[mccorry]

Will a faulty / incorrect power valve cause knock? Timing is dead-on and no matter how much I retart it...the engine still knocks at wot under load. Anything else I should check? Carb jetting, etc?

[drptop70ss]

do you mean knocking like detonation or knocking like a rod knock?

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66/70 chevelle ss396's
69/71 camaros

[tom3]

You can richen up the mixture and help this as will dropping the thermostat temp. Running a higher octane fuel, if possible will help. Go to a colder range spark plug. If running an R45 go to a R44 or R43. Sometimes retarding the timing just a couple more degrees will do the trick.

[Joseph]

I just had this problem with my friend's 442. It turned out to be the balancer. The outer ring had rotated giving incorrect timing reference. If your balancer is old check top dead center, 0 degrees, with a piston stop screwed into the #1 cylinder. Rotated the crank back then forward in the compression stoke and half way in between is true top dead center.

Good luck

Joseph

[mccorry]

Knocking as in detonation. It will only do it under heavy load at 3/4-WOT.

It is a 355ci sb w/ Dart heads/Edelbrock RPM intake/1 3/4" headers. Motor is balaced. Recently replaced cam (was 292H) to a 268H.

I am running a 180 deg thermostat. Timing is set at 4 deg BTDC / 35 deg. at 3K. I run 94 octane fuel with R44 plugs.

[This message has been edited by mccorry (edited 09-13-99).]

[Jay]

Have you checked your vacuum advance? Is it working? How many degrees is it advancing? It could be advancing too much for your engine and causing detonation.

[mccorry]

Right now I have the Vac. advance disconnected. Still knocks.

I am starting to think it may be a power valve problem and the motor is leaning out under load. Comments?

[This message has been edited by mccorry (edited 09-13-99).]

[tom3]

McCorry; Running a similar setup and had to really play with the timing to get it to run best without the WOT detonation. Played around with different mech. adv. weights and springs to limit cent. adv to about 32 degrees with 10 deg. intial. Still get just a little knock. For highway miles I just pull the dist back a little and can run cheaper gas to boot. I've seen several dyno tests that indicate the 36 degree advance doesn't add any real power after detonation sets in and from what I can feel with mine, that's probably true with it. Try limiting total mech adv to about 30 to 32 deg and see what happens. tom

[mccorry]

Tom3...what springs and weights are you now using? Mr. Gasket set?

I feel you are all dead on and I appreciate the good feedback. Based on your comments, I rechecked the timing advance curve (centrifugal). I had the base timing set at 4 deg. BTDC and I found that my timing quickly advanced to 35-36 deg. BTDC at around 2800 RPM. The springs/weights look OK...so I don't know why I am getting SO MUCH centrifugal advance. I, obviously need to look into modifing the advance curve. Any suggestions on recurving kits?

Thanks....

[This message has been edited by mccorry (edited 09-18-99).]

[Jay]

Before you get a recurving kit I would check the distributor's base plate. Look at the oval holes that the weights slide on. It's possible someone could have filed them longer to get more advance. That could be why you're getting so much mechanical advance.

[DjD]

mccorry,
What type dist is it? HEI or points? Before I recurved my HEI I had to keep the inital low as it would make too much total advance. I was told that it is this way for the smog motors it was designed for. A good preformance shop should charge you about 1/2 hr labor to recurve your dist as long as you pull it yourself. They use a dist machine and get it right the first try. I know springs and weights arn't that tough but you might have to go through the drill several times to get it the way you want it.

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...DjD
69RS Ragtop
www.fbody.com/members/topless69

[mccorry]

It is a points type distributor. I will pull the weights tomorrow, Jay, and take a look at the guide slots...don't recall seeing any. That could be my problem.

[Dave Birdwell]

What pistons does the motor have??
It may be too much compression, and to back the timing down enough to eliminate ping, you'll end up overheating.
What carb are you running??
A bad power valve won't cause detonation, but too lean jetting will.

[mccorry]

The motor has cast flat top pistons at 10:1 compression. I am running a Holley 650dp (4150) on top of an Edelbrock RPM manifold.