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Discussion Starter #1
As some of you may know, I’ve been rebuilding the top end of the 410 in our 66 Mercury Monterey. The car runs absolutely great! It idles fine and I can drive all around with no issues. The problem is, there’s like no difference between ¼ throttle and full throttle. The car used to break the tires loose through the first 3 gears and now I can’t even break them loose in 1st on a loose gravel road. My first suspicion is maybe the secondaries aren’t opening? I don’t know. I didn’t touch anything on the carb when I did the rebuild, simply unbolted it and bolted it back on. The carb is (I believe) an Autolite 4100, whatever was factory. I highly doubt vacuum leak because it idles great. I’m just kinda scratchin my head at this point. Ideas?
 

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Sounds like you have a restriction someplace. A shot in the dark for example. Some exhaust pipes are double walled. I had one collapse on me. The exhaust pipe looked fine on the outside when the shop cut it open it was just about completely blocked. My boss wasn't too happy. I was a kid driving the delivery truck and I would get on it the take my foot off. Use to make a big backfire out the exhaust. Then one day after doing that the truck wouldn't go more then about 20 mph.
 

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Discussion Starter #4
Throttle plates opening all the way?
Quick answer: I don’t know. Today, my dad was watching the motor while I was inside the car. I asked him and he said “I don’t know”. I will find out but right now it’s about 30 degrees outside so I’ll have to wait till tomorrow.

Any other ideas would be greatly appreciated. Considering the few replies this has got, I’m thinkin I should have posted this in the Vintage Mustang forum but since this isn’t technically a Mustang, I didn’t (even though an FE motor is an FE motor).
 

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Compression, fuel, ignition. That's it. Verify compression on all cylinders. Then check that you're fueling it sufficiently. Then check the timing and curve. Just like a lawnmower engine, a Beetle engine or anything else. And change only one thing at a time.
 

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Discussion Starter #8 (Edited)
Will check throttle plates and timing today and report back. I doubt it’s compression or I wouldn’t think it’d be running so good at 70 MPH.

We know that the secondaries aren’t opening in the driveway when we rev it up but I read somewhere that it has to be under a load for the secondaries to open. Is that true? It’s a little hard to check that while the car is moving.
 

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The old hot rodders trick to check for secondary opening is to put an old fashioned paper clip on the rod that comes out of the secondary vacuum chamber. Push the paper clip on the rod up against the base of the vacuum chamber. Then go drive the car. When you come back look at the paper clip. The vacuum chamber pulls the rod up into the chamber so if that has happened there will now be a gap between the paper clip and the bottom of the vacuum chamber.
 

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Discussion Starter #10
Thanks for the help so far guys. It turns out I’m busy today and tomorrow, then it’s supposed to rain/snow on Friday and Saturday, so the soonest I will be able to get back on it is Sunday. I’ve made a list of all your suggestions though and will report back as soon as possible.
 

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Discussion Starter #11
So we did get out to the (heated) garage tonight for a few and did some tests without actually having to start the car. The first is that the throttle plates are opening, so check on that.

Then we performed a test my dad found online. Basically, there’s a little brass tube that goes from one of the venturis back to the secondaries and if you blow compressed air down into that venturi it creates a vacuum and the secondaries will open. Well, test performed and passed, the secondaries are opening. So what we don’t know is if the engine is creating enough vacuum while running to open the secondaries. I will perform the paper clip test with a few other tests on Sunday and see where we are from there.
 

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Discussion Starter #12
Problem solved. I did the paper clip test and the secondaries weren’t working. I checked the timing and it was off, I couldn’t even see the mark I’d made on the balancer. Anyway, setting the timing fixed it. In my defense, my dad told me the timing was fine so I never bothered tinkering with it after the rebuild. Lesson learned, never trust anyone, haha.
 
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