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Discussion Starter · #1 ·
When timing my car the other day, I set it at 6 degrees BTDC. When I hooked my vacuum advance line back in, I shot the timing light at it and it went to 30 degrees instantly. Is that normal? My concern is what it will get up to under higher rpms. The problem I am dealing with is an exhaust backfire when you let off of the gas. Anytime the rpms go down it will backfire through the exhaust around 2000 rpms. I was thinking the vacuum advance was bad or something. I just replaced the exhaust from the header gasket all the way to the back to eliminate the exhaust from being the culprit. Any suggestions? The motor is a 390 with an edelbrock performer manifold and edelbrock 600 carb. It has headers and pertronix ignition.
 

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You need to hook it to the other port, its called "ported vacuum" and will be dead at idle

Some guys use manifold vaccum, but Ford didnt, and I find it to make idle erratic at times
 

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Discussion Starter · #4 ·
I tried the timed vacuum port, and it is still doing it. I noticed that if I put it in neutral it stops the backfiring. Anything on a C6 that would do that? Would a bad modulator cause it in some way?
 

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Did you reset the timing with the vacuum unhooked and check your firing order again?

You dont have any exhaust leaks do you? Even small ones? Thats the number on reason for backfiring on decel.

However, let me offer you some advice, dump the 6 degree setting. Put that 390 at 10 it'll be so much happier.
 

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Discussion Starter · #7 ·
I put new plugs in and checked them to see if it was running rich. They look good. I am sure of no exhaust leak. I replaced everything from header gasket all the way back. The wires are only a year old. I did check the timing with the vacuum plugged. I will try it at 10 to see if she likes that. I will double check the exhaust pipes, but they looked very good, maybe there is a little hole somewhere in it close to the motor.
 

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Generally an exhaust backfire is due to oxygen getting in the pipes from a leak and igniting any unburnt fuel during deceleration

However, I do promise you, the car should run a bit better overall with 10 BTDC initial. I never saw an FE that didnt like more timing at idle
 

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Hi there,

If I read your story correctly, That is almost 25 degrees of dizzy timing from just the vacuum advance. Sounds like a lot. Do you have an adjustable vacuum can? It is adjusted from an allen wrench put into the vacuum hole. I would say under 15 degrees vacuum advance is more normal, but every engine is different. Your car does not look like it is for ecomony.

Most vacuum canisters come in near 10 GH. First it sounds like you have your timing on direct manifold vacuum. You should move it to the timmed Vacuum port. It will still advance the same amount, but just will not do it at idle.

Do you know what your mechanical advance amount is?

If not, disconnect and plug your vacuum line. Record down whatever ( the 5 or 10 degrees) initiall that you have at idle.

Then increase the RPM's to 3000 or so, and shoot the timing light again. I would guess it is near 35 degrees??

If so, at partial cruze condition ( high vacuum and RPMs near 3000) You maybe pulling more then 60 degrees. You can measure this by hooking up the vacuum can aagin and reading the advance, ( if your tape goes that high) Or just add the two together. Feel comforatbel with this much advance at partial cruze? Could be your dumping unburned gas into your pipes, and that is the source of your backfire.

Mike
 

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Easy experiment, set the timing and unhook the vac advance, see if it still does it
 

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Discussion Starter · #11 ·
It is just a stock ford replacement advance I got from the auto store last year. I will check that timing again with the rpms up and will also run it without the advance to see if that helps too.
 

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I discovered that with nearly every "reman" Dizzy.
All 289/302 dizzy that my customers bought had a cheap,non adjustable vac that would kick 20deg timing.
Stick that in a pre smog engine and you're asking for trouble!
I say get another vac pot with less advance or even better a good adjustable one.
My Lincoln has the same 10.5 compression from the factory and the original vac pot has only 6 degrees!
If i'd use a dizzy from a 8.0comp Mark4,that would create problems...
Olli
 

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THe port must not have vaccum at idle. If there is vacuum, the timing will advance wehn you put the hose on it. Put it on a point HIGHER than the base plate of the carb. Any vacuum port at or near the level of the butterflies will be constant vacuum.
Any port HIGHER than the butterflies will be regulated by the throttle.

and do the 10* thing, all my 390's liked it a lot.


FE
 
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