I've been chasing this one a while now. Its amazing where troubleshooting will take you.

Here's some background info, engine in question is a L35 396 4 speed car. I have a 3310-4 Holley 750 vacuum secondary carb on it and an Edelbrock performer intake manifold. I have a mallory unilite distributor and a mallory promaster coil. The heads are coded I believe from a '71 402 or 454.

When I bought my '67, the performance of the engine was very lacking. I was told the engine was rebuilt about 2500 miles ago and the heads had an 3 angle job on them and that the cam was one up from stock(whatever that means).

Some many posts ago, I asked about the compression I should be getting on my 396. I performed a compression test on the engine and got values in the 140's and it had about a 10 psi delta. It seemed low so I checked it with a friend's compression gauge and he got very tight 160 psig compression all around.

Also noticed that my choke plate was partially closed. I opened it up all the way and that is when I started having a back firing problem through the carburetor. The engine idles okay, and I can run it up the rpm's, but when it is given a shot of gas, I will get nasty backfiring.

I then tackled the timing and discovered that the distributor wasn't fastened very tightly and was moving. I tightened the clamp and I still could move the distributor. I ended up replacing the chrome hold down clamp with a GM clamp and setting the timing at 12 degrees BTDC. Also, gave it a fresh set of wires and plugs. This still did not alleviate the problem.

I was convinced I had a carburetor problem until today. I went over to a friend's house to try his Holley 750 and the same symptons existed with his carb.

I am now thinking I have either a cam or a lifter problem. Before I tear down top of the engine, I was looking for some feedback regarding scenarios which would cause backfiring only when the gas is quickly applied. Is there anything else I should try before tearing into the engine?

Thanks.

Dave

[This message has been edited by Scooby Doo (edited 02-19-2001).]

It almost sounds like the timing chain is off a tooth. What do you all think?

Could you just be running lean? If you jetted the carb when the choke plate was closed...and the car backfire when the choke is open and when you hit the gas (and advance the timing), then maybe it's lean? Just a guess. laabs

I agree that it sounds like the timing chain is either off or wore out. That will cause the low cranking compression and the backfiring/low power

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375hp 78 Chevy truck
77 Chevy Nova
95 Chevy Lumina 3.4L
and building a 78 Nova

Thanks for the suggestions. I thought I read somewhere on this board how to check if the cam is in alignment with the crank by measuring the valves or something. Could someone describe the procedure or post the link to the thread? Also, I think the compression is okay at around 160 psig. All the cylinders were +/- 3 to 4 psi.

Laabs, I don't think its the carburetor. The symptoms should have changed when I put a different carb on the car. The engine reacted the exact same way.

What's curious is that I can slowly bring the engine to full throttle. If I bring it up too fast, she'll backfire.

Dave

[This message has been edited by Scooby Doo (edited 02-20-2001).]

Try these first:

Take a long flex handle and socket and turn engine to TDC. Rock crankshaft back and forth to measure the number of degrees of slack in timing chain, if more than 8 deg, then replace timing chain. Hook up vacuum gauge to full time intake vacuum, start engine, bring to operating temp, make sure choke is open, observe vacuum reading, should be around 14-18 ins/H20 AND steady, +/- 1 in.
Adjust dist for max vacuum, reset idle mixture screws for max vacuum may have to reset curb idle down to 700-800 rpm. Look down air horn and check to make sure there is no fuel dribbling from vacuum booster, if there is, lower fuel level on carb. Place rag over carb, does eng die? that's good no vacuum leaks. Now stand back and give a quick jab at throttle, hopefully, it'll catch and take off w/out backfiring. Check timing w/timing lite, probably find its too much for hot starting, retard timing and reset curb idle.

If carb been backfiring, you should check power valve for broken diapharm. Replace with one from book/application. If not known, then take vacuum reading and divide by 2, this will be a ballpark figure where to start. Also, install anti-backfire kit into base of carb. If possible, might even swamp distributor just to eliminate

Everett 68/359/PG 11.90/115mph

Did you check the firing order when you put on the new wires. This happened to me on a car I bought. Symptoms just like you describe. Went through the same steps you did. Changed to the new wires one for one never checking the firing order. As it turned out the previous owner had screwed up the order. Wanted to kick myself for not checking in sooner. Just another suggestion.

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Robert

'69 getting better every day... (every pay day)

[This message has been edited by rojo (edited 02-20-2001).]

[This message has been edited by rojo (edited 02-20-2001).]

Thanks for the suggestions. I'll have to check the slack in my chain that way.

I probably need to take a closer look at the distributor advance however, I've verified that the mechanical and vacuum advance is functional.

I'll need to take a closer look at vacuum leaks. I can stall the engine by putting my hand over the carb. It also worked for the 2nd carb. Some of the adjustments suggested are for curing a carb problem which I think I've eliminated a carburetor problem.

Wires are in the proper order.

If the timing gear was off a tooth, I would have a real rough idle to the point of bearly running, wouldn't I?

Dave

Tried to measure any slack in the timing chain. I didn't feel any. That crank is hard as hell to turn in the car.

Pulled all the plugs to check it. I noticed that plug #2 & #3 were carbon fouled. Rest looked normal.

Dave

I did talk to Summit tech last night and they went back to the carburetor. He thought my shooters were too small and I should check the accelerator pump and transfer slots. I asked about swapping carbs and he didn't think that matter since my friends carb wasn't tuned to my 396. I will try playing with the carb some more but I just don't think I should have gotten the exact same symptons with another carburetor.

Dave

If timing was off a tooth, engine would still run smooth, just slow to rev up under load.

Interesting plugs 2&3 carboned fouled. That means that particular side of carb rich. Cyls 2,3,5,8 are fed from the same side. Check for fuel dribbling from venturi booster, lower fuel level.

Next step I'd go is to replace plugs w/next hotter heat range or install a same heat range with a projected tip. Measure the resistance of each plug wire if not replace them w/good quality(brand name).

This would be involved, but ask the seller for the cam card, get a degree wheel, and check the valve timing, then you would know if cam has jumped a tooth or, was installed incorrectly.

Is this a gradual problem or happened overnight? What's vacuum reading? Valves adjusted correctly? Could be a bad ign coil.

When throttle is moved so slightly open, does the accelerator pump immediately pump fuel?

Be sure to check the integrity of the power valve, backfires generally destroy the diaphram in them, Holley book suggests HOL-125-105, 10.5 ins.

Let me how it turns out,

Everett 68/350/PG/11.90/115mph

V8 chevy, drivers side cylinders are 1,3,5,7 and passenger side,2,4,6,8, so cylinders 2 and 3 are on opposite sides of engine.Put in fresh plugs, check timing, adjust for highest rpm or vaccumn, and road test. Unless your engine is very radical, most Holleys work pretty good right out of the box without much tuning or jetting.

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69 rallye green X77 Z/28,1967 SS 396 Conv. 1974 c-10 454 swb

cylinders 2 and 3 are on opposite sides of the engine, but if he has a dual-plane intake, they are both fed from the same side of the carb.

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Justin
-1967 Camaro SS350

www-student.furman.edu/users/j/jtrauner

There are many sources that could be causing this as noted by the above posts. I tend to agree with you that it's not carb related, especially after you swapped carbs. I would zero in on the distributor and the amount of advance you are getting - or not getting. It sounds like when you hit the gas, the timing is not advancing or advancing too much and the motor just craps out. Check anything that has to do with the timing. Start with the correct firing order of the spark plugs, then check the intitial timing and total timing. These are cheap double checks. A car I was having problems with was getting too much vacuum leading to way too much advance so that's a possibility. You may need to pull the distributor and rebuild it. Your thought of swapping your distributor with one you KNOW is good is a great thought. Then go to the timing chain as this is the most involved and expensive. With my car, I finally had to drag it to shop and have a guy run some checks. It cost about $50.00 and has run great since.

Thanks for all the responses. I was out some this week but I will have the weekend to try some more troubleshooting.

I don't know if it makes a difference, but the backfire is very sharp and I'll get a flame through it very often when it backfires.

This weekend, I'm going to check for vacuum leaks, verify my vacuum advance holds(I can hear it when I apply vacuum to it), check for timing chain slack, and watch the springs with the engine running. Unfortunatley, I don't have access to a big block distributor.

I'll keep you posted.

Dave

Well, I played some more with the engine tonight.
I couldn't find any vacuum leaks spraying super starting fluid.

I checked for timing chain slack. Probably 2 or so degrees. It seemed pretty tight.

I verified the vacuum advance works. The distributor is seeing manifold vacuum all the time. Its connected to the port below the passenger fuel/air mixture screw.

Mallory has a voltage test to check the unilite module, performed that and it seemed to check out.

Put the timing light it. Read 10 degrees. Whoops, that was with the vacuum advance on. Pulled out the vacuum advance and plugged the port, timing nearly went to 0 degrees. Advanced it to 10 degrees. I put the vacuum advance back on, timing went to 21 degrees. Total timing was 36 degrees. Drove the car and it responded much better but still backfiring.

Went back to the house and advanced the timing to 14 degrees initial, 24 degrees with vacuum, and 40 degrees total. I haven't test drove it yet, but I can still backfire it through the carburetor revving it.

I did look down at my fuel filter and noticed it was only half full and was bubbling. (I have one of those clear filters). I think thats a separate and different issue but thought I'd mention it. When the engine cools, I'll tighten all the clamps around the fuel filter. I had the fuel filter out last week, so I may not have tightened it enough.

I'm going to get some rocker arm clips tomorrow. I do hear some rattling in the engine so I think I may have misadjusted one of the valves when I did it last week. I want to see the valves moving when the engine is running too.

Dave

Sounds to me that it's lean.Check the accel.pump.I cant see it being a valve spring cause it doesnt backfire,only on snap accel.Does it run smooth at idle?

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68 z28 ,68 rs 327 ,73 454 vette, 2 goofy kids

I could have started this post a couple of years ago. Sounds like the exact same problem I had with a 327. I wound up replacing just about everything. Still did it after a rebuild. Several different carbs and so on.
It wound up being the distributer and timing. Best check would be if you can get another distb. and try it. Check the advance both mech. and vac.

listen, I know that this may be way off base, but are you sure that there are bubbles in your fuel filter. I had the same problem when I put my 360 together. My car did not backfire much, but would not run at high rpm's under a load. I could rev the engine, and such, but no real drivability, and the motor/carb were brand new. Does your car die out at lights or if you let it sit for long periods of time while running? Your problems could be as simple as a bad fuel pump, or a hole in your fuel line. The logic behind this...think of how much gas is displaced by a bubble of air in your mixture... This would account for the car running poorly.

Hope this helps...be sure to let us know.

cfry

1st - Do you have a vacuum gage? How much vacuum at idle? From your description of the engine components you should see at least 15-17" at idle.

2nd - What is your total ignition advance and at what rpm is it all in by. Shoot for 36deg total all in by 2500rpm.

Since your compression is good/even on all cylinders - it sounds to me like carb tuning and/or ignition.

Well, here's an update for you guys.

Someone asked about if my shooters were getting a full shot. Yes they are and I'm able to get just a tad more when I push on the accelerator pump lever at full throttle.

I'm getting around 12 inches vacuum at idle.

I set the lash last weekend and I'm sure I've got the engine all screwed up now cause now I'm backfiring through the exhaust too. Backed off 1/2 turn on all the rocker arms. Now I can hear a little bit of tapping but still backfiring.

I tried playing with the vacuum advance some with no success. I moved the vacuum advance off the manifold vacuum and put it on the port on the side of the carb. I also tried running the car with the vacuum advance unplugged.

I pulled out #1 plug and stuck a screwdriver in it to see if TDC was close. It seems to be accurate.

I did discover my #2 cylinder is not firing. #3 is firing intermittently. I checked by putting the timing light on each wire. These were the same plugs carbon fouled, remember? #2 is worse than #3. I grinded away all the carbon and #2 still didn't fire. I haven't checked the gap but I set them at .040 thousandths when I put the new set in. I've got a set of hotter plugs I plan on putting in.

The gas in the fuel filter still bubbles even after I tightened all the connections. I noticed it only started bubbling when the engine got up to temperature. I looked down and the gas line comes within a 1/4" of one of the exhaust headers. I bet I'm boiling my gas. It sure gets hot down there. Some of the rubber on the boots stuck to the plugs. I've got a new set of wires coming but I wonder if my exhaust is TOO hot.?

Seems like to me, I have multiple problems going on. Aaarrrggghhhh.

Dave

[This message has been edited by Scooby Doo (edited 02-28-2001).]

on your two, or three plug, did you melt the plug wire, and possibly grounding out? are you getting any spark at all to the plug. could test this by disconnecting all the plugs but the one you are testing, put a spark plug in it, and ground out the housing of the plug. if no spark at all go from there.

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67 rs/ss 360 blue w/black vinyl top

Plug fires when out of the hole. Put it back in and its dead. Gap is .040 thousandths. Why would my plug not fire in the cylinder but reacts fine watching it?

Dave

sounds to me like you have bad wires. try putting a new plug and wire on the two cylinder and see if you are still having trouble. Might just me that you have melted the wire and are grounding out, or that there is a break in the wire, and you got lucky when it sparked outside the cylinder. If changing plug and wire does not help, you may need to look to your distributor.

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67 rs/ss 360 blue w/black vinyl top

It takes more spark energy to fire a plug under pressure (compression).
You might have a bad plug wire and it's crossfiring over to a nearby terminal inside the cap.
I'd put on new plug wires and a new cap and rotor.
David

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Check my web page for First Gen Camaro suspension info:
David's Motorsports page (http://ourworld.compuserve.com/homepages/David_Pozzi/)
First Gen Suspension Page (http://ourworld.compuserve.com/homepages/David_Pozzi/first_gen_suspension.htm)
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer

just a thought but is it possible you have a bad exhaust lobe or maybe a bad lifter? probably more likely a lifter if your running hyd. cam. just a thought, I've had that happen to me before. http://www.camaros.net/forum/confused.gif

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69 camaro 327/Th400

Had a real similar problem when I fist started my 396 after rebuilding it.
Try routing your spark wires so they are separate, don't touch each other.
Arley

New plugs & new wires cured my exhaust backfiring!! Engine sounds much healthier too.

I didn't drive the car but I was able to make it backfire through the carb once. I don't think that's gone away but I'll drive the car tomorrow and put a load on it to see.

Dave

Thought I'd let everyone know that I discovered my backfiring problem. I am so happy that I'm halfway to the kitchen now to grab a beer!!

It INDEED was the carburetor. I rechecked the fuel bowls tonight and saw my secondary fuel level was too high. Apparently the needle valve or o-ring is bad cause I had to adjust it just about all the way in! Apparently, I was flooding the gas to the engine and it couldn't take it. I don't know why I didn't see that before?!

Thanks to all for your valuable input. I've learned a lot trying to find this gremlin. Funny, I was so convinced that it was not the carburetor but sometimes we overlook the obvious.

Thank You!!
Dave

thx scooby doo read these post big help , may have 1 of my wires out of order , opps **** happens , probably did it in a hurry when i changed my vavle cover gaskets that was my first problem oil leaking , then the 2nd leak was around my dip stick , fixed them then it started back fireing , got to be my fireing order , **** its cold out in my garage yikes 22 today , ohio , thx eric