This is my first post so I aplogoize if this should be posted elsewhere. I recently purchased a 1968 camaro, 327/210 hp, TH350 automatic, ps, pb, factory A/C, Rochester 2 barrel, factory style dual exhaust. Carb has been recently rebuilt and the fuel pump is new. It has Petronix electronic ignition and coil. The previous owner restored it to stock/original appearance as far as body and drive train. Recently it backfired through the exhaust several times and since then it has been stalling itermittently as well. The first backfire was as I was basically coasting down a large hill in gear at idle. It backfired twice and then stalled and since then it has begun stalling, usually at idle in gear but a couple times under moderate acceleration. When it stalls at the idle it seems to start right back up but the time it stalled under acceleration, it took several pumps of the gas pedal to get it started almost like it was starved for fuel. So far it has only had these problems when the car is warm(running 10 minutes or so.) In general it seems to run a little smoother when the engine isn't as warm. I notice the slightest miss or hesitation when the engine is warm and I am giving it just enough gas to maintain speed. I'm thinking it must be a Carb/fuel problem since it starts right back up after it stalls but I would welcome any thoughts or suggestions. What typically makes the engine backfire like this, too much fuel and not enough spark or something else? When it's not having this intermittent problem, it generally runs and starts great, that's why I'm struggling with finding a reason for this.

thanks,
Tom Harrington

Sound like the camshaft has a rounded lobe possibly. :( I'm no expert but seems I had this problem before. You can check this with a dial indicator or maybe by just observing rockers with valve covers off while someone turns engine over by hand. See what others here think first. It also could be the distributor. By the way welcome to the best Camaro site on the web.

[ 02-12-2005, 11:21 PM: Message edited by: Camaro&HogMan ]

More than likely a carb problem.Unless you are staying stock, change it to a 4 barrel setup.Edelbrock dualplane intake and edelbrock 600 electric choke carb.That would make quite a difference in your performance,If thats not what your looking for then you need to tune your 2 barrel,choke setting,air/fuel mixture,etc.BTW welcome to the site. graemlins/waving.gif

First of all, Welcome to the site.
Maybe the carb float level isn't properly adjusted.

A compression test would be a good first start. Carb problems usually lead to backfire through the carberator along with stalling and hestiation. Exhaust backfire is often a valve related thing or inadiquate ignition in one or more cylinders. Although, if your exhaust is fairly open, most carberated engines will pop and sput when coasting down. But they shouldn't go kerpow!

Welcome aboard. Be sure to check out the tech reference links from the home page. Also get familiar with the search functions. For better or worse, the search engine runs by forum so you might have to hunt around a bit for some topics. Engine related issues often get scattered between the Tech, Performance, Engine, and Troubleshooting forums for example.

Have fun.

In my opinion, do the compression test to see if the valves are sealing. If that checks out, it sounds like textbook carburetor trouble.

Tom you say this carb was just rebuilt? Was it done by a professional? Many times the carb is the first to blame but I beg to differ. The Rochester 2bbl is a fairly simple device compared to a 4bbl and I have rebuilt these myself on an old Impala I owned and never had any problems with it and it had dual exhaust also. I agree with dnult that "Carb problems usually lead to backfire through the carberator". Before I bolted on a new carb I would find the trouble first.

Thanks to everyone who has replied. I appreciate it greatly. I wanted to add a little more background that I didn't add earlier because i thought it might be too confusing. As I said earlier this all seemed to start when I was coasting down a bridge. When I first heard the noise I swore something major broke or I must have run over something because it sounded like something was coming up through the floor board. After it stalled and I rolled off the road, I looked under the car expecting to see something(drive shaft,etc) but it looked ok so I restarted and it seemed ok except for a stall a few minutes later. At home a few days later, I looked up under the back of the car and the transverse muffler was caved in pretty severely on the bottom. I ordered and installed a new muffler from Rick's. I had hoped that perhaps the damaged muffler was causing too much back pressure and that was my running problems but I guess not. Assuming that the smashed muffler may be related to the problems, could the increased backpressure have caused any lasting problems(valvetrain, etc) that wouldn't have gone away with the new muffler installation?

Thanks again,
Tom

Could be. Who knows.

High float level dumping fuel and burning it in the exhaust. He said it was coasting down a hil so this would be high vacume and sucking fuel. Just barly crack the throttle and bang. Check the float level. Also check timing. As stated a compression check is also in order. Carboned up piston or chamber?

Joe

Check your distributor to make sure it's tight. But the 'recently rebuilt carb' sends up a red flag for sure.

Thanks guys. I'll check the distributor and timing. I've searched the archives for float adjustment with no luck. Does anyone have instructions on how to adjust the float? If it's a float problem, wouldn't I see some indication that it's running too rich?

hmmm... what if the backfire skipped the timing chain?

It would run bad all of the time I would think.But possible I have seen it before.I have seen it rip many teeth of a plastic worn factory cam gear.I vote new intake and 4 bbl carb JMO graemlins/thumbsup.gif

Float can be sticky needle and seat dirty?


Joe

As suggested, I checked the distributor and it wasn't loose. I checked the timing with my timing light(that I hadn't used in many years so bear with me). The timing marks on the tab read A O R, I'm guessing the A is advance, the O is TDC and the R is retard. When I checked the timing(enginge warm, vac adv disconected) with the light, the mark on the balancer points to 2 marks on the tab from the O, towards the A. Is that 2 degrees BTDC and is that ok?

The marks on the tab usually represent 2 degrees between marks so yours would be four degrees advance. A stock motor should be 7-10 degrees advance. Make sure you are checking it with the vacuum advance hose disconnected and plugged. I'm not sure if the current setting would be retarded enough to cause it to backfire. Usually retarded timing will cause it to backfire through the carb.

Assuming you are still running points -be sure to check the dwell.

Hi guys, just an update. The carb has checked out ok but the electrical has not. Worked on the problem again last night and got a little more information but not solution yet. When the car is not running but the key is on, 12 volts at battery, 12 volts in and out of the igition switch and 12 volts at the coil. When it's running, 13-14 volts at the alternator and voltage regulator at any rpm but if the car is idling, the coil reads around 11 volts. If I increase the engine rpm's the volts at the coil respond to 13-14???? The Pertronix instructions that I recieved with the car say if your car has a resistor, leave it as is but someone at another forum suggested I remove the cloth covered wire. I removed the cloth covered wire from the coil and the car wouldn't even start(so I put it back). Currently the positive side of the coil has a yellow wire, the cloth covered white wire, a black wire to what I think is a radio capacitor(which I removed and had no effect on problem) and the red wire for the petronix module in the distributor. The negative side of the coil has the black wire for the pertronix module. I'm going to try again tonight to isolate the voltage drop. Any chance that the coil itself could be causing this or is the general consesus that it's probably a bad ground or something like that?

thanks again, guys

Tom

The voltage at the coil is supposed to be lower due to the white resistance wire. If your new distributor needs a full 12 volts, you'll have to replace the white resistance wire section (about 8" long) with regular copper wire.

As far as your backfiring problem, try a new set of spark plugs and check your plug wires for voltage leaks. These would be the simplest, and most obvious things to try first.

Thanks John,

Is that normal for there to be such a fluctation between voltage at the idle and at higher rpm? I'm new to this electonics stuff, what's the difference between the current white wire and replacing it with a copper wire?

Tom

As it is called, the wire "resists" or supresses a portion of the energy traveling along it's length. By replacing it with regular copper wire, the full 'charge' is transmitted with no reduction in current/voltage.

Check out this post
http://www.camaros.net/cgi-bin/forum/ultimatebb.cgi/topic/10/2885.html?
Apparently, what you have to do with a Petronix is hook up the red wire to to a seperate ignition source 12v, and leave the coil wires as they are in stock form. I've never dealt with a Petronix, so I can't say for sure.

The coil is also a Pertronix Flame-thrower coil which I read has built in resistance, for what that's worth. Is the difference in voltage at higher revs relevant?

Just for a test, I'd swap in another distributor without the Pertronix. I have had grief with a Pertronix that gave me much the same symptoms as you have. That will give you an answer or rule out one more thing.