Why and what can I do about the annoying spark knock detonation I have when my a/c is on?

Did do a 140 mile round trip today to the mud bogs in N.C. with the a/c full tilt and that was sure nice riding. But I came home with the air off for fear of damaging the new engine. Just wish I would have heard it on the way there it wasn't till just before I pulled in and the phone rang turned down the stereo and heard it.

This couldn't have been normal back then when new.

 

Check your ignition timing, total advance and base. I have a 76 here at Virginia Beach and have no pinging with the air on or off. The old York compressors do put a load on these engines. I assume your hearing it (knock\detonation) under acceleration, not during normal cruising? Are you running full vacuum advance or partial?

 

am running partial advance the way edlbrock says,pull vac from the passenger side of base plate on carb, and I only know of G.M. using full advance.

YEA now its only at acceleration cause I pulled over and moved the distributor while on the trip but even a few degrees back (retarding) kills the power but looses the knock. Your right I need to start over with timing now that a/c is all hooked up.

Not that the a/c should have changed it but are you suggesting reset timing with a/c on?

Where at the beach?

 

No, setting the timing at base idle without AC on is the way to do it. If your "ratttling" under acceleration then your timing is to far advanced for the rpm at that piont in time. SEVERAL issues can cause this. Are your pistons stock compresion? If not, higher octane is needed. Etc. What size cam are you running? I'd figure RV type based on your mud bug vacation. Is the timing gear set a newer straight up set? Typically, 6-12 degrees advance on these near stock setup's is plenty. Mine pings above 8 degree's with the AC on and under heavy acceleration.

I live by Chimmny Hill.

 

No, setting the timing at base idle without AC on is the way to do it. If your "ratttling" under acceleration then your timing is to far advanced for the rpm at that piont in time. SEVERAL issues can cause this. Are your pistons stock compresion? If not, higher octane is needed. Etc. What size cam are you running? I'd figure RV type based on your mud bog vacation. Is the timing gear set a newer straight up set? Typically, 6-12 degrees advance on these near stock setup's is plenty. Mine pings above 8 degree's with the AC on and under heavy acceleration.

I live by Chimmny Hill.

 

Thanks for your input......on my third tank of gas and hate to hear about stepping up to the higher octane. As far as pistons that was 13 years ago but what I recall standard top pistons with bored out 60 cylinders. For sure the came is just a mild R.V. cam (262?). Long tube L.L. headers with a 750 edelbrock. This timing issue will not get me but what a pain in the rear it seems I have got to move it at different temps. I now thats not right but for example when she's stone cold bammm fire right off ruuning around town/interstate 195 on guage, stop get out to do whatever get back in and it will drag the (high torque) starter right down. Get out move the distributor (retard) just a degree or so and it will fire. Cant be heat from headers cause they are not near the starter I even have a wrap on it. You asked about timming I put the double roller set in and did the straight up timmming. This being I new engine with only a few hundred miles on it am I gonna have a few issues till break in is done I would think not since people dont have these problems with off the lot vehicles.

By the way I was just a spectator at the bog but sure got a lot of pressure to put it in and that aint gonna happen. I guess I have a "mall crawler" as you hear on here from those west coast guys.

Just looked back up to your post and reread that you have some pinging above 8 degrees on acceleration and thats about what I have and thought there is no way someone would accept that when they bought one new. maybe its this sorry crap we have to pay to much for at the pump.

As a side note I own tire stores in the Hampton roads areas and sell new or used tires so if theres something your looking for let me know and you can get it at my cost.

Thanks again
Brian

 

[QUOTE=bigdaddys76;10485339]Thanks for your input......on my third tank of gas and hate to hear about stepping up to the higher octane. As far as pistons that was 13 years ago but what I recall standard top pistons with bored out 60 cylinders.

Brian, this sounds like a fun issue to tackle. If you dont mind, I'd love to look at it and get my butt handed to me or fix it That's how I learn. I love the challenge of the "hard to figure" issues. I'm on travel the next couple of day's. Have tools will travel... Ohh crap, if it get's fixed, I charge 1 12 0z can of diet coke. That is what I charge everybody.

 

Thanks and hopefully we can get together. I decided to get a load of dirt today and turned on the a/c, you know it was only a heat index of 106 out there, and no blower so now I jumped into that feet first so the dash is currently apart. I will be calling on you and will have the diet coke iced down.
Always up for a learning experience as well.

 

I should be back home Sat morn. (hope) Your AC\Blower is not an issue if you have problems with that. Been there done that, mayby 15 times...) PM me if interested in hooking up.


Rich