Can a carburetor that's jetted too lean contribute to pre-detonation at wide open throttle?
Could it be related to my vacuum advance?

Initial timing is not completely stable. Apparently my advance weights/springs allow mechanical advance to start below 800 rpm. This causes a slightly irregular idle.

Initial timing is ~16* at ~850rpm idle.
Mechanical advance is 34* by 2500-3000 rpm.
TricKFlow 23* aluminum heads.
~10.5:1 compression.
91+ octain gas used exclusively.

 

Are you running a vaccum advance and if so what is the total advance of the unit?

 

The vacuum advance unit was included in the SVE KwikCurve Kit that I purhased from Lars and installed back in 2001.
http://forums.corvetteforum.com/revi...7&cat=2&page=4
I do not recall the specs. I suppose I could figure it out using the timing light.

 

If your idle advance is 16 without the vac plugged in. Check with a vacuum gauge and see if you have at least 14 inches of vac. Most vac. advance diaphrams only require 14 inches to pull the arm to max advance. So plug it in and see what your timing jumps to and it will you tell you how much advance your getting.

But I can tell you now that your barking up the wrong tree. You said: WOT. WOT should create very little vacuum.

You can try a couple of things. retard the initial by two degrees at a time and do WOT until the detonation stops.

recourve the distributer for less mechanical advance. Like 14 mech and run the initial at 18 for a total of 32*

Recourve the distributer for higher rpm max mech advance. I have mine on all in at 3200 rpm.

 

If you're running an EGR and it is stuck open that could cause a lean condition as well.

 

EGR shouldn't alter the fuel/air mixture at all. EGR dilutes the incoming fuel/air charge by approximately 7%. Sorta like fooling the engine into thinking it doesn't have quite as much compression as it really does. This lowers NOX emissions by lowering combustion temperatures. At idle EGR should be OFF. At WOT EGR should be OFF. EGR is only ON during cruise conditions. A leaking or stuck open EGR valve will REDUCE the chances of detonation at WOT.

Vacuum advance should be hooked up to ported vacuum not manifold vacuum. With ported vacuum there is no vacuum signal at idle. At WOT there is very little vacuum anywhere but the venturis so there should be no vacuum advance. Vacuum advance is sorta like EGR, it's only there at cruising speeds.

You need to get the problem with the mechanical advance fixed. Mechanical advance should begin somewhere around 1200 to 1500rpm. Depending on the quality of fuel you use it should be all in somewhere between 2500 and 3000rpm. Your initial advance should be between 12 and 18 degrees BTDC.

As far as the carburetor mixture goes richen the secondaries up a little and see if it helps. If this is a Q-jet that will be easy and quick.

Maybe Lars will look in on this one and give some instruction.

Hope this helps.

BigBlockk

Later.....

 

Thanks for the replies.
While I have some time left in the day I'm going to go out and lower my timing a couple of degrees.
-I'm running an Edelbrock Quadrajet carb.
-I'm using the ported vacuum source for my vac advance.
-My EGR valve is blocked off.

My mechanical advance needs work. The weights have a sloppy fit on the pins.

 

Ok, here is the ignition advance data that I just collected using my digital timing light. Please provide feedback.

I went ahead and reduced the inital to 14* to start off.

RPM / degree of timing*
850/14
1200/23
1400/28
1600/32
1800 thru 2800 / 33
3000/32 (oddly, it drobbed back 1 degree from 3000rpm and up!?!)

With vacuum advance connected @ 2000 rpm / 48*
Vacuum at idle is 13.5 in. Hg.

My carburetor is an Edelbrock Q-jet #1904
Please clarify... Even if this isn't the case with my engine, can a lean mixture contribute to pre-detonation?

EDIT to add vac @ idle

 

Yes it could be a lean mixture contributing to detonation.

If it is detonating at WOT above 3000 rpm, the problem is not in the curve. At WOT above 3000 RPM, you are not using vacuum advance or the curve either as all the centrifugal advance is already "in" and there is no vacuum advance at that point. You do not have much advance at 33 degrees so I would look elsewhere for the culprit.

The idle stability will be likely be improved by going up one step higher in tension on one of the centrifugal advance springs. As was said before, you want the centrifugal curve to start roughly 300-400 RPM above your idle speed in order to prevent the unstable "idling on the curve" situation.

Try richening up the fuel and see if it helps.

-Mark.

 

Good point!

 

try a rod step richer on the secondarys. Plug heat range could also be off. The tips can glow at high heat and temp of WOT causing detonation.

 

I'll shop for some jets, rods, etc. at carbsunlimited.com tonight.

It could be the plugs, however, I've been running this same set of plugs ever since I first installed the engine but it hasn't always cattered under WOT. But, it's been doing it for a good while. Maybe the "super unleaded" gas at my regular station isn't so "SUPER" anymore.
My car REALLY COMPLAINS on regular or mid-grade.
Used to run real strong, it's not exactly weak but not like it used to be.

 

Hey sting you said only one spring,could you go a little deeper with that,cause I'm having the same problem

 

I will send you a PM.

 

I'm tired of messing with my old distributor so I ordered a new one. . .

Crane - Street/Race Distributor with electronic advance curves.
http://www.cranecams.com/?show=promo&id=16

HI-6 Performance CD Ignition
http://www.cranecams.com/index.php?s...&lvl=5&prt=130

LX92 Lightweight E-Core Coil
http://www.cranecams.com/index.php?s...&lvl=4&prt=168

 

Great choices! Mike I went with the PS92 for what ever reason. They are fairly heavy.


http://www.cranecams.com/index.php?s...&lvl=4&prt=166


In front of the wipers so it doesn't get RF from the plug wires and it's out in cool air away from the engine. 4 years without even one hickup.

 

I had seen pics of your ignition box mounted in the wiper well. So, I planned on doing the same.

 

How much is the dist? I'd like to see a print out of the advance curves.

 

The distributor was $294 shipped from www.northernautoparts.com
Crane is also offering a $25 rebate thru the end of March.

 

Here's a link to an article from Super Chevy Magazine that has more information about the Crane digital distributor.
http://www.superchevy-web.com/techni.../0411sc_crane/