84MonteSS

iTrader: (2)

hey everyone,

i just installed a 750 holley dp today, and it seems to be a little off. everytime i hit the gas pretty hard (half throttle or more) it cuts out for a half second then takes off. i can ease into full throttle with no problems, but when i just step on it even a little it stumbles everytime. it does this at EVERY rpm, not just idle. is it the jets? lean, or rich? it has 71s primary and 80s secondary. i don't have a clue of its a/f ratio. could it be something else with the carb? i really don't know what it is. it did it when it was cold, and at running temp. ~160deg. any help would be greatly appreciated, and i could offer more details if you need. i hope i can get this worked out!! thanks.

-dave

Studytime

iTrader: (15)

squirter

84MonteSS

iTrader: (2)

primary or secondary? i'm kinda new at tuning carbs..

84MonteSS

iTrader: (2)

i don't wanna sound stupid, but how would i go about it? a bigger squirter? adjust the pump cam? or the spring? i'm kinda lost sorry.

Studytime

iTrader: (15)

There's probably some slack in the linkage. Try to adjust that out first. As soon as the blade is cracking you want the squirter there and starting to shoot fuel into the front venturii.

Once that is done, set the float levels in both bowls and reset the idle mixtures screws. How are you doing those? That should help a whole, whole lot. If it still has stumble to it increasing the squirter would be a good idea. You change it by adding the next larger size, or better yet pick up a few spares and a set of precision drill bits. That way, you can make your own. That's what I used to do.

Also, if you bought it used, change the power valves and then verify you have the stock jets in it. Those will be your best starting point.

Post back up, and I'll help you if you have anything else.

Ben T.

84MonteSS

iTrader: (2)

thanks man, i'll try doing what you said next time i'm home, probably next weekend. i'll make sure there's no slack in the linkage, then set the level in the float bowls. also, how do i know when the idle mixture is good?

smokin2002

iTrader: (10)

you can check your plugs, thats the best poor mans wide band out there.

Studytime

iTrader: (15)

It's way to early for that.

The best way is to go buy a vacuum gauge and use it as a tuning tool. Are the throttle blades in the middle of the transfer slot? That's also important.

Ben T.

PortPros1

You need more fuel sooner; the easiest way to get it is to adjust the accelerator pump cam. The Holley 750 comes with a red cam; it has a second hole in it that will give you more fuel sooner. Just pull the screw and move the cam up, if you still have a problem you can bend the linkage a bit and take up all the slack. If you still have problem get the next bigger cam holly offers, I think it's the black one.

If that doesn’t do it go to a bigger squirter, but 9 times out of 10 the cam will do it.

Strokinit383

iTrader: (22)

I used to have the same problem. It could be a little different for your application, but i advanced my initial timing 1-2 degrees and it solved my problem. My motor was a fresh build too, so if youre timing was already fine then i wouldnt mess with it.

whytryz28

iTrader: (28)

What size power valve do you have for your primary and secondaries?
Did you check your float bowl levels to make sure there at the fill plug?

Heres what im running on my 850 double pumper w/ dual metering blocks and it runs just fine. Not to rich (from plug changes)



Primary Secondary
Power valve 4.5 6.5
Jets 78 88

White accelerator pump cam.

My idle mixture screws are turned 2-1/4 turns out both sides on primary.


Hope this helps somewhat.

84MonteSS

iTrader: (2)

thanks guys for all the input! i am up at school right now, but i'll be heading back home this weekend. i'm gonna try everything you guys said, and i'll keep you updated. any more suggestions would be greatly appreciated. i hope i can get it running snappy by sunday

Nate81Camaro

When it's idling, turn your screws in (leans the mixture) going slowly. Turn one side 1/4 to 1/2 in, go to the other side and do the same. Keep doing this until the motor starts to stumble. Then you'll start to turn the screws out (richen the mixture). Do the same 1/4 to 1/2 turn on each side. After you do this you may need to go to back of the car so you can hear how it's idling. When you hear a difference in the way the exhaust sounds, that should be good. Count the numbers of turns you turned the screws (from when you leaned the mixture to when you richened it) and then put it in the middle.
I was always told a good starting point on a SBC was to turn the screws all the way in, and then 3 HALF turns out (while the engines off).

Hope this helps, if you need further explanation of this, lemme know.