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Discussion Starter · #1 ·
I purchased a brand new Autolite 4100 4V concours carb from Pony Carbs. My car was running great with a Pony Carbs 2100. I am keeping the car all concours and/or period correct.

Of course I upgraded the intake to the correct Ford 4V manifold.

The motor runs OK but not super smooth at idle but when I open the throttle to about 1200 RPM it gets real rough. As I open the throttle more the RPMs decrease and the engine gets even rougher. If I continue to open the throttle the RPMs will pick back up and it runs a bit smoother. The power is pretty good even with all of this going on. Of course except around 1200 RPM.

Now the vacuum is very good at 20-21 inches at 800 RPM. I have checked everwhere for vacuum leaks using the old carb cleaner and listening. I have also checked the ignition exhaustively :(. No problems. I even replaced all plugs ( non of which were worn or fouled, the Pertonix and distributor cap.

Does this sound like a carb problem -- its brand new?
 

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2,871 Posts
Have you asked Pony Carbs? They're customer service is pretty good, especially if you just dropped $600 on one of the concours carbs.
 

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1967 Mustang GT fastback
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4,019 Posts
Sounds like mixture is going off at part revs and then correcting a bit later on. Check with Pony Carbs ASAP. Their carb, their problem.
 

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Discussion Starter · #6 ·
OK. Got it fixed. Talked to John at Pony Carbs. He is the carb king at Pony Carbs. He suggested that the stumble at 1200 RMP was do to too much vacuum advance kicking in. So I temporarily disconnected the vacuum line and the stumble cleared right up.

I retarded the timing a bit and now everything runs perfectly. The carb was perfect from the get go no adjustments needed. Boy it really run well now.

Timing is tricky on these old mustangs because a timing light is usually not going to work right unless you test the timing marks to verify that the metal ring on the harmonic balancer has not shifted. Mine has so I used the technique of adjusting the timing for maximum vacuum at idle. However, this puts the advance too far ahead in my case. So by trial and error I pulled it back some and wow works great now.
 
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