I just recently rebulit my Rochester Carb, changed plugs, oil, etc. We also set the dwell, and timing. The problems is, when I accelerate, and push the clutch in to shift gears, the car backfires (when switching from 1st to 2nd). It doesn't backfire with the 2nd to 3rd shift though. It dosen't backfire on decelleration. Where to begin?????


Any help would be greatly appreciated.

 

Is it backfiring or afterfiring? Backfire is up through the carb. Afterfire is out the exhaust... Causes are different.

 

It is afterfiring....

 

Afterfiring can be caused by timing, but can also be due to carb being a tad rich (discharging unburnt fuel into the exhaust system). Make sure you set the timing up to 36 degrees total, and make sure your vacuum advance control unit is working & hooked up right. On a '74, the idle mixture screws should be set at about 2 turns out, and your float level should be at 3/8". If any of these parameters are significantly off, you may get some afterfiring popping in the exhaust.

 

Thanks lars, I'll check it out when I get home.

 

I have a question Lars... How do I set the timing to 36 degrees with a standard timing light? Is there a way to test the vacuum advance on the dist to see if it fuctioning properly? Last, is timing set with the vacuum advanced disconnected and plugged?

Thanks

 

This could be a simple thing such as a "loose" plug. Check to see you have ALL your spark plugs snugged up. I had this happen once.Only "backfired" under load,which is what you are describing.

 

You can easily set the timing to 36 degrees without an adjustable timing light:

Borrow your wife's sewing measure tape and wrap it around the harmonic balancer to measure the circumference. Divide the measurement by 10 (360 degrees all the way around.. one tenth of 360 is 36). Take the number you get and measure off that distance clockwise from your existing balancer timing mark (as seen from the front of the engine). Place a mark there. That mark is 36 degrees. You can now use your standard timing light to set the engine up for 36 degrees total by aligning the new mark with the "0" point on the timing tab.

Aways set total timing with the hose pulled off the vacuum advance control unit. It doesn't matter if you plug it or not - the vacuum leak will no affect timing as long as the hose has been yanked off. The 36 total should occur when you rev the engine up until you do not get any more centrifugal advance out of it as observed with your timing light. This should ideally occur below 3000 rpm.

Yes, you can check your vacuum advance easily, too. First, hook up a vacuum hose to the vacuum advance unit and suck on it. It should hold a vacuum (no leak). Now, place a mark on your balancer halfway between your stock timing line and the new 36 degree mark. This mark is 18 degrees.

Now, keep in mind that a performance vacuum advance control unit will pull in about 16 degrees of additional advance when fully "sucked in." With that in mind, start the engine without the vacuum hooked up. Using your new timing lines as a reference, (or your original one if possible), note what your base initial timing is at idle. Chances are, the 18-degree mark will be pretty close to lining up to something on the tab: subtract or add the number of degrees that the 18-line is off the "0" mark to read the timing. Note this number. Now, attach the vacuum advance hose to a straight manifold vacuum source so the full advance slams in. This will change idle speed, so adjust your idle speed back down to the same speed as it was before. Now, see how much timing you have at idle: You can probably use the 36-degree mark to interpolate about how much vacuum advance you have coming in. If you're pulling more than 18 degrees of vacuum advance, replace the vacuum advance unit with a shorter curve unit.

For operating purposes you can use either ported or manifold vacuum for your vacuum advance - use whichever source produces best quality idle once you have the 36 total timing established. Depending on how much initial timing you end up with once you get the 36 total, the engine may idle best with either ported or manifold vacuum. Use the sources as a tuning aid in setting your car up for best idle and off-idle response.

PS - Once you're done with your wife's tape measure, wipe all the grease off of it and put it back where you found it. DO NOT tell her you used it on the car.

 

Thanks lars!!! I apprecoate you taking the time to expalin that to me. It sounds fairly straight forward.