pinging when hot

[Ben Garrett]

Hi,  I have an 89 Silverado with a HT383 engine and a 750cfm carb on top.  The engine is pinging under acceleration but only once it warms up.  I have checked the ignition timing and that is good.  To begin with it only did it under heavy load and it does under any acceleration.  Any advice?

[Doug_Scott]

In the area I live our fuel will change with the seasons.  We are now getting our winter fuel that has additives added to help with freezing temps.  I realize you are from the other side of equator and your seasons are 180 off of ours.  Also, you mention 95 octane, what measuring system do you use?  Here we use MON+RON/2 to calculate octane.  Highest we get is 93 at public pumps.  

In order to properly time an engine you need to know what the total advance the distributer is setup to provide.  Having the vacuum advance connected to manifold vacuum will actually retard the timing when vacuum drops(stepping on gas for example), whereas ported vacuum will advance the timing when stepping on gas. Mechanical timing advances with engine rpm up to the limits built into the mechanism.  Do you have the specs for the distributor?  Specifically the amount of mechanical advance and at what rpm is it all in.  Are you setting base timing with or without vacuum advance connected? 

[Jsdirt]

I can't add anything to Doug's post.

 

I was going to ask, has this been ongoing, or did it just start out of the blue? If it's been ongoing, my best guess would be too lean of carb jetting.

[Ben Garrett]

It started out of the blue. I first noticed when i had my camper on the back and accelerated quite hard and now it does it under moderate acceleration with no weight. I have had it running sweet with the current carb. Operating temp is normal. I bought the motor brand new from crate engine depot and i set the timing to the specs in the manual. There is currently on vacuum advance connected. The current carb is a 670cfm (not 750 as previously stated) street avenger Holley with vacuum secondarys. It has been running without fault woth this setup.  From memory the compression is 9.1 and i always run it on 95 octane. If it is carbon build up how would I correct this and how do tou stop it from occurring. I do give it a good blast every so often. Would love to drive i hard all the time but our petrol prices in NZ are really high (equivalent to US$6 per gallon).

[Jsdirt]

I would try retarding the timing a few degrees at a time - see if you can get it to not ping, yet still make decent power.

 

May have been a fuel change that nobody has mentioned - that has happened here in some states in the past, and caused all kinds of driveability issues for older vehicles.

 

Also could be excessive carbon buildup in the engine for whatever reason, causing an increase in compression.

[diyer2]

Fuel system cleaner.

Easy, cheap.

Sea Foam or BG 44K.

[Ben Garrett]

I have tried retarding the timing. I will give the fuel system cleaner a go. Will have to see what products we have available here in NZ

[Doug_Scott]

2 hours ago, Ben Garrett said:

Hi,  I have an 89 Silverado with a HT383 engine and a 750cfm carb on top.  The engine is pinging under acceleration but only once it warms up.  I have checked the ignition timing and that is good.  To begin with it only did it under heavy load and it does under any acceleration.  Any advice?

There are many more causes of pre-ignition than just timing.  Start with running the highest octane fuel you can find.  Look at the operating temp of the engine, the hotter then engine gets, the more likely to get pre-ignition. Any chance of carbon buildup in the combustion chamber?  What brand carburetor?  Is it running too lean?  Note this means jetting, not idle mixture.  You say you have checked timing, but where did you get the timing spec from?  What forms of advance are you using?  If you are running with vacuum advance connected, is it connected to manifold or ported vacuum source?  How much mechanical Advance are you getting?  What is the timing at 3k rpm? 

What is the compression ratio?