Have a moderately built 350 with less than 500 miles. Engine has CompCam (not too aggressive), upgraded stock heads, Perfromer intake, Holley 670 Street Avenger, and stock ignition. Can drive the car one day and it runs fine and then the next time it skips or backfires when I get on the gas, even at cruising speed. It sounds more like a backfire than a skip. Once the engine gets hot and I cut it off, it's hard to start, like the timing is off a little. My dad says it's like the engine is surging. I also noticed a little smoke when I let off to change gears, this goes away once the motor warms up.

Plan on checking the timing and compression this week. It may be time to upgrade the ignition system, may be asking too much from the stock setup.

Suggestions and comments welcomed!

Is it backfiring through the carb?

Backfiring through the exhaust

This backfire happened before and we upgraded the plug wires and it quit, now its back. It almost sounds like excess fuel detonation. Any ideas?

[QUOTE=tim68rsss;1333607]Have a moderately built 350 with less than 500 miles. Can drive the car one day and it runs fine and then the next time it skips or backfires when I get on the gas, even at cruising speed.

See if you can tie this to weather conditions,seeing how it is intermittent.OK one day,not the next.
High humidity like a rainy day will tax your secondary spark voltage.I would have a good long hard look at the inside of your distributor cap for corrrosion and or carbon tracking/cracks etc.
Regardless of what type distributor you use..drill a 1/4" hole in the floor and epoxy some screen in place.You want the corona out and no spiders in.

Burnt plug wire, short, cracked cap, or timing.

If changing the plug wires fixed it the first time and the same problem eventually came back, I would look at the wires again. Start it at night and check for spark jump. It could be that the boots are getting burned by the headers, if you have them.

A couple things come to mind to check, carb float level, mixture, plug wires, and timing.

The puff of smoke or backfire through the pipes sounds like it's running rich or maybe too far advanced. Hard starting when warm definitely sounds like timing issue.

Make sure you plug wires are properly seating on the plugs. A little dab of di-electric grease will help the wires boot slip over the plugs insulators easier. Also helps keep the spark where it should be and any moisture out.

Got new dist cap, rotor button, points/condenser from Napa and plan to install tomorrow, will check the timing as well. Will post results, thanks for the comments.

Ok guys ..... here is an update on my engine problem (engine has only 500 miles since rebuild).

Installed a different dist with an electronic module, rotor button, and rotor cap. Checked the new plug wires and they still look good. The floats in the 670 Street Avenger were off a little so adjusted them and reset the timing.

Pulled compression and all the cylinders were around 180.

Engine backfires under acceleration and will backfire through the carb sometimes if I pop it real good. Smokes when I let off the gas, smells like fuel not oil burn, smoke kind of gray color. Doesn't backfire at idle, actually idles pretty good at 950rpms.

Pulled the plugs which only have about 150 miles on them and was shocked to see how bad they looked, pictures attached. There was oil and gunk on 7 of the 8 plugs. Also had to add quite a bit of oil since last change (250 miles), oil smelled like it had gas in it.

Could I have a cam failure, which doesn't explain the oil on the plugs? Could the oil and gas be mixing, allowing it to pass by the rings and burning when it fires causing the smoke?

Man the plugs all look fouled out. Are you running a PCV and is it sucking oil into the back of the carb. The plugs looks like fuel and oil.

Is the oil on the plugs from a leaking valve cover. If not either rings haven't seated yet or valve giuds are bad or you are sucking in oil from the PVC.

Check timing first.

Defiantly lower the floats levels.

Check fuel pressure.

Mixture screws bring in as far as you can before it wants to stutter then back it out 1/8 turn.

Check valve lash.

PCV is hooked up to the front of the carb and brake booster to the rear. Some small amounts of smoke has always escaped out of the valve cover breather on the right side, not sure why. PCV is on the drivers side front. Valve covers are not leaking. It's weird, engine didn't smoke at all the first few hundred miles. Heads were completely rebuilt with all new guides. Wonder if the valve seals could be bad? Checked timing and adjusted the carb. Stock fuel system. Oil is getting into the cylinders somehow, man I hate to pull the heads. Would plugs fouled out cause the engine to backfire?

PCV is hooked up to the front of the carb and brake booster to the rear. Some small amounts of smoke has always escaped out of the valve cover breather on the right side, not sure why. PCV is on the drivers side front. Valve covers are not leaking. It's weird, engine didn't smoke at all the first few hundred miles. Heads were completely rebuilt with all new guides. Wonder if the valve seals could be bad? Checked timing and adjusted the carb. Stock fuel system. Oil is getting into the cylinders somehow, man I hate to pull the heads. Would plugs fouled out cause the engine to backfire? Check the hose from the PCV to the carb and see if there is any signs of oil getting in to the carb, if there is then that oil would roll down the intake runners and on the plugs and out the exhaust. Also could be a intake gasket or head bolts. Heads are fresh so i would look at the PCV, intake gasket or head bolts/gasket.

As far as the backfiring that would be ignition/timing or valves to tight. Also to light of springs in the distributor would make the engine backfire at cruise including a bad vacuum advance can at a certain rpm. If you put a timing light on it make sure the timing isn't bouncing all over the place, then check total timing to see when it comes in and check vacuum advance. If it advances to early then could be one of your problems as far as the backfire.

Check to see if the distributor is advancing, also if you have a vacuum advance can on the distributor plug it into to constant manifold vacuum off the carb (as long as it has constant vacuum at idle should be best place for the vacuum line).

Other then all that try a new coil.

How do you have the PCV valve hooked to the "front" of an Avenger? Those are small ports, one is full manifold vacuum and the other is ported. I would "T" into the back for the PCV and brake booster. I have my distributor hooked to full manifold vacuum (lower port) and ported is plugged. If you have the PCV hooked to ported, you aren't really getting the full use of the PCV and that is most likely your smoke from your vented cap.

The PCV valve line is connected to the lower port in the front, thought that is where the manual said to connect, will double check. Would be pretty easy to tee into the booster line. As far as the oil getting into the cylinders, whatever it is, it is happening in 7 of the 8. Will check the PCV line first.

Do have a vacuum advance can, from Autozone, not sure how to tell when it comes in or if it's working properly. With the vacuum advance port plugged on the carb the engine likes about 19 degrees at 950 rpms and about 34 degrees at 2500 rpms. Timing isn't bouncing around and springs on the dist seem kind of week to me, stock. Maybe I'm not timing the engine properly.

If you have had this problem since the motor was put together I would suggest checking the cam to see if is off a tooth in relationship to the the crank. I had problems years ago with my '68Z when I first got it. It would not run well at all when set up with the timing and carb adjusted the way it should be. I found the timing chain off a tooth, fixed it, and solved the problem.

Just my thoughts,

Check the PCV hose first and look inside for oil residue, if yes then the next question is how much is getting in the intake runners. Wont hurt anything other then the plugs.

To check the vacuum can just first check initial timing, then unplug the vacuum hose and cap it off with your finger to see if the idle or timing retarded about 10*... If so then its working but also make sure the advance plate with the springs arnt kicking in to soon.

You should be good to go with some new plugs, valve lash correct, timing correct, carb float level correct, mixture screws correct, and the right power valve.

Ok, vacuum can may be off. With it unhooked from the carb and that port sealed off, the engine timing is around 19* at 950 rpms, doesn't change when the hose is hooked back up.

Going to check the PCV hose for oil this afternoon, will post results.

Ok, vacuum can may be off. With it unhooked from the carb and that port sealed off, the engine timing is around 19* at 950 rpms, doesn't change when the hose is hooked back up.

Going to check the PCV hose for oil this afternoon, will post results. The vacuum can may be plugged in to ported vacuum which is why you don't see it change when disconnected. Try to plug the vacuum line into ported vacuum (which is a consent vacuum at idle). But make sure your timing is where it should be 19* small block at idle but what is your total at 3k rpms.

Heres manifold vacuum as you can see at idle its pushing about 10" of vacuum. After some more adjustments i now have 12" of vacuum.
http://i145.photobucket.com/albums/r216/skeeter55/th_firstfireup1113005.jpg (http://s145.photobucket.com/albums/r216/skeeter55/?action=view&current=firstfireup1113005.flv)

Should the vacuum can be plugged into ported or manifold vacuum. Thanks for the video, awesome car! I'll see if I can borrow a gauge to measure the vacuum.

Should the vacuum can be plugged into ported or manifold vacuum. Thanks for the video, awesome car! I'll see if I can borrow a gauge to measure the vacuum. I would plug it into manifold vacuum right off the carb, should have a nipple or two and one of them will have constant vacuum, thats manifold vacuum. The other nipple is ported vacuum only kicks in at a higher rpm then idle.

I would check cam and lifters IMO.
im sure you checked firing order and timing isnt 180 degrees out.
with plugs out and valve covers off spin the motor to see if your rocker arms are moving all the same.
I would also check your valve springs.

No oil residue in the PCV hose. As bad as I hate to admit it, I do remember not putting any sealer on the threads of the head bolts during installation (forgot). With good compression around 180, wouldn't this much oil be coming from the valvetrain?

No oil residue in the PCV hose. As bad as I hate to admit it, I do remember not putting any sealer on the threads of the head bolts during installation (forgot). With good compression around 180, wouldn't this much oil be coming from the valvetrain? Have you dropped some new plugs in and re-tunned the motor. It should have a leak down test done, but the intake could also be where the problem lies. Motor only has 500 miles just broke in and with old plugs, you may just want to start off with fresh plugs, check coolant, check oil and look for signs of fuel (smell it and rub it on your fingers look for a separation with fish eyes in the oil), triple check that timing and the pull a valve cover off like one of the members have mention to make sure the rocker look equal when at full lift.

Replaced plugs a couple hundred miles ago and they looked the same. Oil does seem to have a hint of gas smell. Guess if it's the intake it is leaking on both sides.

Replaced plugs a couple hundred miles ago and they looked the same. Oil does seem to have a hint of gas smell. Guess if it's the intake it is leaking on both sides. Do not run that motor if it smell like gas, it will wash down the cylinder walls/bearing from oiling. If it has gas in the oil then carb needs to be addressed from flooding the motor with fuel.

Some one needs to chime in here to get you straight with a-z so you don't hurt that motor, so far may just be gaskets, bad tunning or valve seals.

Ok guys ..... here is an update on my engine problem (engine has only 500 miles since rebuild).

Installed a different dist with an electronic module, rotor button, and rotor cap. Checked the new plug wires and they still look good. The floats in the 670 Street Avenger were off a little so adjusted them and reset the timing.

Pulled compression and all the cylinders were around 180.

Engine backfires under acceleration and will backfire through the carb sometimes if I pop it real good. Smokes when I let off the gas, smells like fuel not oil burn, smoke kind of gray color. Doesn't backfire at idle, actually idles pretty good at 950rpms.

Pulled the plugs which only have about 150 miles on them and was shocked to see how bad they looked, pictures attached. There was oil and gunk on 7 of the 8 plugs. Also had to add quite a bit of oil since last change (250 miles), oil smelled like it had gas in it.

Could I have a cam failure, which doesn't explain the oil on the plugs? Could the oil and gas be mixing, allowing it to pass by the rings and burning when it fires causing the smoke? From the looks of these plugs and you said oil smells like gas, that would leave to believe ignition/timing and carb problems, also check the cam/valve lash make sure you dont have any bad lobs.

Thanks for the suggestions! Will check the cam lobes next.

Ok guys ..... here is an update on my engine problem (engine has only 500 miles since rebuild).

Pulled the plugs which only have about 150 miles on them and was shocked to see how bad they looked, pictures attached. There was oil and gunk on 7 of the 8 plugs. Also had to add quite a bit of oil since last change (250 miles), oil smelled like it had gas in it.

I too had oil on several of my plugs. I went the drastic route and swapped out the entire heads. Afterward, I pulled the valve springs on the old heads and noticed several of the valve seals were bad. If you are getting oil on the threads, then more than likely you have some bad valve seals. No big deal though. They aren't too hard to replace. Get yourself tool that screws into the plug hole that allows you to attach an air hose to it. This will pump up the cylinder to keep the valves from falling down into the chamber. Also get yourself an overhead style valve spring remover since you won't be able to use the large "C clamp style because you won't have access to the head of the valve. They make several styles of valve seals. Umbrella and a positive seal. Get the positive seal. They are much more durable that the umbrella type. You may need to do some machining on the area around the valve stem to use the positive type if you are running a stock head because of how they install or you could just get new heads, like me, if you are running a stock head.

You may have a bad fuel pump gasket allowing fuel to get in the oil. It is not necessarily the carb. It may be, but not guaranteed.

Talked to my dad, partner in crime, last night and we went over this backfiring issue from the beginning. Engine started backfiring around 300 miles from rebuild, changed the plugs and wires at the same time and it stopped. Now, 200 miles later, it is doing it again. Seems like if I had a cam problem it wouldn't have gone away and then returned. Oil is getting into the cylinders somehow and I don't think it is from the bottom end. We have a Performer intake and I do remember having some intake gasket issues, they didn't seem to look right. I believe I searched TC and found the Felpro gasket # for stock heads and Performer intake. Guess my next step would be to pull the intake, should be able to see if oil is being sucked into the intake runners. Anyone know the correct intake gaskets for this application?

Well you have some good info from Charlie,
I still do a leak down test befor you pull it apart. You may get lucky with just pulling the intake and also replacing the valve seals. But the cam may also be a bigger problem (is it a hydraulic cam) if so what are the specs and are the valve springs set up for this cam.

If the cam looks fine and spring too, then ignition like a bad pick up module our a coil going bad would also sound like a miss/popping/backfiring sound at cruise. Make sure that motor has good grounds from the the motor to the frame.

What kind of cam are you running and whats your distributor gear?

I had a iron gear on my solid roller at the sucker in 150 miles. At around 100 miles into the engines life it would idle weird every start up was different. It started bogging and sputtering at lower rpms and fuel was all over the place. I pulled the gear out and there was teeth that were about to break and other teeth fine.

You do have fuel and oil on plugs though. Keep trying to break it in and seat the rings hopefully thats it but they should have been seated. I would tend to think if it was the valves it would show up on a compression test. Oil in the cylinders though will help seal the rings. You might try it again and squirt oil in each cylinder even the dry plugs to be consistent and see what numbers you get.

Used a CompCam, engine ran fine up until now, 500 miles later. I believe I have the mechanical valve seals, will check. It still could be ignition but that doesn't explain why the plugs are fouling out, 7 of the 8.

How is a leak down test done? Do a compression test and see how long it hold the result?

Well you have some good info from Charlie,
I still do a leak down test befor you pull it apart. You may get lucky with just pulling the intake and also replacing the valve seals. But the cam may also be a bigger problem (is it a hydraulic cam) if so what are the specs and are the valve springs set up for this cam.

If the cam looks fine and spring too, then ignition like a bad pick up module our a coil going bad would also sound like a miss/popping/backfiring sound at cruise. Make sure that motor has good grounds from the the motor to the frame.

Skip may also be on to something with the cam. I thought about this for a bit and I remember when I had a cam go away. It would idle ok in park/neutral, but when I put it in gear, it would idle ok for the 1st minute or so, then it would start to spit and sputter and eventually stall. As for how it did while running, the 1st cam when it went, under WOT, it would make a loud metallic popping. The 2nd cam was the same under idle, but part throttle and WOT were ok because this cam was not as bad as the 1st as far as wiped lobes. A bad cam won't have anything to do with oil burning, but if the cam is too big for the springs you are running on the stock heads, then you will get spring bind and that can break springs and cause valve float, which will affect performance. I can't see the cam being cause for fuel in the oil. It sounds like either the carb is way too rich, in which case you would see black smoke coming out of the pipes both at idle and WOT or a possible bad fuel pump or pump gasket. Maybe the intake gasket, if not installed properly or the incorrect gasket, may be allowing fuel to leak over from the intake runners when they transition from the intake runners to the head ports. Do the easy thing 1st, check the pump gasket and pump. The intake involves a little more work. Not much, but more than the pump and gasket.

What kind of cam are you running and whats your distributor gear?

I had a iron gear on my solid roller at the sucker in 150 miles. At around 100 miles into the engines life it would idle weird every start up was different. It started bogging and sputtering at lower rpms and fuel was all over the place. I pulled the gear out and there was teeth that were about to break and other teeth fine.

You do have fuel and oil on plugs though. Keep trying to break it in and seat the rings hopefully thats it but they should have been seated. I would tend to think if it was the valves it would show up on a compression test. Oil in the cylinders though will help seal the rings. You might try it again and squirt oil in each cylinder even the dry plugs to be consistent and see what numbers you get.

I want to clear this up for him. You say oil and fuel were all over the place. Do you mean oil and PRESSURE or oil and fuel literally? I would think pressure, but I want to be clear for everyone and us slower than others.

I want to clear this up for him. You say oil and fuel were all over the place. Do you mean oil and PRESSURE or oil and fuel literally? I would think pressure, but I want to be clear for everyone and us slower than others.

My fueling was all over the place A/F ratio wise. The cam gear being eaten was throwing off timing affecting vacuum and fuel.

Also I thought about it a little and when i had my valve guides go bad. I was dripping oil into the cylinder even after shut down so on cold starts i would puff out a cloud of oil. IS you engine doing this?

Engine only smokes when I let off the gas. If I rev the engine up real good and left off quickly it smokes pretty good.

Doesn't seem like the valve seals or guides are the issue. If so, 7 of the 8 are bad. Plan on getting a new set of plugs and see how it runs, see if it backfires or skips. If not, pull the plugs and if there is oil, pull the intake and see if I can tell if the intake gasket is leaking. Maybe oil is being pulled into the intake runner.

Does anyone know which is the best intake gasket to use with the Performer intake and stock heads?

UPDATE:

OK guys, put new plugs in and the motor ran great with no backfire or skip, still smoked though. Evidently, the oil in the cylinders fouled out the plugs causing the problem. Now with that determined, I pulled the intake, see pics. There was oil on most of the intake bolts and oil between the heads, gaskets, and intake, don't think that is right. Just ordered a FelPro 1204 gasket kit and plan on using Permatex Right Stuff on the two ends. Not sure the rubber pieces wasn't allowing the intake to seal properly, from the pics does this seem logical.

You are getting it narrowed down now. The end gaskets won't cause this problem. I would change the valve seals now, while you have it down. From what I am seeing now, you either have bad valve seals, worn valve guides, or whoever built the engine installed the piston rings wrong/upside down.

Don't think it is the rings because it just started smoking, 500 miles after rebuild. Looks like if they were in wrong, it would have started smoking right off. I could see oil residue on top of most of the intake valves and in some of the chambers. If it is guides or seals, it is leaking in 7 of the 8 cylinders (7 of the plugs fouled out), guess anything is possible. Just didn't think it should have been that much oil between the intake, intake gasket, and heads, plus there was oil on the intake bolts. Just picked up the FelPro 1204 gasket set and Permatex Right Stuff, plan on reinstalling the intake and see what happens.

Have decided to pull the heads and have them checked out since we are this far into the motor. No big deal, just cost me a set of head gaskets.

Dont use the rubber gaskets on the intake ends. Use RTV instead of the gaskets.

UPDATE:

OK guys, put new plugs in and the motor ran great with no backfire or skip, still smoked though. Evidently, the oil in the cylinders fouled out the plugs causing the problem. Now with that determined, I pulled the intake, see pics. There was oil on most of the intake bolts and oil between the heads, gaskets, and intake, don't think that is right. Just ordered a FelPro 1204 gasket kit and plan on using Permatex Right Stuff on the two ends. Not sure the rubber pieces wasn't allowing the intake to seal properly, from the pics does this seem logical.

If you are running an aluminum intake(Edelbrock Performer or the like), then you MUST NOT use the rubber strips at the front and rear of the intake. Adimately states this in instructions. Also, the 4 most inner bolts on each side of the intake require RTV on the threads. Some use ARP thread sealer, but I have had bad results with that. I use Black RTV. The 4 inner bolts on each side on the intake go into water jackets. You may already know this, I just wanted to state it to be sure.

Have decided to pull the heads and have them checked out since we are this far into the motor. No big deal, just cost me a set of head gaskets.

Prepare yourself for some lifting. Each head weighs about 40lbs. I did head gaskets with the motor still in the car and it sucked. Of course, my heads are studded. If you can, pull the motor. Much easier to sit intake on and much easier to do head gaskets.

Used the rubber gaskets first time, will use Permatex Right Stuff this time. Have help so pulling the heads won't be too bad. Just hate to pull them and find out nothing is wrong with them, but also hate to put intake back and find out it was the valves seals or guides.

make sure you have thread sealant on all head bolts and rocker stud bolts...i had a big block do the same thing.no thread sealant on rocker studs...puffed smoke like a dragon
hope it helps

maybe someone allready said it but couldnt it be just that you had to RETORQUE the headbolts and intake bolts?

Like you said itwas running good the first 500 miles, i had to retorque my intake 3 times.