Severe backfire

Buzz · #1 (**permalink**) 03-09-2002, 03:08 PM

Hi guys. I fired up my Cobra today for the first time since the rebuild began 8 months ago. I'm running a 383 Chevy stroker with a Holley 750 on an Edelbrock Contender intake. The ignition system uses a ballast resistor with a Mallory Unilite distributor. Before shutting down to rebuild the car, the engine was a only little bit testy to start and ran beautifully. The body off rebuild was mostly suspension, brakes, interior, exhaust system and a new wiring harness.
Now, the engine starts real easy, sounds healthy and revs smoothly for the first few seconds and then develops a consistent backfire at idle. When I rev, the engine sounds great and the backfire stops but returns at idle and seems to get stronger and louder the longer the engine runs. Nothing has been changed in the ignition or fuel system except for new spark plugs and wires and of course the connections for the new harness. I also get the occasional flatulent sounding backfire through the carb when it returns to idle after reving.
I NEED HEEEEELP!!!
I ran a search on backfires and most problems stemmed from exhaust leaks on deceleration - nothing about backfires at idle. Any help will be greatly appreciated.

wilf leek · #2 (**permalink**) 03-09-2002, 05:00 PM

Buzz - sticky valve??? Something weird with the cam timing??

Buzz · #3 (**permalink**) 03-09-2002, 05:26 PM

Thanks for the reply, Wilf. I don't know why the timing would be any different than it was before. Nothing to do with timing was altered; but still worth a check. Sticky valve is a possibility however - how should I go about checking?
One thing that hit me as I was browsing Jegs for new carbs is that mine seems to have an electric choke - round black plastic cover on right side with 2 contacts. One is grounded, nothing connected to the other. Could this be part of the problem? What should be connected to the other contact?

wilf leek · #4 (**permalink**) 03-09-2002, 05:35 PM

Sorry Buzz, I guess I should have asked for more info before jumping in. Is this "backfire" through the carb or is it an exhaust noise?
If the latter, are you sure it's not just an exhaust "blow" or leak somewhere? You should be able to feel the gasses coming out at the source if it was that bad.
If it is continuously backfiring throught the carb, then I would suspect a sticking inlet valve - a compression check should show up which cylinder it could be.
I wonder if the valve lash has been set too tight, holding a valve off its seat somewhere?
Sorry - you are going to have to pin this down before you run that engine too much more.
As for the electric choke - I should think that the missing connection would be a power feed through the ignition. Can't see that you are going to need a choke in your climate! I stripped mine completely off my own car here in the UK and don't miss it a bit.
I don't see this as part of your current problem, unless the thing is stuck in the cold start mode, and then you would have a very badly running engine, with lots of black smoke from the exhaust.

Double Venom · #5 (**permalink**) 03-09-2002, 05:58 PM

Buzz,

The one spade you mentioned is for ground, make sure it is actually grounded to the carb. The second is a "key on" hot side. This should be hooked up, even where you're at, just to make sure the butterfly is wide open.

But, the down side, I don't think this does have anything to do with the backfire.

What side is it restricted to? Both?

My guess's:
Exhaust..you did state new exhaust was about the only change to the motor. Look at all the connections close with a bright light. Look for any soot around header flanges, slip tubes, head to header area.

Lifter has failed to totally pump up yet
Lifter is partially collapsed, sticking somewhat open
(Usually, some good running time at moderate rpm with the engine totally warm will solve both the above problems)

You've simply crossed a plug wire

It's the damned Holley! If you try all things mentioned, and nothing works,... it's the damned Holley!

Edelbrock 750, w/electric choke, simple bolt in mod and will work!

Let the flames begin!

DV

Buzz · #6 (**permalink**) 03-09-2002, 06:18 PM

The backfire is through the exhaust and is only on the right side. It can be felt as a pretty good blast of air comming out of the side pipe. Crossed plug wires is another possibility as there was a bit of a mixup when installing two of the new wires (good help is sometimes hard to find). When referenced, the firing order on the mallory dist. seemed vastly different from the stock setup in Chilton's diagram. How should I go about verifying? Thanks again for the help.

wilf leek · #7 (**permalink**) 03-09-2002, 06:46 PM

DV - thanks for jumping in - its 2.30am here, my brain is not functioning as it should. Could be your crossed spark wire suggestion is the simple thing I forgot about!

Buzz- if you know your firing order, (351W is 13726548) then it's just a case of going around the distributor cap in the direction of rotation (anti clockwise as you look down into it)and following the leads, having made sure you got cylinder no 1 right. (Turn engine until you are at tdc on the firing stroke for no 1, see where rotor arm points at). Cylinders are numbered as follows - looking from front of engine, lh bank goes 1234 front to rear, rh bank 5678 ditto.
I made the assumption your engine is a 351wstroker, hope I got that right.

hth

Jack21 · #8 (**permalink**) 03-09-2002, 07:21 PM

Careful here, this is a Chevy motor, not a Ford. #1 cylinder is on left, not right as it is on Fords, and cylinders are numbered 1,3,5,7 on left, and 2,4,6,8 on the right. Check Chilton manual for firing order on Chevy 350, verify dist rotor is pointing at #1 when on balancer timing mark, then follow the numbers around. You could have a plug wire on the wrong plug. You'll really notice it when the engine is under load.

Might also want to get an ignition that does away with ballast resistor. HiPo engines do not like having only 7 volts going to the coil under power.

Electric choke has two leads. One is a ground, and is typically grounded to the carb body. The other is the hot lead that's connected to the ignition on the +12v side of the ballast resistor (not the coil side). Make sure the choke bimetal spring is insulated per instructions or it will short out choke and ignition. A heating coil inside the choke heats the bimetal spring and makes it open the choke when the ignition is on. These black choke caps burn out and need replacing from time to time.

Buzz · #9 (**permalink**) 03-09-2002, 08:11 PM

Wilf, DV and Jack; since the problem is only on one side, I decided to try and isolate the bad cylinder. By removing and replacing the leads one by one I was able to determine that the #8 cylinder was the culprit. With its lead disconnected, the engine runs fine - meaning no backfire. Does this mean anything and if so where should I go from there?
Jack, what can you recommend a good ignition system to switch to?
I owe you guys at least a cold beer but you'll have to come down here to get it! I've got an extra room at the house if any one of you ever decides to visit these parts.

xlr8or · #10 (**permalink**) 03-09-2002, 09:20 PM

I would pull the coil wire and run a compression check on that side. I didn't see it above but the firing order is 18436572 for a small block chevy.
Sounds like a sticky valve to me or hydraulic lifter issue.
May want to pull the valve cover when doing the compression check and watch the valves on that #8 to see if the push rod is going slack.
And definitely check plug wires for correct order it's easy to forget the difference mentioned above about cylinder numbering if you work on both blue oval and bow tie. You may have two of them crossed and that one was just the most obnoxious.
also might want to verify that the #1 wire is on the correct post on the distributor. It is definitely possible to make the car run with them all 1 post over it just makes lots of exhaust pops and carb flashes..

PS: Have you checked the timing? if your way off it may indicate the plug wires are one post off.

wilf leek · #11 (**permalink**) 03-10-2002, 03:16 AM

Jack21 - see what I meant about posting late at night!!! I couldn't even tell the difference between a Ford and Chevy - shows how tired I was.

Buzz - if you pull the plug on the cylinder that you felt was making things bad, what colour is it? If it is black and "wet", then that cylinder is probably not firing. Let us know more when you have made sure of all the things everyone said above (ignore my Ford firing order and cylinder notation!). Lets hope it is just a spark lead problem, because that is going to be far easier to fix than a valve problem.

Buzz · #12 (**permalink**) 03-10-2002, 07:46 AM

Problem solved, Dudes! Thankfully, I established cylinder #1 on the dist, traced the wires around and indeed found two plug wires had been crossed. I sat and let her idle, revved a bit and listened for the first time to the sound through proper (non rusted out) pipes. What a beautiful noise it is! One of my dogs just recovering from the shell shock of yesterday's backfiring took off again and I'm sure my nearest neighbor, cranky old Mr. Atkinson a couple of hundred yards away was far from impressed. Thanks for the help, guys - as soon as she cools down, I will fill and bleed the clutch and brake lines and HOPEFULLY take her out for a short first drive around the neighborhood. If and when I get to that point I will surely reflect on how the kindness, help and support of the people here played a huge role in helping me make this dream a reality!!

Tigertracker · #13 (**permalink**) 03-10-2002, 07:53 AM

Buzz, before you go any further, I would suggest that you pull the plug on that cylinder and swap it with one from the other side of the engine. See if that moves the problem. If it doesn't then swap the plug wires and see if that moves the problem. Could be a bad connection inside the wire or a cracked insulater on the plug.

Buzz · #14 (**permalink**) 03-10-2002, 08:19 AM

Thanks Tiger, you're on track and if you check my previous post you'll see that it was indeed two crossed plug wires causing the problem! Now, onto the brakes and clutch and then hopefully a test drive.

Double Venom · #15 (**permalink**) 03-10-2002, 08:39 AM

YAHOOOOO !!!!! I GOT ONE !

All right Buzz, way to go!

DV

Tigertracker · #16 (**permalink**) 03-10-2002, 09:39 AM

Buzz, if it makes you feel any better, (It probably won't) I have been building engines for many years and I still occasionley mix a wire or two up. (Hard to mix up just one

) Unfortunatly, it will be the last thing you recheck, because you just know that you put them on right the first time. Experience doesn't seem to help, at least it hasn't for me.

wilf leek · #17 (**permalink**) 03-10-2002, 11:26 AM

Buzz - get that thing bled out and ENJOY!!!

You are now on my list of folks to look up if I ever make it to St Lucia. Thats a list so far of one.