[JG700]

Hello. Recently (100 miles) rebuilt 350 sb bored 30 over, mild cam. Auto tranny. Car runs great, except it pings upon hard acceleration (like from a dead stop.) Sounds like a lot of marbles rolling around, small popping sounds that eventually stop after the car gets up to speed.I had the q-jet rebuilt also. I tried retarding the timing from 12btc to 8 btc, and had the carb jets increased from 72 to 73 because it was running very lean. Still pings/pops. What else can I do? Would further increasing the jets help, or adjusting the timing back again? Any ideas/suggestions would be greatly appreciated! JG700

[Blazin72]

What compression ratio does your engine have and what gas are you burning?
Do you have the correct timing tab for the balancer you are using?

[JG700]

No idea about compression ratio (compression check showed 150 per cylinder, if that means anything.) Using 93 octane hightest. Timing tab is correct.

[DHMag]

switch to a colder spark plug, but be assured your ignition system is up to par first.

richening the A/F will also decrease pinging/detonation, as well as retarding timing even more.

[Cetrov]

I took this from another site


Detonation can be prevented by:

The use of a fuel with higher octane rating

The addition of octane-increasing "lead", methylcyclopentadienyl manganese tricarbonyl (MMT), isooctane, or other antiknock agents.
Increasing the amount of fuel injected/inducted (resulting in lower Air to Fuel Ratio)

Reduction of cylinder pressure by increasing the engine revolutions (lower gear), decreasing the manifold pressure (throttle opening) or reducing the load on the engine, or any combination.

Reduction of charge (in-cylinder) temperatures (such as through cooling, water injection or compression ratio reduction).

Retardation of spark plug ignition.

Improved combustion chamber design that concentrates mixture near the spark plug and generates high turbulence to promote fast even burning.

Use of a spark plug of colder heat range in cases where the spark plug insulator has become a source of pre-ignition leading to detonation.

from wikipedia

[jimfulco]

Any chance the distributor weights are sticking in the advanced (outward) position? Happens a lot on older HEI's.

Is there a check valve in the line to the vacuum advance, and if so, it it turned around backwards? I saw this on a friend's car after some yoyo mechanic shop replaced the motor and plumbed the vacuum lines their own special way.

[techinspector1]

I strongly suggest that you do SOMETHING. The rattling you hear is detonation and will ultimately destroy the pistons, followed shortly by everything else.

I suspect the rebuilder pistons that were installed when you had the block bored are the culprits of the whole mess. Oversize pistons are manufactured with a reduced compression height (distance from the centerline of the wrist pin to the crown of the piston) to compensate for the machine shop cutting the deck of the block. If the deck is not cut (suspecting yours wasn't), then the pistons are too far down in the hole at TDC to make an effective squish (or quench) area between the piston crown and the underside of the cylinder head. If the squish were like it is supposed to be (0.035" to 0.040"), this motor wouldn't be rattling at 150 psi, or 160 or 170.

The reason it rattles more at low rpm's is that the pistons/valves are moving too slow to cause turbulence of the mixture, which helps to prevent detonation. At higher r's, the mixture is more turbulent and you don't hear detonation as much.

[JG700]

If that's the case, what's next? Any solutions other than removing the engine and sending it back to the machine shop?

[baddbob]

Use some AC spark plugs in the 43 or 42 heat range, I bet that'll help the problem. You also might be able to switch to steel shim head gaskets if the piston to deck height is .020ish

[JG700]

OK. Retarded timing until the pinging virtually stopped under load. I'm now set at 0*. Initial timing was 12 btc. Is this a problem? Is there such a thing as too retarded (and I don't mean ME!) Or should I use colder plugs and increase the timing? Currently using AC Delco R45ts. Thanks!

[techinspector1]

JG, both DHMag and baddbob suggested changing to a colder plug. Toddle on down to Autozone and get the colder plugs and gap them properly. If using a traditional distributor with separate coil, gap at 0.035". If HEI, gap at 0.045". Pick up one of those little plug gap thingies they keep by the cash register. Looks like a large coin and has a hole to affix to your key chain. Do that and see what happens. Report back with your findings.

Yes, it is possible to retard the ignition.

The problem is that unless you've verified TDC on the outer ring of the harmonic damper where it lines up with the pointer on the front cover, then you are flying blind. Oftentimes, the ring slips on the elastomeric material that is placed between the damper outer ring and the inner hub. If that has happened and you haven't verified the true TDC, then any further attempts to time the motor with a light will prove futile.

[speedydude]

Just my .02.... I agree with Mr. Tech and my gut instinct is that your balancer is off a little. When you can retard back to 0 DEG and your pre-ignition goes away........my first place to look, if it were my engine, is establishing true TDC.

You could try the afore-mentioned stuff but I would want to be sure my timing marks are dead on. Check the weights on the advance and make sure they spring back freely.

I hate to even ask this but you do have the vacuum advance disconnected and plugged when checking the timing right??

[perrib]

Ok no more pinging. Is your total timing now within specs? Then everything should be ok

[JG700]

Switched to a colder plug (r45ts - r44ts). Checked balancer - at 0* tdc#1. With colder plugs, very little pinging (only on really hard acceleration/load.) Yes, timing checked with vacuum plugged, but thanks for reminding me! Should I go colder? Move the timing back a bit more? Or is it ok to have a bit of pinging? Thanks for all of the advice!

[techinspector1]

It is NOT ok to have a bit of pinging.