Pontiac GTO

[nevada]

I just aquired a 1968 GTO 400 with a holly 850 and a competion cam.
When I step on it I sometimes get a "rattle" for a second. I am using premium fuel with a 10 point octane booster. The guy who sold it to me told me it had hardened valves to run unleaded gas. anyone have any ideas?

[MagicRat]

Lots of carburetors have a momentary lean condition when you open the throttle, this is why the accelerator pump is there.
Often a lean condition can cause detonation or 'ping'. As you don't say if you get a hesitation when you step on it, so it can't be all that lean.
Hollys have an adjustable cam for the accelerator pump - there is a little plastic arm attached to the throttle shaft, it should be screwed into a hole on the throttle arm bracked labeled '1'. Try screwing the plastic cam into the adjacent hole labelled '2'.
This will give you a bit more gas through the accelerator pump.

Also there is an accelerator pump adjustment screw at the end of the arm. Turn the screw just enough to eliminate the free play in the arm (as measured just beneath the primary float bowl.

[Pewter'01SS]

My Dad had a Grand Prix back in the day and the guy who sold it to him said to run unleaded only. He never had a problem with it until he tried premium one day and it started knocking like hell. He went back to regular and it was fine. I don't know, it could be a Pontiac thing?

Also, if all you have is a different cam, I think you can lay off the octane a bit anyway.

[MagicRat]

Originally Posted by Pewter'01SS

Also, if all you have is a different cam, I think you can lay off the octane a bit anyway.

Yikes, don't do that. too much octane never hurt anything. It just wastes a bit of money if its not required. Too little octane can be harmful. Most V8 engines from the 60's rattle (ping) like 8 bolts in a coffee can on regular gas.

[MrPbody]

Nevada, As Magicrat hs said, the carb could be responbsible. There is another issue common to Pontiacs. That is, the vacuum advance. Pontiacs like their vacuum can hooked up to "ported" vacuum. That is a place on the carb where vacuum is drawn from ABOVE the throttle plate. This is opposite the Chevy, which requires manifold (below the throttle plate) vacuum. If hooked up to manifold vacuum, timing is already at full advance, and due to the design of the Pontiac intake port and chamber, creates a situation of timing too far advanced. This WILL make it ping.
Another and more serious possibility is that compression is just too high for the fuel you're using. A stock GTO of that era sported an advertised ratio of 10.75 or 10.5:1, depending on options. Beware of octane boosters making unreasonable claims, too. A "10 point" increase means you go from 93.0 octane to 94.0. They fail to explain a "point" means 1/10th. 94 octane will not support 10.5:1. It really won't work with the true 10.1:1 the GTO engines actually had.
Try going to the local source for race gas. Mix 3 gallons of 110 or 114 with 7 gallons of 93. That will eliminate the octane issue. If it also eliminates your pinging, you found the problem.

[Pewter'01SS]

I agree with you, MagicRat, that too much octane never hurt but I just think 103 octane is a bit of an overkill for something with stock compression, but if they really did have 10.75:1 or 10.5:1 that explains ALOT.