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  #16 (permalink)  
Old 07-12-2007, 06:53 PM
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Originally Posted by JG700
Hello. Recently (100 miles) rebuilt 350 sb bored 30 over, mild cam. Auto tranny. Car runs great, except it pings upon hard acceleration (like from a dead stop.) Sounds like a lot of marbles rolling around, small popping sounds that eventually stop after the car gets up to speed.I had the q-jet rebuilt also. I tried retarding the timing from 12btc to 8 btc, and had the carb jets increased from 72 to 73 because it was running very lean. Still pings/pops. What else can I do? Would further increasing the jets help, or adjusting the timing back again? Any ideas/suggestions would be greatly appreciated! JG700
I read thru all these responses and what you're doing and am left with questions leading to more definition.

- The car is a what?
- How heavy?
- What is the transmission (besides being and automatic)?
- What is the rear axle ratio?
- What is the tire size?
- Do you have the cam specs?
- Do you know if the cam gears and chain were timed correctly?
- Is the cam installed with and advance bushing?
- Do you know how much total advance the distributor has, base plus variable which is the centrifugal and or vacuum)?
- Do you know how fast the variable comes in as well as how much?
- Do you know if the original valves were reground or replaced (I'm assuming this is a rebuilt 350 replacing a 350)?
- do you really know the static compression ratio (swept volume and combustion space volume/combustion space volume)?
- Is the carb the same as original to the engine?
- You've changed jet sizes, what about metering rods?
- Which spring is controlling the metering piston that moves the metering rods?
- Did you change the intake?
- Does the intake have a vacuum leak?
- What is the idle Vacuum?
- What is the vacuum under acceleration?

This breaks down into:

- Not enough gears for the weight of vehicle.
- Cam not timed correctly.
- Static compression too high, cylinder pressure measurements aren't a good indicator of this.
- The engine running lean:
- Vacuum leak
- Carb jets too small; metering rods too big; metering piston stuck, metering piston's spring not in synch with engine's vacuum.
- Secondary not coming in properly, there's cams for this and tuning packages for the air valve.
- Fuel pump not delivering enough pressure.
- Float not adjusted correctly.
- The plastic anti-slosh gadget above the needle valve left out.
- Plugged filter, pinched line.
- Too much or improper advance rate:
- Cam and crank not aligned properly.
- Cam too advanced with bushings or off set crank Woodruff key in gear.
- Ignition advance problems:
- The vacuum doesn't drop off fast enough as the centrifugal
comes up.
- The Damper and Timing marks on the timing case cover are
mismatched. Different years and models move these around so
just grabbing parts from here and there can get you into problems.
- The distributor just not installed correctly, or not wired correctly.

- Problems in the combustion chamber.
- Reground valves have had the margin ground too far making them run
hot on the remaining sharp edge.
- Spark Plug heat range too hot.
- Incorrect head gasket or position of gasket:
- Causing improper coolant flow.
- Combustion ring hanging in a chamber and heated to glowing.
- Too thin combining with decked block/milled heads to make
compression too high.
- SMOG open chamber head that when combined with dish piston
can't provide adequate squish or quench.

As you can see, you're just barely touching the possibilities. I can go further myself but need to get on with other things this afternoon, but this ought to be enough for you to cogitate on for a while.

Having to push the timing back speaks pretty loudly to distributor or cam setup issues. If the cam is too advanced you have to compensate by backing the timing down. You need to really know where the cam is relative to the crank. And I don't mean looking a factory alignment dots. You've got to get a degree wheel and TDC finder to know for sure. This isn't a 100% thing maybe not even a 50% thing, but in my so called mind, it's a strong indicator as a place to start looking.


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  #17 (permalink)  
Old 07-12-2007, 10:37 PM
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Put in R43 plugs, 42's will also work just fine but they'll take awhile to warm up when first starting the engine cold. I've got a 350 running pretty lean with poor (.065) quench distance and it likes the 42's.
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Old 07-14-2007, 05:08 PM
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OK, so here is the update: I rejetted my carb from 74 to 77; replaced the distributor springs with thicker ones; set initial timing to 10*(from 12*); replaced spark plugs with colder ones (r44ts). Engine STILL knocks/pings under load. Could I have installed the distributor wrong? As in, off by a tooth? It idles great, and runs smooth under easy acceleration. Could it be the EGR valve? Confounded and frustrated now! Thanks. JG700
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Old 07-15-2007, 12:38 AM
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What heads are on the motor? I believe the R44TS? is a longer reach plug (for the later design heads) than an R44T and if you're using the wrong length plug the exposed threads could be glowing hot and causing the spark knock-possible. Check it out.
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Old 07-15-2007, 10:06 AM
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Car had r45ts, so plugs are virtually the same. New info: I checked the spec sheet from my engine rebuild: they replaced my stock cam with a "mild" cam with the following specs: 214-224 443-465. Is this appropriate for my set up? (1980 Corvette L-82 SB350. Q-Jet, auto tranny, stock heads,new valves 1.94 intake, 1.50 exhaust, 3.07 rear.)
Again, I'm having trouble with detonation and have to turn the timing way back (from 12* to around 6*) to minimize it. Could the problem be this cam set-up? Thanks! JG700
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Old 07-15-2007, 12:42 PM
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Another find: rebuilder installed 1.94 intake/1.50 exhaust valves (new). L-82 version of the 350 (which is what I have) had 2.02 intake/1.60 exhaust valves. Could this be the cause of the detonation/pinging problem?
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Old 07-21-2007, 10:26 AM
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Good news! After getting tdc on #1 timing marks lined up, but rotor was past #1 tower. I reset it to line up, reassembled, started the car, and NO PING! A bit sluggish off the start, but set at 12btc, whereas before I had to turned it back to 5* just to minimize the pinging. Don't know how that happened, unless I installed it off the first time. Thanks for the help!
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