I know knock (ping) is usually caused by excessively advanced timing.

I have a Target Master 305 in my 80 and the original HEI dist. from my 78 Camaro. It is also equiped with an early 70s EGR alum. intake with matching Q-jet and ALL of the emissions equipment from the 78 305. (Catalytic converter, EGR, charcoal can and a half-mile of related vacuum lines.) PS, PB, Cruise(not working)and AC.

I don't know the compression of this engine or what heads are on it. It has not been opened up in 20 years, the carb hasn't been touched in almost as long and the distributor had the cap and rotor changed a few thousand miles ago but no other work has been done on anything else under the hood.

I have never been able to get the timing set up so I can run anything less than 92 octane without a knock (or three) if I mash the throttle between 1000-2500 RPM.

There is no timing tab on the engine to properly set the initial advance. http://www.nastyz28.com/ubb/frown.gif

I have attempted to set it by ear and butt-dyno several times and though it idles great and never knocks from idle, I can't seem to get rid of this mid-throttle tip-in knock. It is most noticeable when the engine is hot, the weather is warm and the AC is on. This is WITH 92 octane!

I tried switching the vacuum advance connection on the carb but that caused a stumble and stall situation so I don't think that is the issue.

I would really like to get it set up so I could use regular gas again.

I am suspecting the centrifugal advance springs in the dist. have gotten weak (they are still original) and giving me too much advance too soon.

Could this be caused by a slight lean-out condition or is this strictly a timing/advance issue?

Where do I start?

Would a compression test or idle vacuum reading help diagnose this problem?

I don't currently have the tools to get either compression or vacuum readings.

I have a simple timing light but it is worthless without a timing tab.

Sorry this is so long.

Thanks in advance for any assistance or ideas you can give!

Yank the vacuum advance canister line and see how it does. I'll bet the part throttle knock goes away and you will end up being able to advance the intial timing quite a bit from where it is now, picking up a ton of WOT power and still no part throttle knock.

This is NOT a solution to the problem, just a diagnostic test. Assuming I'm right......

I'll bet either the EGR system isn't working right for whatever reason or you're right about distributor advance curve problems.

These emissions motors had TONS of vacuum advance- often 24 degrees or more (the amount is stamped into the vacuum advance canister on the flat metal part right behind the canister itself- example: "638 26" is 26* of advance, the last 2 numbers stamped into it). If the EGR system isn't operational or the intial/centrifugal advance gets pushed up much beyond stock you'll often get that annoying part throttle knock, even though you're nowhere near it at WOT.

Those Targetmaster "universal replacement" motors are low compression units, by the way- about 8.5:1.

Thanks Damon!

8.5:1 is the compression that I figured it should be. This thing should run fine with 87-88 octane.

The tank is near empty now, so I'm going to put a few gallons of regular in it for testing tonight.

I'll advance the timing back to where it seems happiest at idle (on regular) and verify the knock problem is still there. Then, I'll pull (and plug) the vacuum advance line and see what happens.

I'll also look at the vacuum advance can to get a better idea of how much vacuum advance I'm dealing with.

Will testing it with the vacuum advance unplugged narrow it down to an EGR problem?

How do you test the EGR to see if it is still functional?

Another possible cheap and easy fix would be a cooler thermostat. Put in a 160 and see if that helps any -

<font face="Arial,Verdana" size="2">Originally posted by El Guapo:
Another possible cheap and easy fix would be a cooler thermostat. Put in a 160 and see if that helps any -</font>

Thanks for the suggestion!

I don't know what is in there for a thermostat now but it would be a good excuse to change the coolant too. It has been a few years and it isn't bright green anymore.

A few things can cause detonation on older smog era cars; excessive carbon build up in the combustion chamber, malfunctioning egr valve, wrong spark plug heat range, poor cooling system performance (engine runs to hot), miss adjusted timing, over lean condition (either caused by carb or vacuum leaks), worn timing chain, frozen advance wieghts, weak advance springs.

I would rule out all the causes one by one starting with the easiest ones first. I would also get a timing tab installed (you can by a cheap one at most any parts store for $4) because your calibrated ear could be off and your intial timing might be to high. Most smog era cars only had 2-8 degrees intial timing, when set that low most engine run crappy till the vacuum advance is hooked up. If your dialing in 16 (where most engine will idle happily) and your engine calls for 6, thats 10 extra degrees and that can cause part throttle detonation. So before you going tearing into your engine get the basics squared away.

Thanks Joekool! More good tips! http://www.nastyz28.com/ubb/biggrin.gif

I'll pull a couple of plugs to check the numbers on them and see how they look for carbon build-up.

The engine only has around 75,000 miles of mostly highway driving so the timing chain/gears should still be good.

I've already checked for vacuum leaks. (shots of starting fluid around every vacuum connection and gasket I could find... no change in idle speed)

I agree that I should get a timing tab on there too... I hope I can see it with PS, waterpump and AC in there.

Which timing cover bolt should it be bolted to?

I don't think you'll be able to get a timing tab on there with the waterpump in place. http://www.nastyz28.com/ubb/frown.gif

There are two tabs, one is for the large 8" balancer that came on high performance cars and HD trucks. The other is a smaller 6 and 3/4" inch that came on every other SBC. Most timing tab kits come with both so bolt the one on that sits the closest to the balancer without rubbing. Most balancers are marked for both styles of timing tabs, most old style SBC's had a timing tab located in the 2 to 3 o'clock postion and the later smog era cars had the tab located in the 12 o'clock postion of the timing cover. Just simply remove #1 spark plug, hold your thumb over the hole and tap the engine over till you feel air push agianst your thumb. That will get you close to TDC for #1 and the timing mark that is visible on the balancer is the one you want to use with the tab that fits.

Okay... I'm feeling a little sheepish here.

The tab and balancer were so covered in crud that I couldn't see it until I shot some brake cleaner down there. It helped when I looked in the 12 o'clock instead of the 2 o'clock position too! http://www.nastyz28.com/ubb/wink.gif
http://img106.echo.cx/img106/4751/timingtab0kc.jpg
(driver's side of car is down.)

I'm assuming the big point is TDC.
Are the other points are 4*, 6* & 8* BTDC?

Specs for this engine call for an initial advance of 4* BTDC & 20* Vac Adv @ 3800.

How much cent. advance is there and what is my total?

I put a few gallons of 89 octane in the car tonight and pulled & plugged the vacuum advance. I still got knocking. http://www.nastyz28.com/ubb/frown.gif

I pulled a couple of plugs and found them to read slightly rich. Not bad, just a little dark black around the center electrode the outer one was tan/gray. Gap wasn't bad yet either. They are AC R45TS which is what is specified for original engine in the car.

I won't have time to work on this project for at least a week, but I've got some good info to work from here!

My plans when I get a chance...

Check EGR operation. (I found the procedure in my Haynes mannual. http://www.nastyz28.com/ubb/biggrin.gif )
Flush and fill coolant
Install cooler thermostat (160*)
Run some seafoam through the carb to help clean out any deposits.
Replace plugs, fuel filter, air filter and clean carb exterior of carb and recheck for vacuum leaks.
Teardown, inspect, rebuild OR replace distributor (160,000+ miles on it. http://www.nastyz28.com/ubb/eek.gif )
Make sure the vacuum can is a 20* unit.

Set timing to 4* BTDC.
Adjust carb to get a good idle.

Then I will test the car on 89 octane gas to see how it runs.

I'll resurrect this thread when I get this stuff done.

All I need to know is how the timing tab is indexed and I should be able to set initial timing!

Thanks much guys!!! You're great to have in my garage!

The large V is 0* or TDC, I usually use some chalk and make a mark on the balancer groove so its easier to see. I also sometimes mark the timing tab at the degree spec. if I dont have a dial back timing light, it makes it easier to line the two up when setting the timing. Youll also want to verify the idle setting, what I do is set the idle, adjust timing, then recheck idle. If it went up or down (which it will if the timing was off) I reset the idle and recheck the timing agian, repeat till both are within spec.

Thanks again, I'm glad I asked about that! Used the notches ("V"s), not the points.

Is this right?
http://img267.echo.cx/img267/1530/timingtab0ku.jpg

Close.

You have 0* (TDC) correctly labeled. It's at the bottom of the big "valley."

The next "peak" (the pointy-outty part, not the bottom of the next "valley") to the left, facing the motor from the front, is 4* BTDC. The next "peak" to the left of that is 8* BTDC and the final "peak" to the left of that one is 12* BTDC.

Where yours says 4* it's actually 6*. Where yours says 6* it's actually 10*. But that's an AWFULLY difficult mess to look at over the internet, so I'm just giving my best guess and what your arrows are pointing at, but I'm about halfway guessing at it.



[This message has been edited by Damon (edited June 25, 2005).]

I'd run 8-10 BTDC advance after I cleaned the EGR valve out and made sure it worked. Or you can unplug it and it effectively richens the A/F mixture http://www.nastyz28.com/ubb/wink.gif If my 9.7 comp. 406 runs on 87 with zero problems I KNOW you can get that 305 running on 87 with 8.5 comp. Email me with what you come up with. With the TC pointer at 12 oclock that means its a late smog engine.

Update time...

Thanks for all of your help so far guys!

The biggest issue was the initial timing. It was advanced off the tab! http://www.nastyz28.com/ubb/eek.gif

I set it to 6* and adjusted the idle and mixture. It idles great and there is no knock even with 89 octane. I don't think I will have any problem running on 87 octane now.

I seem to have lost a little bit of HP judging by the butt dyno so I will try 10* advance to see if that helps without the knock problem recurring.

I installed a 160* thermostat and found that the old one was stuck in the open position. BTW... I found out you can helicoil a stripped thermostat mounting hole with the intake in place because the mounting boss doesn't go through the intake to the air passage. http://www.nastyz28.com/ubb/biggrin.gif

My only issue now is the high end performance... Unloaded, I can run it up to 4500 RPM, it will go higher but I don't want to break anything. Under load, it slowly tops out at 80 MPH in second gear @ 4200 RPM and then it starts to bounce between 4200 & 4000 RPM. I would think I could redline the tach in 2nd gear if I really wanted to.

Can anyone give me a clue about what could cause this 4200-4000 RPM bounce?
Some things I've thought of...

EGR valve opening and closing
Mechanical advance not working correctly
Valve float

Thanks again guys!