Carb to intake vacuum leak

[EngineerDoug]

Hello all,

I've been working to eliminate a heat soak issue with my six pack setup, recently installed a set of 1/4" phenolic spacers between the carbs and the intake. I am using the 0.030" paper gaskets on both sides of the spacers.

So I get the whole thing put together and the idle is too high. I cannot drop the idle speed below about 1300 rpm. This was not the case before the spacers. Seems like I have vacuum leak(s) now that the spacers are installed. The spacers show maybe 0.004" warpage with a straightedge. They are, however, pliable so that little pressure is needed to flatten them.

I will check the carb bases and manifold surfaces as well, but am wondering about the following-

1) Are these spacers problematic in terms of warpage/vacuum leaks?
2) Is there a thicker gasket that might solve the issue?
3) What is recommended torque on the carb mounting bolts?

I know using any sort of sealer is generally not recommended here, but I would like to know if I am missing any tricks that might eliminate this problem.

Thanks!

 

[ykf7b0]

I hope I’m not out of line by asking about your fuel line routing?

 

[Don Frelier]

Phenolic should be really stable.
I usually just tighten carbs or throttle bodies with a nutdriver. But I do have strong hands.
No other vacuum leaks?
Pcv hose?

 

[MoparLeo]

Facytory Service Manual says 200 inch lbs. Carb to manifold bolt. Originally bolts not stud and nuts.

 

[turbine68rt]

Was the throttle cable tension readjusted/confirmed after the spacers were installed?

 

[440+6]

Did you raise the choke .25 inch also?
Or readjust it?
Throttle cable adjustment?

 

[EngineerDoug]

Thanks for the ideas - here's an update:

1) Removed all 3 carbs and found center carb base warped perhaps 3 thousandths. All new carbs BTW. I used some fine grit paper and a sanding block to take out the warp.

2) I also used some fine grit paper, again on a sanding block, to take out the phenolic warpage. These sand easily & smoothly.

As for the linkages & fuel lines, everything is free from binding. I did extend the choke rod to compensate for the elevated carb position.

Once I get it all back together we'll see how it runs.

 

[ykf7b0]

I hope I’m not out of line by asking about your fuel line routing?

. I posted this in reference to the fuel line possibly being too close to the engine and/or head causing the fuel to get hot and percolate.

 

[EngineerDoug]

Understood. I am actually working to convert my existing deadhead system to a return style system. I have a bypass regulator plumbed at the end of the carb feeds, which will route fuel back to the tank. I thought I would add the carb spacers while I am at it.

My issue has not been vapor lock, but rather a change in idle characteristics once the engine reaches full thermal equilibrium. I have observed that the idle quality degrades, and the AFR leans out when this happens. Today's gas being what it is, I've heard a lot about the benefits of keeping the fuel cooler.

Using a mechanical fuel pump that necessarily bolts to the hot engine block doesn't help matters, but I would rather avoid having to switch to an electric pump unless I have no other choice.

 

[Jeff Erwin]

I'm going through this EXACT process right now. Installed the phenolic spacers yesterday, finally managed to get the fuel lines reinstalled (I do hate the 6-pack fuel lines) and started the engine. Idles at about 4000 rpm. Idled perfectly before the spacers went in. I am pretty sure I have a vacuum leak somewhere, will try the propane torch trick to see if I can find it.

I did NOT extend the choke linkage, have to do that this morning. What's involved there?

 

[EngineerDoug]

Jeff,

I just started the engine to evaluate my fixes. Here is what I did:

1) Switched to the 0.060" carb to manifold gaskets.
2) Installed 0.25" thick phenolic spacers. Note that I used a flat sanding block and some 320 grit paper to flatten these out. Easy job.
3) Checked flatness of all 3 carburetor base plates. The center carb was out of flat by about 3 thou. I used some machinists blue and flat sanded this warpage out.

The engine idles well, with about 17 inches of vacuum, but the idle RPM is 1200 or so in neutral. I can't adjust it any lower....

1) I find no vacuum leaks around the carburetors.
2) Plugging the PCV port on the center carburetor does nothing, so no appreciable airflow there.
3) The end carbs are completely shut.

I did, however, put a borescope down the center carburetor and it appears the throttle blades are not completely shut. See the photo attached; there is some gap between the throttle blades and the bores.

I suspect this might be the issue - wonder if anybody else has seen this? These are new carburetors.

As for the choke rod, I made a new one from 1/8" steel rod. Just use some heat to form the two 90 degree bends at the ends. You will need to take the choke off and take it apart to do this. I formed the groove to accept retaining clip/hairpin using a dremel wheel.

 

[EngineerDoug]

OK here's an update - I discovered that the primary linkage was not holding the secondaries open, but it was the other way around. I lengthened the front linkage slightly, and shortened the rear one. This allowed the primaries to close completely. I now have a minimum idle speed of 1000 rpm. Given that the secondary throttle blades have factory-drilled holes in them, would you expect this to be normal? Note I am running 20 degrees initial advance, with another 10 degrees vacuum advance (manifold vacuum).

 

[Jeff Erwin]

That was exactly what my problem was. I wanted to make sure the outer carbs were shut tight at idle, but went overboard to the point where the two linkage bars were preventing the center carb from closing completely. I readjusted both and now everything works correctly. I also found that my idle set screws were loose and actually working themselves closed so the idle was really rough. I put some lock rite on both, we will see if that does the trick.

And my vapor lock issue is gone! I will add the vapor return line next winter now that the urgency is gone.

 

[EngineerDoug]

Jeff,

I also reconfigured my fuel system from a deadhead style to a return style system. All of these changes were motivated by changing idle AFR and behavior as the engine heats up. I have not experienced vapor lock, but I was not happy with unsteady/wavering idle characteristics.

Here is what I did for the fuel system:

1) Added a tee fitting at the fuel feed for the rear carb.
2) Mounted a bypass style regulator (Aeromotive 13301) at this point, with a return line to the tank.
3) Note I am using a Carter mechanical pump that puts out about 10 psi unregulated.
4) Added a schrader valve at the regulator so I can check fuel pressure without having to use a full- time gauge.

I have yet to do much driving with this setup, but I can already tell that the fuel lines and the fuel bowls are cooler. I've seen enough information online to suggest that today's fuels are not well-suited to a carbureted system, so I think these changes will have a big impact.

 

[Jeff Erwin]

Good information.. I have to ask, why didn't you install the vapor separator that came with the stock 440 in the day? I like your solution but the vapor separator return line back to the tank doesn't require a regulator.

 

[EngineerDoug]

I did run the separator for a while, but this still leaves the system as a deadhead after the separator. The fuel waits around in the filter & feed lines to the carbs. By using a regulator after the carburetors, the fuel only lingers in the carb bowls.

The separator did help, and maybe running it with the carb spacers would have been good enough. I do still have the vapor separator if you are interested. Note it is one of the authentic repros, as some apparently are not correct internally.

 

[Jeff Erwin]

Thanks. I already have the separator, new sender with the necessary nipple and new fuel line. Running the fuel line might be tricky, never done that before.

 

[BSB67]

OK here's an update - I discovered that the primary linkage was not holding the secondaries open, but it was the other way around. I lengthened the front linkage slightly, and shortened the rear one. This allowed the primaries to close completely. I now have a minimum idle speed of 1000 rpm. Given that the secondary throttle blades have factory-drilled holes in them, would you expect this to be normal? Note I am running 20 degrees initial advance, with another 10 degrees vacuum advance (manifold vacuum).

I suspect you are missing something much larger. Have you checked your spacer on the center carb without the gasket to see if it fully covers the rear of the carb. Check everything PCV related. Does the car have vacuum assisted power brakes? Is the small vacuum port on the center carb plugged?

Again, your leak is not small. Take a step back and keep an open mind