Backfiring, missing, etc..-need help
Thread Starter
Heel & Toe
Joined: Aug 2006
Posts: 21
Likes: 0
From: Germantown TN
Backfiring, missing, etc..-need help
I have a 77 with what is supposed to be a 383 stroker and it ran fine until the car sat for a month while I was redoing the interior. Then, it ran good for a few miles until the problems started. First, I noticed some backfiring through the exhaust at idle, then it got progressively worse until it was backfiring through the carb when accelerating, missing badly and now not running hardly at all under 2500 rpm. All within @ 10 miles. I looked under the hood at night and saw no sparks. The wires and plugs look good. I replaced the cap, rotor, coil with Accel super and the hei module. The car has a 750 speed demon. Cam gone down? I'm going to look at the static timing next. Can't get it to run well enough to get the light on it. Do these Chevy's have a habit of jumping the timing or distributor gear? Can it be a fuel problem? Any advice will be greatly appreciated.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
If it was sitting outside in the rain check the tank for water.Flashlight only-NO 110volt lights-Check the area around the gas cap and see if it holds water rather than draining.I see this alot and actually I vacumn the bottom of the tank with every tune up.Using a siphon NOT your wifes vacumn cleaner-it will EXPLODE-really.
Last edited by ...Roger...; Sep 23, 2006 at 08:27 PM.
Burning Brakes
Joined: Nov 2005
Posts: 1,143
Likes: 0
From: Leander Texas
Sounds like it is starving for fuel. I would try replacing the fuel filter. it is possible the floats could be stuck from the fuel in the carb evaporating out leaving deposits behind gumming up the carb. Try a new fuel filter first.
Racer
Joined: Apr 2005
Posts: 494
Likes: 2
From: rockwall tx
Originally Posted by DR.Jay
Sounds like it is starving for fuel. I would try replacing the fuel filter. it is possible the floats could be stuck from the fuel in the carb evaporating out leaving deposits behind gumming up the carb. Try a new fuel filter first.
i have seen alot of carburated motorcycles have the needle and seats gum up from deposits making them stick together then running like crap. replace the fuel filter first then maybe run some carb cleaner through it.
Thread Starter
Heel & Toe
Joined: Aug 2006
Posts: 21
Likes: 0
From: Germantown TN
so far...
Got the carb off and found the butterflies on the primary were way up on the transition slot. Pulled one bowl so far, it has 83 jets on that side. Pulled one plug - wet fouled from fuel (rich). Fuel filter - clear.
Going to blow out all of the ports in the carb, baseline set it and try it out. Also will pull all of the other plugs. I'm thinking too cold plugs. This has a 3000 converter and does a lot of idling when street driving. Will post tomorrow. BTW, where is the model # on a demon carb? It looks like a speed demon.
Going to blow out all of the ports in the carb, baseline set it and try it out. Also will pull all of the other plugs. I'm thinking too cold plugs. This has a 3000 converter and does a lot of idling when street driving. Will post tomorrow. BTW, where is the model # on a demon carb? It looks like a speed demon.
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Thread Starter
Heel & Toe
Joined: Aug 2006
Posts: 21
Likes: 0
From: Germantown TN
Bowls off the carb, set floats per BG website, set butterflies per same. Installed and won't run worth anything. Backfires, misses, even worse. Pulled plugs - black and wet. Tried turning the distributer to crank - no good. put distributer back in same place. Groung plug on header - has spark. Finger in #1 hole - on compression stroke lined up timing to tdc - rotor not close to #1 terminal on cap. Pulled valve covers (whoa, crane gold roller rockers!) and brought #1 to compression stroke, went to where both rockers have slack (must be solid or roller cam?) rotor is on #1 terminal on cap, index line on balancer not even in view. It seems like the timing chain skipped. All the rockers go up and down nice. Am I going in the right direction? Haven't done this in years. I also plan to pull the oil filter and look for chunks. If the cam went down, I imagine I'll find some in the filter. Anybody have this happen before?
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by ebbenr
What exactly would I be looking for during the test?
Results of a compression test should have less than 10% variance between cylinders.
I would look toward ignition first and then carburetion
Drifting
Joined: Jul 2002
Posts: 1,943
Likes: 1
From: Rosemount MN
When troubleshooting a Demon carb, use this website:
http://www.barrygrant.com/demon/default.aspx?page=28
It describes your symptoms and gives good advice.
http://www.barrygrant.com/demon/default.aspx?page=28
It describes your symptoms and gives good advice.
Le Mans Master
Joined: Jan 2003
Posts: 6,575
Likes: 1
From: Tuttle OK
Cruise-In VIII Veteran
fixing a car over the internet is sometimes difficult at best. if your car ran fine a month ago and doesn't now, a lot of stuff can happen...i've had burned valves and busted springs, act similar to what you're describing.
i just like a good old compression test because it's cheap, easy, and seperates a lot of stuff.
if you say you're getting spark and your carb is right, why not take 15 minutes and look before you tear into a timing chain?
just my opinion, your mileage may vary..
i just like a good old compression test because it's cheap, easy, and seperates a lot of stuff.
if you say you're getting spark and your carb is right, why not take 15 minutes and look before you tear into a timing chain?
just my opinion, your mileage may vary..
Thread Starter
Heel & Toe
Joined: Aug 2006
Posts: 21
Likes: 0
From: Germantown TN
Avette4me,
I'll check compression dry, then wet. This will tell me how the rings are, correct? If one is way off of the others dry, that means a valve, correct? But what would I see if the timing chain is slipped?
I'll check compression dry, then wet. This will tell me how the rings are, correct? If one is way off of the others dry, that means a valve, correct? But what would I see if the timing chain is slipped?
Le Mans Master
Joined: Jan 2003
Posts: 6,575
Likes: 1
From: Tuttle OK
Cruise-In VIII Veteran
i wouldn't make it too difficult. just run each one dry. if they check out, move on to the next test. if not, you'll know you're on to something. like bobonit said, it could be a waste of time but a much easier one than tearing into a timing chain.
before i went into the timing chain, i would double check that you weren't off 180* on your first timing check...
before i went into the timing chain, i would double check that you weren't off 180* on your first timing check...
Thread Starter
Heel & Toe
Joined: Aug 2006
Posts: 21
Likes: 0
From: Germantown TN
Did the piece of wire in the plug hole to verify the timing. The piston came up on #1 when it was supposed to. The balancer mark is off. Broke #1 plug when I put it in and had to replace it. Still backfiring out the exhaust but not flaming the intake as bad. Will lower the floats, turn the idle mixtures screws to 3/4 turns out and change the plugs next.
Drifting
Joined: Feb 2005
Posts: 1,438
Likes: 2
From: Melrose Florida
My first thought was bad gas...seem to encounter it every so often.
Second thought was stripped top timing gear, factory ones for many engines were nylon teeth on a metal gear, they strip/break, and you lose timing.
Car runs good, then won't restart or runs poorly. Much more common than cam problems.
Steve
Second thought was stripped top timing gear, factory ones for many engines were nylon teeth on a metal gear, they strip/break, and you lose timing.
Car runs good, then won't restart or runs poorly. Much more common than cam problems.
Steve