Setting up a 750 DP

[Stu5766 394]

Hey guys and gals. Hoping to pick some brains.

Just mucking around with the 750 Holley on the XD. It was rich as at idle and constantly stinking of fuel. I got 69 primary's and 80 secondary's with a 6.5 power valve. It went quiet well with only a very slight hesitation when cruising. I checked the vacum and it was pulling about 7 inches at idle when in drive. It has the comp cams XE274 cam in it. As per Holley guide lines I fitted a 3.5 power valve. It now has a major hesitation and is almost undrivable. Do I need a bigger PV or bigger primary jets? Thanks in advance.

[Trev Vaa 1,185]

i "think" we run 68s in the primaries on the XY, which has a similar cam and similar vaccum.... i could be wrong though..... 80s sound too big in the secondaries though....

hesitation can be caused by a lot of things.... not enough fuel or too much.... float level etc have you put fresh plugs in?

got a pic of the old ones? how much did you play around with the idle screws?

 

adjusting the PV may have just made this issue worse. how much of the transfer slot is exposed on the primaries and secondaries? this can be a major cause of overfueling at idle.

 

do you have any other PVs to chuck in? might be something to play with.

[PRO250 1,506]

The jetting seems small as a place to start. but i know nothing about the engine. a 750hp is 75s front and 80s in the rear out of the box. and most hot 351s they will work ok out of the box

hook up a vacume guage and go for a drive it will tell you what the engine is doing while the hesitation is going on

one thing i will ask is now big is the stall? a real small convertor will pull the engine right down in gear making vacume low. does it idle ok in gear?

while im here, i have a ton more vacume then that with more camshaft. can i ask what your brakes feel like? i had just on 6inchs in the last car and the brakes where rubbish

[Stu5766 394]

Brakes a fine mate. It does have the large fairlane booster fitted. It has a 3000 rpm stall converter. Plugs are brand new. BP5FS. The idle screws are 1.5 turns out. It drove pretty good with the 6.5 power valve, it turned to shit with the smaller PV.

[PRO250 1,506]

try bumping the jetting to see if it gets better 

its a real how long is a bit of string thing these things, hard to nut out a tune over a forum






 

[Stu5766 394]

I know, it's a pain in the ass. Thank mate.

[slydog 7,873]

Is it a square bore or spread bore? What size squirters doe's it have in it,is a spacer a option?

 

565 lift and 230-236 duration @ 50 110 lobe sep and 106 centreline.

 

I'd go up in the plug to 6's and what gearing doe's it have?

[Stu5766 394]

G'day sly, it's a square bore. Can't run a spacer cause the shaker won't clear. Not sure on squirter size.

[slydog 7,873]

OK I'd suggest square jetting it too 74's front and back and go to 33-35 squirters.After that adjusting the pump cams and acc arm to suit.You will be able to move the squirters up or down to suit once sorted and I think your 3.5PV will be OK with them mod's.

[Butcha 111]

You might need to change your pump squirter cam to a more aggressive lobe otherwise try bigger squirters. Make sure that your accelerator pump is properly adusted try backing it off until there is no play. Remembering that the bigger difference you have from the idle vacum to the pv size the more fuel it needs from the accl pump it needs until the transition takes place.

 

I wouldnt square jet unless you want to throw out your fuel economy the pv circuit adds roughly 10 to the jet size when in action (full throttle) hence the staggered jetting. Under full load it is essentially square jetting.

 

Just my 2 bob iv picked up over the years

 

 

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[Butcha 111]

A 4.5 or 5.5 pv might be better suited.

 

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[slydog 7,873]

Correct Butcha, all the hp and new dp models are square jetted though and seem to work well.

[Butcha 111]

I havent brought a holley for awhile maybe they got a new box of tricks.

 

Actually I bet the new carbs run pv in the rear now...

 

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[slydog 7,873]

Some do and some have adjustable emulsion jetting as well.Not that any of this is any use to me anymore...webers

[gerg 10,871]

There are lots of differing opinions on power valve selection. Some say you should subtract 2 from your vac reading and go to the closest size PV but that's more for straight out racing. For a street setup, Holley says you should halve your idle vac reading to get your PV size, which you did.

 

However, your vac at idle seems artificially low for that size cam, meaning your setup needs tweaking before arriving at a final number. I should think your vac reading needs to be more like 12 at idle. Needless to say, if it went well with a 6.5, then that's what your engine wants. Jets seem to be on the rich side but if it goes well up top, there's your proof.

 

Like Slydog said, try squaring them up a bit more, like 72/76 or so.

 

Btw double pumpers are not a street carby. They have big power valve channels, big idle feed restrictions and transition circuit designed for all-out performance. They can be made to be efficient with a lot of tweaking but you have to know what you're doing. This explains your rich idle, which has nothing to do with your jet sizes. Maybe play with the air bleeds?

 

My little 302 runs a 270/280@112 cam and pulls at least 15" at idle. You should look at the PCV, transfer slot opening and other sources of vacuum leaks.

 

Check timing: maybe it needs full time vacuum advance to get it all running sweet. It just doesnt sound right how it is.