the way i read it is you need to go back up in jetting on the primaries to like 67 or 68, but maybe i missed something.

 

You are right. That is what I meant to type. I edited/corrected it...do what I mean not what I say right?

 

My afternoon drive yielded a predictably more rich condition (65* 30% humidity). I was getting 15-15.5:1 at cruise instead of 15.5-16:1 that I was getting this morning. I hooked up the vacuum gauge and watch for when I knew the power valve was opening. Vacuum was not getting low enough to open the PV even when under a low cruising up large hills. I forced it lower by accelerating up the hill, but did not press the throttle enough to open the secondaries. I did notice about .5 point richer AFR when the PV should be open.

WhenI got home (50 miles) I took the primary bowl and metering block off. I put one size larger jet in and changed out the PV to a 9.5". I also checked the PVCRs with some feeler gauges to see what the diameter is (no drill bits that small yet). It looks like they are in the neighborhood of .060" in diameter. After installing, I rechecked the vacuum at idle 13-13.5:1 AFR with 11-12" vacuum. I went for a quick spin to check for bad problems. I again saw about .75-.5 point richer when the PV opened, which was quicker now. Idle was the same as before and the cruise was now 14.5-15. It was getting cool again, so I expect this to be 14 or so once get starts getting in the 80s and 90s. It still stumbles around when the AFR gets above 15 and this setup is right on the edge, but should be close for the summer time. I did notice a stumble off of idle and a lean condition when burbing the throttle again which I did not see as much when it was much richer. I upped the primary nozzle from a 28 to a 31, and that helped the stumble when burbing the throttle when checked in the garage. That is not an easy task to do with the carb on the car. I dropped the dang screw a dozen times down into the venturies. The choke got in the way and about sent me over the edge! I did not do another test drive because the dinner bell as ringing.

What surprised me was how high the vacuum is when driving. It is pretty low at idle, but most any other time other than heavy acceleration, it never gets below 10. I was shocked that a 9.5" power valve was what my motor wanted (had a 6.5 from the factory but put a 2.5" in during initial tuning)...I was also shocked at how little AFR changed when it opened. 6-10 jets in this carb on this engine is a ton, but the PV with these PVCRs is only upping the jetting by maybe one size. I guess I could drill them out if I wanted more enrichment, but that is kinda what the secondaries do already. I though the PV would also most flood out the motor if it came on too quickly, but I am not seeing that at all.

 

i think i lost you after the 2nd or 3rd post already. it's a bit much for mee to follow because I've never done those adjustments before. but isn't that exactly what you want a nice and steady AFR? no matter what throttle/load/rpm? Sounds good to me :-)

Kalli

 

Well, Kalli, you are right that you want a steady AFR, for the most part, but under high load, you want to increase the fuel to air ratio (lower afr). A power valve and accelerator pump help with that, so that you can run a leaner more fuel efficient setup when the motor does not need the extra fuel. Race cars can block off the power valve and just jet for WOT, but when I did that a while back (basically was doing with a 2.5"pv), my fuel economy dropped almost 2 miles per gallon. I am not trying to create a commuter car, but I also do not want to foul plugs and have gas dripping out of the tail pipe when I finished idling in traffic.

What has me scratch my head is how little the power valve circuit adds to the afr on this setup, and how much one jet size affects the AFR. Neither of which is normal based on what I have read.

BTW, cprstreetmachines, that Holley book you posted is fantastic. My wife calls it my bible b/c I read up on something in bed every night before I go to sleep.

 

i always thought their purpose is to add fuel so you end up on correct AFR (avoid leaning out), opposed to richen up the mixture to end up rich for accellerating.
meaning if my engine would be happy full throttle at a specific AFR then i'd try to tune it to the same constant AFR in all scenarios (accel, cruise, idle, stopping, pushing ...) or am I completely off my book here ...
again, this is just for learning purpose. never done that before

 

No, you are right that opening the throttle can produce a lean condition depending on what the setup is. The main thing is you get the best performance from running 15-20%, but under light load conditions, your car only needs 14.7:1 ratio to completely burn the burn. Fuel economy verse performance is the best way to say it, I guess. If you ran 14.7 all the time regardless of what your load was and what your throttle position was, I think you would be leaving some performance on the table. If you ran 12.5:1 all the time, your performance would be great, but you would have issues with plugs and terrible fuel economy.

 

still seems to me you need to go one more jet size larger in the front. this isn't a gas sipping daily driver, its a performance machine. i'd be much happier in the mid 13's (good blend of power/efficiency), and even when weather conditions change you should still be in the 12.5-14.5 range.

 

The PV is not always going to show a huge jump. Just variables in the PV opening itself, you may not have opened the PV yet. But DP's usually have very small PVCR's at about .039 because they are meant for race engine, and are about 5 jets sizes too high in the front for normal street driving. I've noticed that holds true for about every DP except for Demons. Those things are just LEAN. But a bog in a DP with tiny PVCR's, is pretty hard to get a bog in an engine that big.

I LOVE that book. I did the same thing. Then I got hooked on Turbos, read books on that. As long as it keeps my interest, I'll read it all the time.