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plumpcars
09-12-2011, 07:50 AM
Spent all my life turning spanners but this one is doing my brain in. Trying to think what my head has missed on a friends small block Chevy which has pre ignition. The engine was a stock low mileage ZZ4 which ran fine. Heads were changed to Dart Pro1, cam to a 270 Extreme Energy hydraulic roller with high quality valvetrain and carbs to a pair of Edelbrock 500's on one of the new Edelbrock 2x4 air gap intakes. Ignition is a vacumn MSD distributor with a digital 6 box. The car runs perfectly well except it has pre ignition( running on) It is not caused by the MSD back feeding but is true pre ignition. On the street the car is quick with no hesitation or detonation. Totally tractable and a great all around package. It's quite heavy but runs consistent 14.2 in the mid nineties using auto upshifts. Cam was installed correctly and ignition set at 38 degrees total with the curve coming in quickly (light springs) giving really good bottom end performance.
The idle speed is about 700rpm, carbs respond to mixture screws and no signs of air leaks or running on the secondaries. The compression ratio is now slightly lower than it was stock at just under 10:1. Have tried colder plugs to no avail. Have tried various fuels and octane booster.
The other day it was driven at 60mph in the pouring rain for 60miles...no running on. Yesterday it was driven hard for 3 miles...ran on. It seems to be related to heat but water temp is steady all day long at around 180.
Can anybody please think of what I have missed?

tmoble
09-12-2011, 11:50 AM
what's the alternator set up like? sounds to me like it's feeding back through the alt warning light circuit.

bigblock261
09-12-2011, 12:30 PM
I have always thought of preignition as that rattling sound the engine makes under acceleration, often called ping or valve clatter.

To me, running on means the engine doesn't die when ignition is ceased, but continues to attempt to run in the absence of ignition. Meaning there is no electrical ignition being provided by any means, intended or otherwise.

Running on in my experience makes the engine sound like it will knock the rods out, speeding up and slowing down, usually terminating with a wheezing sound. Since it is not receiving properly timed ignition, the engine sounds terrible. Shutting the engine off in gear helps it die. My '70 Impala 350 had a throttle solenoid that completely closed the carb when the ignition was shut off. It takes air, fuel, and spark to make an engine run. Chevy engineers removed the air from the equation.

The unwanted source of ignition is something in the combustion chamber becoming ultra hot, probably due to low octane. It can be carbon, the edge of a valve, a sharp edge on the dome of a piston, a gasket edge, etc.

The only thing correctible without disassembly is carbon removal. It is possible that running a higher grade of fuel will remove said carbon. Chrysler dealers, in the eighties, provided a liquid additive from their parts counter that was actually poured in the carburetor with the engine running, to remove carbon. It worked, often overloading the catalytic convertor or muffler.

crankshaft
09-12-2011, 01:12 PM
Take a look at the carbs. If the throttle plates are not closing properly on one of the carbs it will cause these symptoms. They obviously will not completely close but they must are staying open too wide on shut down it will cause run -on.:anim_25:

plumpcars
09-12-2011, 01:31 PM
The car does not detonate when being driven and it runs on only for a couple of wheezes, enough to be annoying and not good for the internals. If you turn off in drive then it does not do it. The idle speed in neutral is about 750rpm so I felt it was slow enough out of gear to not be a throttle blade issue. It does not appear to be drawing anything through the secondaries at idle.
The car had the MSD/electric fan/wiring all in place for a year with the stock ZZ4 and no running on at all. My experience of an MSD backfeed was that they ran on as if the ignition was still switched on?
The problem only started after changing heads/cam/carbs and manifold.
Thanks for the suggestions so far.

Detonation is pinging under load. We get absolutely none of that.
Pre-ignition is running on when you turn the key off.

bigblock261
09-12-2011, 01:49 PM
[QUOTE=plumpcars;683055]My experience of an MSD backfeed was that they ran on as if the ignition was still switched on?QUOTE]

Correct, with backfeed you could drive the car on a 50 mile cruise.


[QUOTE=plumpcars;683055]The problem only started after changing heads/cam/carbs and manifold.
QUOTE]

Could the spark plugs possibly reach too far into the chamber, exposing threads that could be over heating?

Chevynut
09-12-2011, 02:06 PM
There's something that's getting hot enough to cause ignition when the ignition is turned off. If there was electrical backfeed, it would stay running. The carb is going to be the same position with or without the ignition on. All you're doing is eliminating the ignition when the key is off.

This isn't really pre-ignition, it's "auto-ignition" or sometimes called "dieseling". A diesel doesn't have spark plugs and it ignites because the air gets hot enough to burn the fuel when it's injected.

I would guess something is getting hot in one or more cylinders. It could be anything sharp, or even some carbon deposits. One way to deal with it is to install a throttle solenoid so the plates close completely when the ignition is turned off. That's how the OEMs did it.

plumpcars
09-12-2011, 02:32 PM
I agree that something is getting hot and igniting the mixture. What's throwing me is that the engine is very low mileage and the piston crowns were perfect when the heads were changed. The heads are brand new Dart's.
I'm thinking along the lines of getting the car on a dyno to see afr throughout the range. The heads/cam/ carbs are really not extreme pieces and I thought the carbs would be close out of the box. If it were down to me I would be running a single Holley on the car but this set up was the owners choice.

I'm aware of the throttle solenoids and over here we had cars fitted with solenoids which dumped the vacuum from the intake to stop the problem back in the eighties. We could fit either but it's annoying me that it does it at all as the package is so mild!

crankshaft
09-12-2011, 03:21 PM
Sharp corners or objects in the cylinder will cause detonation also and you say there is none of that. Extra fuel in the cylinder will ignite at shut off even at normal cylinder temps.

Typhoontx
09-12-2011, 03:56 PM
What spark plug are you running ?


Regards,

Robert

Chevynut
09-12-2011, 03:57 PM
Sharp corners or objects in the cylinder will cause detonation also and you say there is none of that.

What you describe is pre-ignition...ignition of the furl/air charge before the timed ignition event. Detonation is caused by fuel that burns too fast and uncontrollably....i.e. low octane fuel in a high compression engine or under high loads where cylinder pressures are high.

Extra fuel in the cylinder will ignite at shut off even at normal cylinder temps.

If that was the case, the engine would never stop running. Temperatures and pressures in a gas engine are not normally high enough to cause auto-ignition without a spark.

plumpcars
09-12-2011, 04:44 PM
Having to run Accel shorty plugs due to header style. Started with 416 and now running 414, the silver tip ones. I know some people frown on the Accel's but until the headers are replaced this winter we are forced to use them.

crankshaft
09-12-2011, 04:52 PM
What you describe is pre-ignition...ignition of the furl/air charge before the timed ignition event. Detonation is caused by fuel that burns too fast and uncontrollably....i.e. low octane fuel in a high compression engine or under high loads where cylinder pressures are high.



If that was the case, the engine would never stop running. Temperatures and pressures in a gas engine are not normally high enough to cause auto-ignition without a spark.

In a laboratory you may be correct, in the real world, not so much.
Have seen and corrected exactly what I described, otherwise I would not have commented.

bigblock261
09-12-2011, 05:06 PM
Having to run Accel shorty plugs due to header style. Started with 416 and now running 414, the silver tip ones. I know some people frown on the Accel's but until the headers are replaced this winter we are forced to use them.

I'm suspicious some component of the spark plug threads might be the culprit. Once you swap the headers, maybe try a shorter or longer reach plug. If the reach is too short, it might be exposed spark plug threads of the cylinder head casting heating up.

56owner
09-12-2011, 05:07 PM
I agree with crankshafts reply. Make sure the seconday carb linkage is adjusted to close the butterflies completely and turn the idle screws all the way in so no idle fuel goes thru secondary carb. I think you are getting fuel thru the secondary carb to cause the run on.



:sign0020:

tmoble
09-13-2011, 12:16 AM
turn the idle speed down 50 RPM, see if it helps. if it does try 50 more.