Ad Radar
Item Posts    Sort Order

backfires through the carb when reving

 
twofastferu
I love my Chevy Chevette!
Posts: 10
Joined: 08/09
Posted: 08/05/09 10:40 PM

My friend has a 69 chevelle with a 454 and it idles fine but when we rev the engine it backfires through the carb. we replaced the timing chain, the carb is a brand new edelbrock 750 cfm and we adjusted the timing 6 way from sunday with no success.
my  friend leaves for bootcamp this sunday and we would love to get the car running right so he can drive it before he leaves. please help.  

Post Reply
85_trans_am
I have an SS396 tatoo
Posts: 252
Joined: 05/09
Posted: 08/06/09 07:39 AM

check the plug wires. sounds like there is a wire crossed.  

Post Reply
elanum01
I love my Chevy Chevette!
Posts: 11
Joined: 04/09
Posted: 08/06/09 08:30 AM

I would say you have a wire crossed or make sure that is dropped in right and you didnt time it on the #6 hole, it will idle and run but you will have some problems  

Post Reply
68scott385
I have an SS396 tatoo
Posts: 325
Joined: 07/09
Posted: 08/06/09 11:54 AM

i have seen excessively worn cam lobes (exhaust) cause similar problems

hope that isn't your/his problem though  
68scott385 68scott385 68scott385

Post Reply
Pontiacman2
Moderator
Posts: 891
Joined: 01/08
Posted: 08/06/09 12:09 PM

My edelcrap carburetor did that and it was new also if the things the other posters said to try does not work then try adjusting the accelerator pump it is easy to do.  
Pontiacman2
Pontiacman2

Professional Hi-performance engine builder

Post Reply
68scott385
I have an SS396 tatoo
Posts: 325
Joined: 07/09
Posted: 08/06/09 12:12 PM

come to think of it, i have two of those edelbrock 750's and they seem to me to be prone to back-firing when all seems well otherwise

pulled a holley of unknown origin (double-pumper w/milled choke horn...supposed to be a 650...no numbers...who knows, don't personnally know any holley gurus) and dropped the edelbrock in its place and it liked to back fire...been quite a while since then but seems like i tinkered with the timing a little, of course checking all the settings of the carb helps, i.e., float settings (fuel level, float drop)...these carbs can't handle much pressure either, like 5-6psi max  
68scott385 68scott385 68scott385

Post Reply
waynep712
I have an SS396 tatoo
Posts: 358
Joined: 12/07
Posted: 08/06/09 05:57 PM

2 fast for you....    does the big block have a dual plane intake or a single plane intake.????/

single plane intakes require larger squirter nozzles...  perhaps a different lever hole for the pump rod...

on holleys there are a hand full of different pump cams that can go on and multiple positions for each one..

when the throttle opens... the pump shot has to be instant... INSTANT...   if the check balls in the accelerator system are worn, wrong...   if the pump height is not right... it won't shoot enough gas into the intake plenum.. it will go lean... and back fire.

it could also be the idle transition restrictors...    they might need to be checked and verified that they are open and the air bleeds are also open..

which model edelbrock did you get...   carb number. ???   14??

so after running the motor for a minute or two.. but before it gets too hot to lay on.... with the engine off...  climb up on top so  you can see straight down the primarys... work the throttle.. a tiny amount... see if the accelerator pump shoots fuel out as soon as you move the throttle..   not a few degrees after you open it.. instantly...

let up know what you find...     and you have to really pull the primary boosters.. to properly check the idle feed tubes,  restrictions , bleeds...   do so... with tiny wire... and a blast of brake cleaner or carb cleaning spray....   i have usually gotten tiny tiny strong wires from wire brushes...

there are a bunch of other things it could also be... this is just what i have found... and i could be totally wrong.... sometimes i am..  usually i am....  

oh... and where is the vacuum advance hooked up to ..?????? manifold vacuum or ported vacuum.....  

Post Reply
Grizzly.Bear0
I love my Chevy Chevette!
Posts: 9
Joined: 07/09
Posted: 08/10/09 10:13 PM

I'm sorry to say but it sounds like your cam is wiped.  

Post Reply
unclepsycho
I love my Chevy Chevette!
Posts: 8
Joined: 08/09
Posted: 08/11/09 09:54 PM

Lots of good info here.  2-fast never got back here to tell what the results were.
When I crossed up #5 and #7 on my 350 I got the same backfire 2-fast described.
would be interesting to know what was found.  

Post Reply
twofastferu
I love my Chevy Chevette!
Posts: 10
Joined: 08/09
Posted: 09/09/09 10:10 PM

the exhaust lobe was wiped right off on cylinder 2. We replaced the cam and it ran fine until today. again the same problem I think we need to tear the whole thing down and start over. who ever my buddy bought the car from screwed him over really good  

Post Reply
waynep7122
Everything in my driveway’s a Chevy
Posts: 556
Joined: 08/09
Posted: 09/10/09 01:01 AM

i would have you make a lifter rack with a wood drill bit and a chunk of 2x4 or 2x6....  16 holes for lifters...   with a mark up front to say which end if the front...

examine the lifters ... look for scratches...    try each lifter in it's bore...  perhaps.. when the engine was torn down previously...   i have seen people drive the lifters out with a rod...   putting a scratch in the lifter bore.. that was enough to keep the lifter from rotating freely...

i would also say... put super magnets in the bottom corners of the oil pan...   catch any metal...   remember the oil pump is not filtered.. only a screen... replace it... (super magnets are usually neodynium, and are really really strong. radio shack sells them cheep. you can also pry them out of old computer hard drives..)

wash the block out... use oil galley brushes...       you can get a ball hone for the lifter bores...

you might also stick the cam back in... and look down the lifter bore... and see if the offset on that lifter bore verses cam lobe is the same...   there was a batch of 305s that had the lifter bores so far off.. you could not keep a cam in them...   someone made a wide lobe cam just for that motor.. i have not seen it on a big block.. but you never know..


there are also lifters that have an edm burned hole through the lifter face.. squirts a bit of oil right out the face and onto the cam lobe..  

Post Reply
hydro11
I love my Chevy Chevette!
Posts: 2
Joined: 12/09
Posted: 12/31/09 07:03 AM

Icon Quotewaynep7122:
i would have you make a lifter rack with a wood drill bit and a chunk of 2x4 or 2x6....  16 holes for lifters...   with a mark up front to say which end if the front...

examine the lifters ... look for scratches...    try each lifter in it's bore...  perhaps.. when the engine was torn down previously...   i have seen people drive the lifters out with a rod...   putting a scratch in the lifter bore.. that was enough to keep the lifter from rotating freely...

i would also say... put super magnets in the bottom corners of the oil pan...   catch any metal...   remember the oil pump is not filtered.. only a screen... replace it... (super magnets are usually neodynium, and are really really strong. radio shack sells them cheep. you can also pry them out of old computer hard drives..)

wash the block out... use oil galley brushes...       you can get a ball hone for the lifter bores...

you might also stick the cam back in... and look down the lifter bore... and see if the offset on that lifter bore verses cam lobe is the same...   there was a batch of 305s that had the lifter bores so far off.. you could not keep a cam in them...   someone made a wide lobe cam just for that motor.. i have not seen it on a big block.. but you never know..


there are also lifters that have an edm burned hole through the lifter face.. squirts a bit of oil right out the face and onto the cam lobe..


Confused sounds like a tight valve....  

Post Reply
adamallen
I love my Chevy Chevette!
Posts: 2
Joined: 01/10
Posted: 01/12/10 02:58 PM

i had a 383 stroker do the same thing if it has an electric fuel pump on it you need to make sure its not puting out anymore than 7 psi if it is it will just keep pumping fuel into the carb and cause
the problems your having a cheap fuel pressure regulater should fix your problem  

Post Reply
55Guy
Administrator
Posts: 1153
Joined: 07/06
Posted: 01/13/10 02:17 PM

What kind of cam are we talking here, flat tappet or roller? If it's a flat tappet, was it broken in properly, and was break-in lube added with the engine oil for a safe break-in procedure?  

Post Reply
musclecarjeff
I love my Chevy Chevette!
Posts: 3
Joined: 01/10
Posted: 01/14/10 01:37 AM

First you have to get to the actual problem. Did you do anything else, i.e. remove or change the plug wires, remove a vacuum line etc.  If all you did was R&R the carb then the problem is in the carb. If the problem existed before and you were hoping the carb would solve it than you need to look elsewhere. The problem could be the following. Bad plug, bad plug wire, carbon tracing inside the distributor cap, low coil output, low primary voltage to the coil, worn cam lobe, lean running due to a vacuum leak. Eliminate all the possibilities and it should lead you to the actual problem. Don't get too exotic with the trouble shooting, 9 times out of 10 it's something simple but get back to the basics. Good luck  

Post Reply
TO TOP