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I'm new here and was told this is the place to ask my questions. Here it goes... I have a 68 xr7 gt, she's got a 390 "stock & rebuilt", and new Holly Street Dominator intake, Hooker Headers, 3 1/4 Flowmaster exhaust, Holly 750, Mallory Coil + Ignition + Distributor, and all the other goodies are new also (alt,voltage reg, ect)
So here's my question if you bring up #1 cylinder on compression stroke & the timing mark is @ tdc and you set the dizzy so the rotor points to #1 on the cap, why would she start & drive like a bat outa hell but apon slowing down to the legal speed limit backfire through the exhaust? It's not dumping fuel, it's like a late detonation and will backfire "exhaust" in the shop when I bring it up to cruising rpm to set the timing.

I have NO vacume leaks or exhaust leaks either my plugs are new + wires & I'm running premium fuel.

Help?
 

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itool?, is that you? No, didn't think so, but I had to ask...

How is the engine? Cam, heads/valves?
 

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Don't know for sure if this applies to your car, but recently ran into a similar situation that turned out to be too lean on the idle circuit, which is what the carb reverts to on deceleration. I too have heard that an exhaust leak will cause this to happen, but in my case it was not applicable.
 

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Jim may be on to something. My bike did that on decel, turned out I lost a vacuum cap on the carb, but ran fine other than the decel popping. Lean condition or a plug or 2 not firing well, causing raw fuel to get into the exhaust.
 

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Can you describe exactly what is happening, does it back fire when you get back on the gas after deceleration?

What does this mean? "will backfire "exhaust" in the shop when I bring it up to cruising rpm to set the timing."
 

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It doesn't even have to be a big leak in the exhaust to cause that problem. Had the same problem with my torino. small crack in the header big bummer
 

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So here's my question if you bring up #1 cylinder on compression stroke & the timing mark is @ tdc and you set the dizzy so the rotor points to #1 on the cap, why would she start & drive like a bat outa hell but apon slowing down to the legal speed limit backfire through the exhaust? It's not dumping fuel, it's like a late detonation and will backfire "exhaust" in the shop when I bring it up to cruising rpm to set the timing.

I have NO vacume leaks or exhaust leaks either my plugs are new + wires & I'm running premium fuel.

Help?
After you did this, did you use a timing light to set the timing? If so what is it set to at initial? You cannot set timing accurately by setting the rotor at #1 with te engine off.

Matt
 
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