Strange Pinging, incorrect vacumm advance?? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

grayw0lf · 06-25-2004, 12:43 AM

Hey Guys,

I've been hearing a strange pinging while driving around my '70 C-10. I have suspected it to be detonation, as the previous owner set the timing strictly by ear. I disconnected the vacuum advance & plugged the hose at the carb side.

(Should I have plugged it at the distributor side as well?) I don't know the exact timing for a 305 from an 84 Camaro w/ HEI, but I figure its between 8 - 12 degress BTDC. I set it to about 10.5 btdc. I reconnected the vacumm advance & it immediately went to about 25 degrees (possibly more) btdc at idle in Park.

Is this normal, or could this be my culprit?

Thanks,
John
P.S., I swear to the Lord above that it had a 305 when I bought it!

Jeepster376 · 06-25-2004, 02:45 AM

That is normal when connecting to a full manifold vacuum port. Technically it's supposed to be connected to the timed vacuum port on the carburator,(no advance at idle), but either way doesn't make much difference as long as your Idle mix and RPMs are set accordingly.

The vaccum advance increases timing advance during part throttle (high Vac) and reduces advance during high load (low Vac).

If your pinging occurs during part throttle driving you have too much vacuum advance. If it pings under heavy throttle, you have too much initial timing advance.

jimfulco · 06-25-2004, 04:40 AM

Got EGR? If the EGR valve is disconnected or removed from an engine that has a carb calibrated for EGR use, it will tend to ping at part-throttle under load.

grayw0lf · 06-25-2004, 11:39 AM

It does seem to happen more when warm, in drive when stopped at a light or under light acceleration.

My EGR is blocked off. The carb is a Holley 4175, i don't think it has any EGR stuff to it.

Suggestion?

68speedalert · 06-25-2004, 11:54 AM

Spark knock at idle isn't usually spark knock. under light accel could be the EGR, not at idle. The '84 Camaro's had all eletronic advance, so base timing per spec is worthless. For grins, take off the distrib. cap and cheap the mechaical advance by grabbing the rotor and turn it counterclockwise. It should turn a few inches and then spring back when released.

BTW, jimfulco is dead on.

Steve

Steve

grayw0lf · 06-25-2004, 09:47 PM

Thanks guys for all the good info.

So here is the next question...

If it is my carb being tuned for an EGR which is no longer present... What can I do w/ my carb or how can I tune it to work otherwise?

Thanks,

John
Holley 4175 series, model 0-9923 in specific. 650 cfm spreadbore, 4 bbl on a 305.

06-25-2004, 12:43 AM	#1
grayw0lf Registered User Join Date: Apr 2004 Location: Clanton, AL Posts: 1,253	Strange Pinging, incorrect vacumm advance?? Hey Guys, I've been hearing a strange pinging while driving around my '70 C-10. I have suspected it to be detonation, as the previous owner set the timing strictly by ear. I disconnected the vacuum advance & plugged the hose at the carb side. (Should I have plugged it at the distributor side as well?) I don't know the exact timing for a 305 from an 84 Camaro w/ HEI, but I figure its between 8 - 12 degress BTDC. I set it to about 10.5 btdc. I reconnected the vacumm advance & it immediately went to about 25 degrees (possibly more) btdc at idle in Park. Is this normal, or could this be my culprit? Thanks, John P.S., I swear to the Lord above that it had a 305 when I bought it! __________________ 85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal!

06-25-2004, 02:45 AM	#2
Jeepster376 Registered User Join Date: Feb 2004 Location: Massachusetts Posts: 269	That is normal when connecting to a full manifold vacuum port. Technically it's supposed to be connected to the timed vacuum port on the carburator,(no advance at idle), but either way doesn't make much difference as long as your Idle mix and RPMs are set accordingly. The vaccum advance increases timing advance during part throttle (high Vac) and reduces advance during high load (low Vac). If your pinging occurs during part throttle driving you have too much vacuum advance. If it pings under heavy throttle, you have too much initial timing advance. __________________ 72 Cheyenne Super 4x4 SWB

06-25-2004, 11:39 AM	#4
grayw0lf Registered User Join Date: Apr 2004 Location: Clanton, AL Posts: 1,253	It does seem to happen more when warm, in drive when stopped at a light or under light acceleration. My EGR is blocked off. The carb is a Holley 4175, i don't think it has any EGR stuff to it. Suggestion? __________________ 85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal!

06-25-2004, 11:54 AM	#5
68speedalert Semi-Senior Member Join Date: Feb 2003 Location: Washington, MO Posts: 390	Spark knock at idle isn't usually spark knock. under light accel could be the EGR, not at idle. The '84 Camaro's had all eletronic advance, so base timing per spec is worthless. For grins, take off the distrib. cap and cheap the mechaical advance by grabbing the rotor and turn it counterclockwise. It should turn a few inches and then spring back when released. BTW, jimfulco is dead on. Steve Steve __________________ '68 50th Anniversery LWB Custom Fleetside, '77 305 v-8, turbo 350, factory speed alert (still works), '71 drivetrain w/front discs. Some call it ugly yellow, others call it Ochre '83 Jeep CJ-7, <80,000 miles, original owner '04 Chevy Suburban, 4x4, 2500, 6.0 ltr, Predator Diablo programmer '95 GMC K1500 Extended cab

06-25-2004, 09:47 PM	#6
grayw0lf Registered User Join Date: Apr 2004 Location: Clanton, AL Posts: 1,253	Thanks guys for all the good info. So here is the next question... If it is my carb being tuned for an EGR which is no longer present... What can I do w/ my carb or how can I tune it to work otherwise? Thanks, John Holley 4175 series, model 0-9923 in specific. 650 cfm spreadbore, 4 bbl on a 305. __________________ 85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal!

06-25-2004, 04:40 AM	#3
jimfulco Registered User Join Date: May 2003 Location: Shreveport LA Posts: 3,170	Got EGR? If the EGR valve is disconnected or removed from an engine that has a carb calibrated for EGR use, it will tend to ping at part-throttle under load.



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