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Holley 1945 missing fuel vapor port connector
https://slantsix.org/forum/viewtopic.php?t=58771		 Page 1 of 1
Author:  rich006 [ Fri Dec 04, 2015 4:47 pm ]
Post subject:  Holley 1945 missing fuel vapor port connector
My car came with a newly installed carb, but the old one was in the trunk. The new one leaks when the accelerator pump is activated ( I assume it's the accelerator pump--what I've actually seen is fuel leaking out below the gasket when I manually open the throttle while the car isn't running), and my idea is to rebuild the old carb and replace the new one with the rebuilt one. I have two questions.

1. On the new carb, there is a metal tube sticking out of the fuel vapor port, but on the old one this tube is not there (see pics). This tube is where you connect the air hose that leads to the charcoal canister. Can I remove this tube from the new carb and press it into the old one? If so, are special tools required?

2. Presumably there was some problem with the old carb, or the guys who sold the car to me wouldn't have had to replace it to get the car running (it previously sat for several years). Is there anything obvious I should look for before rebuilding, to make sure I'm not wasting my rebuild kit on a carb that will never work right? I've read that the throttle shaft can become worn, requiring new bushings. Mine has some play in it but hardly any. Should it have zero play? How do you measure that?

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Author:  Louise76 [ Fri Dec 04, 2015 5:45 pm ]
Post subject: 
There are folks on this forum that can rebuild that carb in their sleep, but I have done several myself, mostly awake. My experience is that little tube comes loose often, as it is just pressed into place. I've actually cheated by scarring up the inserted end with tools, then tapped it back into place coated with a "bonding agent". If your throttle shaft appears tight, it should be OK for a daily driver. (I tend to not get too picky with trying to make everything better than new.) If you had the engine running with the old carb, one test of the throttle shaft would be to squirt some fizz-can lube on the shaft bushings at idle and listen for a change in engine RPM, as this could be an intake leak that would affect the efficiency of the carb. Accelerator pump: "fuel leaking out at the gasket"? If fuel leaks out at the carb-to- intake mounting flange gasket when you operate the throttle /accelerator pump, this means that the accel pump is working correctly. But it also means that the flange gasket is leaking- usually because someone re-used the old one, didn't re-tighten the 2 nuts after running it a few times, or the nuts were over-tightened so badly that the carb flange is warped into a curve instead of being absolutely flat. If you look at a new flange gasket assembly, you will see that the two steel bushings are shorter than the thickness of the gasket. This is to allow for "crush" when you torque the two attach nuts, and that's why you can't re-use the gasket. If you do elect to re-build the very old carb, I recommend buying a new set of floats, as the old ones may have deteriorated to the point of filling with fuel, thus sinking somewhat and throwing off your whole fuel metering system. :)
Author:  rich006 [ Sat Dec 05, 2015 8:55 am ]
Post subject: 
Louise, thanks for the info. It's interesting you should mention the need to retighten the bolts after driving a few times. The guys who sold the car (sons of the previous owner who had died) installed the new carb to get it running, but I know for a fact they didn't put more than a couple of miles on it before I bought it...and I have not touched the bolts. That being said, the FSM says they should be at 30 in-lb, and they were already much tighter than that. I tried tightening them some more anyway, but it didn't help. I'm guessing the over-tightening warped the flange as you suggested...or maybe they did reuse the old gasket.

So I suppose if everything is working correctly, the fuel squirted by the accelerator pump would (if the engine is not running) drain down into the intake manifold, where it would evaporate. The fuel vapor would somehow make its way to the charcoal canister? I'd like to understand how that works.

I can get a brass float from Mike's Carburetor Parts, where I got the kit. But I'd still like to hear if there are any obvious red flags to look for in the old carb before I rebuild it.
Author:  SlantSixDan [ Sat Dec 05, 2015 9:48 am ]
Post subject: 
I agree, find and fix the trouble with the new carb rather than messing with the old one. Also, your bowl vent (large hose nipple at top of new carb) is capped off. That should go to the charcoal can's "BOWL" or "CARB" port. See here.
Author:  rich006 [ Sat Dec 05, 2015 11:44 am ]
Post subject: 
@Dan,
The "CARB BOWL" connection on the canister is also capped off...just like the first picture on the Allpar article you linked to. I take it my old carb was internally vented, since it doesn't have the connection for that hose. Being internally vented, it did not need to use that connection...but the new installed carb has the vent, and so I should use it.

Speaking of the linked Allpar article, it says the purge line is teed into the distributor vacuum advance, but that's not the case on my car or in my FSM. Was that only true pre-1974?
Author:  SlantSixDan [ Sat Dec 05, 2015 11:58 am ]
Post subject: 
Quote:
the new installed carb has the vent, and so I should use it.
Things're happier that way.
Quote:
Speaking of the linked Allpar article, it says the purge line is teed into the distributor vacuum advance, but that's not the case on my car or in my FSM. Was that only true pre-1974?
The carbs with a built-in purge port use the throttle plate as a purge valve. It's cheap and nasty, but it sorta works and saved Chrysler a penny or two per car. You can use the built-in purge port or cap it and set up with the purge valve and distributor line tee-in as described.
Author:  rich006 [ Sat Dec 05, 2015 2:42 pm ]
Post subject: 
Dan, you make it sound like I have a choice. Clearly my carburetor has the (always-open?) purge port above the throttle--that's what I pointed to in the original post. But do I also have the vacuum-controlled purge valve? If so, where is it?

Also, here's a recap of what I think are the differences between the two systems. Please correct me if I'm wrong.

In the 1972 design, there was a vacuum-controlled purge valve in the carburetor that would open at speeds above idle, letting the carburetor suck in vapors from the purge line of the canister. So that this valve would only be open above idle, its control vacuum was teed into the distributor advance line.

In the 1973+ design, the throttle itself controls whether vapor is sucked in from the canister. This is accomplished by connecting the purge line to an always-open port above the throttle. It's less precise because the throttle doesn't provide consistent vacuum. So the amount of fuel vapor being sucked through the purge line is not consistent, meaning the fuel-air mixture going to the manifold is less predictable.
Author:  SlantSixDan [ Sat Dec 05, 2015 4:01 pm ]
Post subject: 
Quote:
Clearly my carburetor has the (always-open?) purge port above the throttle--that's what I pointed to in the original post. But do I also have the vacuum-controlled purge valve?
No. Again, the throttle plate acts as a cheap-and-crude purge valve.
Quote:
In the 1972 design, there was a vacuum-controlled purge valve in the carburetor
No. The purge valve is external to the carburetor. It's either a standalone, or built onto the charcoal can.
Quote:
So that this valve would only be open above idle, its control vacuum was teed into the distributor advance line.
Yes. (Also yes to the rest of what you wrote)
Author:  rich006 [ Mon Feb 29, 2016 6:40 pm ]
Post subject: 
I'm still fiddling with this--I now think it may be the cause of the fuel smell in the garage after parking. I hate to be dense, but I really want to understand how this works.

1. In the system that uses the throttle as a purge valve, what keeps the canister vapors from simply evaporating through the purge port above the throttle?
2. In the carburetors that have the internal bowl vent, where does the bowl vent to?
3. In general, what attracts the vapor to the canister? Why doesn't it just evaporate from the bowl up into the air cleaner?
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