Ottawa, Ontario
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We had the privilege of tuning a gorgeous, unrestored, alloriginal 1969 Mustang Shelby GT500 with a 428 Cobra Jet engine. This classic came in with only 32,300 original miles on the odometer and even featured an original interior in mint condition.

The owner even had the original sales receipt for $5,115.40. The receipt shows that the original purchaser ordered this GT500 in Wimbledon White, with a TractionLox differential, sport deck rear seat, forced ventilation, and AM radio and XHD suspension. A Marti report proves that of the 1,982 fastbacks Shelbys produced in 1969, the trim package on this car was oneofakind with its Wimbledon White paint, 4284V Cobra Jet V8, 4speed close ratio manual transmission, black knit vinyl Mach 1 bucket seats, 3.50 TractionLox rear axles, sport deck rear seats and power ventilation. Built in Dearborn, Michigan, on May 8th, 1969, it was shipped by rail and delivered to the customer just 5 days later.

But service records show that from day one, this car blew a haze of smoke out the exhaust. Despite repeated attempts by previous owners and the current owner to resolve this issue, the problem persisted, which was the primary reason it was brought to us.

As always, we started with a few baseline passes to see what kind of power was being made and to collect some AFR data. During the testing, we saw clouds of smoke billowing from the exhaust, which increased with every pass. The power output of the best baseline pass was 292.95 lbsft of Tq and 216.58 HP at the wheels. This car was initially rated for 335 HP at the crank, which translates to about 280290 HP at the wheels so right off the bat, we saw performance wasn't even close to where it should be.

During the baseline testing, we also measured the AFR at 100 RPM increments from 2100 to 4500 RPM. The data showed a corrected AirtoFuel Ratio range of 12.39:1 at 2200 RPM to 11.51:1 at 4500 RPM. These results were not too far off and probably shouldn't have amounted to the billowing we saw; nevertheless, we'd need to pull some fuel from this overlyrich mixture.

On our first set of adjustments, we reset the lowspeed air adjustments, set the front float to 90 degrees, changed the front main jets from number 69s to 66s, replaced the gasket on the power valve with a thinner one, and replaced the power valve itself with a 6.5. We also checked the timing and found the values to be within tolerance.

After these changes, we ran her on the dyno again and saw slightly less smoke from the exhaust, some improved Tq and wHP numbers, a more consistent power curve, and leaner fueling. Our next step was to adjust the timing from 10 and 28 degrees to 14 and 32 degrees and make another pass on the dyno.

This time, the dyno test showed a more significant performance improvement, clocking in 358.68 TQ and 252.70 HP at the wheels. A respectable gain from the baseline of 65.73 (22.4%) Tq and 36.12 (16.7%) wHP. We also observed that the smoke wasn't coming into play until about 3800, so we removed the rear bowl and swapped the number 79 main jets to 75s.

Our third testing pass demonstrated even more power at 394.85 lbsft Tq and 292.41 wHP gains of 101.89 lbsft and 75.83 wHP from the baseline. That's a 35% improvement for both torque and horsepower. Translating this wheel HP to crank HP, we were now just above factory spec at about 340 cHP. While these changes eliminated the smoking at the lowmid range, a haze continued to appear at 4200 RPM. We decided to install a set of smaller accelerator pump nozzles the thinking being that once the engine gets up to speed, it draws fuel like a venturi on the accelerator pump nozzle and drips into the combustion chamber.

After installing a smaller 20 cc nozzle and backing it off a little at the lowspeed NO SMOKE! The final dyno test results were 403.01 lbsft Tq and 299.64 wHP with AFR now ranging from 12.6:1 to 12.8:1 with timing at 14 and 32 degrees. The overall performance improvement was a respectable 110.05 lbsft and 83.06 wHP or 37.6% and 38.4% respectively.

Just another day at the office and another happy customer!

LINK How to Tune a Carburetor:    • How to Tune a Carburetor | stepbyst...  
WEBSITE: https://internationaldynoauthority.com
ARTICLE: https://www.autoevolution.com/news/pr...

0:00 intro
1:08 under the hood
2:04 baseline testing
2:30 baseline analysis
4:42 initial steps
9:33 1st test passes & results
11:47 timing adjustments
12:11 2nd test passes & results
13:49 more tuning adjustments
14:02 3rd test passes & results
16:00 4th test passes after tuning
16:12 Final results & conclusion