SpeedTalk Store - Opinion Columns

Backfiring through carb..why?

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Backfiring through carb..why?

Postby c4mo » Thu Dec 29, 2011 6:59 pm

I recently put together a sbc, in which I am having a problem with the engine wanting to back fire through the carb right before it starts. The timing is 16 degrees initial, and I have verified the lash and other mechanical aspects. I do know that the carb is rich, and this is making me
Think it's possibly the problem. Any ideas? More info can be given as needed. Thanks
Morgan Allen
c4mo
Member
Member
 
Posts: 154
Joined: Fri Feb 27, 2009 9:28 pm
Location: ky

Re: Backfiring through carb..why?

Postby JohnsnsFord2@aol.com » Thu Dec 29, 2011 7:15 pm

firing order correct?
Stephen Johnson #2162
Horace Johnson #2167
SS/D 427 Ford Fairlane NHRA-IHRA
Ex-Jim Van Cleve
1985 Mustang GT N/A 331
JohnsnsFord2@aol.com
Member
Member
 
Posts: 94
Joined: Mon Jun 27, 2005 9:22 pm

Re: Backfiring through carb..why?

Postby c4mo » Thu Dec 29, 2011 7:27 pm

Yes, once the engine starts it is fairly smooth, until a high no load rev, when it will pop, but not backfire.
Morgan Allen
c4mo
Member
Member
 
Posts: 154
Joined: Fri Feb 27, 2009 9:28 pm
Location: ky

Re: Backfiring through carb..why?

Postby FastBuick » Thu Dec 29, 2011 7:37 pm

Yes a rich condition will definitely cause that. You are actually lighting the excess fuel that is laying in the intake. A lot of guys have that problem trying to start a race car in the cold weather by pumping the throttle too much when starting it or taking too long to start it which also floods it or causes a lot of excess fuel to be in the intake.
FastBuick
Pro
Pro
 
Posts: 496
Joined: Fri Nov 12, 2010 9:39 am
Location: Pennsylvania

Re: Backfiring through carb..why?

Postby Dodge Freak » Thu Dec 29, 2011 7:41 pm

I would double check the lash.

What do you mean the carb is rich, s the exhaust smoke black, does it just seem rich, if its a non heated intake and its fairly cold outside the exhaust will tend to stink more till it warms up.

Why not just drop the jets down 2 numbers or jet it at stock settings and go from there
Dodge Freak
Guru
Guru
 
Posts: 1708
Joined: Sun Oct 21, 2007 5:56 pm

Re: Backfiring through carb..why?

Postby ProPower engines » Thu Dec 29, 2011 8:14 pm

c4mo wrote:Yes, once the engine starts it is fairly smooth, until a high no load rev, when it will pop, but not backfire.



Is the pop continuous or just a single pop?
Also how much time is on the engine? Is it possible an intake valve is not sealing completely causing the pop or backfire issue.
Lastly have you confirmed the valve lift is correct on all the exhaust valves? Heaven forbid you are loosing an exhaust lobe has that been on your check list? A bad valve seal or exhaust lobe issue could also cause the issues you decribe.
Real Race Cars Don't Have Doors
ProPower engines
Guru
Guru
 
Posts: 5574
Joined: Sun Jan 21, 2007 1:16 pm
Location: VICTORIA BC CANADA

Re: Backfiring through carb..why?

Postby raynorshine » Thu Dec 29, 2011 10:01 pm

popping/backfiring thru intake, carb.....

1) too lean..
2) valve(s) adjusted too tight..
3) removing lobe(s) from camshaft..usually exh lobe..
4) timing incorrect..
5) bad plug wire(s), sparkplug(s), cap, rotor, etc..
6) rinse, repeat..
7) and likely many more..
Use it up
Wear it out
Eat it all!
raynorshine
Expert
Expert
 
Posts: 869
Joined: Tue Mar 06, 2007 1:59 pm
Location: BC, Canada

Re: Backfiring through carb..why?

Postby c4mo » Thu Dec 29, 2011 10:28 pm

The engine is fresh, just got done with the cam Breakin, I used Valvoline racing 20w-50 with Lucas Breakin for 20 min at 2000 rpm, once started it down not pop, except at high rev it will pop out the exhaust. It is a single plane, not heated. Fastbuick what you said is basically what's going on. I'll recheck the lash, and then take some jet out of it. It smells rich, Thats why I thought it was, I have a wideband, I'll hook it up also, and go from there.
Morgan Allen
c4mo
Member
Member
 
Posts: 154
Joined: Fri Feb 27, 2009 9:28 pm
Location: ky

Re: Backfiring through carb..why?

Postby Matt Gruber » Fri Dec 30, 2011 7:07 am

had what sounds like the same problem 12 yrs ago.
friend said it is quite common on his low buck rebuilds.
he said just drive it around.
i did, and within 50 miles it broke in (valve seat to valve) 1st 10 miles were awful :lol: i've since seen stubborn guys take off theheads and have them touched up. :roll:
the problem was, i bought bare heads, and the pre-installed seats had a slight distortion from being pressed in. being a low buck build, i wasn't about to take them to a shop.
.
.
tame a lumpy cam for the street, more street torque! see my article, archived in the waybackmachine.
https://web.archive.org/web/20130707064 ... TGRU/carb/
Great manners equals more fun.
Matt Gruber
Guru
Guru
 
Posts: 1623
Joined: Fri Mar 19, 2010 6:32 am
Location: near Daytona Beach FL


Return to Engine Tech

Who is online

Users browsing this forum: Bing [Bot], fast_crew, machinedave, melancia, racear2865, SpeedtalkHD, SupStk, Truckedup, Yahoo [Bot] and 42 guests