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Old 05-25-2009, 03:15 PM
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Chevy 305 backfire

I have a chevy 305 that was a tbi engine that has been changed over to a carberator 600 cfm edelbrock performer with dual plane intake.... This thing has a msd street fire hei dizzy new plugs and wires.....

Things I have checked
Timing set at 6 deg btdc (with the vacume advance I have 49 deg total @ 3000...)
I think the total timing is to much but I cant seam to get it any lower unless I disconnect the vacume advance and then I only get 23 deg total....

this thing caught the carberator on fire when trying to take off from dead stop...

Compression is 175 - 180 all the way around...


I am getting a backfire through the carb that is so bad it blows my pcv valve connector right off of the carb port....

it will fire and run but is does not run smooth

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Old 05-25-2009, 03:24 PM
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Back the dizzy up see what it does . 49 to much it should call for like 40 total or alittle less. You are running stock heads right. I would back it off and see what happens .Also make sure you dont have a wire crossed. Cole

Last edited by eloc431962; 05-25-2009 at 03:31 PM.
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Old 05-25-2009, 03:32 PM
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stock heads

yes the heads are stock but they are the tbi heads..... And I have played with the timing trying to get it down but with no luck.... it doesnt seam to help anything lowering the timing.... I am wondering about a flat lobe on the cam or even a broken valve spring or something of that sort? would I still have good compression if it were timing chain and gears or camshaft failure?
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Old 05-25-2009, 03:51 PM
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Quote:
Originally Posted by jeff79
yes the heads are stock but they are the tbi heads..... And I have played with the timing trying to get it down but with no luck.... it doesnt seam to help anything lowering the timing.... I am wondering about a flat lobe on the cam or even a broken valve spring or something of that sort? would I still have good compression if it were timing chain and gears or camshaft failure?
Yea you would still have good compression on the cylinders. Pull the valve covers and check the rockers. And yes it could be a lobe off the cam. check the rockers and let us know what you found so we can help. It sounds like timing but it can be one of the other things i mentioned. Cole
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Old 05-25-2009, 08:11 PM
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rockers

I will pull it apart tomorrow night when I get home from work.... I am going to bring home a dial indicator from work and check everything out.... I will get back to you as soon as I have some resaults.... Thanks for the help.....
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Old 05-25-2009, 10:31 PM
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random thoughts.

I guess you already verified plug wires are not crossed like Cole said.



I don't think the following is your problem but it may intensify the back fires:

If the carb step up spring is too Strong your edelbrock the carb will stage rich early which could intensify a backfiring situation. If your vacuum is erratic or weak then your carb may stage rich with a strong spring. I would at least do a test run with weakest blue 3" spring to see if it reduces back fire intinsity.

Total advance of 49 BTDC is ok on some cars. Some fords have total advances of 45-55 BTDC. On most SBF engines you have to start being careful when mechanical centriical advance exceeds 36 BTDC. A typical performance SBF is set up between 10-20 BTDC initial advance making very sure your mechanical advance is calculated to avoid going over 36 BTDC until you are ready to fine tune.

You can get adjustable dist vac advance units so you have some control on when and how much vacuum advance is added.




I hope the following is not your problem:

timing chain jumped.
worn cam.

Last edited by 001mustang; 05-25-2009 at 11:14 PM.
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Old 05-26-2009, 02:01 AM
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Sounds like it is just too low. If you have 23 degrees total with the vacuum advance unhooked, then it is too low, should be in the 32-38 degree range. When the vacuum advance is connected it should jump to 55 degrees or slightly more, this is normal. Vacuum advance should be hooked to a manifold, or full time, vacuum source and not a ported vacuum source.

The vacuum advance amount is not generally included in the total timing figures people quote. They are referring to just the mechanical advance added by the advance wieght and springs mechanism and the initial static timing. Initial + mechanical = total. The vacuum advance is extra added for gas mileage purposes after the total is set.

Last edited by ericnova72; 05-26-2009 at 02:07 AM. Reason: more info
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