labor knocking

[390owner]

I have a rebuilt 390 in my 79 bronco. I have the spark advance unhooked for the time being. I had a tank full of 93 octane gas and have the timing set at 11 degrees. The total timing is about 34 degrees. I was going down the interstate about 70  and stepped on the gas and it labor knocked like crazy. What is the deal. I was having trouble with this before that is why I unhooked the spark advance. I have a one wire dist and the engine has around 2000 miles on the rebuild by a machine shop. It has a small comp cam. If I slow the timing down any more it is sluggish

[BattlestarGalactic]

Have you checked the timing curve?  Maybe it is coming on too fast?  What RPM is 70 mph?

[427LX]

What compression do you have? If it's over 9.75 with old iron heads and a small cam your cylinder pressure may be to high
for pump gas. Retarding timing will cause overheating issues too.

[machoneman]

Slipped outer ring on the balancer? A common issue with old balancers.

[wayne]

 Can your tell us more about your setup heads carb ect by the way a full size bronco with a fe is as good as it gets.

[390owner]

2600 at 70 mph. I have not checked the timing curve. As far as the balancer slipping,maybe but if I time it by ear I really cant go any slower because it is sluggish.  It has stock heads with some exhaust valve work for the cam, a cast iron 4 barrel intake, bored 60 over and a 600 edelbrock carb and headman headers. Yes it is a good engine for a 79 bronco. It tows my 24 foot camper well

[My427stang]

2600 at 70 mph. I have not checked the timing curve. As far as the balancer slipping,maybe but if I time it by ear I really cant go any slower because it is sluggish.  It has stock heads with some exhaust valve work for the cam, a cast iron 4 barrel intake, bored 60 over and a 600 edelbrock carb and headman headers. Yes it is a good engine for a 79 bronco. It tows my 24 foot camper well

I would check initial, total, and what RPM you get to total.  Could be a broken spring, could be that you have too much advance, who knows

One point though, although you are right that being sluggish "by ear" can indicate too little initial,  it doesn't tell you what total is doing.  That's why we recurve, stock distributors are meant to have a ton of advance.  If you have access to a dial back light, mark TDC clearly, and check every 250 RPM or so until it stops advancing. 

[Thumperbird]

Now I know a term to describe how my body feels when I go to work.

[390owner]

Total is 34 at 3000 rpms

[BattlestarGalactic]

So your total is just over your cruise RPM.  Is this with no trailer?   You might need to tighten it a touch to move it up a little higher.

[plovett]

No way 34 degrees total at 3000 should knock, in my opinion.  Something besides the timing is wrong.  Sorry I don't know what it is.

paulie

[e philpott]

I agree 34 degrees  shouldn't knock on anything . I wonder what jets and Metering Rods are in the Edelbrock ?? What model number # 600 carb is it , Econo calibration or Performance Calibration ?

[wayne]

I think mr Philpott is right its lean edelbrock 600 is a good carb but are set up for good mpg get a kit it has new springs and rods its easy to work on.I had a 69 f 100 with a 65 390-300 with a small cam stock cj intake and a 3310 holley.It never knocked even with a car on a trailer I bought a 89 bronco that knocked so bad you almost could not drive it on 93 octane. The 302 should have run on squirrel piss it took a long time to find it the holes in the intake from the egr valve was pluged bad. Had to use a drill to open them up after that it never knocked on 87 octane it was lean even with injection.           

[390owner]

The carb is a 600 (1406) with the electric choke. The primary jet is .098 with 75 x 47 rods and the yellow springs. I think the secondary jet is 95. So you think maybe to lean at full throttle. It does labor knock when pulling my 24 foot camper but not to bad usually when I just start up a hill.

[wayne]

Edelbrock site has a chart to help you tune it i think i would start with a clear step up spring and see if that helps. If you have one if it works then you can go to a softer spring when it starts to knock go one spring stronger as a place to start you may not need to change rods.