70 L46 pinging and run on

[Bill L.]

I have been reading that an L46 should be able to run at factory specs with todays gas. Motor is original L46 with 120k miles.

However, the only way I have been able to eliminate pinging, run on, and rough idle is to mix in a little CAM2 racing fuel. Runs flawlessly with the extra octane. I typically put in 3-4 gallons of CAM2 and fill the remainder with 93 pump gas. No smoking or tapping. Starts right up. Idle kicks down just as it should when new. Idles smooth as silk.

If I run straight 93 Octane the idle is a little jumpy, it pings on even moderate acceleration, and will run on after turning the key off. The acceleration is not the same either. I get a some hesitation when the secondaries open up too.

New plugs, wires, original carb that has been bushed and re-kitted, new points, new cap, idle, dwell, and timing are right to factory specs. Original distributor that has been rebuilt and set up to factory specs. Replaced timing chain about 2 years ago.

It would ping even worse until we brought everything back up to factory specs.

Love to hear thoughts. It would sure be nice to run on pump fuel so I could drive a little further from home.


Bill

[Patrick H.]

Re: 70 L46 pinging and run on

How is the vaccum from the carb to the distributor routed? Have you tried running it drectly from the carb to the distributor and bypass the TCS solenoid? Did you check the curve on the distributor when rebuilt, and if so what are they? These answers will help.

[Timothy B.]

Re: 70 L46 pinging and run on

Doesn't that carburetor have a idle solenoid that dis-engages to close throttle blades when the key is turned off. I think run-on is usually caused buy the throttle blades slightly open and HOT cylinder temperatures like ported vacuum and low initial timing. Advancing the initial will speed the idle RPM but you have to be sure the centrifugal advance curve does not allow more than 36* WOT timing (initial and centrifugal).

[Michael W.]

Re: 70 L46 pinging and run on

93 octane is already 2 points higher than required when the car was new. They were designed for 91. Octane rating has no effect on idle quality, hesitation or power output of the engine.

[Edward J.]

Re: 70 L46 pinging and run on

Bill, what sometimes happens with engines after they run for 120,000 mile is carbon build up on valves and pistons, This will this will cause a engine to run lean and the carbon will cause pre ignition when the car is shut down. I would say that if you are sure of the engine spec.s especially the Distributor vacuum advance curve and mechanical advance are in spec.s. test the engine vacuum with a gauge to see what you have at idle on a warm engine, then test you vacuum advance to see how much vacuum is required to fully deploy it. Ed

[Bill L.]

Re: 70 L46 pinging and run on

So it should run fine on 93 octane.

Now the fun begins. The distributor was redone by an extremely reputable shop in Gaithersburg Maryland. I am pretty sure it is done correctly.

No idle stop solenoid on a 70 . One year only carb with the pot on the front.

I will double check but I recall setting total advance at 34* WOT Timing. I have also been debating whether or not to try and de-carbon with the GM top end cleaner. Read where it may do more harm than good sometimes.

TCS is hooked up and working. What will bypassing a working TCS system accomplish? I read it might help mpg a little bit.

It will take me a while to get to these things but I will start with checking the vacuum as Edward suggests.

Ideally it might be nice to tear down the motor and rebuild, but it runs so well with the extra octane. Right now a rebuild is not going to happen due to finances.

This will be yet another good learning experience


Thanks,


Bill

[Timothy B.]

Re: 70 L46 pinging and run on

Bill,

Where is the initial timing set? if you have 34* total WOT timing (initial and centrifugal) I would bump the initial timing up 4-6* from where it's set. This will speed idle rpm and you will need to close throttle blades to get back to the desired idle RPM. Closing the blades will help with the run-on.

If the car has ported vacuum advance using the TCS switch, the extra initial timing will also help cool the engine. Could it be possible the distributor is set up with a curve that's more suitable for full manifold vacuum advance. Did anyone tell you what curve the distributor is set to provide.

You certainly don't need to tear down a good motor and $$$, just do some research on timing and vacuum advance and experiment for what the engine likes best. FWIW, my 67 300hp engine has 6* initial timing but with the full manifold vacuum advance (15*) the engine sees approx 21* idle timing (15* V/A and 6* initial).

[William C.]

Re: 70 L46 pinging and run on

OK, when the distributor was "done" did you get a graph showing the advance curve? If so, how does it compare to the original curve that should be in the distributor? For reference, the '490 distributor should have an advance that does not start until 900 rpm, and should be all in at 2550 rpm, giving 32 degrees of advance in the centrifugal advance, which with your 4 degrees of initial advance should work just fine, with 19 degrees of vacuum advance at 17 inches of vacuum for good fuel economy under light (cruising highway) conditions. I'm betting the timing mark on the balancer is off, or the distributor curve is off...

[Duke W.]

Re: 70 L46 pinging and run on

So it should run fine on 93 octane. The distributor was redone by an extremely reputable shop in Gaithersburg Maryland. I am pretty sure it is done correctly.Bill

Uh... did you talk to them?

I just love it! A "reputable shop" set up your distributor and you are "pretty sure it is [sic] done correctly", but the engine rattles like a coffee can half full of marbles in the back of a buckboard on a washboard road, and, of course, you have no data. We've seen this thread many times before, and like the others, your logic defies logic.

You have an emission controlled engine. The spark advance map is severely retarded for emission control reasons. This heats up the combustion chamber boundaries, which can lead to detonation and run-on.

Your engine will run best with about 26-30 degrees total idle advance, which means you need full time vacuum advance, but the OE VAC has too much advance and doesn't meet the Two-Inch Rule. Install a 16*/12" B26 VAC routed to full manifold vacuum and set the initial at 10-12 degrees. This gets you into the correct total idle advance ballpark and 36-38 degrees maximum WOT advance, but with the OE springs, it's not all in until 5000.

Once you make these changes go through the pre-emission idle speed/mixture adjustment with a target idle speed of about 750.

You can likely get the centrifugal in quicker than the lazy OE curve, but if it's too quick, detonation could be a problem.

So you need to understand the OE spark advance map, what an optimum detonation-free spark advance map looks like, and how your "reputable shop" set up the vacuum and centrifugal advance on your distributor. I bet it isn't close to what I recommended above.

Google my name, San Diego, Corvette, and read the presentation from the 2012 National Convention.

Duke

[William C.]

Re: 70 L46 pinging and run on

I used to make my drinking $$ by repairing distributors and vacuum setups done by shops that didn't understand how the distributor and emissions systems interacted back in the day...Bottom line is you need to KNOW exactly how yours is setup, what curve, what specs on the vacuum advance and what is the vacuum source and how is it controlled (switched, full-time, or "ported")

[Bill L.]

Re: 70 L46 pinging and run on

I hear what is being said. I will take what is being said to the shop regarding the distributor. They are EXTREMELY reputable and have always done good by me. Let me also say you all are way above my knowledge curve. I did talk to them but did not know exactly what info I was seeking. I just wanted it set up as it was originally. Obviously I now know that I have no way of knowing the curve is correct without a graph.

Am I also correct in saying that with the OE vacuum advance I may have a problem based on what you are saying Duke? If so than that would mean that it will not run properly on today's pump gas without changes. If that is the case I totally understand. What I wanted to know is if you were able to buy a brand new L46 and put today's 93 octane gas would it run as the factory intended off the showroom floor in 1970 emissions and all? I should have been more specific in how I asked the question.

Sounds like I need to at least start with the vacuum advance recommended, adjust timing and first establish the distributor curve. Then set up the distributor so the full vacuum advance comes in before 5000 RPMs ideally.

Sound about right?

I may be a little slow but I really need to understand exactly what I need to do.

I gotta say with a little extra octane it runs absolutely like a top. I always assumed it was just today's gas causing the problem. After reading here and on another forum I started thinking that maybe it had a MAJOR problem since it would not run well on pump gas. If adjustments are required that are other than what the factory provided new, I think it is fair to say that it will not run well on todays pump gas without making recommended adjustments.

I hope to have the rear suspension back in it later this month. Then I will begin to tackle this issue.


Thank you for all the help!


Bill

[Duke W.]

Re: 70 L46 pinging and run on

If the engine was exactly to OE spec it would most likely run detonation-free on today's 93 PON fuel.

You appear to not fully understand the concept of a "spark advance map", but it's not rocket science.

Did you download, read, and absorb the presentation I referenced?

Duke

[Bill L.]

Re: 70 L46 pinging and run on

I frankly have no clue what a "spark advanced map" is Duke. I learn as I go as best I can.

I will download the paper.

Thanks as always!

Bill

[Timothy B.]

Re: 70 L46 pinging and run on

Bill,

I think you are starting to get the picture about engine timing.

The extra octane in the fuel makes the fuel burn slower so the extra timing advance does not cause a ping (your situation). The ping is caused by the point of maximum cylinder pressure happening BEFORE the piston reaches top dead center and starts down in the bore. What you try to time the engine for is the point of maximum cylinder pressure just after TDC at approx 15* down in the bore so it pushes hard on the piston.

The factory specs for you engine are a starting point, there has to be a starting point and you can tune from there to get better performance but if you bring the advance in to quickly the result is the pinging you experience.

Stay with it and you will learn and get the engine to run better, there is probably more information on the net about timing, manifold vacuum advance, ported vacuum advance etc. than you care to read but you will get the best info right here..

[Bill L.]

Re: 70 L46 pinging and run on

Thanks Timothy,

At least now I have a plan

Bill