detonation - Team Camaro Tech
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post #1 of 23 (permalink) Old Oct 7th, 01, 11:02 AM Thread Starter
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i have a 69 302 new cam, carb and dist. im gettind a detonation when i back off the gas no matter what i do i cant seem to get rid of it. it runs strong all the way through the rpm range but when i back off the gas it sounds like hell. could it be valves or springs?
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post #2 of 23 (permalink) Old Oct 7th, 01, 11:37 AM
 
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What is your initial timing? total timing with mech advance added? what is total with dist vacuum added in?

What kind of distributor (points / electronic) - do you have an adjustable vacuum can on the dist?



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post #3 of 23 (permalink) Old Oct 7th, 01, 11:47 AM Thread Starter
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its a mallory electronic, new. it runs the best at 20 deg. i know that it sounds weird but there isnt much play. it has a vac advance on the dist. it goes to 24( i think) whan i plug in the vac advance. the mech advance goes to 34 when rev.
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post #4 of 23 (permalink) Old Oct 7th, 01, 01:22 PM
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Are you sure it's not back pressure rattling your exhaust or something? That mallory should have adjustable mechanical advance but I think it's set at 24 from the factory. If you have it set at 20 initial that would be 44 total. It the mallory new or used? What port is the vacuum advance hooked to? Is it adjustable?

Maybe your mechanical advance isn't releasing when you take your foot out of it!! Double check that everything moves freely and take a reading at idle with the vacuum disconnected. Then take the total timing (where ever it stops at when you rev it). Subtract the initial from the total to verify how much mechanical is built into the dist.

Your 302 should like no more than 32deg total on pump gas. You also don't want too much vacuum advance 6-8 max or you will have part throttle ping...
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post #5 of 23 (permalink) Old Oct 7th, 01, 01:40 PM Thread Starter
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it did the same thing with the exhust off. its a new mallory, and yes its adj. the vac is hooked to the top port of the dist. when on the bottom the idle doesnt want to come down to norm. ive been dealing with this for a month now. i gave some thought to weather the springs were to weak ill check the total again tomorrow. the mech springs are under the plate. it seems to rotate correctly.
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post #6 of 23 (permalink) Old Oct 8th, 01, 06:56 AM Thread Starter
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well, i rechecked the timing


at 900 rpm vac disconnected and plugged
timing at 20 deg.

with vac connected at 900 rpm the timing is at 35 deg

at 1500 rpm the timing goes above 60 deg
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post #7 of 23 (permalink) Old Oct 8th, 01, 08:15 AM
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YaowwwwZAAA!!! You have 15+ degs of vacuum advance from a manifold source. Add in the 24 degs mechanical built into the dist and an initial setting of 20 degrees and it's gonna be around 60 at part throttle. You'll drop the vacuum at WOT for about 40 total... Your rod bearings must be getting hammered!!!

Set your initial to 10 degrees (should equate to 34 total) with the vacuum disconnected. and go for a drive.... Don't hook the vacuum back up unless you can adjust it to 6-8 degs at the most!!!

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post #8 of 23 (permalink) Old Oct 8th, 01, 09:43 AM
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He says it only detonates on DECELERATION not acceleration. I've never heard of that.

Are you sure it isn't excessively rich causing backfiring in the pipes on decel?

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post #9 of 23 (permalink) Old Oct 8th, 01, 09:59 AM
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I agree Jody... But he said it did it without pipes also!! I can't imagine a 302 not detonating with 40+ degs total advance on pump gas though. Throw in the vacuum at part throttle and you have the makings for disaster... Maybe damage has already been done and what he is hearing when he takes his foot off it is rod knock or other internal rattling...

beattie z28 does it make the same sound when in neutral and you rev it and let is idle back down? What cam did you install? New lifters and pushrods to go with? How about the rocker/valve adjustment?

I'd start by geting the timing under control first...
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post #10 of 23 (permalink) Old Oct 8th, 01, 11:15 AM Thread Starter
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its a comp cam replacement for the 3030. i replaced the lifters, rockersand pushrods. when i rev it inneutral it detonates. im not sure its a backfire. its more like poping now that i have an exhust. but it only happens while backing off fast.
i put 68 jets in the new holly and i replaced the plugs before i took it out yesterday. thay had the right color R45s
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post #11 of 23 (permalink) Old Oct 8th, 01, 04:35 PM
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Does it have the A.I.R. system still on it? When the diaphragm in the diverter valve fails (or isn't getting vacuum), it dumps air into the manifolds all the time, which causes constant backfiring on run-down. If that's not it, I'm e-mailing you a procedure for adjusting the valves on a 30-30 cam (which requires a different process than any other solid-lifter cam). Set them per the process, set your total timing (no vacuum connected) at 32 degrees (ignore whatever the base timing turns out to be), and let us know how it runs.

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post #12 of 23 (permalink) Old Oct 8th, 01, 05:05 PM
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Careful with the valve adjustment!! John - if it's a comp cam from their "Nostaliga Plus Series" from what I remember the lash is .012" not .030" like a GM cam... beattie z28 you need to check your cam card!!
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post #13 of 23 (permalink) Old Oct 8th, 01, 09:28 PM Thread Starter
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the cam is called the factory muscle. 26-28 on the adj. the main dif between the cams is lobe sep 114 on the factory and 110 on the the replacement. wow, you guys are persistant! i took off the vac line that goes from the carb to the air pump in my state of confused desperation 2 months ago, and never hooked it back up. im going to hook it back up when i get home today and check it out. the car is from calif. and they reinstalled the air system with the headers.
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post #14 of 23 (permalink) Old Oct 9th, 01, 05:26 AM
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Just trying to help!!
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post #15 of 23 (permalink) Old Oct 9th, 01, 09:42 AM Thread Starter
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well, well, well... i hooked up the smog line to the carb and took it for a ride. about half the detonation was there. so i removed the check valves and plugged them with iron plugs. not one hint of detonation!
but a lack of power. went back plugged the vac advance turned up the idle and set the timing at 32, reidle and left the vac plugged. it launchs at take off, shifts smoother and feels like 50 more hp.
i cant thank you guys enough. i came within minutes of tearing this motor apart.
thanks again, george
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