holley 750 dp

[chad]

I got a holley 750 on my 383. The motor has the purple 484 shaft in it, ported 906 heads, its a solid 10-1 motor. Set the timing at 36 total. Now this carb has always been funky, I had a buddy of mine redo it. He was supposed to be a carb guru, but now the carb doesn't respond a whole lot with the idle air screws. I tried setting it with the vacuum guage to no avail. The funny thing with this carb is , if I set the idle speed a ,900 rpms and go for a ride. The rpm will shoot up to 1200-1300 at a stop light unless I ride the clutch a little and load the motor and push clutch back in it will idle a 900 rpm fine. When I am shifting gears the rpms don't come down. Almost like it has a vacuum leak. But it doesn't. But now I noticed I unplugged the vacuum advance line and there is some fuel coming out of it. Could it be he has the gasket in the mettlering block backwards ? Also if I turn the idle speed screw its really touchy. A quarter turn make a 300 rpm change

 

[Sonny Black]

Sonny had almost the identical problem!
...did that done that!!!..etc....jets, springs, rebuilt the carb (not me) but a so called expert...had my neighbor look at it(owns a NAPA store, builds cars)....here is what saved me, I laughed BUT IT worked!

long story short..---->.full tank 91 octane NO Ethanol gas, don't laugh (I did),

ONE--> 2.49 oz bottle, Gumout Fuel Injector & Carburetor Cleaner 6 Oz...within 3 to 5 miles of driving.....the carb is more responsive, idles correct, and I was shocked...I thought this stuff was for dummys BUT maybe it is, I bought it!!!!

when the guy at advance told me to try it, "on Sale:iamwithstupid:I thought you don't know your *** from''''''

so I thought can't hurt...well the rest is history..

may I be struck dead if I am lying!! (so if I do not post here again, well...)


I donot have a holley DP on it....I have this thing...small 610 4 BBL

also have the 484 purple

 

[chad]

Ya this is driving me crazy. Another thing this motor only pulls 7" of vacuum with this cam and these cams aren't all that wild.

 

[67 coronet]

there should be almost no way for raw fuel to come out vac advance hookup unless,gaskets are installed wrong(never seen it,but who knows)or the body of the carb is cracked.

 

[chad]

That's what I was thinking the gasket was backwards

 

[Sonny Black]

Sonny thought the same thing...put new gaskets etc,:eusa_think:

..........I'm telling you, try Gumout fuel additive...they don't sell millions of bottles for fun!

worth a try for a couple of bucks

 

[lewtot184]

7" of vacuum will need a 3.5 or maybe a 4.5 power valve. ignition curve is probably too slow and not enough initial timing. typical problems with hydraulic cams. i think the 284/.484 is lot more cam than some folks think it is.

 

[chad]

The power valve is a 3.5. The timing is all in by 2400. It has a petronix in it. Initial timing is around 16

- - - Updated - - -

Sonny thought the same thing...put new gaskets etc,:eusa_think:

..........I'm telling you, try Gumout fuel additive...they don't sell millions of bottles for fun!

worth a try for a couple of bucks

Will giver a go

 

[mentor70]

if that dont work, maybe disconnect all the throttle cable and springs and see if the butterflies are hanging up in the bores when you slowly move the throttle off idle and back again.

 

[Meep-Meep]

7" of vacuum will need a 3.5 or maybe a 4.5 power valve. ignition curve is probably too slow and not enough initial timing. typical problems with hydraulic cams. i think the 284/.484 is lot more cam than some folks think it is.

Exactly right! That cam was always the kiss of death around my parts. The hemi grind (.474/280) is way better. The .484 has more overlap than the hemi cam so naturally he won't have any vacuum. Checking float levels can't hurt either.

 

[chad]

Float levels are spot on.

 

[Garys1969RR]

Carbs are always fun to work on. I would have another expert take a look at it. Its not all that difficult. Especially a 750 dbl pumper. We are working on a 650 dbl pumper, and it calls for 67 primary jets. The 70 Bee (383 auto) would stumble badly when you open the throttle even half way. So we went to 75 primary jets and it works great now! So I do a lot of experimenting to see what works best. Sometimes metering blocks get swapped, etc. Ask another expert to check it out or do it your self. Good luck!

 

[chad]

I am gonna run the gum out through it today. Just for curiosity sake now. I always always put good gas in it. So I can rule that out. Then I am gonna tear into the carb. There's no vacuum leaks, timing is good, I did pull the plugs, they are little sooty. But every once and a while it will pop back through the carb which to me is a lean problem. But the idle doesn't come back down at a stop light or when I shift gears. So maybe something is gummed up in the carb and hanging it open a tad.

 

[Sonny Black]

I am gonna run the gum out through it today. Just for curiosity sake now. I always always put good gas in it. So I can rule that out. Then I am gonna tear into the carb. There's no vacuum leaks, timing is good, I did pull the plugs, they are little sooty. But every once and a while it will pop back through the carb which to me is a lean problem. But the idle doesn't come back down at a stop light or when I shift gears. So maybe something is gummed up in the carb and hanging it open a tad.

I also had a problem with my throttle cable hanging up .....replaced it (cable) and with duel return spring.....it was hanging up.
...new cable also was a help.

 

[chad]

Well I tried the gum out ran a tad better. So I broke the carb down. The carb expert had the gasket in backwards. Oh well everyone makes mistakes. Seems good now. Just a bit of a hesitation of the line we gotta work out now.

 

[lewtot184]

I am gonna run the gum out through it today. Just for curiosity sake now. I always always put good gas in it. So I can rule that out. Then I am gonna tear into the carb. There's no vacuum leaks, timing is good, I did pull the plugs, they are little sooty. But every once and a while it will pop back through the carb which to me is a lean problem. But the idle doesn't come back down at a stop light or when I shift gears. So maybe something is gummed up in the carb and hanging it open a tad.

backfiring thru the carb will damage a power valve.

 

[chad]

I checked the power valve when I had it apart. Seems OK. Think I might need to go up on the squirter size.

 

[lewtot184]

if the plugs are sooty, rich, why would you up size the squirter?

 

[Gpuller]

I have the same cam in a 440. Got best results buy not running the vac advance. It likes lots of initial timing, like 20+deg. Limit your mechanical advance to 36 total.

 

[chad]

if the plugs are sooty, rich, why would you up size the squirter?

The plugs were sooty because the carb was flooding it self out. I changed the plugs after I went through the carb. And the motor will pop every once and a while. The plugs look like the motor is running a little lean now.