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There is no connection between the vacuum secondary diaphram and passages and the fuel distribution circuit on your 4100. Are you sure that your father didn't mistakenly remove the accelerator pump diaphram which is on the opposite end on the carb housing from the secondary diaphram?

The rubber sec. diaphram can deteriorate and harden over time by dry rot. An old diaphram may have been installed from the beginning. Dry rot is the same thing that causes tires to show cracks in the side walls. This will cause it to split. Sometimes a diaphram can be ruptured when one or more of the cover screws grabs the diaphram when the cover is being installed. This problem can be prevented by coating the screw's threads with some silicone grease.

The reason for using silicone grease is that it won't rot the rubber like a petroleum based grease such as Vasoline. The silicone grease is the same as what is used for disc brake caliper lubrication at the points where the caliper slides. It is available in small pouches usually found at check out counters at auto part stores.

You need to double check which diaphram it is that has the split. As I see it, there's no way that fuel makes contact with the sec. diaphram in any way. I'm surprised that the Pony Carb. people didn't mention this to you, but then again this company has had customers less than pleased with them in the past.
 

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As I said before, the Pony Carb people aren't that great when it comes to dealing with customers. The tech support in particular, so it doesn't surprise me that he didn't mention anything about fuel being behind the sec. diaphram. You are lucky that they answered the phone. A friend of mine tried to get in contact with a real person and all that he kept getting was an answering machine. The messages that he left were never returned. I don't know if this has been remedied. Maybe it has.

I still maintain that you won't find any fuel behind this diaphram. One thing that needs checking is if the ball check is missing in the vacuum passage to the diaphram. The passage is at the top of the sec. dia. mounting flange and is installed before the diaphram, return spring and the cover is placed into position.

A return spring that is too heavy will prevent the secondaries from opening when they should. Holley's return springs work on the 4100. The springs are available in a kit that contains several color coded springs that give different opening rates. You may need to go with a lighter spring to get the secondaries to work at the rate that's correct for your engine.

If you do decide to get another carb., Summit is now offering a carb that is very similar to the Autolite 4100. Many Holley parts fit these carbs. The 600 CFM vacuum secondary with electric choke sells for only $250. I bought one and found the quality to be very good. These are brand new carbs, not rebuilds.
 
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