My friend has a 70 SS396 350 hp and he's having a strange problem w/his Q-jet. Pulling up to a light, he pushes in the clutch, steps on the brake, and the idle rises from the normal 700 rpm to 900-1000 rpm. If he blips the throttle, it will not idle down - he has to let the clutch out a tad and energize the drivetrain a little and it will idle down.

He has checked for vacuum leaks and all seems ok. The carb was rebuilt a couple years ago and the guy epoxied the usual areas. He didn't have this problem until after he had the engine rebuilt recently and it cropped up.

He can't find where any of the linkage is binding up, and he didn't mess with any of that while the carb was off the car, anyway.

He played with the weights and springs in the dist. 'cause a friend said he might have bad springs - tried some new springs to no effect. He lubed the rubbing parts, but that didn't help either.

He also installed a new throttle cable - no change. And had the primary throttle shaft rebushed because it was a little loose - nada.

So, he is stumped.

Any ideas are most welcome. Thanks! :D :thumbsup:

This might sound odd but it might be the throttle shafts leaking.

As I said, he had the primary shaft rebushed. I guess the secondary shaft usually doesn't need rebushing, since it doesn't get used that much.

Wehn idle ins when hitting brakes that a vauum related issue with respect to possibly aproblem with vacuum adv in dist and or not running enough base timing so tbase timing is retarded so afectine idle vacuum casuing unstable isle esp if vac adv cut in vacuum lvl is right about where the motors idlevacuum is.

anyway,if running an afermarket cam of any kond try running 16-18 deg base timing to see if that helps.

also try hooking the vac adv to full int vac all the time too after yu set base timing to 16-18 deg. vac adv needs to be fully activates at 2" below lowest idle vac in gear woth auto trans and running 16-18deg base timing.

look for 36-38 total tops,will likely need to imit mech adv in dist to 20 deg max in by 2800 roms ,then 16-18 base + 20 deg mech with get you 36-38 total which isa god place to start with many perf cams.

Could also collectively be weak springs in dist mech adv along with retartded base timing and vac adv not hooked to full int vac all, the time.

But its vacuum related becasue the idle only changes when he hits the power brakes which activates the power brake booster so there migh be an issue there too thats leaking vacuum like when yu pull a pcv valve out of v-cover and the idle increases too so chekc that out too.

could 1st try just take the hose off the brake booster and plugging it to see if that stops the idle from inc when break is hit. If that stops it you now its vacuum related and to go after what i suggest 1 thing at a time untill yu get it sorted out becuse you likely have more then 1 thing going on at this time.

Scott

Scott, thanks. :D I appreciate it.

He has a new brake booster and the cam is very close to stock. I will pass on your advice tomorrow and we'll see what happens.

Dave,even a very mild perf cam close to stock 396 needs at least 16 deg base timing ,maybe even 18 deg base if its a hotter cam then you thnk it is.

Let us know how you make out when you removing/plugging the vacuum line to the brake booster along with also trying more base timing like 16-18 deg too & attaching the vac adv to full int vacuum too.

Scott

Dave,even a very mild perf cam close to stock 396 needs at least 16 deg base timing ,maybe even 18 deg base if its a hotter cam then you thnk it is.

Let us know how you make out when you removing/plugging the vacuum line to the brake booster along with also trying more base timing like 16-18 deg too & attaching the vac adv to full int vacuum too.

Scott

Will do - thanks again, Scott. :thumbsup: