just dont get it, started backfiring
Pro
Joined: Oct 2008
Posts: 645
Likes: 1
From: Hayward Wi
You know the more i think about it. could the High RPM jets Fuel Flow have been messed up some how because it would explain it. you could have an Vac leak to.
have you tryed messing with the HIGH RPM flow screws and just to see what it dose.
another thing ot think about is could it be a few small things that are all adding up.
Ryan
have you tryed messing with the HIGH RPM flow screws and just to see what it dose.
another thing ot think about is could it be a few small things that are all adding up.
Ryan
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Drifting
Joined: Dec 2002
Posts: 1,890
Likes: 376
From: Montgomery NY
I have had cars idle smooth then pop back thru intake when throttle is opened no noise or rough idle with a flat exhaust lobe. If the lobe is not completely gone the charge can leave at an idle but the opening is not big enough at higher rpms, so it waits till the intake valve opens and blows it back thru the intake.A motor is a compressor air goes in and has to get out and will seek any means possible.
Melting Slicks
Joined: Mar 2007
Posts: 2,520
Likes: 11
From: Hinckley OH
I have had cars idle smooth then pop back thru intake when throttle is opened no noise or rough idle with a flat exhaust lobe. If the lobe is not completely gone the charge can leave at an idle but the opening is not big enough at higher rpms, so it waits till the intake valve opens and blows it back thru the intake.A motor is a compressor air goes in and has to get out and will seek any means possible.
Been there, done that also.
Thread Starter
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
ok pulled both valve covers
all rocker arms are moving from the eyeball test the same
i dont have a dial indicator to get exact measurements but most definitely there is enough lobe to move the rockers
i am going to go with plug wires to start with. its just wierd that it will idle fine with no problems unless there is a load on the engine.
at first i thought vacuum leak but with a gauge it is reading a steady 18-19 in/hg. there is no way it would ready that high of a vacuum with a big enough leak to make it backfire.
gonna spray the plug wires tonight and see what happens
***15 minutes later**********
no popping or sparking on plug wires
i am going to get a dial indicator and check the cam tomorrow
all rocker arms are moving from the eyeball test the same
i dont have a dial indicator to get exact measurements but most definitely there is enough lobe to move the rockers
i am going to go with plug wires to start with. its just wierd that it will idle fine with no problems unless there is a load on the engine.
at first i thought vacuum leak but with a gauge it is reading a steady 18-19 in/hg. there is no way it would ready that high of a vacuum with a big enough leak to make it backfire.
gonna spray the plug wires tonight and see what happens
***15 minutes later**********
no popping or sparking on plug wires
i am going to get a dial indicator and check the cam tomorrow
Last edited by baxsom; Mar 25, 2009 at 12:16 AM.
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Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
ok pulled both valve covers
all rocker arms are moving from the eyeball test the same
i dont have a dial indicator to get exact measurements but most definitely there is enough lobe to move the rockers
i am going to go with plug wires to start with. its just wierd that it will idle fine with no problems unless there is a load on the engine.
at first i thought vacuum leak but with a gauge it is reading a steady 18-19 in/hg. there is no way it would ready that high of a vacuum with a big enough leak to make it backfire.
gonna spray the plug wires tonight and see what happens
***15 minutes later**********
no popping or sparking on plug wires
i am going to get a dial indicator and check the cam tomorrow
all rocker arms are moving from the eyeball test the same
i dont have a dial indicator to get exact measurements but most definitely there is enough lobe to move the rockers
i am going to go with plug wires to start with. its just wierd that it will idle fine with no problems unless there is a load on the engine.
at first i thought vacuum leak but with a gauge it is reading a steady 18-19 in/hg. there is no way it would ready that high of a vacuum with a big enough leak to make it backfire.
gonna spray the plug wires tonight and see what happens
***15 minutes later**********
no popping or sparking on plug wires
i am going to get a dial indicator and check the cam tomorrow
Last edited by midyearvette; Mar 25, 2009 at 08:09 AM.
Thread Starter
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
i bought one of those dragonfire HEI distributers to make sure that the problem isnt in there somewhere.
new coil and plug wires are going in with that (wires obviously)
when i get the wires from what i understand i can use the wires for a 75 C20 pickup to work with my BB.
also, i know i didnt jump a cam tooth. after i watched the rocker arms spray oil all over the engine bay for a few minutes and decided that they were good i checked TDC
how does this sound. i pulled the #1 plug, stuck my finger in the hole and turned the motor until my finger was blew out. the timing mark was about 2 inches before the 0 mark.
i advanced the crank until the timing mark was dead on the zero.
the rotor was right on the #1 wire just before the vacuum canister.
i am thinking if i jumped a cam tooth it wouldnt be near the #1 wire right?
i think until the new distributer comes in i am not even going into the garage just to let it go for a bit
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
This backfire; is it coming through the intake (opposed to the exhaust) and is it one pop or a series of pops? Does the engine recover after the backfire or does it stumble all the way through the rpm band? I think you're wasting your time checking the cam.
Thread Starter
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
the inside of the carb is black, the bottom of the air cleaner is black.
if i ease into the throttle there is a constant mild stumble but no backfiring.
if i stomp it once it reaches about 2500 rpms bang usually one sometimes 2 before i let go of the pedal. i havent been brave enough to hold it down after that but as soon as i let off the gas the engine recovers and it starts idling smoothly until i stomp it again.
more and more i am thinking ignition.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Seems more like ignition, but is it load or speed sensitive? If you slowly bring the rpm up from idle to around 5k rpm, what happens? If you are at idle, in gear, and stop the throttle but lift before 2.5k rpm, what happens?
Forgot to add: Have you hooked up a vacuum gauge and what reading do you get from idle and no load up to 3k rpm?
Forgot to add: Have you hooked up a vacuum gauge and what reading do you get from idle and no load up to 3k rpm?
Melting Slicks
Joined: Jul 2006
Posts: 2,285
Likes: 487
From: Salmon Arm, BC
When you listed all the new ignition parts replaced, spark plugs were not listed. I'd also look at or replacing those. I've had experience with plugs working fine at low power, but totally breaking down under high load (cylinder pressure).
Team Owner
Joined: Jul 2008
Posts: 23,419
Likes: 22
From: In limbo
St. Jude Donor '13-'15, '17 thru '22
It's firing up through the carb? Nobody said that before! Maybe it is just my humyuk upbringings showing through, but when it fired out the exhaust, we always called that a "backfire", firing up through the carb is what we called a "blowback" if I am remembering correctly.
I also think you are wasting your time checking cam lobes. Blowback is always caused by the intake valve being open when gas is getting burned. It has to be because there is no other route to get back up to the carb. This is definitely an ignition timing issue IMO. Since it is happening when you stomp on it, it is smelling like a vacuum advance problem. Is your car automatic or manual?
I also think you are wasting your time checking cam lobes. Blowback is always caused by the intake valve being open when gas is getting burned. It has to be because there is no other route to get back up to the carb. This is definitely an ignition timing issue IMO. Since it is happening when you stomp on it, it is smelling like a vacuum advance problem. Is your car automatic or manual?
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
The first item to check is the ignition system.
The second item is the valve train - Before you go spinning to higher rpm, check all of the valve springs carefully for broken coils and dampers. Use a small mirror to check all around each spring.
The third item is timing accuracy - locate TDC first with a TDC stop through the spark plug hole (you just can't assume that it's correct), and then recheck the ignition timing.
The second item is the valve train - Before you go spinning to higher rpm, check all of the valve springs carefully for broken coils and dampers. Use a small mirror to check all around each spring.
The third item is timing accuracy - locate TDC first with a TDC stop through the spark plug hole (you just can't assume that it's correct), and then recheck the ignition timing.
Pro
Joined: Oct 2008
Posts: 645
Likes: 1
From: Hayward Wi
i would get new plugs while you are at it just to get the whole ignition system out of the way. if you do lots of city driving ie stop and go i would be there bad Ryan
Thread Starter
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
its getting there.
just for grins while i wait for my HEI to come in i decided to get rid of the proform center body.
i totally rebuilt the carb using a holley rebuild kit.
secondaries are set so that the transfer slots are a perfect square.
all passages blew out with compressor etc etc.
fired it up and once i got the idle and mixture pretty decent i took it for a ride.
now normal driving gets no pops.
it only pops when i got from a dead stop to WOT.
if i hammer the pedal it pops once and then the tail chases the front end until i shifts gears.(i am 35 and thats still cool to me)
i think at this point it is a pump cam tuning issue.
what do you guys think
just for grins while i wait for my HEI to come in i decided to get rid of the proform center body.
i totally rebuilt the carb using a holley rebuild kit.
secondaries are set so that the transfer slots are a perfect square.
all passages blew out with compressor etc etc.
fired it up and once i got the idle and mixture pretty decent i took it for a ride.
now normal driving gets no pops.
it only pops when i got from a dead stop to WOT.
if i hammer the pedal it pops once and then the tail chases the front end until i shifts gears.(i am 35 and thats still cool to me)
i think at this point it is a pump cam tuning issue.
what do you guys think