Max fuel pressure for Carter 3705 carbs?

[fmahannah]

I have a max wedge tribute car with factory wedge crossram and two Carter 3705s. Carbs have been rebuilt and I am still way rich at idle (but this could be the 509 cam's low vacuum at idle). Mechanical fuel pump (factory style replacement) is putting out 6.5 lbs of pressure. Could that be too much for the Carters and if so does anyone know of a return-less regulator that can be installed on top the wedge manifold where the max wedge fuel lines run (2 lines, 2 filters, etc)? I don't really want to change the way the factory fuel lines are set up.

Thanks!
Forrest

 

[steve340]

I can't answer all your questions but 6.5 psi should not be too much.
However if you had a faulty needle valve it could be the problem.
Also have you actually put an accurate gauge in-line or are you just quoting pump specs?
By the way there are many reasons the car can run rich at idle.

 

[fmahannah]

Yes 6.5 was from an actual holley gauge reading. Everything inside the carbs has been replaced and checked and adjusted twice. I have already tried increasing initial timing to 18 to get more vacuum at idle from the 509 cam but best I can get is 6 inches at idle at 1000 RPM. Step up springs are the blue ones and are pulled all the way down at idle now. They were never down with the orange springs it had in it and just bounced. I thought for sure that would solve the over-rich condition but it did not. Jets and rods are set to what Dvorak recommended in his magazine article.

Because of the low vacuum I know the transfer slot is coming into play at idle, making it not run off the idle circuit, but I don't want to drill holes in the throttle plates on these 3705 carbs to get it to idle and not having to overexpose the transfer slot.

 

[WileERobby]

Is the cam retarded, straight-up, or advanced ? I've run 509's in the past, not the best for vacuum, but don't recall it that low.

 

[toolmanmike]

I think 6.5 is max pressure recommended. I like to see around 5 myself. I didn't see a idle speed listed. Do you have a big cam and can't get it to idle slower. Are the metering rods not seating at idle? (lighter springs) Transfer slot issue? Just throwing some things out here a bit.

 

[steve340]

I will admit to knowing nothing about that camshaft - but 6 inches is very low and will likely be a bit hard to tune around.
Those carbs are probably designed to see vacuum of say 12 - 14 or somewhere near it minimum at idle.

 

[Geff McCarthy]

My Edelbrock 4bbl leaked fuel, refused to start until i installed a regulator. I now run 4.5psi and it performs normally.

 

[fmahannah]

Yeah I am going to add a pressure regulator tomorrow just in case, at least that would eliminate pressure forcing the needle open as the cause. I know these cams have real low vacuum at idle, had one in my other 63 Polara and had similar issues. Had to live with that one running rich at idle since I don't want to idle at 1200 rpms to get it leaner. The 3705 carbs are what came on them from the factory in 1964, so is the cam as far as I know. But the max wedges were not really intended for idle and low speed cruising LOL

 

[fmahannah]

Is the cam retarded, straight-up, or advanced ? I've run 509's in the past, not the best for vacuum, but don't recall it that low.

Cam was supposedly installed straight up. With timing at 12 initial I only get 3-4 inches of vacuum. With it at 18 initial I have 6 inches barely. Some guys go even higher initial as a way to get more vacuum at idle so carb tuning is easier. I guess they make a 509 cam that is better for street use, but this one is a Mopar P4120237 292/292 509

 

[Geoff 2]

6.5 PSI is not excessive & will not cause a rich idle.

With 6" of vacuum & a cross ram intake that produces a poor low speed signal, I would expect those carbs would be idling off the transfer slot [ pri t/blades opened too far ]. This will cause a rich idle. Another symptom would be idle mixture screws screwed all the way in &/or mixture screws have little or no control over idle.

 

[fmahannah]

6.5 PSI is not excessive & will not cause a rich idle.

With 6" of vacuum & a cross ram intake that produces a poor low speed signal, I would expect those carbs would be idling off the transfer slot [ pri t/blades opened too far ]. This will cause a rich idle. Another symptom would be idle mixture screws screwed all the way in &/or mixture screws have little or no control over idle.

Yes that is what is happening as mentioned above. Is it possible to open the secondaries slightly on a Carter like you can on a Holley to allow air bypass so the primary throttle plates can be closed enough to get correct transfer slot exposure?

How far can idle ignition timing be advanced safely to get the idle where it needs to be without opening the primaries as far as they are now? I am at 18 initial 36 total now.

I will drill the primaries as a last resort, but I won't give up the sound of a 509 cam idling and change the cam. It is worth the aggravation LOL

 

[fmahannah]

I will pull the carbs again today and take pics of the transfer slot exposure the way it is now with it idling at 1000 RPM and 18 degrees initial timing. From what I saw when I rebuilt the carbs it has been running way rich for a very long time. Previous owner had idle cranked up to 1500 rpms.

 

[fmahannah]

OK pic is blurry but definitely too much transfer slot exposed at idle. Over an eighth of an inch, not square. Both carbs are like this. Other than drilling holes in primaries what other options are there? Thanks

 

[toolmanmike]

A buddy had the same type if problem with his 383. When he switched to his 440 he used the same carb and fuel pump. Same problem on the new engine. We checked fuel pressure and it was around 7 psi. Installed a new pump and that one was about 10 psi. Installed regulator set at 4 and all was good.

 

[Don Frelier]

The Carter literaure I've read say no more than 6.0 psi at idle.
They recommend 5.5 psi.

 

[fmahannah]

A buddy had the same type if problem with his 383. When he switched to his 440 he used the same carb and fuel pump. Same problem on the new engine. We checked fuel pressure and it was around 7 psi. Installed a new pump and that one was about 10 psi. Installed regulator set at 4 and all was good.

Yeah if it ever gets here today I will install a regulator just to be sure. I know edelbrocks like 4-5 psi, not sure what Carters like

 

[fmahannah]

The Carter literaure I've read say no more than 6.0 psi at idle.
They recommend 5.5 psi.

View attachment 1259532

Yeah I am at a steady 6.5 at idle and this is with mechanical fuel pump.

 

[Don Frelier]

Yeah if it ever gets here today I will install a regulator just to be sure. I know edelbrocks like 4-5 psi, not sure what Carters like

An Edelbrock is just a copy of a Carter

 

[toolmanmike]

Yeah if it ever gets here today I will install a regulator just to be sure. I know edelbrocks like 4-5 psi, not sure what Carters like

Carter/Edelbrock are pretty much the same. You are safe with 4# on about any carb. Holley's can tolerate more up to about 7. It's the supply volume that is the key.

 

[Dragon Slayer]

Not really when it comes to floats. Carter used larger float for chrysler and normally a smaller seat then you may find on an edlebrock. Carters should have no problem handling 6 to 7 psi. Your service manual has the fuel pump spec so you can check it. You can check that your floats are set correct, but I don't think that is your problem.

How do you know you are rich, how is your ignition system performance? Who did your carbs? Did they make sure all the bleeds were spec'd to orig.