Topic: BB detonation '70
in Forum: C3 Engines
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Houston, TX - USA
Joined: 2/7/2008
Posts: 93
Vette(s): 1970 454 4 speed A/C power everything else
My motor (70 454) is stock except for a mild cam upgrade, Comp 286 extreme energy. When cold it runs fine with no detonation, but as it warms to operating temperature it will detonate on slight acceleration through the gears (4 speed).
The initial timing is unknown, as i cant se the timing pointer and balancer unless I lay across the front of the motor. I set the timing by a vacuum gauge at 15 inches. It is the original distributor with pertronix I.
I was thinking it would help if I could get some colder air into the motor..........or of course retard the timing, maybe a colder plug, I have autolite 42's.
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Lifetime MemberHOWELL, NJ - USA
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Vette(s): 1979, Targa Blue (72 Color), Pace Car rear spoiler, L88 hood, Dark blue factory interior, 525HP 406, HD 700R4, 370 gears,Steeroids, composite rear spring, TT IIs wrapped in T/A Radials.
Sounds like timing. Without thowing a light on it, you'll never know...
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Duncanville, TX - USA
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Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
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#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
or questionable fuel quality...
Back the timing down a hair at a time until the detonation goes away. Does it start easily when hot?
Back the timing down a hair at a time until the detonation goes away. Does it start easily when hot?
Joel Adams
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Former Member
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Houston, TX - USA
Joined: 2/7/2008
Posts: 93
Vette(s): 1970 454 4 speed A/C power everything else
Yes, it starts fine when hot.......I buy Exxon 93...........I added two bottles of octane boost (Lucas) with no difference.
I really cant see the timing pointer without laying across the fender. It has power steering and a tensioner pulley right in the way.
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Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
You may have a vacuum advance can that isn't working properly, or an EGR valve issue. Either could cause a slight ping under a light acceleration load, without effecting the hot start. Mechanical timing that is too far advanced will cause the ping, but it also normally makes them hard to start("kickback") when hot.
You may need to test the vacuum for the EGR and see if it is working right. Do you have a service manual for your car?
You may need to test the vacuum for the EGR and see if it is working right. Do you have a service manual for your car?
Joel Adams
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Former Member
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Houston, TX - USA
Joined: 2/7/2008
Posts: 93
Vette(s): 1970 454 4 speed A/C power everything else
I dont have an EGR. The distributor has been recurved with a new vacuum can, and i dont have a service manual. Im going see if I can turn the motor until its on number one and look at the timing mark.
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Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
[QUOTE=Fairlane514]
The distributor has been recurved with a new vacuum can, [/QUOTE]
Did you have this pinging problem before that was done? Or, was the distributor worked on because of the pinging?
Adams' Apple 2009-11-20 11:36:01
The distributor has been recurved with a new vacuum can, [/QUOTE]
Did you have this pinging problem before that was done? Or, was the distributor worked on because of the pinging?
Joel Adams
C3VR Lifetime Member #56
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(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Former Member
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Houston, TX - USA
Joined: 2/7/2008
Posts: 93
Vette(s): 1970 454 4 speed A/C power everything else
I had the recurve because I couldn't get it to idle without it eventually stalling out after about a minute of idling. It seemed to lose timing the longer it idled and the vacuum advance didnt compensate. I thought maybe the original vacuum can wasn't responding to 15 inches of vacuum so I had a guy (recommended) recurve and find a can that would respond to 13 inches of vacuum at idle.
I think now after all the troubleshooting and carburetor issues and pertronix, it was not idling well because of carb loading up.
As far as I know total timing is in at about 2500 rpm.
I guess what you're thinking is too much timing too soon?
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Houston, TX - USA
Joined: 2/7/2008
Posts: 93
Vette(s): 1970 454 4 speed A/C power everything else
OK........I was finally able to check the timing with a light risking a new haircut in the process. With the vacuum advance disconnected at about 900 rpm I was at 25 advanced according to my dial back light. I dropped the timing to 15...............that should help. I think I will also leave the vacuum advance disconnected.
As far as total timing, I dont know (it will just have to be whatever it is)........I can barely see the zero mark on my pointer
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Wow...25@ idle is waaaaay up there! 15 will be better, for sure, but it may still be too much. It may not run very well with the vac disconnected, tho...might have a little hesitation at cruising speed when you get on it. Let us know how it works with it at 15. 
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
in Forum: C3 Engines
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