Backfire threw Carb - Team Camaro Tech
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post #1 of 21 (permalink) Old Dec 31st, 09, 07:00 PM Thread Starter
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Backfire threw Carb

I am getting a backfire (with flame) threw the carb. It will backfire right off idle at about 2000 rpm in neutral. Lower rpm when in gear.


Stock'ish 350 with a Holley carb. Timing is very close to ideal. Squirter is squirting. No vacuum leaks that I can find.

I did some searching on the forum and decided to look into a whipped cam. When I pulled the sparkplug wires one at a time and revved the motor the backfire went away when I pulled the #7 wire. So I did a compretion test and all the cylinders were right around 130 including #7. Next I pulled the valve covers and all of the valves seem to be moving the same amount. I swapped out the #7 plug and still the backfire.

Any ideas???

My Junk.

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post #2 of 21 (permalink) Old Dec 31st, 09, 07:05 PM
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Alan
 
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Re: Backfire threw Carb

Bad spark plug?

alan
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post #3 of 21 (permalink) Old Dec 31st, 09, 07:09 PM
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Tom
 
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Re: Backfire threw Carb

bad cam. #7 exhaust valve not opening much. That's something I've had before.

'67 rs - ordered new by my Grandfather
327 L30, K-K, Deluxe int, tach & gauges, 12 bolt posi, 4 speed.

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post #4 of 21 (permalink) Old Dec 31st, 09, 07:10 PM Thread Starter
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Re: Backfire threw Carb

I swapped out the #7 plug with no change.

My Junk.

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post #5 of 21 (permalink) Old Dec 31st, 09, 07:14 PM Thread Starter
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Re: Backfire threw Carb

Quote:
Originally Posted by Melrose RS View Post
bad cam. #7 exhaust valve not opening much. That's something I've had before.
I watched the rocked arm move on the intake and exhaust side and they seem to be moving the same amount as the other cylinders.

My Junk.

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post #6 of 21 (permalink) Old Dec 31st, 09, 07:18 PM
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Tom
 
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Re: Backfire threw Carb

Can you measure the lift with a dial indicator or some improvised method?

Could also be a burnt/bent/sticking intake on #7 but not sure what you would get in the compression test in this case.

How about that #7 wire. Is it good? no burns or cracks?

'67 rs - ordered new by my Grandfather
327 L30, K-K, Deluxe int, tach & gauges, 12 bolt posi, 4 speed.

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post #7 of 21 (permalink) Old Dec 31st, 09, 07:32 PM Thread Starter
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Re: Backfire threw Carb

Quote:
Originally Posted by Melrose RS View Post
Can you measure the lift with a dial indicator or some improvised method?

Could also be a burnt/bent/sticking intake on #7 but not sure what you would get in the compression test in this case.

How about that #7 wire. Is it good? no burns or cracks?
I did not swap the plugwire but I did put my timing light as close to the plug as I could get and it did fire off the light.

I will swap out the wire next year.

My Junk.

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post #8 of 21 (permalink) Old Dec 31st, 09, 07:35 PM
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Tom
 
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Re: Backfire threw Carb

I hope I'm wrong and it's not your cam. Good luck

'67 rs - ordered new by my Grandfather
327 L30, K-K, Deluxe int, tach & gauges, 12 bolt posi, 4 speed.

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post #9 of 21 (permalink) Old Dec 31st, 09, 10:45 PM
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Re: Backfire threw Carb

Lean condition............

Accelerator pump or timing will cause it off idle....

A lean #7...........Power brakes are usually taken from the #7 intake runner on top of intake,or back of carb.These effect #7 the most.

Cap it off and reconnect #7 plug,see if the problem goes away......

Remember,I'm not making this stuff up!

1969 rs/ss 396 350hp/4spd conv a/c,pw,ps,foldown
1968 Z/28 crossram- j/l8 conv pw,ps,foldown
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post #10 of 21 (permalink) Old Jan 1st, 10, 10:24 AM
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Re: Backfire threw Carb

Three things (perhaps more) can cause backfire through the carb. One is a lean condition. The other is timing problems. Make sure your harmonic balancer's 0* mark aligns with TDC. I've seen many that have slipped and lead you off in the weeds when setting the timing. The third thing is valve issues such as a damaged valve, or a valve that has been adjusted improperly causing it to hang open slightly. To complicate matters, often times problems are caused by two or more things working together.

Dave
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68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #11 of 21 (permalink) Old Jan 1st, 10, 12:10 PM Thread Starter
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Re: Backfire threw Carb

Quote:
Originally Posted by wagonman View Post
Lean condition............

Accelerator pump or timing will cause it off idle....

A lean #7...........Power brakes are usually taken from the #7 intake runner on top of intake,or back of carb.These effect #7 the most.

Cap it off and reconnect #7 plug,see if the problem goes away......
I caped off the power brake booster as soon as I started to diagnose the problem.



Quote:
Originally Posted by dnult View Post
Three things (perhaps more) can cause backfire through the carb. One is a lean condition. The other is timing problems. Make sure your harmonic balancer's 0* mark aligns with TDC. I've seen many that have slipped and lead you off in the weeds when setting the timing. The third thing is valve issues such as a damaged valve, or a valve that has been adjusted improperly causing it to hang open slightly. To complicate matters, often times problems are caused by two or more things working together.
The motor does not have a timing pointer so I have the timing set so the motor runs as good as possible. If anything it may be slightly advanced. I did pick up a pointer today just to be sure. I will have to check the valve adjustment.

Could it be a wiped cam? If the cam was bad wouldn't that cause the intake valve not to open?

My Junk.

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post #12 of 21 (permalink) Old Jan 1st, 10, 12:27 PM
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Re: Backfire threw Carb

Keep in mind that a motor will idle fine with lots of advance. You can get it too far advanced easy enough at idle. I wouldn't think too much advance would cause a back-fire though. Did you adjust your valves at all? We've seen lots of guys over adjust the valves trying to remove all pushrod movement or by not properly rotating the engine between lashings.

Dave
========================
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #13 of 21 (permalink) Old Jan 1st, 10, 02:27 PM Thread Starter
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Re: Backfire threw Carb

I checked the valve adjustment and it is good. I can spin the pushrod with the exhaust valve open and vise versa.

I pulled the motor out of truck that ran enough to put it on a trailer and then the garage to pull it. The motor then sat for 2 years. I put in the distributor and installed the carb.

My Junk.

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post #14 of 21 (permalink) Old Jan 1st, 10, 05:57 PM Thread Starter
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Re: Backfire threw Carb

I spent $30 bucks and the problem seems to be gone. I will be able to give it a test drive tomorrow.

My Junk.

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post #15 of 21 (permalink) Old Jan 1st, 10, 06:25 PM
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Re: Backfire threw Carb

18436572.
Notice how close 5 is to 7 on the same bank.
Nothing like a little crossfire under the distributor cap or some emi going on with plug wire routing.

George
68 Camaro
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