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Old 11-01-2014, 11:21 AM   #1
joeaxial
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Default Detonation issue '75 500

I've had a 1975 500 installed in a 1980 chevy 1/2 ton 2wd pickup for years. It sat around mostly unused for around 5 years. Last winter I restored the body and interior and started driving it again. I am having problems with excessive detonation or pinging. This occurs at light throttle and at WOT. Overdrive transmission. Engine runs 2200 rpm at 70 mph cruise.

The long block is a stock rebuild with the exception of a cam I purchased from PAW years ago specs unknown. It idles smooth.... Edelbrock intake, stock '75 quadra-jet professional rebuild with throttle shaft bushings, HEI distributor with Pertronix flame thrower module and coil and adjustable advance can. I disassembled distributor and re-lubed advance so that is working properly. Autolite 646 plugs. (I tried AC R43XLS and pinging seemed worse) 180 degree thermostat.

If I use any vacuum advance at all the engine has light detonation at cruise, So I've got the vacuum can disconnected.

It also pings on hard acceleration with the initial timing set at only 4*. when I adjust the timing back far enough not to detonate it has very poor power, idles like crap and the initial timing is less than 0*

I used a piston stop to find TDC and the timing marks are exactly on.

I put timing tape on the balancer after determining the timing marks are correct. The mechanical advance seems to be operating properly.

Today my tank was empty so I filled up with 92 octane and it still knocks on hard acceleration.

This thing is only 8.5:1 so there should be no issue with detonation. The high compression 1968 472 in my other truck does not give any problem.

Any thoughts or ideas? Spark plugs run very white. Tail pipes are clean. (too lean?)
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Old 11-03-2014, 04:57 AM   #2
Schurkey
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1. Has the EGR been disabled? Lack of EGR without tuning the carb and distributor can cause massive ping/detonation.

2. Run engine at ~2000 rpm, fully warmed-up. Drizzle a "urine stream" of plain tap water back and forth between the primary venturis. This will "steam-clean" any carbon from the combustion chambers, WITHOUT creating angry neighbors from using a petroleum-based cleaner such as Seafoam, or GM Top Engine Cleaner. You'll want to change oil afterward, as this tends to put some moisture in the crank case. I will typically drop a couple of gallons of water down the carb--but slowly enough that the engine speed comes down, but without misfire.

3. Overly-lean fuel mixture will cause detonation. Are you SURE the carb is performing properly? Adequate fuel pressure/volume?
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Old 11-03-2014, 11:06 AM   #3
joeaxial
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Hi Schurkey, Thanks a for the advice! I did try a different carb over the weekend. Its an Edelbrock performer 750 that I had sitting on the shelf. This helped A LOT with the detonation issue. Off idle response is not as nice as with the quadrajet, but at WOT there is more power and I heard no detonation. It wasn't pinging at cruise either. I want to stick with the quadrajet, so I guess I'll have to find a "carb guy" to help me out with that. I understand that JET performance will do re-cals on quads. Anyone got other suggestions? I'm in rural ND and the only chassis dyno near me is staffed by diesel engine guys who know very little about tuning a gasoline engine.
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Old 11-03-2014, 12:53 PM   #4
Schurkey
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