,fuel pressure came up with boost as designed.csu carb with 71 jets front,72 rear.dual brpv's and .062 pvcr's front and back.during the run I was hooked up in 2nd and 3rd gear and ran it out to 5000rpm.afr was 11.8-11.9 pulling thru 2nd and 3rd.intake air temp out of turbo got to 175.intake air temp in the intake manifold was 91,so the intercooler was working.my question is if I go up to say 10 or 12lbs boost do i go up in size on my pvcr's or am i thinking wrong?should I jet up one or two and get the afr closer to 11.5?I would like to do some plug check runs.how long should i run new plugs before i do a test hit?

Historically people would leave the primary jetting alone, increase the Primary and Secondary Power Valve restrictions to a max of .087" and then increase jet on the secondary from that point. Sneak up on the size of the PVCRS until you get to your best drive and performance/afr on the primary side.

Tom V.

Like Tom says up the secondary jets. I would pull some more timing out until your done dialing in that carb. What fuel are you using, octane?

Fuel
 

Straight gas.non ethanol 90 octane

Congrats!!! Cool you got it done. I forgot what heads you run and compression. But as stated before, drop the timing a little, I have never been under 25 degrees though. Best way to read the plugs is, do a pull in second or third until the max rpm you want. Shut the car down ignition off, don't lock the wheel!!! Pull the plugs, all of them too. I was only pulling a few but one time my 2 rear cylinders were not running like the fronts and I broke 2 piston lands.
If my A/f goes into the 12.0+ range I let off to be safe. You might find with more boost the a/f will still stay in that range. If it does not let off quick. Then make adjustments.
Keep us posted!

Carb jetting
 

I understand all but the jetting up only on the secondaries. why only jet up on the secondaries.wouldn't that lead to a lean condition in the forward cyl's?maybe I don't understand what's happening as far as fuel/air distribution under the carb and in the intake plenum.somebody school me on this.thanks

The pvcr's are part of the main jets channels by way of the power valve. They are there to serve as a supplementray way of adding fuel so the main jets can be made as small as needed to be to achieve the best fuel economy . So the method for tuning with them is : Get the cruse and part throttle to the correct AFR by trimming your front main jets down. Then once you got that under control , now and only now you start the WOT adjustments by changing the pvcr's as needed ..

Hope this clears it up some for you...

Theres more to it but thats the short version ..

For a drag race situation the time between the Primary Main jets fueling the part throttle/cruise circuit and the rest of the circuits is very short in duration.

Say you are like Charlie66 and drive the car a long distance vs just doing drag race passes.
Under those conditions you would want the right fuel under passing modes on the freeway so that the air/fuel was not too lean during that vehicle mode.

So you set the Primary Power Valve Enrichment for the best air/fuel ratios under that mode.
You can do this by disconnecting the secondary link that would open the secondary throttle blades and wiring the blades closed while you were doing that tuning.

You would also engage your Brain and keep telling your brain I am not going to race someone during this mode of testing. It is embarassing when your boosted two bbl carb is not the winner but still makes a lot more power than you expected. So you get the air fuel ratio on the Primary side nice at all throttle levels even under some boost levels.

Then once you remove the secondary lock-out device, you can repeat some of the heavier throttle events and see what the secondary jetting needs to be like hooked up.

Then you try to remove again some of that secondary main jetting by making the Secondary Power Valve Channel Restrictions larger and lower the secondary jetting a bit.
(ALL OF THIS IS ASSUMING THAT YOU HAVE A TRUE BOOSTED CARB DESIGNED WITH CALIBRATED BOOST REFERENCED POWER VALVES ON BOTH ENDS OF THE CARB.)

You can get pretty close with no PVCRS on the secondary side (PV Plug) and larger Jets on the rear but at some point on higher boost calibrations you run out of main jet size on the secondary without going to alcohol jets. The assumption here is Gasoline for fuel).

Same process but bigger orifices with E-85 type calibrations. The PVCRs on the secondary side really help with E-85 fueling demands. Think Charlie66 and 38 psi of boost modes. LOL

Hope that helps. Charlie66 is a very sharp E-85 carb tuner. he has done all of the mods mentioned over the years on his carb stuff prior to going to EFI.

There are other mods to the carb like the way to turn on the power valves under higher boost modes and how you supply fuel to the PVCRs especially on the secondary side.

Tom V.

Unless you are a carb guy pay the money for a Proper Boost Carb and the technical help they offer to get you close on tuning the thing.

When i was tuning with a carb i disconnected the secondarys and wire them shut. Its surprised me when it didnt really feel any slower. In fact i dont think it really was slower . Having the extra fuel bowl for the higher power levels is where the real benefit is in having a 4 barrel. I think if the manifold is seeing X-amount of boost ,thats all the motor knows and it will produce accordingly. Its was always funny when i raced someone and they asked me after i beat them what i had in it. I would say a 198 ci 4 cyl . They would say " yeah right " Then id say, Yes and it only has a 2 barrel on it! They would always walk away scratching there head with there friends laughing at them. haha..

Nice!