Hello all,
I know that there are many very knowledgeable folks on this forum so
I am posting this in an attempt to solve an issue with my '67 coupe.
Here are the details.
L79, completely stock with the exception of a Tremec 5 speed.
I just finished a complete tune up on it.
New parts installed are: correct points, condenser, R43s plugs, new OEM AC Delco plug wires, new ac delco correct coil, new correct coil wire.
I have a new distributor cap that I have not yet installed, but the old one appears to be fine.
Timing set at 12, dwell at 30.
The car starts and runs just fine. The issue is when I drive it. Just driving halfway around the block in my neighborhood, it will backfire loudly from the passenger's side. (Side pipes)
I have no idea what is causing this. I've tried adjusting the timing a bit in both directions, to no avail.

Any suggestions?

Thanks!

 

Sounds like either a lean carb or a timing problem. When was the last time the carb got some attention? Have you checked your timing at 2800 RPM? Is the bushing installed on the vacuum advance can still working? Is the vacuum advance can still working? Just a few things to check. Others will chime in. Jerry

 

R43s are too cold.

 

How are your ignition wires? Look at cyl 6 and 8 by the boots.

 

Hi Jerry,
the carb is actually a new Holley I installed last December while the factory Holley was being rebuilt by Holley.
The car runs so good with it, that I never put the original Holley back on the car when Holley shipped it back!!
They had given me an eight week window for return and I didn't want to go that long not being able to drive my car,

I have not checked the timing at 2800, only at idle.
The vacuum advance was working fine before I did the tune up, so I'm hoping it's still ok.

Thank you for your input!!
Looking forward to driving my car again.

Which do you recommend? R45S ?

 

All the wires are brand new. Will check them though!!

Thank you!

 

I have no idea......................but one time my heat riser got stuck and my 67 ran bad.

 

Check for exhaust leaks around the manifolds.

 

Thank you.

Thank you.

 

Your timing should be set to 10* BTDC, not 12*

It also sounds like you have two plug wires switched. My guess is cylinders 4 & 6

 

just because parts are new does not make them good or better then what you replaced. what was the reason for all the whole sale changes? of all five classic cars my dad and I have they all have 50 odd year old coils. plugs and wires should be good for 50k miles. unleaded gas will make your plugs run a long time. points are good for at least 10-15k miles.

 

Thanks for your question and perspective.
You make a good point and I don't disagree, but in my case new IS better.
I just want my car to run smoothly.

It's wonderful that you enjoy your classic cars with your dad.
I sure love my '67.
I love tinkering with it, cleaning it, and most of all driving it!
I've had it for over 18 years. The coil and wire were on there and old when I bought it so I don't really know how old they were.
I started to just clean up my 12 year old plugs, then installed new points and a condenser.
The clip from the coil wire that goes into the points was just barely hanging on so it was time for a new coil wire.
I needed to remove the distributor to do that. It then became a case of "as long as I'm here" along with I've got the time and it'll be fun, so why not?
It was a snowball effect. Gotta admit, I enjoyed it.
I'll get this little hiccup sorted out!
The car is therapy for me..



Thank you,

 

beside the coil wire being bad I see no reason to change everything else those parts don't go bad sitting around. tinkering is a very bad habit. just drive it until it breaks. then fix it and move on. preventive maintenance is a good thing too. if it was me I would double check the plug wires and timing like said above. then swap back the coil and see what happens

 

I have found that it is not good to do so many changes at once because when there is a problem pop up , you don't know which one to start looking at. Replace one thing at a time and drive it for a while . Yes , tinkering can get out of control !

 

You said you removed the distributor it could be off a tooth. I would also check spark plug wires as mentioned above.

 

I would swap back in your old coil, I have had 2 of the new coils go bad.

 

If only the right side is affected a compression test might be done to check for a possible leaky exhaust valve after firing fuel mixture in the exhaust pipe when in decel.
John

 

After firing in the exhaust is typical of too rich a fuel mixture. Backfiring through the carb is often indicative of a lean carb mixture. Of course other issues can contribute - timing, ignition weakness, etc.

if it’s after firing, try increasing your idle fuel screw on that side 1/2 turn out and see if it improves any. If no help then you may be into the primary circuit when it happens and need increase your jet size a step.

It’s a pretty blurry line between tinkering and preventive maintenance. Often tinkering or probing around finds something that needs maintenance or repair. So tinker away.

Wow - wrote this before my morning coffee. I had a lean carb condition/ backfire last year and had to increase the primary jetting to correct. Guess I had that in the back of my mind when I answered. Actually, If it is after firing due to a rich mixture, depending on if you are in the the transition circuit, or more likely the primary, you would need to reduce the fuel to air ratio - probably step down a couple jet sizes and see if it helps.

Need to start coffee regimen earlier.

 

Lots of good advice here.
The fact that it is backfiring only on the passenger's side and that it just set for an extended period of time, leads me to agree you should check the heat riser valve in the exhaust on the passengers side to assure it is not frozen closed.
Also the plug wires being crossed is plausible as the shieling makes it very difficult to verify correct routing under the shielding. I use my VOM meter to verify the correct wire.
A backfire would indicate unburned gas in the exhaust, but can be from either excessive fuel from a dead cylinder on the passenger's side due to electrical or fuel delivery. A cracked distributor cap, arc trace on the rotor, etc. Yet these should give you a rough idle also. Sticking mechanical advance weights or vacuum advance might contribute, but not isolated to one side of the engine.
Burned or misadjusted valve, sticking lifter, timing chain slack, etc. should give you an audible warning.
I had a Dodge one time that would misfire and die when turning a corner to the left. The coil was shorting out to the firewall on left hand turns. Have you removed your shielding to verify it is not shorting out the ignition?
As NowWhere Man stated, I went thru the parts changing shot in the dark wrenching on R66 trying to find a misfire. (carb rebuild, plugs, points, cap, wires. etc.) Only to find the problem was idling in the shop carboned up the AC43 plugs. I took it out for an aggressive 10 minute drive and it idled like a kitten. I know better than throwing parts, but got stupid with frustration. I also agree the AC43 plugs are much too cold for the street. I run R45S in both of our cars.
New parts can be defective, especially with new CHINA parts in my opinion. Thus, changing the parts one at a time and reinstalling the used part that makes no improvement is generally a good way to isolate a problem.

 

I agree. After I had done some tune work I had the same issue. Turned out to be a blown exhaust manifold gasket. I never heard the ticking noise because after I was done I never ran the car with the hood open. Then one day...