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Jun 15, 2005 | 10:14 AM

rexx78

Thread Starter

Burning Brakes

Joined: Dec 2002

Posts: 1,069

Likes: 2

From: Sherwood Park AB

Pinging under load

I have some pinging at low rpm's under load. The timing has been set to 36* BTDC at 3000 rpm for a while with no detonation, but I have done several changes to the Q-Jet since then. The last few changes I made were:

1: lowered float level
2: changed power piston spring
3: adjusted idle air screws
4: replaced needle and seat with high flow one

I am going to trouble shoot tonight, I just wanted to get an idea where to start.
I have an adjustable vacuum control unit that I am planning on putting in. This may help right?

Jun 15, 2005 | 10:35 AM

flood

Race Director

Joined: Dec 2000

Posts: 10,896

Likes: 2

From: CO

Whats your setup?

Jun 15, 2005 | 11:04 AM

rexx78

Thread Starter

Burning Brakes

Joined: Dec 2002

Posts: 1,069

Likes: 2

From: Sherwood Park AB

I guess that's important. :o

78 vette with 400 ci, 462 heads, XE 262 cam, performer intake, stock distributor, new ignition components (cap, coil, wires, coil, module, etc.) new mechanical advance with the light springs, 79 L-82 Q-jet with "68" jets and "40" rods (as measured with calipers since they are not stamped). 4 speed and 3.70 rear end.

Jun 15, 2005 | 11:12 AM

gerry72

Safety Car

Joined: Aug 2000

Posts: 3,711

Likes: 43

From: San Antonio TX

You might be a wee bit on the lean side due to lower the float. Float adjustments are a good way to make slight changes, lean or rich, to the A/F ratio.

It could also be your primary spring if it's holding down the piston for your load. If you went to a heavier spring, you should be fine since this provides earlier enrichment.

It too could be your idle mixture adjustment since idle and transition circuits contribute to the total mixture. Not a lot but some.

And it could be nothing more than the conditions you initially tuned it to no longer exist. Higher underhood air temps and such.

Jun 15, 2005 | 11:14 AM

vettfixr

Le Mans Master

Joined: Jul 1999

Posts: 9,206

Likes: 17

From: Sewell NJ

From your description of your mods it looks like the springs in the distributor may be too light making the mechanical advance come in too soon. It could also be a vacuum advance that is pulling in too many degrees of advance. With most performance ignition curves the vacuum can should be replaced with one that gives only about 10 to 12 degrees of advance as opposed to the typical 18 to 22 degrees that a stock can gives. First try heavier springs. If that doesn't cure it try running with the vacuum advance disconnected. If the problem goes away check Lars paper for the correct vacuum advance can.

Jun 15, 2005 | 04:59 PM

NHvette

Le Mans Master

Joined: Jun 2002

Posts: 8,339

Likes: 24

From: I can walk to MA

Well, it sounds like he has the ignition curve pushed to the wall,
which is not a bad thing. Also, sounds like it was working well
for some time - until the carb changes.

So, I'd guess that the most likely culprit would be the lowered
float level - giving a leaner (and hotter burning) mixture.
backing off thye timing a few degrees couldn't hurt. 36 is a bit aggresive.

Jun 15, 2005 | 08:37 PM

stingr69

Le Mans Master

Joined: Mar 2004

Posts: 7,450

Likes: 1,479

From: Little Rock AR

I made a few assumptions and came up with about 11:1 compression with the 262 cam?!? Then you lean it out, with soft advance springs. Sounds like you are probably over the limit on dynamic compression. This one could be difficult to tune out.

Fatten it back up, install some stiffer advance springs and see if it helps. You might want to run the numbers on the DCR as I would be looking there first. I would run it for you but I do not have the cam seat to seat timing numbers.

-Mark.