Can't stomp my 396 - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

Reply
 
LinkBack Thread Tools Display Modes
post #1 of 30 (permalink) Old Feb 19th, 01, 03:46 PM Thread Starter
Senior Tech
Dave
 
Join Date: Sep 2000
Location: Katy, TX
Posts: 482
Post

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).]
Scooby Doo is offline  
Sponsored Links
Advertisement
 
post #2 of 30 (permalink) Old Feb 19th, 01, 04:14 PM
Senior Tech
 
Join Date: Dec 1999
Location: Meridian, Idaho
Posts: 321
Post

It almost sounds like the timing chain is off a tooth. What do you all think?
camaroguy is offline  
post #3 of 30 (permalink) Old Feb 19th, 01, 05:40 PM
Senior Tech
 
Join Date: Mar 2000
Location: Crofton, MD
Posts: 126
Post

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
laabs is offline  
 
post #4 of 30 (permalink) Old Feb 19th, 01, 08:26 PM
Senior Tech
 
Join Date: Jan 2001
Location: skiatook, ok
Posts: 3,692
Post

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

------------------
375hp 78 Chevy truck
77 Chevy Nova
95 Chevy Lumina 3.4L
and building a 78 Nova
travis is offline  
post #5 of 30 (permalink) Old Feb 20th, 01, 12:55 AM Thread Starter
Senior Tech
Dave
 
Join Date: Sep 2000
Location: Katy, TX
Posts: 482
Post

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).]
Scooby Doo is offline  
post #6 of 30 (permalink) Old Feb 20th, 01, 01:28 AM
Super Moderator
Bess-68's rule
 
Everett#2390's Avatar
 
Join Date: Sep 2000
Location: Va Beach VA
Posts: 31,620
Garage
Cool

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
Everett#2390 is offline  
post #7 of 30 (permalink) Old Feb 20th, 01, 03:50 AM
Gold Lifetime Member
Robert
 
Join Date: Oct 2000
Location: Simpsonville, SC
Posts: 3,358
Post

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.

------------------
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).]
rojo is offline  
post #8 of 30 (permalink) Old Feb 20th, 01, 12:17 PM Thread Starter
Senior Tech
Dave
 
Join Date: Sep 2000
Location: Katy, TX
Posts: 482
Post

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
Scooby Doo is offline  
post #9 of 30 (permalink) Old Feb 20th, 01, 01:48 PM Thread Starter
Senior Tech
Dave
 
Join Date: Sep 2000
Location: Katy, TX
Posts: 482
Post

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
Scooby Doo is offline  
post #10 of 30 (permalink) Old Feb 21st, 01, 01:03 AM Thread Starter
Senior Tech
Dave
 
Join Date: Sep 2000
Location: Katy, TX
Posts: 482
Post

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

Scooby Doo is offline  
post #11 of 30 (permalink) Old Feb 21st, 01, 01:24 AM
Super Moderator
Bess-68's rule
 
Everett#2390's Avatar
 
Join Date: Sep 2000
Location: Va Beach VA
Posts: 31,620
Garage
Cool

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
Everett#2390 is offline  
post #12 of 30 (permalink) Old Feb 21st, 01, 02:34 AM
Senior Tech
 
Join Date: Nov 2000
Location: Pass Christian, MS
Posts: 294
Post

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.

------------------
69 rallye green X77 Z/28,1967 SS 396 Conv. 1974 c-10 454 swb
pipeman is offline  
post #13 of 30 (permalink) Old Feb 22nd, 01, 12:39 PM
Senior Tech
 
Join Date: Dec 1999
Location: Winston Salem, NC
Posts: 297
Post

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.

------------------
Justin
-1967 Camaro SS350

www-student.furman.edu/users/j/jtrauner
jrt67ss350 is offline  
post #14 of 30 (permalink) Old Feb 23rd, 01, 05:01 AM
Senior Tech
 
Join Date: Jul 1999
Location: denver, co USA
Posts: 407
Post

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.
denverRS/SS is offline  
post #15 of 30 (permalink) Old Feb 23rd, 01, 11:48 AM Thread Starter
Senior Tech
Dave
 
Join Date: Sep 2000
Location: Katy, TX
Posts: 482
Post

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

Scooby Doo is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Team Camaro Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address.
NOTE we receive a lot of registrations with bad email addresses. IF you do not receive your confirmation email you will not be able to post. contact support and we will try and help.
Be sure you enter a valid email address and check your spam folder as well.



Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome