I just bought a 750 VS carburetor to replace the 650 double pumper I was using. I'm having trouble getting the carb to work right though. The car seems to be running extremely rich. I think this is due to the fact that my cam doesn't pull much vaccum at idle and has forced me to open up the primaries to much to get a decent idle. My fast idle doesn't seem to work at all, no matter how I adjust it. What is the best way to go about tuning this? Would opening up my primaries a little fix my problem. Anyone running the 280 Magnum cam?

 

The 750Vs carb should idle just as good as your old 650DP.
Replace the power valve with a "4.5". If you still have the
Tork-Link intake on your car you won't be able to set the idle up
untill it is fully warmed up to operating temp because this intake is not heated. If you set it up before the manifold is fully warmed itwill be over rich once warmed up. After you have done this, remove the carb, flip it over and look at the butterflies. The idle transision slots should only be partially
exposed (.040" or less ) The secondaries can be opened
a little to allow you to close the primaries a little if nessessary.
The idle should be shared by all 4 barrels.

The power valve needs to be changed because you don't have enough manifold vaccuum to keep it closed at idle causeing an over rich idle.

 

With my '57, I don't have quite that much cam, and do have a bigger displacement, but I found the 4.5 PV to be a dangerous piece. The off-idle stumble was so bad, I came very close to having some intersection "incidents".

Go to the Holley web site (link is in the Sticky at the top of this forum) and read what they have to say about PV selection. Following those instructions, I put in an 8.5 PV, and it finally runs great (I might even be able to go with a 9.5, but this seems fine for now). At this altitude, I only have about 10 in Hg idle vacuum, but that does not cause a rich idle condition even with that high of a number PV.

I do agree with the initial throttle blade setting recommendations, but not with the PV statements. The PV circuit doesn't get any vacuum to pull fuel through it until well off-idle, so even if the PV is opening, it's not causing a rich idle condition unless the throttle is open too much.

 

Can't compare the power valve requirements of a car with a big
rough idle cam with low manifold vaccuum with that of a car with a milder high vaccum cam.
The power valve must remain closed at idle and just off idle
or it will cause extra fuel to flow , enrichening the idle curcuit at the wrong time. On an automatic set the car in gear and check manifold vaccuum. with a manual trans, start the car moving
and let the car idle along with the clutch all the way out
and throttle at idle and observe the manifold vaccuum
with a vaccuum gauge. Select the power valve with a rating
lower than the lowest observed reading.
EG. if your manifold vaccuum reads 5.5" on the guage get
a 4.5" power valve.
The 750vs comes stock from holley with either a 6.5 or a 10.5,
both which are wrong for a car with a 280 magnum cam.
Jetting should be 72 primary and either #21 secondary metering plate or #76 to #80 secondary jets.
The tork-link intake is quite big internally and may need a bigger accelorator pump shooter and a different pump cam.
The stock accelorator pump shooter on a 750vs is .025"
if you need more try a .035 or .038" shooter.

 

Thanks for your advice both of you. I will start out working on trying to get the right power valve. I am extremely rich at idle and can't seem to lean it out, so that I think is definitely one of my problems. I think the pump on the carb is fine, cause no off idle hesitation or stumble seems to occur.

 

What happens if you close the idle mix screws all the way?

If you can close them all the way and it keeps running, then there is an internal leak somewhere. Blown PV, PV opening at idle, badly adjusted floats, or a blown needle and seat. Thats about it. If your bowl level is good, then change the PV to one at least 2" lower than your vacuum at idle in drive.

 

When I was playing around with it yesterday i turned the idle screws all the way in and it kept running, but then i remebered I opened up the secondaries while playing around with it, so I closed them a little and the car died. I hope I don't have a blown power valve, the carburetor is not even a week old. I will check the level and get a vacuum reading.

Also, do you think this will cure the problem I am having with my fast idle? It is so annoying heading out to work in the mornings at 5:30 and having to jump from pedal to pedal to keep the car from dying till it warms up.

 

I hooked up the vacuum gauge to it, and it was reading about 9 inches. I haven't gotten to check my float level yet because I seem to always park the car on an incline. I will take care of that this weekeend though. Any suggestions on which power valve to purchase with this vacuum reading?

 

How did you obtain the reading of 9" vacuum?
While standing idling in neutral or while idling along in gear.

With a manual transmission you have to read the vacuum guage
while idling along in 1st gear with the clutch all the way out.
not while standing idling in neutral.

At any rate most 750vs carbs come with a 10" power valve which is too high for your car and opening at idle causing a rich idle.
Get a 4.5" power valve.
setting up the float level would be a good idea.

 

The reading was taken when the car was in neutral idling. I followed the instructions that holley had on their website. I did however put in a 4.5 power valve today, and it runs great. For some reason after disassembling and reassembling my carb to install my quick change spring kit the choke seems to be working fine now. I have no off idle hesitation or stumble whatoever, and I'm still trying to get it tuned right. I may try going down to a 3.5 if I can't lean it out. I appriciate everyone's help.

 

Changing the power valve from 4.5 to3.5 will have no effect
on your idle now.

A closed power valve is a closed power valve.

Eliminate all chances of a internal leak in the carb
Try and adjust the idle mixture screws for best idle
work slowly. Be sure that the idle is being shared by all four
barrels. The transition slot near the butterflies should be just barley exposed when viewed from the bottom. Adjust the secondary idle stop as nessessary. Be sure to hook up the PCV
valve and a breather on the other cover. This will lean out the idle a bit.
After you have done all this, the Proform carb body comes with
changable screw in air bleeds. You can buy them from holley.
Work only with the idle air bleeds (outside pair)
larger will lean the idle curcuit. Use this as a last resort
as your carb airbleeds should be just about right "out of the box".