Swapped #5 and #7 in firing order

[William H.]

Working on my stock 65 350HP last few days. Have been having problems with flooding and poor idle. Worked them out trash in needle/Seat and rest of idle circuits. But ever since I have owned it the car has run you out of the garage with a really bad smell after just a few minutes of running. I just always wrote this off as the side pipes and L79 Cam. My 68 L89 with aftermarket cam has always been the same way.
But during troubleshooting last night after I put in a new B26 VAC I was removing one wire at a time at idle and getting a drop from about 800 down to 4-500 RPM. This was consistent all the way around the cap until I got to #5 and the idle speed INCREASED about 50-100 when I pulled the wire out! did the same thing when I pulled out #7! SO then I swapped them and the speed went up another 50-100 to about 900-1000!
So, apparently For the last 8 years the wires have been crossed! I have not had time to remove the lower shields and ring out the wires to make sure it is currently correct. But I don't think I would get these results if they were correct originally and I now have them reversed.
So, I am wandering if the reversal had one of the cylinders so retarded or advanced that it was creating the bad smell? Now I wander if they are reversed on my L89?
I was just really surprised because I thought there would be way worse problems if they were reversed?
Although I know you have to be careful about running those cylinders wires close together as the spark from one can fire the other and hurt the motor because the spark is way to early or late.

Anyway the car sure runs allot better with the functional advance and the correct firing order
initial timing is about 12. At 2800 rpm without vacuum advance i HAVE 38 the Vac is adding 15-16 Degrees Crankshaft.

Any Comments?
Thanks
Bill

[William H.]

Re: Swapped #5 and #7 in fireing order

Hello again,
One other question. After the above work I went out for a test drive and had to race a 72 NOVA. I went up in smoke off the line and then couldn't get into second gear on first try so I lost !
I would have had him off the line though if I wasn't spinning so bad. I just left at idle dumped the clutch and stood on the gas at the same time, because I wasn't real serious so I really didn't expect to spin so bad. I guess I have a lot more torque now than I did.
I have factory wheels and hubcaps. BF goodrich radials that I think are 205/70 or 75/ 15
Any Idea about a radial tire that would fit the stock rim and get some bite? OF course these tires are 8 years old too and have only about 50 miles on them. So I may just need to get them hot a few times and shave off some tread to get some bite out of them.
I know before you say it no tire should be on the street if they are over 5 years old. BUT they have no cracking at all. have been inside all their life and only 50 miles. I don't think I can toss them yet. At least not untill I start to drive the car on a weekly or daily basis.
Thanks
Bill

[Duke W.]

Re: Swapped #5 and #7 in fireing order

Reversing the 5 and 7 wires would obviously cause both cylinders to misfire and should be obvious to most, but "if that's always the way it's been", like from the time you bought the car it could be misconstrued at normal.

It sounds like you've optimized the spark advance map. Getting all the centrifugal in at 2800 with a 12" B26 VAC without detonation is about as good as it gets, and with this setup the L-79 has a excellent combination of low end torque and top end power and revs with the convenience of hydraulic lifters, which is why I consider it to be the best "all around" small block configuration, and with air conditioning, it's a quick responsive car that you can drive anywhere, any time.

You should check idle speed/vacuum (VAC connected). Assuming a real L-79 cam best idle should be about 14-15" @ 750 with a slight lope, and the OE Holley idle screws should be about one turn out from the seat for best idle quality at 750.

The typical S-rated 205/75R-15 tires that most guys run have rock hard compounds (wear rating of typically well over 300) and lack the safety of a spiral wrapped nylon cap belt that will keep the tire from disintegrating if there is an internal structural failure. That's why I call them "van tires". The rock hard tread compound may last 50K miles, but they have little grip both in terms of cornering traction and off the line traction.

Most guys seem to be totally absorbed with cosmetic considerations and give little thought to safety and performance.

I am aware of only two available high performance tires in sizes that fit OE wheels that have about the same revs per mile as the OE tires. The V-rated 205/70R-15 Avon CR6ZZ, and V or H-rated Vredestein 215/70R-15 Sprint Classic, and both have the spiral wound nylon cap belt with relatively soft tread compounds for high grip. The Avon is considered a DOT legal racing tire with a wear rating of 80 and should be good for 10K normal street tires depending on how much time you spend attacking canyon roads or doing drag race launches. It's also available in a 215/70, which is only 0.1" larger OD so revs per mile are about the same as the 205, but it has about a inch wider tread and may interfere with the wheel well lips on OE wheels.

You can find more by doing a Web search to get the spec sheets and distributors/retailers.

There is no arbitrary age limit for tires. That's a myth! The age limit of a tire begins with the quality of construction and materials, and a high speed rated tire with the spiral wrapped nylon cap belt is a more durable design using more durable materials and is much safer over time than a cheap van tire. Then it comes down to service environment. For a typical vintage car that spends most of its time in a garage, the tires will have very limited exposure to high temperatures and sunlight, which slowly degrades tires (UV). The other enemy is ozone, so the car should not be stored near heavy electrical equipment.

There's no reason why under the above conditions a tire like the Avon could not be run to cord (assuming no wet road driving) even if it takes 20 years or more.

Duke

[William H.]

Re: Swapped #5 and #7 in fireing order

Thanks Duke!
I'm still thinking about Idle speed. Right now Throttle about 1/2 turn from fully closed it is idling 800-900 rpm. I am not sure as the car tach reads 800 and was just calibrated. Tune up tach says 900 but is old and not sure if it works with my ignition.(TI distributor connected to Hei module to trigger TI coil. Anyway the mixture screws are two turns out as this is where Holley says to start. IF I turn them in the idle speed will drop to 750 i am sure but I thought we want them to be turned open until no more increase in idle speed/vacuum. I believe that right now I have about 15" vacuum at the current idle speed. SO I don't know if I should reduce the idle speed by turning in the mixture screws or maybe retarding the timing some. I will play with it some more and advise of the idle speed /vacuum readings.
Thanks for the tire info Ill look into it some more. I'm not really planning on doing allot of racing. But sometimes opportunities arise and i don't like to loose. Of course I have no Idea what was in the nova. It sounded healthy and had much wider tires. Probably a 350 but maybe a BBC or LS who knows.
Thanks so much for all the fine advice you give here. I really enjoy your posts and always learn a great deal from them. Please keep it up and don't be discouraged by people asking the same things over and over again. Sometime we have to have information presented to us at just the right time for it to "Click" and sink in. You are a great asset to our community. Keep it up!
Bill

[Dick W.]

Re: Swapped #5 and #7 in fireing order

Reversing the 5 and 7 wires would obviously cause both cylinders to misfire and should be obvious to most, but "if that's always the way it's been", like from the time you bought the car it could be misconstrued at normal.

It sounds like you've optimized the spark advance map. Getting all the centrifugal in at 2800 with a 12" B26 VAC without detonation is about as good as it gets, and with this setup the L-79 has a excellent combination of low end torque and top end power and revs with the convenience of hydraulic lifters, which is why I consider it to be the best "all around" small block configuration, and with air conditioning, it's a quick responsive car that you can drive anywhere, any time.

You should check idle speed/vacuum (VAC connected). Assuming a real L-79 cam best idle should be about 14-15" @ 750 with a slight lope, and the OE Holley idle screws should be about one turn out from the seat for best idle quality at 750.

The typical S-rated 205/75R-15 tires that most guys run have rock hard compounds (wear rating of typically well over 300) and lack the safety of a spiral wrapped nylon cap belt that will keep the tire from disintegrating if there is an internal structural failure. That's why I call them "van tires". The rock hard tread compound may last 50K miles, but they have little grip both in terms of cornering traction and off the line traction.

Most guys seem to be totally absorbed with cosmetic considerations and give little thought to safety and performance.

I am aware of only two available high performance tires in sizes that fit OE wheels that have about the same revs per mile as the OE tires. The V-rated 205/70R-15 Avon CR6ZZ, and V or H-rated Vredestein 215/70R-15 Sprint Classic, and both have the spiral wound nylon cap belt with relatively soft tread compounds for high grip. The Avon is considered a DOT legal racing tire with a wear rating of 80 and should be good for 10K normal street tires depending on how much time you spend attacking canyon roads or doing drag race launches. It's also available in a 215/70, which is only 0.1" larger OD so revs per mile are about the same as the 205, but it has about a inch wider tread and may interfere with the wheel well lips on OE wheels.

You can find more by doing a Web search to get the spec sheets and distributors/retailers.

There is no arbitrary age limit for tires. That's a myth! The age limit of a tire begins with the quality of construction and materials, and a high speed rated tire with the spiral wrapped nylon cap belt is a more durable design using more durable materials and is much safer over time than a cheap van tire. Then it comes down to service environment. For a typical vintage car that spends most of its time in a garage, the tires will have very limited exposure to high temperatures and sunlight, which slowly degrades tires (UV). The other enemy is ozone, so the car should not be stored near heavy electrical equipment.

There's no reason why under the above conditions a tire like the Avon could not be run to cord (assuming no wet road driving) even if it takes 20 years or more.

Duke

Tell that to one of my friends that put new (to him) Michelins on his pickup. Loaded is with about 1,000# and blew one in less than 300 miles. Put the spare on and blew another tire in less that another 100 miles. Went back to the tire dealer, come to find out they had been stored somewhere and were over nine years old. My experience has been anything over five or six years, and driving at interstate speeds, you are on your own. I have blown several Michleins over the years, all were aged out in my opinion.

[Mark E.]

Re: Swapped #5 and #7 in fireing order

It's incredible that reversing #5 and #7 didn't cause an obvious, intolerable misfire. The engine would probably run better with both cylinders disconnected.

[Duke W.]

Re: Swapped #5 and #7 in fireing order

Tell that to one of my friends that put new (to him) Michelins on his pickup. Loaded is with about 1,000# and blew one in less than 300 miles. Put the spare on and blew another tire in less that another 100 miles. Went back to the tire dealer, come to find out they had been stored somewhere and were over nine years old. My experience has been anything over five or six years, and driving at interstate speeds, you are on your own. I have blown several Michleins over the years, all were aged out in my opinion.

Michelin makes cheap, low speed rated "van tires" that lack a spiral wound nylon cap belt, too, and truck tires are a different animal than a Corvette because trucks can be easily overloaded, and most people never give a thought to regularly checking tire pressure and using an appropriate pressure when carrying heavy loads.

I've experienced three tread separations on race tracks. In all cases the car developed a serious vibration, so I went back to the paddock and found large bulges in the tread area. Obviously there had been internal structural failures, but they were all H or V speed rated and had the spiral wound nylon cap belt that keep the tire in one piece holding air.

That's why I always harp about buying high speed rated tires (at least H) with a spiral wound nylon cap belt because it will usually keep the tire from disintegrating in the event of an internal failure... doesn't matter if you never intend to achieve the speed rating. It's a safety issue, but most guys are primary concerned with cosmetic issues and don't give a thought to safety.

Duke

[Duke W.]

Re: Swapped #5 and #7 in fireing order

In my experience typical L-79 idle behavior is 750 @ 14- 15" with a slight lope and the OE carb idle mixture screws one turn out from the seat and total idle advance in the mid to high twenties. If you increase idle speed to achieve at least about 16" then the idle smooths out.

If you can't achieve this then I suspect either a different cam with more overlap or some mechanical issue.

The idle speed/mixture adjustment process is iterative. Set the emulsion screws at the shop manual recommended initial setting. Then set idle speed to your target, and that should be "the lowest speed that yields acceptable idle quality" which is a subjective call, but 750 is a good target if the engine has a real L-79 cam.

Move both screws in an out in quarter turn increments to find the highest idle speed/manifold vacuum. If idle speed changes, reset to the target and do the procedure again. Once there is no change in idle speed you're done at both idle speed screws should be out the same number of turns from the seat.

Duke

[Mark E.]

Re: Swapped #5 and #7 in fireing order

Thanks for the tire lesson, Duke. Sounds like good advice.

Now I need to Google "spiral wound nylon cap belt". ��

[Duke W.]

Re: Swapped #5 and #7 in fireing order

I don't think that will get you what you want, but if you Web search the size, like 215/70R15 you should get some hits. Then go down the list and look for H or V in the service index. Virtually all tires speed rated H and above have the spiral wound nylon cap belt.

You can also search that size on the big retailer Web sites like The Tire Rack and manufacturers Web sites.

You might find a couple of H-rated tires in that size, but they may be brands you've never heard of.

And there's always the previously mentioned Avon or Vredestein to fall back on.

Duke

[Mark E.]

Re: Swapped #5 and #7 in fireing order

I don't think that will get you what you want, but if you Web search the size, like 215/70R15 you should get some hits. Then go down the list and look for H or V in the service index. Virtually all tires speed rated H and above have the spiral wound nylon cap belt.

You can also search that size on the big retailer Web sites like The Tire Rack and manufacturers Web sites.

You might find a couple of H-rated tires in that size, but they may be brands you've never heard of.

And there's always the previously mentioned Avon or Vredestein to fall back on.

Duke

Google suggests a nylon cap belt is not uncommon. Is that what you're looking for? Or is "spiral wound" a unique feature?

[Duke W.]

Re: Swapped #5 and #7 in fireing order

Spiral wound means the belt is a continuous nylon multi-filament thread wound around the entire tread section in a spiral manner, so there are no butt joints. Pound for pound nylon is stronger than steel and its this strength and the continuous wind that will keep the tire from disintegrating in the event of an internal structural failure. Steel belts are heavy, and if not contained can separate due to centrifugal force at high speed, which is why nearly all tires speed rated H or above have one or two spiral wound nylon cap belt(s).

Any tire spec sheet like what you find at The Tire Rack and manufacturers' Web sites should include the summary construction architecture. It's also molded into the tire.

The service description after the size designation contains two or three numerals, which is the "load index" followed by the speed rating letter. You need a conversion table to convert the load index into the actual maximum load, but the actual max load spec at max cold tire pressure is also clearly spelled out in the spec sheets and molded into the tire sidewall, often in both English and metric units.

The third piece of useful information is the UTQG (Uniform Tire Quality Grading) code - numerals followed by two letter codes. The numeral is the wear index and if you're looking for high grip, the lower the better. A good summer high performance tire should be no more than about 300 and DOT legal racing tires are usually 100 or less.

The next field is the wet braking traction rating -- AA, A, B, or C, and the following letter is the temperature rating, A. B, or C which is the ability of the tire to tolerate high temperature, and any H or above tire should be A.

A few years ago I bought what were probably the last new 225/70R15 Firestone PV41 police pursuit tires on the planet, and the following is molded into the sidewall:

225/70R15 100V
Tread 2 ply polyester + 2 steel + 2 nylon / Sidewall 2 ply polyester
Treadwear 340 Traction A Temperature A
Max load 1755 lbs @ 44 psi max cold pressure.

Note that four of these tires max load capacity is nearly double the gross weight of a typical C2 Corvette, or what I call 100 percent load margin. The higher the better, and I like to see at least 50 percent on all my cars.

You can pretty well characterize a tire's performance characteristics and designed in safety features by reading spec sheets and looking at the data molded into the tire's sidewall.

Duke

[William H.]

Re: Swapped #5 and #7 in fireing order

Hi Duke, I finally got around to looking at my BF goodrich's
They are 215/70R15 97T not 205 and the tread wear is 640!!! no wander they spun. Temp and traction are both A so that is good and they have 1 nylon plie on the tread so I guess they are OK until I can afford some better ones like the AVON's you mentioned.
Bill