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Discussion Starter #1 (Edited)
I'm haveing a but of an issue with my Holley, and the more I read, the less clear it gets. Seems everyone has a different opinion, so I need to ask you guys here. When I go to WOT, wether from idle or cruising at 3000rpm, I get a lean condition, a pfftt type backfire thru the carb, then the car goes. This tells me I need more fuel for this second during transition. But here's where I'm having issues. It's an 850HP carb. The secondary side has a 35 squirter, an agressive pump cam on the number 2 hole, and a 50cc pump. It runs 87 rear jets with the PV blocked. Now should I just start goin big on the squirter size and hope this helps, or should I put a secondary PV in the carb for that instant enrichment. That's where I stumble when reading this. I usually leave the PV in, but I read alot about blocking it as well. Now wether I;m just cruising, idling alot, or just got done with a WOT blast, my mixture's close, the plugs look great. So what do you all think. Squirters or PV?

Oh, and since I know you'll ask, my timing is 15 initial and 36 total, so I don't beleive it's timing related. My timing could be off a little yet for max power, but not enough for this carb issue. Now once it catches and goes, it runs really strong.
 

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You say it's an 850 H.P. I don't see such a carb listed..is it the Street H.P. #510-0-80531 ? That one came with a secondary PV and is calibrated for 502 crate motors. It would have 82 jets with the power valve, so you are probably a couple jet sizes lean with 87's.
 

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Im with what bowtie says. Need to know more about the carb and such. As far as timing goes, you may need to increase the rate of timing. Depending on the cam sometimes the advance curve needs to come in faster. Try advancing the timing and see if that helps. What stall converter do you have? gears? whats done to the engine? what jet/PV is in the front?

FWIW I had a 850 #4781-2 that I ran 72 jet with a 6.5 PV and 84 jet in the rear with PV blocked. Stock 30cc pumps, 31 squirters, white cams on both squirters. This was on my 12 to 1 406 with a Victor jr. HEI and small roller cam. BTW my timing was 22 initial and [email protected],000.
 

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Discussion Starter #4
Timing's coming in fast. It's all in by 2600rpm. As for the rest, 10.5 440cid big block, Victor Jr., 249/255 @ .050 solid roller, Summit ignition with box, 4 speed, 3.73 gears.

The carb is an 850HP that was originally a Keith Dorton modified carb.I don't have the carb right in front of me, but the main body casting has 850HP stamped right into it where Holley marks them. I run 74's in the front with a 30cc pump and a 31 squirter, and like I said the rear runs 87's, no PV, and a 35 squirter with a 50cc pump. The front PV is a 6.5. The front runs a red cam, and I'm not 100% on the rear right now, like I said it's not in front of me. I want to say I have the white cam in the secondary, but it may be the pink. Like I said, once I get past that initial pop, the car runs great. The plugs have a nice tan color to them, no black sooting. There are also no signs of detonation on the porceline either, just need to tune out that lean condition. Now I already have the primary's tuned. The car was sluggish and I had to go down on the squirter size. It had a 35 as well. Once I did that the engine was much more responsive. Now I just need to tune the secondary side, but don't want to spend all day chasing my tail.

The one thing I don't seem to find an answer for, is why do you eliminate the secondary power valve? If it acts like an aditional squirter, why not keep it? If I increase the secondary jet size to compensate for the blocked power valve to something like a 92 or so, now I'll probably be running rich on the secondary side, so why would you want that all the time? If I enlarge the seconday jets to compensate and it does run much richer, now i have to mess with the high speed air bleeds to get the ratio back right? I know there's lots of way to look at this, I guess I'm just trying to find the simplest solution to my problem.

Oh, btw, the car sees more "spirited driving" than anything else, lots of on and off the throttle. I like to autocross it too when I get the chance, and it will go down the strip on occasion as well. Kind of an all around car. That's why this tune is important to get right, because it's not for just one kind of driving.
 

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Im sure I'll get blasted for this but I was told a long time ago that at 7,000 RPM's the engine has no vacuum due to being WOT but starts to build up like 1 or 2 lbs pressure and can cause the PV to flutter and cause a high RPM miss. Again thats what I was told by a guy that I truely trust and has vast experience tuning at the dragstrip. I had PV's in my carb and took them out at the track because I had a high RPM pop. Maybe cuz it was jetted lean with the PV's and corrected it by blocking off and rejetting but hey it worked. In the secondaries all the fuel contol is now by the jets for more precise plug readings and tuning is my guess.

I'm sure there is somebody out in nova land that can give a better more eloquent explanation.

P.S. if the cam on the secondary is set on #2 maybe try it on #1? have it come in sooner with all that shot?
 

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so why would you want that all the time?
How often do you have the secondaries open? If it's jetted correctly, why would you feel it's too rich? And using the air bleeds to fine tune it sounds reasonable to me.
 

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Most racers block the secondary power valve because the PVCR (power valve channel restrictions) are higher than the jets and under acceleration they are uncovered by the fuel climbing the back of the bowl. I think the 50cc pumps need certain cams to work right. Brown or yellow I think. Any other cams will act like a 30cc pump because of less lift. Don't touch the air bleeds. Tune it with jets.
 
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