I just put dual exhaust on my 76.(no cat.s) Now it sounds really good, but idles very rough and does not accelerate well. I am assuming that I need to tune it differently due to a change in back preasure.

Can anyone give me some input on what I need to do?


[Modified by TJRed76, 10:40 PM 5/7/2003]

 

If everything else is stock, then I suppose what you are experiencing is better cylinder exhaust scavanging. My guess would be that it's running rich since there won't be any more air in the cylinders when the intake opens. Just to be sure while the engine is running at idle adjust the idle mixture screw out 1 turn...if it starts to die then it's too rich...bring it back to where it was. Then turn it in 1/4 turn and wait a moment to see if it changes, if not then turn another 1/4 turn...keep going until the engine RPM's start to die off...this will be the correct idle mixture...some people will back it out 1/4 turn after the RPM's die off...it depends on how the engine responds to it...leaner is good for fuel economy and carbs with a nice strong accelerator pump, while richer is good for idle to WOT transfers.

Also, be sure the idle is at the correct rpms for the idle system to be working...around 550 to 900 rpm...anymore and you may be getting into main metering and the idle mixture won't have any effect.

 

TJ -
Reducing exhaust back pressure will lean you out. If your '76 Q-Jet is set up to its stock specs, it is running too lean on the idle & cruise portion of the metering circuit. Running true duals without a cat will also allow you to quicken up your advance curve for much improved performance. So do this:

First. set up and verify that your total timing is hitting 36 degrees (with vac adv disconnected) at a useable rpm (around 3000). You should be able to get 36 degrees total out of a '76 distributor with about 16 degrees of initial advance at idle, but you need to check it.

Next, hook up your vacuum advance to direct manifold vacuum after you verify that it's working (suck on it to check it). This will give you a bunch of initial timing right off idle, and will quicken up your throttle reponse.

Finally, pop the top of your carb and set your float level to .410". While you're in there, check the primary jet and rod sizes against the spec for your carb number. Make sure the jets are no snaller than the stock spec size, and make sure the primary rods are no bigger than the stock size. Once you have verified and corrected all this, pop it back together. Install a new set of standard resistor Autolite spark plugs for your car (do not use Rapidfires or Platinums) and go for a 5-minute cruise. Pull it over without allowing it to idle any more than a few seconds and yank a plug to do a plug reading. If you're pure white, you need to richen it up a bit more to compensate for the new exhaust. If this is the case, bump the primary jet size up 2 sizes.

 

Lars,
A few quick questions:

1. Why not Rapidfire plugs? What problems do they cause?

2. I'm not familiar with the jet sizes, is a larger number a larger size, or is it the reverse, like wire sizes?

3. Is there a special tool to check the float level?


[Modified by Captain Morgan, 9:55 PM 5/7/2003]

 

1. The Rapid Fire plugs and the exotic metal plugs all run so clean that you cannot get an accurate "plug reading" off of them: you can be off on your mixture several steps either rich or lean, and the Rapid Fires and the Platinums will look exactly the same. The standard resistor plugs are more sensitive to correct mixture, so they will display a corresponding color on their insulating nose. So use the standard plugs to determine mixture through a plug reading (correct mixture will leave the insulating nose a light tan color - not pure white, and not sooty). Once you have verified that you're running correctly, you can switch over to the exotic plugs for best performance and plug life.

2. The size on the jet is the diameter of the hole in thousands of an inch. Thus, a bigger number is a richer jet. Likewise, the number on the primary rods is its "fat" diameter in thousands of an inch. I bigger number is a fatter rod. Fatter rods run leaner than skinny rods in the same jet.

3. You can check the float level with a small scale. I use a set of calipers. To check, pop the top off the carb. Place one finger on the metal hinge pin clip to keep it firmly seated in its groove. Use another finger to lightly press the needle arm of the float down to fully seat the needle into the seat. Measure the distance from the top of the float bowl casting to the top, rear edge of the float. Set yours at .410" - .420". Let me know if you need directions for popping the top off the carb and re-installing it.




[Modified by lars, 10:32 AM 5/8/2003]

 

Thanks Lars. I'll be installing headers this weekend and I'll want to make sure I'm not too lean afterwards. I know you're a busy guy so I'll search corvettefaq for any papers you've written on the subject and also check out the GM manual first. If I have any questions I'll send you an e-mail.