Rough Idle

[Tom S.]

I have a 69 L-46 with a very rough idle problem. I have put in new plugs,points ,wires, condensor, a rebuilt carburator ,set the timing and dwell and double checked it. I can't seem to find any vacuum leaks, it seems to smooth out above 1000 rpms. I have the idle set at 750 as called for any ideas will be greatly appreciated.

Tom Stanton

[Duke W.]

Re: Rough Idle

L-46 is a SHP engine with a more aggresive cam than L-79. It's my opinion that Chevrolet's recommended idle speeds for SHP engines are artificially low, and they need higher idle speed to achieve reasonably acceptable idle quality and stability.

My rule of thumb is to set idle speed at the "lowest commensurate with acceptable idle quality and stability". This criterion is subjective, so what is "acceptable" will vary between individuals. Some like a rough, lopey, "big cam" idle. Some lope is acceptable to me, but I demand stability.

The vacuum advance has a big impact on idle quality and stability, and Chevrolet did not always choose an appropriate vacuum advance. Also, since your engine has emission controls, the ignition map was selected primarily to reduce emissions and may be in comflict with best performance and economy.

Since I don't have specifications for you engine, if you provide the vacuum advance specs, I may be able to be of some assistance, and read the numbers on the vacuum can so we can determine if it is the OE or OE spec can. Also, I need to know if the vacuum can signal line is "ported" or full manifold vacuum.

Is the "rebuilt" carburetor correct for the engine, and have you set the idle mixuture properly?

Duke

[Tom S.]

Re: Rough Idle

Duke
The carb is supposed to be correct according to the rebuilder. It is not # matching as I have not been able to find a core for rebuilding.I had a holley intake and carb when I bought it and changed the intake backto a 186 and got as close as I could on the carb. I'm still looking for a 7029207 carb.I will check the vacuum lines and let you know on that. I think you are correct on the idle though, if I turn it up to about 825-900 it smooths right out. I was trying to keep it as correct as possible.Thanks for the input!

Tom

[Roger Legge]

Re: Rough Idle

I have a '68 L-79 with a 7029207 Q-jet that came on the car. I believe 7029207 was sold for some time as an over the counter replacement for a number of applications. I was having a problem getting it to pass local emissions testing - high HC at idle. I compared jets, primary & secondary metering rods and rod hanger to original '68 specs and my primary metering rods were one number leaner than original specs. Original spec primary metering rods and it passed fine. Guess is the problem was a lean idle miss. Maybe verify the primary metering rods and the jets, at least as a start. There are a whole number of ways for a Q-jet to vary from original specs.

Roger

[Jeffrey S.]

Re: Rough Idle

Tom:
If you have a correct carb for your application, the vacuum advance is ported. As soon as you increase the RPM's you open the the port to off idle allowing vacuum to the advance can. Try hooking the vacuum line from the A.I.R. port which is full vacuum at idle to the advance can and see if it smooths out. I installed a taller cam in my '69 small block and found the same problem. When I used full vacuum from the AIR port the engine smoothed right out at idle since I got an additikonal 10 degrees of advance at idle. If the signal is ported at the advance port there is a permanent fix for this.
Hope this helps.
Jeff

[Tom S.]

Re: Rough Idle

Thanks to all
I found a smaal slit in the bottom of a vacuum line. I had put on all new vacuum hoses when I installed the intake so it took a me by suprise to find a leak here.I also bumped the idle to about 825 and it runs a lot better.Thanks again for all the help.

Tom

[George J.]

Re: Rough Idle

Tom,

Also check the idle rpm with an electronic device. My tach reads a couple hundred rpm high and I've since checked others that read a couple hundred too low. This makes a big difference at idle.

George

[Tom S.]

Re: Rough Idle

Roger
You should have a 7028219 carb on yours if it is manual trans. L79 engine. If you find the correct carb I would be interested in buying yours.I checked the idle electronicly and it was within 50 rpms. Thanks again

Tom