Holley Jetting question

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Dustedu2

Dustedu2

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I am trying different carbs just to learn a little more. I have a Holley DP 870 CFM running 74 square. WOT seems fine at 12.5 to 13.3 A/F but cruising speed high 10's low 11's A/F should I jet down primary say 72 or am I off track. Just learning about carb's. seems to have decent response otherwise. I did get my fuel pressure dialed in at 6.5, up from 4.5 after I broke my $200 pump but let's not get into that. lol
 
Read the plugs. You cannot go wrong.
 
Jetting alone will not fix the rich cruise situation.

You need to learn about the idle feed restrictors, Idle air bleeds, power valve channel restictors to get a handle on the cruise situation.

It's some boring reading, but, the innovative site has some good tuning suggestions/theory for holleys in their forums.

Chances are you need a much smaller IFR, bigger PVCR and other tweaks to get it to come around. The newer holley DP race series carbs are really bad for this issue.

Take some no-doze before reading this stuff... LOL
http://www.innovatemotorsports.com/forums/showthread.php?p=71662

http://www.innovatemotorsports.com/forums/showthread.php?t=3134&highlight=avenger+870

Same basic stuff you need to deal with and plenty of theory to put you to sleep.
 
if you are going to leave it square jetted you need to take the power valve out and put in a block off
 
sounds like you have a A/F meter, you could try jetting down, do you have vacuum advance hooked up? Whats your timing at cruse rpm?
 
call the holly tech line,,some oen should give u answers to get u in the ball park,,,
 
crackedback said:
Jetting alone will not fix the rich cruise situation.

You need to learn about the idle feed restrictors, Idle air bleeds, power valve channel restictors to get a handle on the cruise situation.

It's some boring reading, but, the innovative site has some good tuning suggestions/theory for holleys in their forums.

Chances are you need a much smaller IFR, bigger PVCR and other tweaks to get it to come around. The newer holley DP race series carbs are really bad for this issue.

Take some no-doze before reading this stuff... LOL
http://www.innovatemotorsports.com/forums/showthread.php?p=71662

http://www.innovatemotorsports.com/forums/showthread.php?t=3134&highlight=avenger+870

Same basic stuff you need to deal with and plenty of theory to put you to sleep.
Click to expand...

Excellent advice. If you drop the jet sizes your WOT AFR will increase (lean out). In the meantime I would check vacuum at cruise and make sure the PV rating is below that of your cruise vacuum. Once that is established (PV rating should be 2 numbers below idle vac in drive at idle IIRC) I would step up to the square window high flow PV and drop your mains by 2.

Quick Fuel makes an excellent metering block which IMHO is much eaisier to tune and more accurate than the Holley stock block. Just food for thought.
 
If you have fix air bleeds and the idle circuit is rich try putting a small strand of wire bent in an L ( a single strand from a 14 to 18 ga. wire) in the idle feed restriction in the metering block until you achieve a 14to15 AFR (go with a thicker strand to lean thinner to richen). If it is already lean you have to drill out the idle feed in small increments to richen. If it has air bleed jets(bigger leans smaller richens). Then lean out your main jet till you get your cruise rpm up to high 13 to high 14 AFR then open up your power valve channel restriction in small increments to get back to your original WOT AFR ( best to unhook secondaries on carb till you get the cruise RPM fixed then hook back up for WOT). If you get a flat spot off idle or going to WOT adjust shooter size and or pump shot duration.
 
fstfish66 said:
call the holly tech line,,some oen should give u answers to get u in the ball park,,,
Click to expand...

I wouldn't bother.

I've spoken with them many times and they are not worth the time to call. On one situation i got told to put a smaller carb on the engine and it had the exact same characteristics as the the larger 950hp to 750hp, still fat as snot in cruise and the "tech" was worthless. Couldn't even answer questions regarding IFR/emulsion sizes, I had to open the carb up and pin gauge the stuff to get sizing.

You may get lucky, but, I wouldn't count on it.

Your mileage may vary. :yawinkle:
 
Check out mcmaster-carr for the parts you need to tune the carb

brass set screws in 6-32 and 10-32 threads
tapered and bottoming taps same sizes for modding the metering block.
Drill sets in 1-60 and 61-80 sizes

As 19dart66 mentioned, find some wire and put it in the holes to get a general idea of what is needed to reduce an existing opening. Get used to the formula, pi times radius squared... It's required to figure out the area of the opening. A common misconception is if you have a .050 hole and you stick a .020 wire in it you now have a .030 sized hole... which is VERY wrong.

Since you have an AF meter it should be easier for you to get a handle on it.
 
EL5DEMON340 said:
sounds like you have a A/F meter, you could try jetting down, do you have vacuum advance hooked up? Whats your timing at cruse rpm?
Click to expand...
MSD Distributor, 14° bushing initial set at 22° light springs. not sure of cruse timing
 
i would try to jet down the pri to 70 or so and if it goes lean on the top bump the sec up to 76 or 78. I'm not sure why its jetted square with a PV. as a comparative my 750 race holley is jetted 68 pri 80 sec with a 5.5 PV on the primary only sec is plugged, but i have annular boosters which give a stronger signal and don't require as large a jet.
 
EL5DEMON340 said:
i would try to jet down the pri to 70 or so and if it goes lean on the top bump the sec up to 76 or 78. I'm not sure why its jetted square with a PV. as a comparative my 750 race holley is jetted 68 pri 80 sec with a 5.5 PV on the primary only sec is plugged, but i have annular boosters which give a stronger signal and don't require as large a jet.
Click to expand...
I'll try that and see what happens. 70 pim, 78 sec
Thanks all!
 
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