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Discussion Starter · #1 ·
I run a Holley 950HP which came 79 squared 6.5 PV front and back
I blocked the back and went up to a 87.

This carb on old combo which ran 7.50's was borderline lean car was DEADLY consistant didn't move more than one or two numbers on race day and only moved 5 or 6 numbers from good air to bad air.

On my new combo which runs 6.90's the carb is rich moved a tenth from the good air in early srping till now. On race day car has run out 2 weeks in a row, it is moving 5 or 6 numbers.

How could this carb be lean on the old motor and fat on this one.

Was thinking of jetting down from 79/87 to 77/85, or should i go down one jet at a time and see what happens.

What about timing, someone suggested bumping timing up 2 degress said that would lean it out

Looking for opinions/suggestions.
 

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Is thisa strip only car or does it see street duty too?

What's the currnet motor setup like size cid/cam/etc and is this a strip only motor/car?

If this motor has a lrg cam and is run on the street the 6.5 p/valve can definately be part of the problem of with running rich due to low intake vacuum. It could even affect a strip only motor when just strolling back from a run if your running enough gear to cause you to have to apply enough throttle to cruise at 30mph pushing the prim throttle blaldes out of/past the idle circut .

What's the intake vacuum at idle in Gear with auto trans and while at sloe low load part throttle cruise?

If you know its really running fat on both sides of carb then stepping down 2 jet size is ok foryour 1st tuning attempt.

But but if the 2 step down in jet isstill a smidge fat then only go down 1 jet size at a time from there becasue your likely close to where you need to be at that point and you dont want to go too lean by accident.

Also,since you asked about tweaking the timing what base/total timing are you running?

Scott
 

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Discussion Starter · #4 ·
The old combo typical run was 7.50 at 91.5Mph, new combo 6.98 98.4 Mph

Run a Rpm intake on both.

Strip only footbrake car
383
Edelbrock RPM heads
10.8:1
RPM intake 950HP
hydro flat-tappet 239/247 @50 507/530 106lsa
1 5/8 headers
Th350 ATI 8"
4.56 28x10.5 slicks
3200w/driver

I don't know the vaccum
Run 34 total timing everyone i have talked to says that less is more with the RPM heads.
 

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WELL WITH APPROX 240 DEG @.05 CAM ON A 106 LSA in only 383 cubes that cam will not mak much intake vac so i would suggest a 4.0-4.5 prim p/valve but the 6.5 is not right at all for all that cam in a 383.

Since this is a strip only motor I would also suggest plugging off the sec p/valve and try maybe 80-82 for a starting place along with 75-76 for prim side as a good starting place. Since this is a strip only motor with open headers etc you will likely need more jet/esp sec but you can inc those as as required.

Then you can tune up/down in jetting as required from there and as for the timing just try the 2 deg diff to see if that helps,if not put timing back to where you started or where you know it runs best.

Scott
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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you're not far off, I wouldn't be making any big changes with out proof that it's needed, like a log from a WBO2 or similar.

Take one size out of the secondaries and see how it goes. if it helps take one size out of the primaries. if either of these causes a drop in trap MPH, put it back like it was.

You might find out that the new engine is just plain more sensitive to changes in the DA.

BTW, I disagree with Scott on the PV stuff. I doubt if your car ever has enough vacuum to close the PV except at idle in the pits so it probably doesn't make a lot of difference what value PV you run.
 

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I just got a Prosystems HP 950 for my 408 sbc. Its jetted 76P/86S if that helps at all. What did you do to the combo to pick up from 91mph to 98 mph???? Thats a TON to pick up in the 1/8 NA. Im jealous!
 

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Hey Tom,guess what,surprise surprise i disagree with you this time arround too(LOL!!!!)

Anyway,i say that because he doesnt have much vacuum like you stated an i have seen this before many times over the yrs guys running stock holley's with the usual 6.5 p/valve with fairly lrg cams running into problems because the 6.5 valvre is just not a good match in that case for clean primry fuel calibration eso if the car is going to see some street use.

I have had many times where i changed the p/valve from a 6.5 to a lower one like 4.5-5.0 thats better suited for a lrg cam/low vacuum and walla thier lower end/rpm rich issues went away and the people were happy.

Do i know for sure if its going to fix his problems no because its all depends on how well these people describe the problems they are having and on how well they rept back on the follow thru diagnaositcs they do when they try the suggestion we give them.

This is just my experience with doing this stuff for the past 37 yrs and i get the impression you have also been at this stiuff a long time too so if thats the case your getting to be an old fart like me too(LOL!!!!).

Scott
 

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Boldly procrastrinating
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You only count as old if you were around to see God bring in the first wheelbarrows of dirt. :)
 

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Discussion Starter · #10 ·
I went from a 355 to a 383 to pick up MPH


I currently have a 6.5PV in front and block in the back jetted 79/87
So you think going 77/85 is to big a jump, should i go to 78/86 and run it like that for a week then if still rich go to 77/85
 

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Tom,that's funny(LOL!!!!)

Scott
 
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