1965 L79 Just started back firing

Mr. Cricket

Mr. Cricket

Committee Member
teamzr1 said:
IF the spark plug model # has an R in it, then it has a resister
Also keep in mind sparkplug wires can come with also an RFI reducer, so not mix R with each other

At the moment the spark jumps the gap, it causes a high frequency burst of energy, known as RFI (radio frequency interference). RFI, as its name suggests, creates static on your radio and interference with other electronic equipment, including the vehicle’s on-board electronic control units (ECUs) and be heard with radio output
The resistors are there to smooth out the “arc” that a spark plug makes as it fires.

Resistor plugs were developed in the 1960s to suppress some of the spark energy, thus lowering RFI to an acceptable level.
Most resistor spark plugs use a monolithic resistor, generally made of graphite and glass materials, to filter the electrical voltage as it passes through the center electrode.

Since resistor type plugs actually “resist” some of the spark energy, non-resistor type plugs actually deliver a more powerful spark.
It is for this reason that most racing plugs are non-resistor types.
However, in most automotive applications, a resistor plug is required for proper vehicle operation.
Use of non-resistor plugs in vehicles that call for a resistor type can result in rough idling, high-rpm misfire, and abnormal combustion.

Typically, the resistor in the center electrode of a resistor spark plug will be around 4000 to 6000 ohms.
To check the spark plug resistance, you can use a multimeter and measure the ohm reading by placing the leads on the terminal and the tip.

So in short if the car has limited electronics and want the highest spark then do not use R type plug or wires
If the RFI affects older car radios then use R rated plugs or plug wires but not both at the same time
Click to expand...
Thanks Team. That's pretty much what I found on line this morning and clearly being a '65 it doesn't have any electronics to worry about and I doubt I'll ever use the radio or if it even works. In the conversation with Pertronix he was adamant that with the flame thrower 2 installed in the car it should have the non resister plugs and he sold me a set of HT leads as well to be sure as I couldn't answer what type are on there already.
 
teamzr1

teamzr1

Supporting vendor
Mr. Cricket said:
Thanks Team. That's pretty much what I found on line this morning and clearly being a '65 it doesn't have any electronics to worry about and I doubt I'll ever use the radio or if it even works. In the conversation with Pertronix he was adamant that with the flame thrower 2 installed in the car it should have the non resister plugs and he sold me a set of HT leads as well to be sure as I couldn't answer what type are on there already.
Click to expand...

As you see the GM design specs ( Attached below as full engineering spec PDF) show non resistor plugs and wires
65c2.jpg

Read/see all design for 65 Vette
 

Attachments

  • 1965Corvette.pdf
    3.3 MB · Views: 1
UK67

UK67

CCCUK Member
Mr. Cricket said:
Only owned it a couple of months so only managed a couple of short trips but it ran flawlessly. Then just like flicking a switch it's now difficult to start, to keep running and huge back fires going on so much so I'd be reluctant to drive it to my local garage.

New house and garage and no heat yet in the garage and one day last week the place was ringing wet through with condensation which I've now got a handle on with a small dehumidifier.

Checked all plugs leads are on.
Checked the distributer and no signs of any dampness.
Installed new battery.
Checked the fuel to make sure I hadn't put diesel in and I haven't and can't see any signs of water in the tank.

And thinking back when I discovered the condensation issue I ran this and another Corvette for maybe 40 minutes to get rid of any dampness I couldn't wipe down myself and they ran fine.

Any other ailments that would show as a back fire? Suppose I should check / change the fuel filter?

Stuart
Click to expand...
I started having a similar problem with my L79 engine last October. The odd back fire plus, like you, had to just feather the pedal to just to keep moving. Although it would always start okay and run fine for maybe 10 minutes sometimes 20 mins. I always use the Esso premium fuel.
I started checks which didn't cost anything i.e. blocked fuel filter, carb off stripped and thoroughly cleaned, blew air through the fuel feed to the tank, looked in the tank with a torch for obvious gunk.
I then changed just about all the ignition side in steps each time going for a test drive :-
Plugs 😣
Plug leads😣
Distributor cap☹️
Coil😩
Highly unlikely but running out of ideas so changed Pertronix unit.😢
Nothing worked 😤

Winter approaching so with an additional cost of just a few gaskets I decided to remove the heads and give them an overhaul with new stem seals etc
My short test drive proved okay but will not be convinced until a longer one!
Maybe I had a vacuum leak at the inlet I don't know or maybe it was contaminated fuel ??
All I can conclude is I know what wasn't causing the issue 😬
 
Mr. Cricket

Mr. Cricket

Committee Member
UK67 said:
I started having a similar problem with my L79 engine last October. The odd back fire plus, like you, had to just feather the pedal to just to keep moving. Although it would always start okay and run fine for maybe 10 minutes sometimes 20 mins. I always use the Esso premium fuel.
I started checks which didn't cost anything i.e. blocked fuel filter, carb off stripped and thoroughly cleaned, blew air through the fuel feed to the tank, looked in the tank with a torch for obvious gunk.
I then changed just about all the ignition side in steps each time going for a test drive :-
Plugs 😣
Plug leads😣
Distributor cap☹️
Coil😩
Highly unlikely but running out of ideas so changed Pertronix unit.😢
Nothing worked 😤

Winter approaching so with an additional cost of just a few gaskets I decided to remove the heads and give them an overhaul with new stem seals etc
My short test drive proved okay but will not be convinced until a longer one!
Maybe I had a vacuum leak at the inlet I don't know or maybe it was contaminated fuel ??
All I can conclude is I know what wasn't causing the issue 😬
Click to expand...

Well that does sound awfully familiar and hopefully you've cracked.

Good news that the new Pertronix module due 7th Feb is now coming tomorrow so that completes the set:
New battery - check
New coil - check
New module - check
New plugs - check
New HT leads - check
Timing light - check
Endoscope camera - due tomorrow
A mate joining me that has been around the block on these things and restored his SWC
 
Roscobbc

Roscobbc

Moderator
If there are any issues with plug fouling etc the resistor plugs can potentially amplify any issues with HT distribution.
On the basis that electricity (and especially HT) follows the path of least resistance, in my experience issues like HT jumping the gap, say between heat damaged spark plug boot and a header flange (coupled perhaps with high resistance from one or more fouled spark plugs) can cause the spark to one or more plugs to be 'shared' between the fouled plug and where ever the HT lead is earthing to. The net result is a significantly reduce spark to the fouled plug or plugs and even damage within the distributor cap as the HT struggles to find an earth to dispel its energy.
Even with 'good' plugs the additional fuel loading within a cylinder when accelerating can be enough to severely reduce, even extinquish a 'shared' spark within a cylinder.
My engine with its 'lopey' cam really needs quite a rich mixture to idle properly - I routinely change the plugs every couple of years irrespective of mileage as they will carbon-up. I will get an odd one causing backfiring in the exhaust system untill it either clears or goes open circuit. Not saying any of this is the issue with yours but it would be interesting to crank the engine over in the dark to see if there is any 'tracking'.
 
teamzr1

teamzr1

Supporting vendor
If wanting to reduce carbon buildup from valves, plugs, intake manifold and cylinders is installing a water/methanol kit
All my Corvettes since early 1980s have been using this
The waterside will steam clean while the Methanol will increase octane level, colder charge to cylinders and being overall
this reduces engine knock would allow adding more timing to induce more torque.

For older carb engines, the water/meth can be triggered by vacuum and nozzle installed to face downward of carb

Best case running a 50/50% mix of water and Methanol and can even user cheaper blue colored winter windshield washer fluid
that has 38% methanol in it
In fact, I use the windshield washer tank to wash windows and inject the water/meth
 
Forrest Gump

Forrest Gump

CCCUK regional rep
Roscobbc said:
If there are any issues with plug fouling etc the resistor plugs can potentially amplify any issues with HT distribution.
On the basis that electricity (and especially HT) follows the path of least resistance, in my experience issues like HT jumping the gap, say between heat damaged spark plug boot and a header flange (coupled perhaps with high resistance from one or more fouled spark plugs) can cause the spark to one or more plugs to be 'shared' between the fouled plug and where ever the HT lead is earthing to. The net result is a significantly reduce spark to the fouled plug or plugs and even damage within the distributor cap as the HT struggles to find an earth to dispel its energy.
Even with 'good' plugs the additional fuel loading within a cylinder when accelerating can be enough to severely reduce, even extinquish a 'shared' spark within a cylinder.
My engine with its 'lopey' cam really needs quite a rich mixture to idle properly - I routinely change the plugs every couple of years irrespective of mileage as they will carbon-up. I will get an odd one causing backfiring in the exhaust system untill it either clears or goes open circuit. Not saying any of this is the issue with yours but it would be interesting to crank the engine over in the dark to see if there is any 'tracking'.
Click to expand...
Do you use non resistor plugs then Ross?
Looking at the “Plug finder” for some of the spark plug manufacturers, it seems to always bring up a “resistor“ plug for Corvette applications, even if you put in a 1960’s Corvette.
I wonder if the manufacturers are trying to phase out not resistor plugs, logic being that they can cause more problems with electronics interference, and resistor plugs will work in just about all applications with negligable performance loss.
I bet thousands of cars with Pertronix ignition are running resistor plugs.
 
Mr. Cricket

Mr. Cricket

Committee Member
Forrest Gump said:
Do you use non resistor plugs then Ross?
Looking at the “Plug finder” for some of the spark plug manufacturers, it seems to always bring up a “resistor“ plug for Corvette applications, even if you put in a 1960’s Corvette.
I wonder if the manufacturers are trying to phase out not resistor plugs, logic being that they can cause more problems with electronics interference, and resistor plugs will work in just about all applications with negligable performance loss.
I bet thousands of cars with Pertronix ignition are running resistor plugs.
Click to expand...
A long-time member and C2 owner that also has changed from points to electronic called me today to say he has always run non resistor plugs without issues. The Pertronix guy I spoke to was adamant that resistor plugs would first break down the module and then the coil.

I'm committed now so we'll see. One tip I got from plug man was clean your old plugs with oven cleaner x 3 and rinse with water. Use no abrasive materials as the electrodes aren't glazed these days. Probably a good idea to make sure they're properly dry before putting them to use though 💥
 
Roscobbc

Roscobbc

Moderator
Forrest Gump said:
Do you use non resistor plugs then Ross?
Looking at the “Plug finder” for some of the spark plug manufacturers, it seems to always bring up a “resistor“ plug for Corvette applications, even if you put in a 1960’s Corvette.
I wonder if the manufacturers are trying to phase out not resistor plugs, logic being that they can cause more problems with electronics interference, and resistor plugs will work in just about all applications with negligable performance loss.
I bet thousands of cars with Pertronix ignition are running resistor plugs.
Click to expand...
IMG_20230124_203306.jpg
www.gsparkplug.com

NGK BCP7ES 5030 Spark Plug Copper Core

Buy NGK BCP7ES 5030 Spark Plug Copper Core (14mm Diameter) Today. See our great range of NGK Products Online Here.
www.gsparkplug.com www.gsparkplug.com
Not saying this is suitable for any other engines - just what I have found best for more mine - did originally use same family plug but two heat ranges colder.....
 
Forrest Gump

Forrest Gump

CCCUK regional rep
Mr. Cricket said:
A long-time member and C2 owner that also has changed from points to electronic called me today to say he has always run non resistor plugs without issues. The Pertronix guy I spoke to was adamant that resistor plugs would first break down the module and then the coil.

I'm committed now so we'll see. One tip I got from plug man was clean your old plugs with oven cleaner x 3 and rinse with water. Use no abrasive materials as the electrodes aren't glazed these days. Probably a good idea to make sure they're properly dry before putting them to use though 💥
Click to expand...
Oh yes, definitely follow their advice, perhaps the wrong plugs have indeed damaged your ignition.
 
Mr. Cricket

Mr. Cricket

Committee Member
Well bit of a mystery as we only changed the plugs to the non resister type. Graham came armed with old school points, condenser, starter motor but none were needed. Chris brought a 1965 carb technical guide. No change at first but one last crank before the start to change other parts with the go peddle hard down and it spluttered into life then ran sweet. Put it all back together and went for a short 5 mile trip and it ran absolutely fine fingers crossed 🤞

Massive thanks to Chris Sale & Graham Knott both C2 owners and members.

 
Mr. Cricket

Mr. Cricket

Committee Member
teamzr1 said:
That is good news !

You think then it was the type of spark plugs installed ?
Click to expand...
we're pretty stumped. The lead into it getting as bad as it did back firing like crazy was one or two pops that then got progressively worse so I'll need a few miles of no back firing to be confident with it. Timing was spot on just slightly advanced.
 
Roscobbc

Roscobbc

Moderator
Might be worth while checking fuel pressure. Previous use of the car with ethanol fuel content could partly damage diaphragm within the pump......not sufficient to stop pump partly working under normal low demand conditions but would perhaps fail to deliver sufficient fuel when giving it stick.....or maybe when cranking engine over with empty fuel bowls......had it happen to me half way up a Kop Hill climb run.......
 
You must log in or register to reply here.
Top