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My drag car is bogging when leaving the line. I leave at 1800-2000 RPM. If I floor it it will bog then ketch but if I leave at the same RPM and just rapidly depress the pedal it doesn't bog. Running two 1000 CFM HP holleys with 30CC front accelerator pumps and two 50cc rear accelerator pumps.
Would it help to change the rear pumps to 30cc? I'm running 30 degrees timing on a MSD 6al2 programmable.
Please note: It is not backfiring through the intake, it is bogging as in to rich but I have not noticed any black smoke.

Dart 434
Bore: 4.155
Stroke: 4.00
Compression: 8.6
Comp Cams roller cam
Intake lift .637
Exhaust lift .631
Duration @ .020 intake 300 exhaust .302
Valve timing @.050 intake open 21 BTDC close 63 ABDC
Exhaust open 71 BBDC close 13 ATDC
Installed @ 111.0 center line
Duration @ .050 intake 264 exhaust 264
Lobe lift intake .4210 exhaust .4210
Lobe separation 115.0
Brodix track 1 cnc 227cc heads with manley valves and springs
Weiand 6/71 blower .12 under drive
Twin Holley 1000 CFM HP carbs blocked power valves #94 jets all around
NGK 4554 R5671A-8 plugs
 

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Even a 50cc pump will only pump as much as the pump cam and pump nozzle dictate. Depending on what pump cam and what nozzle is there now, it may be a cam too big for a 30cc pump, which is maxed out with the Pink cam in hole #1/link hole #1....Pink hole #3 in link hole #2(Pink hole #2 is never used, don't ask me why Holley says this IDK) is too big for a 30cc pump and will bottom it out, as will a Brown or Yellow cam which are 50cc use only.

If you feel the pump shot is too big, put a smaller pump cam on the secondary, along with maybe one size smaller nozzles. This will reduce the initial volume of the shot, but balance to keep the length of time the shot is active the same.

Pump cam decides total volume of the pump shot, while the pump nozzle determnes if it will be a lot of fuel early in the throttle opening event but of short duration (large nozzle), or if it will be a smaller amount of fuel initially but stretched out over more time (smaller nozzle).

It's a trial and error process. It may turn out that just a smaller pump cam is all that is needed, so that with the current nozzle the beginning of the shot will be the same, it will just last through less of the throttle opening event (time).
You're going to have to set yourself up with as couple of pump cam assortment kits, maybe even 4 kits if you get into tuning all four pumps and they turn out to alll need the same cam....same thng with pump nozzles.
It can al get pretty expensive as far as a large nozzle assortment goes, when tining 4 pump nozzles.
 

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You really need an Wide band O2 A/F meter on this....it could save you a lot of money in tuning parts.....bacause it may be too lean but the blower covers it up and prevents the typical backfire.
 

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You really need an Wide band O2 A/F meter on this....it could save you a lot of money in tuning parts.....bacause it may be too lean but the blower covers it up and prtevents the typical backfire.

Doh, good point. Never heard of that, but sounds like it could create a mess.


Can't - how about vid?
 

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My 600 dp would go way rich on the pump shot creating a bog and the dyno showed this on the A/F readout (tailpipe sniffer). I went with the smallest squirters Holley offers and decreased the pump shot a bit, and this helped.
 

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Dtill the front shooter to the next drill size, then the back. Repeat until it leaves. 35/47 final sizes on my 351C with a 650DP. Edpecislly with your low staging RPM. Also try 200-400 more rpm if possible.
 

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Discussion Starter #11
Dtill the front shooter to the next drill size, then the back. Repeat until it leaves. 35/47 final sizes on my 351C with a 650DP. Edpecislly with your low staging RPM. Also try 200-400 more rpm if possible.
What drill kit do you use?
 

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What drill kit do you use?
#'s 61 thru 80 size drill set.

Same size set used to make small air bleeds or idle feed restrictions/emulsion bleeds.
#61 is .039", #80 is .0135"

Next size set larger from that point is #1 thru 60....#80 is .040", #1 is .228".
Numbers 1 thru 80 fill in gaps left in the fractional size chart.
 

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I use both the 80-60 and a 60 up set. The shooters on the methanol 750 I use are .067, 50cc pumps with cams set for max delivery. I use drilled screws in all carb, without regard to shooter size. With a 351C, 4V heads, big single plane I've run .040 or better on a 750 DP. RPM and initial timing help. Even with the blower on there, there is still a lot of air space which usually mean fuel.
 
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