First my question … are there higher rated distributor centrifugal weight springs available?
Now the background I’ve been battling with pre-ignition for some time. I have diligently followed Lars tech bulletins on timing and vacuum can selection yet my problem persists. I have tried backing off timing in 2 degree increments with marginal improvement. During the set up I followed Lars instructions and ended up with the silver and black spring combination suggested from the Mr Gasket Kit 927/928. After several test drives I am beginning to wonder if the centrifugal advance is coming in too soon. When combined with the initial advance and the vacuum advance and some premature centrifugal advance it was just too much and hence the pre-ignition in 1st & 2nd gears at low rpm. I have just rerun the rpm test to see when the centrifugal starts and at what rpm it is “all in”. The first test with black/silver springs showed “all in” coming in really quickly, it seemed so far off I didn’t bother noting the curve. I removed the black/silver combo and replaced them the stiffest springs in the kit, gold/gold. Centrifugal timing started to come in at about 1,000 rpm and was “all in” at about 2,000 rpm. This curve still seems too low, I’m thinking it needs to advance 800 rpm or so. Still stiffer springs might accomplish this, or perhaps something else is going on? BTW I have a 1970 350/350 (L46).

 

I have the same 70 350/350 as you and don't have any detonation problems. I think something else is out of order.

My base timing, no vacuum, is about 10* or 12* degrees. I use the lightest springs and get 36* right around 2800rpm.

Is your dampner in the correct position or moving around?
I don't understand how you could get your timing all in using the heavy springs by 2000rpm. Doesn't make much sense.

Pull the #1 plug, stick your thumb over the hole, have someone bump the engine, when you feel the compression stroke, look and see if the timing mark on the dampner is aligning with the timing tab. It should be close. If the timing mark is nowhere near the timing tab you will have a timing issue.

 

On my new (well not new now), MSD HEI distributor I was getting all in at about 2200 RPM with the heaviest springs, I wanted to get it a bit higher so I cut one of the springs (cut a coil off), that got me up to right about 2500 RPM's so you might try cutting a coil or two off of the end of one spring to see where that get's you at.

 

Thank you that's certainly an option for consideration.

 

Thank you for your suggestions, like you I agree it doesn't make much sense.

In the engine rebuild when I removed the body I did replace the harmonic balancer because the sandwiched dampening material looked as if it was failing. I purchased a replacement from a national catalog that sold it as a direct replacement. The new one looked correct, timing marks seemed the same. The new cam timing chain lined up with the dots and the balancer lined up with #1 cylinder at tdc. I think that's alright but I'll check.

As I mentioned at the start I have been wrestling with this for some time. By following Lars tech bulletins I have significantly improved things but pre-ignition (knock) still remains mostly in 1st & 2nd in low rpm under load. From a cold start it seems fine then after about 10 minutes the knock starts. I have considered fuel mixture and or little burrs inside that glow and cause misfire. Apart from the choke and idle mixture perhaps there are additional adjustments. The original carb was professionally rebuilt as was the engine 2,000 miles ago. During the rebuild the heads were cleaned, block bored, new pistons and rings, valve springs seals etc.

As a result of following Lars procedure I replaced the vacuum can on the distributor which significantly helped the idle and low rpm end of the range, the points were new as were the wires, plugs, cap and rotor. Following the directions I set the advance at 36* all in at around 2700 rpm with black and silver springs ... the knock continued. Per the instructions back off the timing 2* and try again, knock continued so I did another 2* ... same result. Now I'm all in at 32*. I thought that there might be some overlap now between idle advance and the vacuum so I tried bringing up the centrifugal range by adding stiffer springs, this may have helped a bit but the knocking continued.

I thought perhaps the ignition side was weak so I opted to upgrade the distributor with a Petronix 71181 ignitor III electronic replacement of the points along with the matching Flame-Thrower III coil. When the distributor was removed I adjusted the gear drive float by adding a shim along with checking for bearing wear and refurbishing the upper bearing grease tray. This set-up significantly increases the ignition spark so I opened up the plug gaps by .005.

Following Lars procedure again I set the engine at 36* all in with 2700 rpm with black and silver springs. Test drive ... still knocking as before.
So far I have backed off a total of 7* in stages without a change in knock.

I again looked at what was going on at low rpm with the black and silver springs fitted. With idle at around 800 rpm timing was steady, as soon as rpm increased the centrifugal started coming in. I'm not an expert in these things but the original specs for the distributor have the centrifugal timing starting between 1000 and 1700 rpm. Hence the thought to install stiffer springs to bring the centrifugal range higher and not overlap with the idle and vacuum advance.

Well these are my ramblings, don't feel that you are duty bound to reply, perhaps others have a thought ... at least for a moment it makes me feel a little better!

 

Carbon build-up in the engine?

 

Knock is a frustrating problem. But there are other things you can try to get rid of it.

First you might want to try reading the plugs and try to see what they are telling you about your jetting and timing. You may have to start with new plugs to get a good read. The easiest plugs to read are the NGK's.
This is the best article on reading plugs I have found.

http://honda-tech.com/showthread.php?t=3063102


1.) Enrichen the carb jetting. Install richer jets in the carb. If your running 71's now try 72's.

2.) What temp thermostat are you running? hotter water = hotter intake manifold = hotter intake charge = greater chance of knock.

3.) where is your intake air coming from? If it's under the hood air then that air is super heated before it even gets to the carb putting the engine much closer to knock.

4.) Try a cooler plug heat range. 6 or 7 heat range in an NGK maybe

 

A subtle but important point- 'pre-ignition' and 'detonation' (knock) are VERY different things. The terms are not interchangeable.

Adjusting timing and advance curves will do little or nothing for pre-ignition.

 

what octane gas are you using? what temperatue thermostat? what spark plug number and manufacturer are you using?

 

Have you disconnected/plugged the vacuum advance and driven it to see if the problem is still there? That should take out 16* or so of advance at cruise and give you a better idea of whether the problem is timing related.

 

Total timing all-in at 2000 rpm is too quick. You have to install stiffer springs to make it come in at about 3000. Try one stock spring and one gold spring from the MRG kit - that will get you in the ballpark.

Lars

 

That could be possible but I don't think probable. It's only 2,000 miles since the engine rebuild, block bored, new pistons, rings, heads cleaned etc.

 

93 octane, tried different brands and gas stations.
Started with AC Delco R-45 and changed to R-43 a few months ago (no real change).
At engine rebuild (2000 miles ago) I switched from a 195 to 180 to reduce carb boil on shut down.

 

Yesterday I decided to start over and reset from scratch. Best I could tell following your instructions I was "all in" at 2500 rpm with one spring at about 2650 with a black and silver spring set. After driving for about 10 mins the knock started again in 1st and 2nd gear. Out of frustration I just went to the two gold springs to see if it made a difference ... it didn't. When hot, to pull away in 1st I needed to slip the clutch to build rpm beyond the knocking ... hope that makes sense?

When comparing the original GM specs for my distributor (70, L46) I see some differences.


GM Spec Lars Spec Difference

Advance BTDC @ idle 8* 12* (1) 4*

Vacuum advance @ inlet -2 12* 16* (2) 4*

Cent adv @ 1000 rpm 0*
Cent adv @ 1700 rpm 10*
Cent adv @ 2750 rpm 36* (3)
Cent adv @ 5000 rpm 26*

Total advance 46* 52* (4) 6*


1, value after setting "all in" at 2500 rpm and measuring at 750 idle rpm
2, vacuum can recommended
3, "all in" centrifugal advance established at 2500 rpm with one spring.
At the low rpm range if I'm understanding this correctly it looks as if I'm currently 8* over the original GM spec and +6* over all. Could this 8* be the cause? A previous person suggested plugging the vac as a quick test to take out 16* and see if it makes a difference with the knock, do you think this makes good sense? If reset the ignition at 750 rpm idle with vac connected to measure 20* BTDC (8*+12*) this would bring me back to the GM spec at idle. Leaving the vac connected makes it easier to maintain 750 rpm idle ... is this worth a try?

 

Good thought, I'll give it a try.

 

When did this pre-ignition start? What was the last thing you did to the car before it started? It certainly seems like you understand the timing process/principals. There must be something else going on other than just incorrect timing.

 

You're correct when you question the 8* extra advance. As Lars said, install stiffer springs and determine if the problem persists. Go with the stiffer stock GM springs and work slowly to looser springs until the problem begins again, then go one step stiffer. That will be what your engine can use-regardless of what the "books" recommend. Check the condition of the stock springs. Hook them together and stretch them to confirm that both stretch the same amount. I had a set of stock springs that were worn out and allowed full advance below 2000 RPM.

 

Almost the same here, 8* and 26* 2800rpm, no vacuum.
1969 L46, 94 US octane gas (99 EU octane)

 

Thanks for the in put, this is 4* lower and 10* lower than I'm currently at without vacuum.

 

I had a small talk (mail) with Lars regarding my L46 engine.
This is what he told me, and I am 112% sure this is right.

" An 11:1 engine will require a minimum AKI of 100 octane, which in your EU RON system would be about 106 octane. You cannot run your engine on 100 EU octane fuel - it will detonate. Your 99-octane is about the same as our standard 94 octane pump gas, which will not support an 11:1 engine. You need to lower your compression ratio."