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Author Topic:   Backfiring through carb when engine is cold
skips69
Gearhead

Posts: 228
From: Catlin, IL USA
Registered: Mar 2001

posted 05-27-2003 11:31 PM     Click Here to See the Profile for skips69   Click Here to Email skips69     Edit/Delete Message   Reply w/Quote
I have about 1,000 miles now on my rebuild (in a 1979 Ford F150 4x4 with C6 auto). It is a 400, .030" over, stock pistons (mid eight CR range), a little larger cam (Comp High Energy "268H" - .494 lift, Cloye's double roller timing chain and gears, Edelbrock Perfomer 4V Intake, and Edelbrock 600 CFM 4V carb (w/ electric choke). Heads were completely rebuilt and include all new components and larger valve springs. I retained the standard exhaust manifolds, added dual exhaust (without cats), and the distributor, coil, plugs, wires , etc are all new and stock OEM replacements. Everything touching or inside of the engine was replaced with new components. The problem has been, and continues to be with the engine cold, it wants to backfire through the carb just as the truck is about to shift from 1st to 2nd, and from 2nd to 3rd (C6 automatic also stock with fresh fluid and filter change). Once the truck warms up (about 5 minutes or so), it runs beautiful and I have no issues with re-starting, overheating, lack of performance, bogging, cutting out, pinging, dieseling, etc - Runs flat out great and is an excellent daily driver once it is warmed up. I have the vacuum advance going to the LH side of the carb (as you look into engine bay from grill), and it does advance the timing when you accelerate. The timing is set at 10 degrees BTDC now. Before, it was set at about 15 or 16, and I have started to back it off a little, but the truck diesels when you shut it off if I go any lower than 10, so I am staying put there as of now. I have 2 options to take off when it is cold: Crawl, or stomp on it a little until it is well into 3rd gear and the RPM's are up a little. Just doesn't seem right. Any thoughts, or is this a case of just being "cold natured", and live with it? To date I have not tried to adjust the carb because it seems to run so good where it is at (and warmed up). I have only been playing with the timing.

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KULTULZ
Gearhead

Posts: 217
From: Rockville, MD
Registered: Oct 2002

posted 05-28-2003 10:08 AM     Click Here to See the Profile for KULTULZ   Click Here to Email KULTULZ     Edit/Delete Message   Reply w/Quote
Try enriching the choke circuit (tighten the cap and allow it to stay on a little longer). You might also want to consider a choke stove and OEM air cleaner assembly (they can be modified/decorated to look fast) that will have a flapper valve to run the engine off heated air while cold and outside air once warm.

You are just displaying a lean condition on cold start.

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skips69
Gearhead

Posts: 228
From: Catlin, IL USA
Registered: Mar 2001

posted 05-29-2003 07:59 AM     Click Here to See the Profile for skips69   Click Here to Email skips69     Edit/Delete Message   Reply w/Quote
Thanks for the input. I just last week picked up a factory air cleaner from a 1978 F150 equipped with an original 460. This should fit my 4 bbl carb just fine, and I can go back to the junk yard and see what else I can scrounge up to get the flapper connected and working properly. I sand blasted the original valve covers and re-painted them Corporate Blue anyway, and the air cleaner will keep with the stock theme I have going. It's a nice little sleeper, and runs really nice other than this little annoyance. Thanks again!

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MLariviere
Moderator

Posts: 3237
From: Biddeford,Me.USA
Registered: May 99

posted 05-29-2003 10:41 PM     Click Here to See the Profile for MLariviere   Click Here to Email MLariviere     Edit/Delete Message   Reply w/Quote
Does the problem lessen when you advance it a few degrees? If it does,and it doesn't ping,then I'd say run it like that.

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skips69
Gearhead

Posts: 228
From: Catlin, IL USA
Registered: Mar 2001

posted 06-03-2003 08:22 AM     Click Here to See the Profile for skips69   Click Here to Email skips69     Edit/Delete Message   Reply w/Quote
It didn't run any different at 15 degrees BTDC, as it does now at about 10 degrees BTDC. I didn't think I could (or should)advance it even that much to tell you the truth, so I didn't go any further than 15. I started out at 0 initially, but it idled poorly and backfired through the carb both cold and warm. Right now, it runs about as good as I could hope for once warm, and I am content to say the least. I haven't got the air cleaner done yet, but I did mean to run through my carb manual as well and see what they recommend for this. I did want to see what other member's might have run across with a similiar condition, so at least it doesn't sound as if I am hurting anything and I have gotten pretty good at taking off without making the carb backfire. It just needs a little tweaking still I guess.

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MLariviere
Moderator

Posts: 3237
From: Biddeford,Me.USA
Registered: May 99

posted 06-03-2003 10:36 PM     Click Here to See the Profile for MLariviere   Click Here to Email MLariviere     Edit/Delete Message   Reply w/Quote
Steve gave me a magazine with a carb tuning article,and the wizard in it said he likes to see the initial timing at about 20*. I tried my 351C there,and it was pinging. The spec is 16*,but I can get away with 18*.

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skips69
Gearhead

Posts: 228
From: Catlin, IL USA
Registered: Mar 2001

posted 06-04-2003 08:18 AM     Click Here to See the Profile for skips69   Click Here to Email skips69     Edit/Delete Message   Reply w/Quote
My Valve Cover decal called for 4 BTDC, when it was stock and all smog was hooked up. I thought 10-15 would be the ideal range from what I read other's were running their 400's at with similiar builds. No one has mentioned the backfiring though when the engine is cold.

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MLariviere
Moderator

Posts: 3237
From: Biddeford,Me.USA
Registered: May 99

posted 06-04-2003 09:57 PM     Click Here to See the Profile for MLariviere   Click Here to Email MLariviere     Edit/Delete Message   Reply w/Quote
I think the preheat tube on the stock aircleaner will help alot.

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