General Category > Quadrajet Carb Talk and Tips
cam selection
Kenth:
Are you already using full manifold vacuum to the vacuum advance?
If not try, engine will run smoother and this will often cure run-on (diesling) at shut off because the engine will run with more closed throttle blades.
It certainly wont hurt anything.
tayto:
I am running ported. will try full vac the next or 2 and report back,I suspect my in gear vacuum will go up a few points.
Cliff Ruggles:
Timing can be a big player with idle quality.
Some engines like a LOT of timing at idle, others not so much.
I would get the engine fully warmed up, adjust the idle mixture screws for best idle quality and highest vacuum.
Then loosen up the distributor so you can turn it. Advance the timing till it runs dead smooth and highest RPM's, then start retarding it until it just starts to slow down and the exhaust note develops a slight deep/heavy sound.
Take a look at where the timing is at. That's going to be pretty close to the ideal setting for your engine. If it's back around 10-14 degrees there will be no need to run manifold vacuum to the advance at idle.
Most well thought out engine combos will not like, want, need or ever respond well to a lot of timing at idle. This assumes the tuner has the carb set up to deliver adequate fuel to the mixture screws from rich to lean, and that there is no nozzle drip from the carb so adequate bypass air.
Almost every engine I've built here in the past 25 years or so has been fine with about 10-14 degrees timing in it.
I've tuned some troubled engines brought up here that needed a lot of timing at idle. In every single case the cam was simply poorly chosen for the total combination of parts (low vacuum at idle speed). I consider running the timing clear off the scale at idle a "crutch" for poor parts selection and just as often poor tuning skills.......FWIW......
Kenth:
I have found it is an undeniable fact that most 1967 and earlier engines used full manifold vacuum for the vacuum advance. 26-30° timing advance at idle was the norm. These engines were in no way a result of poor engineering, this was a way to make the engines work as efficiently as possible. In 1968, there were legal requirements for reduced exhaust gas values and one way to achieve this was to delay the ignition, even to the negative side of the scale, so that the cylinder heads would be heated up at the end of the combustion cycle by the mixture still burning when the exhaust valve was opened to reduce NOX. This did nothing to improve the efficiency of combustion, quite the contrary. When the throttle blades opening became larger with later ignition, idle bypass air was introduced in the Quadrajet to cure problems that arose with the later ignition, e.g. run-on/dieseling. With a hotter camshaft than standard, however, it can be difficult to get enough vacuum to fully activate the vacuum advance and then it can be advantageous to use a ported vacuum source for the vacuum advance and help the carburetor deliver a sufficiently combustible mixture and add more idle bypass air to avoid having the dampers open more than necessary and that the dieseling at shut-off. If anything, i think the "crutches" started 1968, and in the end the level of needed "crutches" to have a proper running engine depends on the skills of the engineer/designer/tuner.
JMHO
77cruiser:
You ever see how much timing a TBI 350 idles with? About 30 degs. till you pull the tan wire.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version